Wednesday, August 27, 2008

Homology biology


Homology biology



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In evolutionary biology, homology has come to mean any similarity between characters that is due to their shared ancestry. There are examples in different branches of biology. Anatomical structures that perform the same function in different biological species and evolved from the same structure in some ancestor species are homologous. In genetics, homology can be observed in DNA sequences that code for proteins genes and in noncoding DNA. For protein coding genes, one can compare translated aminoacid sequences of different genes. Sequence homology may also indicate common function. Homologous chromosomes are nonidentical chromosomes that can pair synapse during meiosis, and are believed to share common ancestry.Shared ancestry can be evolutionary or developmental. Evolutionary ancestry means that structures evolved from some structure in a common ancestor for example, the wings of bats and the arms of primates are homologous in this sense. Developmental ancestry means that structures arose from the same tissue in embryonal development the ovaries of female humans and the testicles of male humans are homologous in this sense.Homology is different from analogy. The wings of a maple seed and the wings of an albatross are analogous but not homologous they both allow the organism to travel on the wind, but they didnt both develop from the same structure. This is called homoplasy. But structures can be homologous and analogous. The wings of a bat and a bird are homologous, in that they both developed from the pectoral fins of fish. They are also analogous, in that the forelimbs of the ancestors of birds and of bats developed into organs of a similar new function independently. Thus evolution can be initially divergent, giving rise to homologous structures, and subsequently convergent, causing the structures to become analogous again.Homology among proteins and DNA is often concluded on the basis of sequence similarity, especially in bioinformatics. For example, in general, if two or more genes have highly similar DNA sequences, it is likely that they are homologous. But sequence similarity may arise from different ancestors short sequences may be similar by chance, and sequences may be similar because both were selected to bind to a particular protein, such as a transcription factor. Such sequences are similar but not homologous. Sequence regions that are homologous are also called conserved. This is not to be confused with conservation in amino acid sequences in which the amino acid at a specific position has changed but the physiochemical properties of the amino acid remain unchanged.The phrase percent homology is sometimes used but is incorrect. Percent identity or percent similarity should be used to quantify the similarity between the biomolecule sequences. For two naturally occurring sequences, percent identity is a factual measurement, whereas homology is a hypothesis supported by evidence. One can, however, refer to partial homology where a fraction of the sequences compared are presumed to share descent, while the rest does not. For example, partial homology may result from a gene fusion event.






Chromosomes are organized structures of DNA and proteins that are found in cells. A chromosome is a singular piece of DNA, which contains many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNAbound proteins, which serve to package the DNA and control its functions. The word chromosome comes from the Greek χρῶμα chroma, color and σῶμα soma, body due to their property of being stained very strongly by some dyes.Chromosomes vary extensively between different organisms. The DNA molecule may be circular or linear, and can contain anything from tens of kilobase pairs to hundreds of megabase pairs. Typically eukaryotic cells cells with nuclei have large linear chromosomes and prokaryotic cells cells without defined nuclei have smaller circular chromosomes, although there are many exceptions to this rule. Furthermore, cells may contain more than one type of chromosome for example mitochondria in most eukaryotes and chloroplasts in plants have their own small chromosomes.In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin. This allows the very long DNA molecules to fit into the cell nucleus. The structure of chromosomes and chromatin varies through the cell cycle. Chromosomes may exist as either duplicated or unduplicated—unduplicated chromosomes are single linear strands, while duplicated chromosomes copied during synthesis phase contain two copies joined by a centromere. Compaction of the duplicated chromosomes during mitosis and meiosis results in the classic fourarm structure pictured to the right.Chromosome is a rather loosely defined term. In prokaryotes, a small circular DNA molecule may be called either a plasmid or a small chromosome. These small circular genomes are also found in mitochondria and chloroplasts, reflecting their bacterial origins. The simplest chromosomes are found in viruses these DNA or RNA molecules are short linear or circular chromosomes that often lack any structural proteins.Visual discovery of chromosomes. Textbooks have often said that chromosomes were first observed in plant cells by a Swiss botanist named Karl Wilhelm von Nägeli in . However, this opinion has been challenged, perhaps decisively, by Henry Harris, who has freshly reviewed the primary literature. In his opinion the claim of Nägeli to have seen spore mother cells divide is mistaken, as are some of his interpretations. Harris considers other candidates, especially Wilhelm Hofmeister, whose publications in include plates which definitely show mitotic events. Hofmeister was also the choice of Cyril DarlingtoThe work of other cytologists such as Walther Flemming, Eduard Strasburger, Otto Bütschli, Oskar Hertwig and Carl Rabl should definitely be acknowledged. The use of basophilic aniline dyes was a new technique for effectively staining the chromatin material in the nucleus. Their behavior in animal salamander cells was later described in detail by Walther Flemming, who in provided a superb summary of the state of the field. The name chromosome was invented in by Heinrich von Waldeyer. However, van Benedens monograph of on the fertilised eggs of the parasitic roundworm Ascaris megalocephala was the outstanding work of this period.

Homologous chromosome



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Homologous chromosomes are chromosomes in a biological cell that pair synapse during meiosis, or alternatively, nonidentical chromosomes that contain information for the same biological features and contain the same genes at the same loci but possibly different genetic information, called alleles, at those genes. For example, two chromosomes may have genes encoding eye color, but one may code for brown eyes, the other for blue.Nonhomologous chromosomes representing all the biological features of an organism form a set, and the number of sets in a cell is called ploidy. In diploid organisms most plants and animals, each member of a pair of homologous chromosomes is inherited from a different parent. But polyploid organisms have more than two homologous chromosomes.Homologous chromosomes are similar in length, except for sex chromosomes in several taxa, where the X chromosome is considerably larger than the Y chromosome. These chromosomes share only small regions of homology.Humans have pairs of homologous nonsex chromosomes called autosomes. Each member of a pair is inherited from one of their two parents. In addition, female humans have a homologous pair of sex chromosomes Xs males have an X and a Y chromosome.A sexdetermination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. In many cases, sex determination is genetic males and females have different alleles or even different genes that specify their sexual morphology. In animals, this is often accompanied by chromosomal differences. In other cases, sex is determined by environmental variables such as temperature or social variables the size of an organism relative to other members of its population. The details of some sexdetermination systems are not yet fully understood.Each person normally has one pair of sex chromosomes in each cell. Females have two X chromosomes, while males have one X and one Y chromosome. Both males and females retain one of their mothers X chromosomes, and females retain their second X chromosome from their father. Since the father retains his X chromosome from his mother, a human female has one X chromosome from her paternal grandmother, and one X chromosome from her mother.Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. The X chromosome contains about genes compared to the Y chromosome containing genes, out of the estimated , to , total genes in the human genome. Genetic disorders that are due to mutations in genes on the X chromosome are described as X linked.The X chromosome carries a couple thousand genes but few, if any, of these have anything to do directly with sex determination. Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in nearly all somatic cells cells other than egg and sperm cells. This phenomenon is called Xinactivation or Lyonization, and creates a Barr body. Xinactivation ensures that females, like males, have one functional copy of the X chromosome in each body cell. It was previously assumed that only one copy is actively used. However, recent research suggests that the Barr body may be more biologically active than was previously supposed.





The Xchromosome is a remarkably genepoor region. It is composed primarily of repeated segments of DNA which do not code for proteins or any known function. Only .% of the chromosome encodes for any functional proteins at alllowest density of genes to dateand the genes themselves are very short compared to the length of the average human gene. It is estimated that about % of the genes encoded by the Xchromosome are associated with a family of CT genes, so named because they encode for markers found in both tumor cells in Cancer patients as well as in the human Testis in healthy patients. These CT genes found on the Xchromosome are estimated to account for about % of all the CT genes encoded within the human genome. Due to their relative abundance, it is thus hypothesized that these genes and thus the Xchromosome confer evolutionary fitness to human males. It is theorized by Ross et al and Ohno that the Xchromosome is at least partially derived from the autosomal nonsexrelated genome of other mammals evidenced from interspecies genomic sequence alignments.The Xchromosome is notably larger and has a more active euchromatin region than its Ychromosome counterpart. Further comparison of the X and Y reveal regions of homology between the two. However, the corresponding region in the Y appears far shorter and lacks regions which are conserved in the X throughout primate species, implying a genetic degeneration for Y in that region. Because males have only one xchromosome, they are more likely to have an xchromosome related disease.Chromosomes are organized structures of DNA and proteins that are found in cells. A chromosome is a singular piece of DNA, which contains many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNAbound proteins, which serve to package the DNA and control its functions. The word chromosome comes from the Greek χρῶμα chroma, color and σῶμα soma, body due to their property of being stained very strongly by some dyes.Chromosomes vary extensively between different organisms. The DNA molecule may be circular or linear, and can contain anything from tens of kilobase pairs to hundreds of megabase pairs. Typically eukaryotic cells cells with nuclei have large linear chromosomes and prokaryotic cells cells without defined nuclei have smaller circular chromosomes, although there are many exceptions to this rule. Furthermore, cells may contain more than one type of chromosome for example mitochondria in most eukaryotes and chloroplasts in plants have their own small chromosomes.In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin. This allows the very long DNA molecules to fit into the cell nucleus. The structure of chromosomes and chromatin varies through the cell cycle. Chromosomes may exist as either duplicated or unduplicated—unduplicated chromosomes are single linear strands, while duplicated chromosomes copied during synthesis phase contain two copies joined by a centromere. Compaction of the duplicated chromosomes during mitosis and meiosis results in the classic fourarm structure pictured to the right.Chromosome is a rather loosely defined term. In prokaryotes, a small circular DNA molecule may be called either a plasmid or a small chromosome. These small circular genomes are also found in mitochondria and chloroplasts, reflecting their bacterial origins. The simplest chromosomes are found in viruses these DNA or RNA molecules are short linear or circular chromosomes that often lack any structural proteins.

Homology of structures in evolution

Shared ancestry can be evolutionary or developmental. Evolutionary ancestry means that structures evolved from some structure in a common ancestor for example, the wings of bats and the arms of primates are homologous in this sense. Developmental ancestry means that structures arose from the same tissue in embryonal development the ovaries of female humans and the testicles of male humans are homologous in this sense.Homology is different from analogy. The wings of a maple seed and the wings of an albatross are analogous but not homologous they both allow the organism to travel on the wind, but they didnt both develop from the same structure. This is called homoplasy. But structures can be homologous and analogous. The wings of a bat and a bird are homologous, in that they both developed from the pectoral fins of fish. They are also analogous, in that the forelimbs of the ancestors of birds and of bats developed into organs of a similar new function independently. Thus evolution can be initially divergent, giving rise to homologous structures, and subsequently convergent, causing the structures to become analogous again.A primate is any member of the biological order Primates Latin prime, first rank, the group that contains lemurs, the Ayeaye, lorids, galagos, tarsiers, monkeys, and apes, with the last category including humans. With the exception of humans, which now inhabit every continent on Earth, most primates live in tropical or subtropical regions of the Americas, Africa and Asia. Primates range in size from the g ounce Pygmy Mouse Lemur to the kg pound Gorilla. According to fossil evidence, the primitive ancestors of primates may have existed in the late Cretaceous period around Ma million years ago, and the oldest known primate is the Late Paleocene Plesiadapis, c – Ma. Molecular clock studies suggest that the primate branch is even more ancient, originating in the midCretaceous period around Ma.The Primates order is divided informally into three main groupings prosimians, monkeys of the New World, and monkeys and apes of the Old World. The bodies of the prosimians most closely resemble those of the early protoprimates. The bestknown of the prosimians, the lemurs, are located on the island of Madagascar and to a lesser extent on the Comoros Islands, isolated from the rest of the world. The New World monkeys, which include the familiar capuchin, howler, and squirrel monkeys, live exclusively in the Americas. With the exception of humans, the rest of the Old World monkeys and the apes inhabit Africa and southern and central Asia, although fossil evidence shows many species existed in Europe as well.Primates are adapted for a treedwelling lifestyle. Anatomical adaptations support their reliance on vision, their dominant sensory system, rather than smell, the dominant sensory system in most mammals. In some primates, threecolor vision has developed. Most primates also have opposable thumbs and some have prehensile tails. Many species are sexually dimorphic, in that males and females have different physical traits, including body mass, canine tooth size, and coloration. Primates have slower rates of development than other similarly sized mammals, and reach maturity later but have longer lifespans. A variety of locomotion techniques are used, including leaping from tree to tree, walking on two or four limbs, knucklewalking and swinging between branches of trees known as brachiation. Similarly, primates use a variety of social systems. Some species are solitary, others are monogamous, and others live in groups of up to hundreds of members.

The Primates order lies in a tight clustering of related orders the Euarchontoglires within the Eutheria, a subclass of Mammalia. Recent molecular genetic research on primates, colugos, and treeshrews has shown that the two species of colugos are more closely related to the primates than the treeshrews, even though the treeshrews were at one time considered primates. These three orders make up the Euarchonta clade. This clade combines with the Glires clade made up of the Rodentia and Lagomorpha to form the Euarchontoglires clade. Variously, both Euarchonta and Euarchontoglires are ranked as superorders. Also, some scientists consider Dermoptera a suborder of Primates and call the true primates the suborder Euprimates.In modern, cladistic reckonings, the Primates order is also a true clade. The suborder Strepsirrhini, the wetnosed primates, is generally believed to have split off from the primitive primate line about Ma million years ago, although an earlier date may be possible. The seven strepsirhine families are the four related lemur families and the three remaining families that include the lorises, the Ayeaye, the galagos, and the pottos. Older classification schemes wrap the Lepilemuridae into the Lemuridae and the Galagidae into the Lorisidae, yielding a threetwo family split instead of the fourthree split as presented here. Other lineages of lower primates inhabited Earth. During the Eocene, most of the northern continents were dominated by two groups, the adapids and the omomyids. The former is considered a member of Strepsirrhini, but it does not have a toothcomb like modern lemurs. The latter was related closely to tarsiers, monkeys, and apes. Adapids survived until about Ma. Omomyids, on the other hand, perished about million years earlier.Ringtailed Lemur, a strepsirrhine primateRingtailed Lemur, a strepsirrhine primateThe Ayeaye is difficult to place in Strepsirrhini. Theories have been proposed that its family, Daubentoniidae, is either be a lemuriform primate meaning its ancestors split from lemur line more recently than the lemurs and lorises split or that it is sister to all of the other strepsirrhines. Research confirms that the Ayeaye family, Daubentoniidae, is a lemuriform.The suborder Haplorrhini, the drynosed primates, is composed of two sister clades. The prosimian tarsiers in family Tarsiidae monotypic in its own infraorder Tarsiiformes, represent the most primitive division at about Ma. The Simiiformes infraorder emerged about Ma, and contains the two clades the parvorder Platyrrhini that developed in South America and contains New World monkeys is one, and the parvorder Catarrhini that developed in Africa and contains the Old World monkeys, humans and the other apes in the other. A third clade, which included the eosimiids, developed in Asia but went extinct millions of years ago.A few new species are discovered each year, and the evaluation of current populations as distinct species is in flux. Biological anthropologist Colin Groves listed about species of primates in Primate Taxonomy in . The recently published third edition of Mammal Species of the World MSW lists species. But even the latter list falls short of current understanding as its collection cutoff was in , and a number of publications since then have pushed up to species. Notable new species not listed in MSW include the Bemaraha Woolly Lemur Avahi cleesei, named after British actor and lemur enthusiast John Cleese and the GoldenPalace.com Monkey whose name was put up for auction.

Homology of sequences in genetics

Homology among proteins and DNA is often concluded on the basis of sequence similarity, especially in bioinformatics. For example, in general, if two or more genes have highly similar DNA sequences, it is likely that they are homologous. But sequence similarity may arise from different ancestors short sequences may be similar by chance, and sequences may be similar because both were selected to bind to a particular protein, such as a transcription factor. Such sequences are similar but not homologous. Sequence regions that are homologous are also called conserved. This is not to be confused with conservation in amino acid sequences in which the amino acid at a specific position has changed but the physiochemical properties of the amino acid remain unchanged.The phrase percent homology is sometimes used but is incorrect. Percent identity or percent similarity should be used to quantify the similarity between the biomolecule sequences. For two naturally occurring sequences, percent identity is a factual measurement, whereas homology is a hypothesis supported by evidence. One can, however, refer to partial homology where a fraction of the sequences compared are presumed to share descent, while the rest does not. For example, partial homology may result from a gene fusion event.Transcription is the synthesis of RNA under the direction of DNA. RNA synthesis, or transcription, is the process of transcribing DNA nucleotide sequence information into RNA sequence information. Both nucleic acid sequences use complementary language, and the information is simply transcribed, or copied, from one molecule to the other. DNA sequence is enzymatically copied by RNA polymerase to produce a complementary nucleotide RNA strand, called messenger RNA mRNA, because it carries a genetic message from the DNA to the proteinsynthesizing machinery of the cell. One significant difference between RNA and DNA sequence is the presence of U, or uracil in RNA instead of the T, or thymine of DNA. In the case of proteinencoding DNA, transcription is the first step that usually leads to the expression of the genes, by the production of the mRNA intermediate, which is a faithful transcript of the genes proteinbuilding instruction. The stretch of DNA that is transcribed into an RNA molecule is called a transcription unit. A transcription unit that is translated into protein contains sequences that direct and regulate protein synthesis in addition to coding the sequence that is translated into protein. The regulatory sequence that is before, or , of the coding sequence is called untranslated region UTR, and sequence found following, or , of the coding sequence is called untranslated region UTR. Transcription has some proofreading mechanisms, but they are fewer and less effective than the controls for copying DNA therefore, transcription has a lower copying fidelity than DNA replication.As in DNA replication, RNA is synthesized in the → direction from the point of view of the growing RNA transcript. Only one of the two DNA strands is transcribed. This strand is called the template strand, because it provides the template for ordering the sequence of nucleotides in an RNA transcript. The other strand is called the coding strand, because its sequence is the same as the newly created RNA transcript except for uracil being substituted for thymine. The DNA template strand is read → by RNA polymerase and the new RNA strand is synthesized in the → direction. RNA polymerase binds to the end of a gene promoter on the DNA template strand and travels toward the end.

Transcription is divided into stages preinitiation, initiation, promoter clearance, elongation and termination.One strand of DNA, the template strand or noncoding strand, is used as a template for RNA synthesis. As transcription proceeds, RNA polymerase traverses the template strand and uses base pairing complementarity with the DNA template to create an RNA copy. Although RNA polymerase traverses the template strand from → , the coding nontemplate strand is usually used as the reference point, so transcription is said to go from → . This produces an RNA molecule from → , an exact copy of the coding strand except that thymines are replaced with uracils, and the nucleotides are composed of a ribose carbon sugar where DNA has deoxyribose one less oxygen atom in its sugarphosphate backbone.Unlike DNA replication, mRNA transcription can involve multiple RNA polymerases on a single DNA template and multiple rounds of transcription amplification of particular mRNA, so many mRNA molecules can be produced from a single copy of a gene. This step also involves a proofreading mechanism that can replace incorrectly incorporated bases.Prokaryotic elongation starts with the abortive initiation cycle. During this cycle RNA Polymerase will synthesize mRNA fragments nucleotides long. This continues to occur until the σ factor rearranges, which results in the transcription elongation complex which gives a bp moving footprint. The σ factor is released before nucleotides of mRNA are synthesized.In Eukaryotic transcription the polymerase can experience pauses. These pauses may be intrinsic to the RNA polymerase or due to chromatin structure. Often the polymerase pauses to allow appropriate RNA editing factors to bind.Bacteria use two different strategies for transcription termination in Rhoindependent transcription termination, RNA transcription stops when the newly synthesized RNA molecule forms a GC rich hairpin loop, followed by a run of Us, which makes it detach from the DNA template. In the Rhodependent type of termination, a protein factor called Rho destabilizes the interaction between the template and the mRNA, thus releasing the newly synthesized mRNA from the elongation complex. Transcription termination in eukaryotes is less well understood. It involves cleavage of the new transcript, followed by templateindependent addition of As at its new end, in a process called polyadenylation.Active transcription units are clustered in the nucleus, in discrete sites called ‘transcription factories’. Such sites could be visualized after allowing engaged polymerases to extend their transcripts in tagged precursors BrUTP or BrU, and immunolabeling the tagged nascent RNA. Transcription factories can also be localized using fluorescence in situ hybridization, or marked by antibodies directed against polymerases. There are ~, factories in the nucleoplasm of a HeLa cell, among which are ~, polymerase II factories and ~, polymerase III factories. Each polymerase II factory contains ~ polymerases. As most active transcription units are associated with only one polymerase, each factory will be associated with ~ different transcription units. These units might be associated through promoters andor enhancers, with loops forming a ‘cloud’ around the factory.

Homologous chromosome sets

Synapsis also called syndesis is the pairing of two homologous chromosomes that occurs during meiosis. It is a form of chromosomal crossover. Synapsis takes place during prophase I. When homologous chromosomes synapse, they come closer together until they are connected by a protein complex called the synaptonemal complex, which contains central and lateral elements. While autosomes undergo synapsis during meiosis sex chromosomes usually remain unpaired.When the nonsister chromatids intertwine, segments of chromatids with the same sequence break apart at and are exchanged in a process known as genetic recombination or crossingover. Recombination exchanges genetic material between homologous chromosomes and increases the genetic variability of the offspring. This exchange produces a chiasma, a region that is shaped like an X, where the two chromosomes are physically joined.In biology or life science, meiosis pronounced myohsis is a process of reduction division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes. The word meiosis comes from the Greek verb meioun, meaning to make small, since it results in a reduction in chromosome number in the gamete cell.Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes including singlecelled organisms that reproduce sexually. A few eukaryotes, notably the Bdelloid rotifers, have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis. Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as mitosis or binary fission. Each cell has half the number of chromosomes as the parent cell.During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells. Each of these cells contain one complete set of chromosomes, or half of the genetic content of the original cell. If meiosis produces gametes, these cells must fuse during fertilization to create a new diploid cell, or zygote before any new growth can occur. Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization. Because the chromosomes of each parent undergo genetic recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. Together, meiosis and fertilization constitute sexuality in the eukaryotes, and generate genetically distinct individuals in populations.In all plants, and in many protists, meiosis results in the formation of haploid cells that can divide vegetatively without undergoing fertilization. In these groups, gametes are produced by mitosis.Meiosis uses many of the same biochemical mechanisms employed during mitosis to accomplish the redistribution of chromosomes. There are several features unique to meiosis, most importantly the pairing and genetic recombination between homologous chromosomes.

Meiosis was discovered and described for the first time in sea urchin eggs in , by noted German biologist Oscar Hertwig . It was described again in , at the level of chromosomes, by Belgian zoologist Edouard Van Beneden , in Ascaris worms eggs. The significance of meiosis for reproduction and inheritance, however, was described only in by German biologist August Weismann , who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained. In the American geneticist Thomas Hunt Morgan observed crossover in Drosophila melanogaster meiosis and provided the first true genetics.Meiosis is thought to have appeared . billion years ago. The only supergroup of eukaryotes which does not have meiosis in all organisms is excavata. The other five major supergroups, opisthokonts, amoebozoa, rhizaria, archaeplastida and chromalveolates all seem to have genes for meiosis universally present, even if not always functional. Some excavata species do have meiosis which is consistent with the hypothesis that excavata is an ancient, paraphyletic grade. An example of eukaryotic organism in which meiosis does not exist is euglenoid.Meiosis occur in eukaryotic life cycles involving sexual reproduction, comprising of the constant cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell division. In multicellular organisms, there is an intermediary step between the diploid and haploid transition where the organism grows. The organism will then produce the germ cells that continue in the life cycle. The rest of the cells, called somatic cells, function within the organism and will die with it.Cycling meiosis and fertilization events produces a series of transitions back and forth between alternating haploid and diploid states. The organism phase of the life cycle can occur either during the diploid state gametic life cycle, or during the haploid state zygotic life cycle, or both sporic life cycle, in which there two distinct organism phases, one during the haploid state and the other during the diploid state. In this sense, there are three types of life cycles that utilize sexual reproduction, differentiated by the location of the organisms phases. In the gametic life cycle, the species is diploid, grown from a diploid cell called the zygote. In the zygotic life cycle the species is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete. Humans, for example, are diploid creatures. Human stem cells undergo meiosis to create haploid gametes, which are spermatozoa for males or ova for females. These gametes then fertilize in the Fallopian tubes of the female, producing a diploid zygote. The zygote undergoes progressive stages of mitosis and differentiation, turns into a blastocyst and then gets implanted in the uterus endometrium to create an embryo.In the gametic life cycle, of which humans are a part, the living organism is diploid in nature. Here, we will generalize the example of human reproduction stated previously. The organisms diploid germline stem cells undergo meiosis to create haploid gametes, which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism. Mitosis is a related process to meiosis that creates two cells that are genetically identical to the parent cell. The general principle is that mitosis creates somatic cells and meiosis creates germ cells.In the zygotic life cycle, the living organism is haploid. Two organisms of opposing gender contribute their haploid germ cells to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the organism. Many fungi and many protozoa are members of the zygotic life cycle.Finally, in the sporic life cycle, the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the alternation of generations. The diploid organisms germline cells undergo meiosis to produce gametes. The gametes proliferate by mitosis, growing into a haploid organism. The haploid organisms germ cells then combine with another haploid organisms cells, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become the diploid organism again. The sporic life cycle can be considered a fusion of the gametic and zygotic life cycles.

Homologous chromosomes

The origins, early forms, and early development of the Hellenic language family are not well understood, owing to the lack of contemporaneous evidence. There are several theories about what Hellenic dialect groups may have existed between the divergence of early Greeklike speech from the common IndoEuropean language not later than BC, and about BC. They have the same general outline but differ in some of the detail. The only attested dialect from this period is Mycenaean, but its relationship to the historical dialects and the historical circumstances of the times imply that the overall groups already existed in some form.The major dialect groups of the Ancient Greek period can be assumed to have developed not later than BC, at the time of the Dorian invasions, and their first appearances as precise alphabetic writing began in the th century BC. The invasion would not be Dorian unless the invaders had some cultural relationship to the historical Dorians moreover, the invasion is known to have displaced population to the later AtticIonic regions, who regarded themselves as descendants of the population displaced by or contending with the Dorians.The ancient Greeks themselves considered there to be three major divisions of the Greek people, into Dorians, Aeolians, and Ionians including Athenians, each with their own defining and distinctive dialects. Allowing for their oversight of Arcadian, an obscure mountain dialect, and Cyprian, far from the center of Greek scholarship, this division of people and language is quite similar to the results of modern archaeologicallinguistic investigation. This is very important to realize because of the content and the change that has occurred.Greek, like all of the older IndoEuropean languages, is highly inflected. It is highly archaic in its preservation of ProtoIndoEuropean forms. In Ancient Greek nouns including proper nouns have five cases nominative, genitive, dative, accusative and vocative, three genders masculine, feminine and neuter, and three numbers singular, dual and plural. Verbs have four moods indicative, imperative, subjunctive, and optative, three voices active, middle and passive, as well as three persons first, second and third and various other forms. Verbs are conjugated through seven tenses the present, future and imperfect tenses are imperfective in aspect the aorist tense perfective aspect a presentperfect, pluperfect and future perfect all with perfect aspect. Most tenses display all four moods and three voices, although there is no future subjunctive or imperative. There are infinitives and participles corresponding to the finite combinations of tense, aspect and voice.Genetics from Ancient Greek γενετικός genetikos, “genitive” and that from γένεσις genesis, “origin”, a discipline of biology, is the science of heredity and variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. However, the modern science of genetics, which seeks to understand the process of inheritance, only began with the work of Gregor Mendel in the midnineteenth century. Although he did not know the physical basis for heredity, Mendel observed that organisms inherit traits in a discrete manner—these basic units of inheritance are now called genes.NA, the molecular basis for inheritance. Each strand of DNA is a chain of nucleotides, matching each other in the center to form what look like rungs on a twisted ladder.DNA, the molecular basis for inheritance. Each strand of DNA is a chain of nucleotides, matching each other in the center to form what look like rungs on a twisted ladder.Genes correspond to regions within DNA, a molecule composed of a chain of four different types of nucleotides—the sequence of these nucleotides is the genetic information organisms inherit. DNA naturally occurs in a double stranded form, with nucleotides on each strand complementary to each other. Each strand can act as a template for creating a new partner strand—this is the physical method for making copies of genes that can be inherited.

The sequence of nucleotides in a gene is translated by cells to produce a chain of amino acids, creating proteins—the order of amino acids in a protein corresponds to the order of nucleotides in the gene. This is known as the genetic code. The amino acids in a protein determine how it folds into a threedimensional shape this structure is, in turn, responsible for the proteins function. Proteins carry out almost all the functions needed for cells to live. A change to the DNA in a gene can change a proteins amino acids, changing its shape and function this can have a dramatic effect in the cell and on the organism as a whole.Although genetics plays a large role in the appearance and behavior of organisms, it is the combination of genetics with what an organism experiences that determines the ultimate outcome. For example, while genes play a role in determining a persons height, the nutrition and health that person experiences in childhood also have a large effect.Although the science of genetics began with the applied and theoretical work of Gregor Mendel in the mids, other theories of inheritance preceded Mendel. A popular theory during Mendels time was the concept of blending inheritance the idea that individuals inherit a smooth blend of traits from their parents. Mendels work disproved this, showing that traits are composed of combinations of distinct genes rather than a continuous blend. Another theory that had some support at that time was the inheritance of acquired characteristics the belief that individuals inherit traits strengthened by their parents. This theory commonly associated with JeanBaptiste Lamarck is now known to be wrong—the experiences of individuals do not affect the genes they pass to their children. Other theories included the pangenesis of Charles Darwin which had both acquired and inherited aspects and Francis Galtons reformulation of pangenesis as both particulate and inherited.The modern science of genetics traces its roots to Gregor Johann Mendel, a GermanCzech Augustinian monk and scientist who studied the nature of inheritance in plants. In his paper Versuche über Pflanzenhybriden Experiments on Plant Hybridization, presented in to the Naturforschender Verein Society for Research in Nature in Brünn, Mendel traced the inheritance patterns of certain traits in pea plants and described them mathematically. Although this pattern of inheritance could only be observed for a few traits, Mendels work suggested that heredity was particulate, not acquired, and that the inheritance patterns of many traits could be explained through simple rules and ratios.The importance of Mendels work did not gain wide understanding until the s, after his death, when other scientists working on similar problems rediscovered his research. William Bateson, a proponent of Mendels work, coined the word genetics in . The adjective genetic, derived from the Greek word genesis γένεσις, origin and that from the word genno γεννώ, to give birth, predates the noun and was first used in a biological sense in . Bateson popularized the usage of the word genetics to describe the study of inheritance in his inaugural address to the Third International Conference on Plant Hybridization in London, England, in .After the rediscovery of Mendels work, scientists tried to determine which molecules in the cell were responsible for inheritance. In , Thomas Hunt Morgan argued that genes are on chromosomes, based on observations of a sexlinked white eye mutation in fruit flies. In , his student Alfred Sturtevant used the phenomenon of genetic linkage to show that genes are arranged linearly on the chromosome.

chromosomes

Although genes were known to exist on chromosomes, chromosomes are composed of both protein and DNA—scientists did not know which of these was responsible for inheritance. In , Frederick Griffith discovered the phenomenon of transformation see Griffiths experiment dead bacteria could transfer genetic material to transform other stillliving bacteria. Sixteen years later, in , Oswald Theodore Avery, Colin McLeod and Maclyn McCarty identified the molecule responsible for transformation as DNA. The HersheyChase experiment in also showed that DNA rather than protein was the genetic material of the viruses that infect bacteria, providing further evidence that DNA was the molecule responsible for inheritance.James D. Watson and Francis Crick determined the structure of DNA in , using the Xray crystallography work of Rosalind Franklin that indicated DNA had a helical structure i.e., shaped like a corkscrew. Their doublehelix model had two strands of DNA with the nucleotides pointing inward, each matching a complementary nucleotide on the other strand to form what looks like rungs on a twisted ladder. This structure showed that genetic information exists in the sequence of nucleotides on each strand of DNA. The structure also suggested a simple method for duplication if the strands are separated, new partner strands can be reconstructed for each based on the sequence of the old strand.Although the structure of DNA showed how inheritance worked, it was still not known how DNA influenced the behavior of cells. In the following years, scientists tried to understand how DNA controls the process of protein production. It was discovered that the cell uses DNA as a template to create matching messenger RNA a molecule with nucleotides, very similar to DNA. The nucleotide sequence of a messenger RNA is used to create an amino acid sequence in protein this translation between nucleotide and amino acid sequences is known as the genetic code.With this molecular understanding of inheritance, an explosion of research became possible. One important development was chaintermination DNA sequencing in by Frederick Sanger this technology allows scientists to read the nucleotide sequence of a DNA molecule. In , Kary Banks Mullis developed the polymerase chain reaction, providing a quick way to isolate and amplify a specific section of a DNA from a mixture. Through the pooled efforts of the Human Genome Project and the parallel private effort by Celera Genomics, these and other techniques culminated in the sequencing of the human genome in .Horizontal gene transfer HGT, also Lateral gene transfer LGT, is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism. By contrast, vertical transfer occurs when an organism receives genetic material from its ancestor, e.g. its parent or a species from which it evolved. Most thinking in genetics has focused on the more prevalent vertical transfer, but there is a recent awareness that horizontal gene transfer is a significant phenomenon.Horizontal gene transfer was first described in Japan in a publication that demonstrated the transfer of antibiotic resistance between different species of bacteria. However, the significance of this research was not appreciated in the west for another ten years. Michael Syvanen was among the earliest western biologists to explore the potential significance of lateral gene transfer. Syvanen published a series of papers on horizontal gene transfer starting in , predicting that lateral gene transfer exists, has biological significance, and is a process that shaped evolutionary history from the very beginning of life on earth. Artificial horizontal gene transfer is a form of genetic engineering.As Jain, Rivera and Lake put it Increasingly, studies of genes and genomes are indicating that considerable horizontal transfer has occurred between prokaryotes. see also Lake and Rivera, . The phenomenon appears to have had some significance for unicellular eukaryotes as well. As Bapteste et al. observe, additional evidencesuggests that gene transfer might also be an important evolutionary mechanism in protist evolution.There is some evidence that even higher plants and animals have been affected and this has raised concerns for safety. However, Richardson and Palmer state Horizontal gene transfer HGT has played a major role in bacterial evolution and is fairly common in certain unicellular eukaryotes. However, the prevalence and importance of HGT in the evolution of multicellular eukaryotes remain unclear.

Due to the increasing amount of evidence suggesting the importance of these phenomena for evolution see below, molecular biologists such as Peter Gogarten have described horizontal gene transfer as A New Paradigm for Biology.It should also be noted that the process may be a hidden hazard of genetic engineering, as it may allow dangerous transgenic DNA which is optimised for transfer to spread from species to species.Horizontal gene transfer is a potential confounding factor in inferring phylogenetic trees based on the sequence of one gene. For example, given two distantly related bacteria that have exchanged a gene, a phylogenetic tree including those species will show them to be closely related because that gene is the same, even though most other genes have substantially diverged. For this reason, it is often ideal to use other information to infer robust phylogenies, such as the presence or absence of genes, or, more commonly, to include as wide a range of genes for phylogenetic analysis as possible.For example, the most common gene to be used for constructing phylogenetic relationships in prokaryotes is the s rRNA gene, since its sequences tend to be conserved among members with close phylogenetic distances, but variable enough that differences can be measured. However, in recent years it has also been argued that s rRNA genes can also be horizontally transferred. Although this may be infrequent, validity of s rRNAconstructed phylogenetic trees must be reevaluated.Biologist Gogarten suggests the original metaphor of a tree no longer fits the data from recent genome research therefore biologists should use the metaphor of a mosaic to describe the different histories combined in individual genomes and use the metaphor of a net to visualize the rich exchange and cooperative effects of HGT among microbes.Using single genes as phylogenetic markers, it is difficult to trace organismal phylogeny in the presence of horizontal gene transfer. Combining the simple coalescence model of cladogenesis with rare HGT horizontal gene transfer events suggest there was no single most recent common ancestor that contained all of the genes ancestral to those shared among the three domains of life. Each contemporary molecule has its own history and traces back to an individual molecule cenancestor. However, these molecular ancestors were likely to be present in different organisms at different times.Uprooting the Tree of Life by W. Ford Doolittle Scientific American, February , pp contains a discussion of the Last Universal Common Ancestor, and the problems that arose with respect to that concept when one considers horizontal gene transfer. The article covers a wide area the endosymbiont hypothesis for eukaryotes, the use of small subunit ribosomal RNA SSU rRNA as a measure of evolutionary distances this was the field Carl Woese worked in when formulating the first modern tree of life, and his research results with SSU rRNA led him to propose the Archaea as a third domain of life and other relevant topics. Indeed, it was while examining the new threedomain view of life that horizontal gene transfer arose as a complicating issue Archaeoglobus fulgidus is cited in the article p. as being an anomaly with respect to a phylogenetic tree based upon the encoding for the enzyme HMGCoA reductase the organism in question is a definite Archaean, with all the cell lipids and transcription machinery that are expected of an Archaean, but whose HMGCoA genes are actually of bacterial origin.

List of homologues of the human reproductive system

The List of homologues of the human reproductive system shows how indifferent embryonic organs differentiate into the respective sex organs in males and females. Mullerian ducts are also referred to as paramesonephric ducts, and Wolffian ducts as mesonephric duct.In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, urinary, and usually genital tracts of certain animal species. The word comes from Latin, and means sewer. All birds, reptiles, and amphibians possess this orifice, from which they excrete both urine and feces, unlike placental mammals, which possess two separate orifices for evacuation. Marsupials and monotremes also possess one in marsupials and a few birds, the genital tract is separate. In contrast, each individual among most species of placental mammals and bony fishes has, in lieu of a cloaca, a specialized opening for at least one of these tracts. This is one of the features of marsupials and monotremes which suggest their primitivity, as the reptiles from which mammals evolved possessed a cloaca, and the earliest animals to diverge into the mammalian class would have had this feature too.In birds the cloaca is also referred to as the vent, and among falconers the word vent is also a verb meaning to defecate. Birds also reproduce with this organ, this is known as a cloacal kiss.xcretory systems with analogous purpose in certain invertebrates are also sometimes referred to as cloacae.cIn birds the reproductive system must be reengorged prior to the mating season of each species. Such regeneration usually takes about a month. Birds generally produce one batch of eggs per year, but they will produce another if the first is taken away they have the ability to produce more. For some birds, such as some species of swans and ducks, the males do not use the cloaca for reproduction but have a penis.The cloacal region is also often associated with a secretory organ, the cloacal gland, which has been implicated in the scent marking behavior of some reptiles, amphibians and monotremes.Some turtles, especially those specialized in diving, are highly reliant on cloacal respiration during dives. They accomplish this by having a pair of accessory air bladders connected to the cloaca which can absorb oxygen from the water. Sea cucumbers also extract oxygen from water in a pair of lungs or respiratory trees that branch off the cloaca just inside the anus.There are also a variety of fishes, as well as polychaete worms and even crabs, that are specialized to take advantage of the constant flow of water through the cloacal respiratory tree of sea cucumbers while simultaneously gaining the protection of living within the sea cucumber itself. At night many of these species emerge from the anus of the sea cucumber in search of food. In humans, the ureters arise from the renal pelvis on the medial aspect of each kidney before descending towards the bladder on the front of the psoas major muscle. The ureters cross the pelvic brim near the bifurcation of the iliac arteries which they run over. This pelviureteric junction is a common site for the impaction of kidney stones the other being the ureterovesical valve. The ureters run posteroinferiorly on the lateral walls of the pelvis. They then curve anteriormedially to enter the bladder through the back, at the vesicoureteric junction, running within the wall of the bladder for a few centimeters. The backflow of urine is prevented by valves known as ureterovesical valves, pressure from the filling of the bladder, and the tone of the muscle in the bladder wall.

The ureter has a diameter of about millimeters, and the lumen is starshaped. Like the bladder, it is lined with transitional epithelium, and contains layers of smooth muscle, thereby being under autonomic control.The epithelial cells of the ureter are stratified in many layers, are normally round in shape but become squamous flat when stretched. The lamina propria is thick and elastic as it is important that it is impermeable.There are two spiral layers of smooth muscle in the ureter wall, an inner loose spiral, and an outer tight spiral. The inner loose spiral is sometimes described as longitudinal, and the outer as circular, this is the opposite to the situation in the gastrointestinal tract. The distal third of the ureter contains another layer of outer longitudinal muscle. The urachus is an embryological canal connecting the urinary bladder of the fetus with the allantois, a structure that contributes to the formation of the umbilical cord. The lumen inside of the urachus is normally obliterated during embryonic development, transforming the urachus into a solid cord, a functionless remnant. The urachus lies in the space of Retzius, between the transversalis fascia anteriorly and the peritoneum posteriorly.The vesicourethral portion of the urogenital sinus absorbs the ends of the Wolffian ducts and the associated ends of the renal diverticula, and these give rise to the trigone of the bladder and part of the prostatic urethra. The remainder of the vesicourethral portion forms the body of the bladder and part of the prostatic urethra its apex is prolonged to the umbilicus as a narrow canal, which later is obliterated and becomes the median umbilical ligament urachus. Note The two medial umbilical ligaments are the obliterated umbilical arteries.vStress urinary incontinence SUI is essentially due to pelvic floor muscle weakness. It is loss of small amounts of urine with coughing, laughing, sneezing, exercising or other movements that increase intraabdominal pressure and thus increase pressure on the bladder. Physical changes resulting from pregnancy, childbirth, and menopause often cause stress incontinence, and in men it is a common problem following a prostatectomy. It is the most common form of incontinence in men and is treatable.The urethra is supported by fascia of the pelvic floor. If the fascial support is weakened, as it can be in pregnancy and childbirth, the urethra can move downward at times of increased abdominal pressure, resulting in stress incontinence.Stress incontinence can worsen during the week before the menstrual period. At that time, lowered estrogen levels may lead to lower muscular pressure around the urethra, increasing chances of leakage. The incidence of stress incontinence increases following menopause, similarly because of lowered estrogen levels. Most lab results, such as urine analysis, cystometry and postvoid residual volume are normal.