Homology (biology)

	Email		Print
	Bookmark		Link

Homology (biology)

In evolutionary biology

Evolutionary biology

Evolutionary biology is a sub-field of biology concerned with the origin of species from a common descent and descent of species, as well as their evolution, multiplication and diversity over time....
, homology refers to any similarity between characteristic

Characteristic

Characteristic has several particular meanings: *in mathematics** characteristic function ** Euler characteristic** characteristic ** characteristic subgroup...
s that is due to their shared ancestry

Common descent

A group of organisms is said to have common descent if they have a common ancestor. In modern biology, it is generally accepted that all living organisms on Earth are descended from a common ancestor or ancestral gene pool....
. The word homologous derives from the ancient Greek

Ancient Greek

Ancient Greek is the historical stage in the development of the Greek language spanning across the Archaic Greece , Classical Greece , and Hellenistic civilization periods of ancient Greece and the classical antiquity....
?µ????e??, 'to agree'. 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

Genetics

Genetics , a discipline of biology, is the science of heredity and Genetic 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....
, homology can be observed in DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetics instructions used in the development and functioning of all known living organisms and some viruses....
sequences

Sequencing

Discussion

Ask a question about 'Homology (biology)'

Start a new discussion about 'Homology (biology)'

Answer questions from other users

Full Discussion Forum

Encyclopedia

In evolutionary biology

Evolutionary biology

Characteristic

Common descent

Ancient Greek

Genetics

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetics instructions used in the development and functioning of all known living organisms and some viruses....
sequences

Sequencing

In genetics and biochemistry, sequencing means to determine the primary structure of an unbranched biopolymer. Sequencing results in a symbolic linear depiction known as a sequence which succinctly summarizes much of the atomic-level structure of the sequenced molecule....
that code for proteins (genes) and in noncoding DNA. For protein coding genes, one can compare translated amino-acid sequences of different genes. Sequence homology may also indicate common function. Homologous chromosome

Homologous chromosome

Homologous chromosomes are chromosomes in a biological cell that pair during meiosis, or alternatively, non-identical chromosomes that contain information for the same biological features and contain the same genes at the same locus but possibly different genetic information, called alleles, at those genes....
s are chromosomes with the same genes, but non-identical nucleotide sequences that can pair (synapse

Synapsis

Synapsis is the pairing of two homologous chromosomes that occurs during meiosis. It is a form of chromosomal crossover. Synapsis takes place during prophase I....
) during meiosis

Meiosis

Homology of structures in evolution

Shared ancestry can be evolution

Evolution

In biology, evolution is change in the heritability trait of a population of organisms from one generation to the next. These changes are caused by a combination of three main processes: variation, reproduction, and selection....
ary or developmental. Evolutionary ancestry means that structures evolved from some structure in a common ancestor; for example, the wings of bat

Bat

Bats are mammals in the order Chiroptera. The forelimbs of all bats are developed as wings, making them the only mammals naturally capable of sustained flight ....
s and the arms of primate

Primate

A primate is a member of the biological order Primates , the group that contains lemurs, the Aye-aye, Lorisidaes, galagos, tarsiers, monkeys, and apes, with the last category including humans....
s are homologous in this sense. Developmental ancestry means that structures arose from the same tissue in embryo

Embryo

An embryo is a multicellular organism ploidy eukaryote in its earliest stage of development, from the time of first cell division until birth, Egg , or germination....
nal development; the ovaries

Ovary

The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in females are homology to testicle in males, in that they are both gonads and endocrine glands....
of female humans and the testicle

Testicle

The testicle is the male gonad in animals. This article will concentrate on mammalian testicles unless otherwise noted.The etymology of the word is somewhat colorfully based on Roman law....
s of male humans are homologous in this sense.

Homology is different from analogy

Analogy (biology)

Two structures in biology are said to be analogous if they perform the same or similar function by a similar mechanism but evolved separately....
. 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 didn't 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 of sequences in genetics

Homology among proteins and DNA is often concluded on the basis of sequence similarity, especially in bioinformatics

Bioinformatics

Bioinformatics is the application of information technology to the field of molecular biology. The term bioinformatics was coined by Paulien Hogeweg in 1978 for the study of informatic processes in biotic systems....
. 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 also arise without common ancestry: 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

Transcription (genetics)

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....
factor. Such sequences are similar but not homologous. Sequence regions that are homologous are also called conserved

Conserved sequence

In biology, conserved sequences are similar or identical Sequence that may occur within nucleic acid sequences , peptide sequence, protein structures or polysaccharide within multiple species of organism or within different molecules produced by the same organism ....
. 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 physio-chemical 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.

Many algorithms exist to cluster protein sequences into sequence families, which are sets of mutually homologous sequences. (See sequence clustering

Sequence clustering

In bioinformatics, Primary structure clustering algorithms attempt to group sequences that are somehow related. The sequences can be either of genomic, "transcriptomic" or protein origin....
and sequence alignment

Sequence alignment

In bioinformatics, a sequence alignment is a way of arranging the sequences of DNA, RNA, or protein to identify regions of similarity that may be a consequence of functional, structural biology, or evolutionary relationships between the sequences....
.)

Homologous sequences are of two types: orthologous and paralogous.

Orthology

Homologous sequences are orthologous if they were separated by a speciation

Speciation

Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages....
event: when a species diverges into two separate species, the divergent copies of a single gene in the resulting species are said to be orthologous. Orthologs, or orthologous genes, are genes in different species that are similar to each other because they originated from a common ancestor. The term "ortholog" was coined in 1970 by Walter Fitch.

The strongest evidence that two similar genes are orthologous is the result of a phylogenetic analysis of the gene lineage. Genes that are found within one clade

Clade

A clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.The term "monophyletic group" is used in this article in the conventional sense of "an a...
are orthologs, descended from a common ancestor. Orthologs often, but not always, have the same function.

Orthologous sequences provide useful information in taxonomic classification studies of organisms. The pattern of genetic divergence can be used to trace the relatedness of organisms. Two organisms that are very closely related are likely to display very similar DNA sequences between two orthologs. Conversely, an organism that is further removed evolutionarily from another organism is likely to display a greater divergence in the sequence of the orthologs being studied.

Paralogy

Homologous sequences are paralogous if they were separated by a gene duplication

Gene duplication

Gene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposon event, or duplication of an entire chromosome....
event: if a gene in an organism is duplicated to occupy two different positions in the same genome, then the two copies are paralogous.

A set of sequences that are paralogous are called paralogs of each other. Paralogs typically have the same or similar function, but sometimes do not: due to lack of the original selective pressure upon one copy of the duplicated gene, this copy is free to mutate and acquire new functions.

Paralogous sequences provide useful insight to the way genomes evolve

Evolution

Translation (genetics)

Translation is the first stage of protein biosynthesis . Translation is the production of proteins by decoding mRNA produced in Transcription ....
myoglobin

Myoglobin

Myoglobin is a Tertiary structure globular protein of 153 amino acids, containing a heme prosthetic group in the center around which the remaining apoprotein folds....
and hemoglobin

Hemoglobin

Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of vertebrates, and the tissues of some invertebrates....
are considered to be ancient paralogs. Similarly, the four known classes of hemoglobins (hemoglobin A

Hemoglobin A

Hemoglobin A is the most common human hemoglobin tetramer.It consists of two alpha chains, and two beta chains....
, hemoglobin A2

Hemoglobin A2

Hemoglobin A2 is a normal variant of hemoglobin A that consists of two alpha and two Delta hemoglobins and is found in small quantity in normal human blood. Hemoglobin A2 may be increased in beta thalassemia....
, hemoglobin B, and hemoglobin F

Fetal hemoglobin

Fetal hemoglobin, or Foetal haemoglobin in British English, is the main oxygen transport protein in the fetus during the last seven months of development in the uterus and in the newborn until roughly 6 months old....
) are paralogs of each other. While each of these genes serves the same basic function of oxygen transport, they have already diverged slightly in function: fetal hemoglobin (hemoglobin F) has a higher affinity for oxygen than adult hemoglobin.

Another example can be found in rodents such as rats and mice. Rodents have a pair of paralogous insulin

Insulin

Insulin is a hormone with extensive effects on both metabolism and several other body systems . Insulin causes most of the body's cells to take up glucose from the blood , storing it as glycogen in the liver and muscle, and stops use of fat as an energy source....
genes, although it is unclear if any divergence in function has occurred.

Paralogous genes often belong to the same species, but this is not necessary: for example, the hemoglobin gene of humans and the myoglobin gene of chimpanzees are paralogs. This is a common problem in bioinformatics: when genomes of different species have been sequenced and homologous genes have been found, one can not immediately conclude that these genes have the same or similar function, as they could be paralogs whose function has diverged.

Ohnology

Ohnologous genes are paralogous genes

Gênes

G?nes is the name of a d?partement in France of the First French Empire in present Italy. It was named after the city Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa....
that have originated by a process of whole-genome duplication (WGD). The name was first given in honour of Susumu Ohno

Susumu Ohno

Susumu Ohno , was an Asian American geneticist and evolutionary biologist, and seminal researcher in the field of molecular evolution....
by Ken Wolfe. Ohnologs are interesting for evolutionary analysis because they all have been diverging for the same length of time since their common origin.

Xenology

Homologs resulting from horizontal gene transfer

Horizontal gene transfer

Horizontal gene transfer , also Lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the Reproduction of that organism....
between two organisms are termed xenologs. Xenologs can have different functions, if the new environment is vastly different for the horizontally moving gene. In general, though, xenologs typically have similar function in both organisms.

Gametology

Gametology denotes the relationship between homologous genes on nonrecombining, opposite sex chromosomes. Gametologs result from the origination of genetic sex determination and barriers to recombination between sex chromosomes. Examples of gametologs include CHDW and CHDZ in birds.

Homologous chromosome sets

Homologous chromosome

Synapsis

Synapsis is the pairing of two homologous chromosomes that occurs during meiosis. It is a form of chromosomal crossover. Synapsis takes place during prophase I....
) during meiosis

Meiosis

In biology or life science, meiosis is a process of reductional division in which the number of chromosomes per cell is halved. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores....
. Except for the sex chromosomes, homologous chromosomes share significant sequence similarity across their entire length, typically contain the same sequence of genes, and pair up to allow for proper disjunction during meiosis

Meiosis

In biology or life science, meiosis is a process of reductional division in which the number of chromosomes per cell is halved. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores....
. The chromosomes can also undergo cross-over at this stage. There may be some variations between genes on homologues giving rise to alternate forms or alleles. Sex chromosomes have a shorter region of sequence similarity. Based on the sequence similarity and our knowledge of biology, it is believed that they are paralogous.

Homology (biology)

Homology of structures in evolution

Homology of sequences in genetics

Orthology

Paralogy

Ohnology

Xenology

Gametology

Homologous chromosome sets

See also

Further reading

External links