Homologous, Paralogous and Orthologous Genes

In this is homologous, paralogous and orthologous genes post we have briefly explained about homologous genes, paralogous genes and orthologous genes.

Homologous, Paralogous and Orthologous Genes

Homology is the core structure of comparative biology. The term “homologous” refers to a trait described as homolog (also known as homologue). In genetics, homolog is a term used to describe a trait “homolog” is used both to refer to homologous proteins and for the genetic code (DNA sequence) which encodes it.

Similar to anatomical structures, homology between DNA or protein sequences is determined by the common ancestral lineage. Two DNA segments could share ancestry due to a speciation event (orthologs) or an event of duplication (paralogs). Homology between DNA and proteins is often interpreted incorrectly by relying on sequence similarity. The words “percent homology” and “sequence similarity” are frequently employed in conjunction.

Like anatomical structures, high-sequence similarity may be due to the concurrent process or, like shorter sequences, due to chance. These sequences are comparable. However, they are not homologous genes. The homologous genes sequences are also known as conserved. This should not be confused with the conservation of amino acid sequences. Another has replaced the amino acid located at an exact location with physicochemical properties that are functionally comparable. It is possible to be referring to partial homology in which certain sequences that are compared (are believed to) have common descent, but the rest do not. A case is that partial homology could result from a gene fusion.

Homologous Orthologous and Paralogous Sequences

Example of homologous genes: This is the sequence alignment of a homologous protein from two different species The “*” represent a conserved amino acid in the two proteins.

Homologous gene sequences can be considered orthologous genes if separated by an event of speciation. When the species split into two distinct species, the copies of the same gene found in both species are considered orthologous. Orthologous genes, also known as orthologous genes, are genes from different species that were derived by vertical descent from one gene belonging to the common ancestral.

For example, the plant Flu regulatory protein can also be found within Arabidopsis and Chlamydomonas. This Chlamydomonas variant is much more complicated: it passes through the membrane twice, not only once, has additional domains, and is subject to alternative splicing. It can, however, fully substitute for the less complex Arabidopsis proteins when transferred from algae to plants by genetic engineering.

The significant sequence similarities and the functional domains that share commonality suggest that the two gene families are orthologous genes inherited from the same ancestral species. The orthologous genes sequences can be useful for taxonomic classification as well as the study of phylogenetic relationships of organisms. Patterns of divergence in genetics could be used to identify the relationship between organisms. The closely related organisms will likely exhibit identical DNA sequences between two orthologs. However, an organism that is further separated from another is more likely to show more variation in the sequence of the orthologs that are being investigated.

Homologous sequences are considered paralogous genes if they are separated due to the duplication of genes. When a gene of one organism duplicates to fill two distinct positions within an identical genome, the two copies are both paralogous. Paralogous genes often belong to one species; however, this isn’t necessary.

For instance, the haemoglobin gene of human beings and myoglobin gene from the chimps are both related genes. Paralogs are divided into intra-paralogs and out-paralogs. Between species, out-paralogs are a set of paralogs between two organisms because of duplicated genes before speciation. Out-paralogs within species refer to pairs of paralogs present in the same species, but their duplication happens after the speciation.

Paralogs usually perform the same or a similar function but occasionally doesn’t. Due to the lack of initial selective pressure on just one version of the duplicated gene, the copy can change and gain new functions. Paralogous genes sequences can provide valuable insights into how genomes develop. Myoglobin and haemoglobin are believed as ancient paralogs.

In the same way, the four recognized types of haemoglobins (haemoglobin A, haemoglobin A2, B and haemoglobin) are each a paralogs of the other. Although each performs the same purpose of transporting oxygen, they’ve already differed in their function slightly in that the foetal haemoglobin (haemoglobin F) has a greater affinity to oxygen than mature haemoglobin. However, their function is not always preserved. For instance, human angiogenin diverged from the ribonuclease, and even though the two paralogs are identical in their tertiary structures, their roles within cells are quite distinct.

Take Away

Homologous genes (or homolog) is a gene that has been inherited in two species through an ancestor who is common to both species. Although homologous genes may be alike in sequence, identical sequences aren’t necessarily homologous.

Orthologous genes are homologous genes families where one gene diverges following the event of speciation, but the gene itself and its primary purpose are preserved.

Suppose an individual gene is duplicated in an animal. In that case, the duplicated genes are the paralogs of one another, even though they may change in their sequence composition and function with time.

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