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Homology among DNA, RNA, or proteins is typically inferred from their nucleotide or amino acid sequence similarity. Significant similarity is strong evidence that two sequences are related by evolutionary changes from a common ancestral sequence. Alignments of multiple sequences are used to indicate which regions of each sequence are homologous.
Homologous sequences are orthologous if they are descended from the same ancestral sequence separated by a speciation event: when a species diverges into two separate species, the copies of a single gene in the two resulting species are said to be orthologous. The term "ortholog" was coined in 1970 by the molecular evolutionist Walter Fitch. [41]
In contrast, less condensed chromatin (Euchromatin)—which tends to be rich with guanine and cytosine and more transcriptionally active—incorporates less Giemsa stain, and these regions appear as light bands in G-banding. [3] The pattern of bands are numbered on each arm of the chromosome from the centromere to the telomere.
The main diagonal represents the sequence's alignment with itself; lines off the main diagonal represent similar or repetitive patterns within the sequence. In bioinformatics a dot plot is a graphical method for comparing two biological sequences and identifying regions of close similarity after sequence alignment. It is a type of recurrence plot.
The two pathways for homologous recombination in eukaryotes, showing the formation and resolution of Holliday junctions. The Holliday junction is a key intermediate in homologous recombination, a biological process that increases genetic diversity by shifting genes between two chromosomes, as well as site-specific recombination events involving integrases.
Branch migration is the process by which base pairs on homologous DNA strands are consecutively exchanged at a Holliday junction, moving the branch point up or down the DNA sequence. [1] Branch migration is the second step of genetic recombination, following the exchange of two single strands of DNA between two homologous chromosomes. [2]
Gene conversion is the process by which one DNA sequence replaces a homologous sequence such that the sequences become identical after the conversion. [1] Gene conversion can be either allelic, meaning that one allele of the same gene replaces another allele, or ectopic, meaning that one paralogous DNA sequence converts another.
The use of the same nucleotide sequence to encode multiple genes may provide evolutionary advantage due to reduction in genome size and due to the opportunity for transcriptional and translational co-regulation of the overlapping genes. [12] [21] [22] [23] Gene overlaps introduce novel evolutionary constraints on the sequences of the overlap ...