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Mechanisms of Evolution

Beyond Darwin and Neo-Darwinism

mispairing

Mispairing refers to the presence of at least one nucleotide in one strand of a DNA molecule, which is not the complement of the nucleotide at the corresponding locus in the other strand.

Slipped strand mispairing occurs during DNA replication. Regions of DNA that are capable of assuming hairpin-like secondary structures are particularly prone to this error because displacement of the strands disrupts alignment of bases. Mispairing results in the repeated replication of the same stretch of DNA, and provides an explanation for satellite DNA.

Deletion and insertion mutations, indels, often occur in repetitive sequences. For example, deletion of "AT" from the sequence "ATAT" in the CFTR gene. Such mutations are most often caused by a "replication slippage", where the new strand mispairs with the template strand at repetitive sequences.

Tandem repeat sequences are particularly prone to high mutation rates because of mispairing and replication slippage. Slippage can cause mispairing of several repeats. Forward slippage results in deletion mutations, while backward slippage results in insertions. Replication slippage results in mispairing not because the nucleotides are altered, but because the number of repeats may vary between one strand and the other. Such repeats are retained in sections of the genome that can tolerate variability.

Replication slippage is mainly responsible for microsatellite polymorphisms, which are also called short tandem repeats (STR) or simple sequence repeats (SSR). In microsatellites, the repeat unit comprises only 1 to 6 bp and the whole repetitive region spans less than 150 bp, while minisatellites range from 1 to 20 kb, and satellites span from 100 kb to more than 1 Mb. SSRs occur throughout the genome, while tandem repeats occur in telomeres and centromeres. The nucleotide sequence of repeats is fairly well conserved across a species, but variation in the length of the repeat is common, as in VNTRs, or variable number tandem repeats.

Once an alteration (such as a point-mutation) occurs within a tandem repeat, the cell's replication machinery is able align the two DNA strands and the alteration will be replicated and inherited. Subsequent deletion of the alignment-promoting alteration would restore slippage. Thus, purity of repetition indicates a lengthy history of deletion and expansion. Strands of repeated sequences can code for polypeptides with repeated amino acids, and the ratio of such poly-amino-acid chains can regulate transcription factors. Polyglutamine can increase the rate of transcription, while polyalanine reduces it. Polyadenylation is a stage of RNA processing in which the 3’ end of the pre-mRNA is cleaved before a stretch of adenosines are added to the end of the molecule.

Animation of slipped strand DNA mispairing DNA Animation :
 Table Mechanisms of Biological Evolution :  Gene Regulation in E.coli :

External : Tandem repeats and morphological variation

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12:11 PM  
Blogger qtr said...

Allopatric speciation occurs when a geographical barrier sub-divides a parent species, resulting in geographic and reproductive isolation such that the descendent species can no longer interbreed upon removal of the barrier.


Anagenesis differs from cladogenesis in that one species progressively transforms into a replacement species when sufficient gene mutations fix in the descendant population. At this point, the ancestral species has become extinct. This mechanism is distinct from the increase in numbers of species generated by cladogenetic branching events.

Cladogenesis is the mechanism of speciation in which one or more lineages (clades) arise from an ancestral line. Such speciation events increase the variety of plants or animals through branching of the phylogenetic tree. Cladogenesis is differentiated from anagenesis, which is the in toto replacement of one species by an anatomically distinct species.

Monophyletic taxon or clade: an accurate grouping of only (opp. polyphyletic) and all (opp. paraphyletic) descendents of a shared common ancestor. A monopyletic group is genetically homogeneous and reflects evolutionary relationships.

Paraphyletic taxon or clade: a monophyletic group that excludes one or more discrete groups descended from the most recent common ancestral species of the entire group. Other descendent species of the most recent common ancestor have been excluded from the paraphyletic taxon, usually because of morphologic distinctiveness.

Phenetic system: groupings of organisms based on mutual similarity of phenotypic (physical and chemical) characteristics. Phenetic groupings may or may not correlate with evolutionary relationships.


Phylogenetic system: groups organisms based on shared evolutionary heritage. DNA and RNA sequencing techniques are considered to give the most meaningful phylogenies.

Phylogenetic separation into evolutionary relationships (clades), based on comparison of genomes is likely to supplant phenotypical (phenetic) taxonomies of the prokaryotes.

Peripatry (paripatry) is a subset of allopatry in which an isolated group has a smaller population than the parent group. Ernst Mayr introduced the term. Peripatric speciation occurs when the smaller sub-group of a species enters a novel niche within the range of the parent species, becoming geographically and reproductively isolated. Peripatric speciation (paripatric) is distinguished from allopatric speciation by the smaller size of the isolate group, and from sympatric speciation, which involves no barrier to breeding.

Polyphyletic taxon: opposite to monophyletic taxon: A polyphyletic group is mistakenly or improperly erected on the basis of homoplasy.—characteristics that have arisen despite not sharing a common ancestor. Homoplasy arises because of convergent evolution, parallelism, evolutionary reversals, horizontal gene transfer, or gene duplications. Polyphyletic taxa are genetically heterogeneous because members do not share a common ancestor.

Neontology is a branch of biology that emphasizes the study of modern biota (living or recent organisms) rather than fossilized organisms (paleontology).

Numerical Taxonomies are a common approach to phenetic taxonomy that employ a number of phenotypic characteristics to generate similarity coefficients that may be mapped in dendrograms. Groupings based on numerical taxonomy may or may not correlate with evolutionary relationships.

Taxonomies aim to group organisms according to shared characteristics against the background of biological diversity.

Sympatry involves no geographical separation of sub-populations of individuals. Sympatric speciation events occur most often in plants by the mechanism of polyploidy in which the number of chromosomes is doubled or tripled. John Maynard Smith proposed a model called disruptive speciation, in which homozygotes might have greater fitness than heterozygotes under some environmental conditions.

4:59 PM  

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