Genome Evolution Evolution of Gene clusters

Why are genes are arranged in clusters? Many genes are arranged in groups of related genes along a chromosome - gene clustersMany genes are arranged in groups of related genes along a chromosome - gene clusters Eukaryotic ribosomal RNA genes: tandem repeatsEukaryotic ribosomal RNA genes: tandem repeats ITS28SITS28SNTS18SETSNTS 45S

Gene Families Related genes may be organized is several clusters at different locations These are known as gene families e.g. Globin genesRelated genes may be organized is several clusters at different locations These are known as gene families e.g. Globin genes

Gene Families

Gene duplication and misalignment The misalignment of genes during recombination is the most likely cause of gene duplication and clustering.The misalignment of genes during recombination is the most likely cause of gene duplication and clustering. Once repeats exist the probability of misalignment increases.Once repeats exist the probability of misalignment increases.

Misalignment - simple case A A B B A A B B

Misalignment - Globin

But not all DNA codes for genes Single copy DNA can be used as a rough guide to the amount of the DNA that actually codes for genes.Single copy DNA can be used as a rough guide to the amount of the DNA that actually codes for genes. % of single copy DNA ranges from 95% in some arthropods to 12% in an Amphibian (newt)% of single copy DNA ranges from 95% in some arthropods to 12% in an Amphibian (newt) Human have 64% SC DNAHuman have 64% SC DNA

Types of repetitive DNA Tandem RepeatsTandem Repeats –Microsatellites (repeats 2-5 b pairs) 100 repeats per loci –Minisatellites (repeats 15 base pairs) average length nucleotides –Satellite DNA (5-100 base pairs) found in blocks of 1000 or more repeats

Types of repetitive DNA Scattered repeats - Longer sequences (100 or more base pairs) scattered through the genome usually as single copiesScattered repeats - Longer sequences (100 or more base pairs) scattered through the genome usually as single copies

Selfish DNA - tandem repeats Most non-coding DNA is selectively neutralMost non-coding DNA is selectively neutral It will passively replicate and may be duplicates as with coding tandem repeatsIt will passively replicate and may be duplicates as with coding tandem repeats It will continue to accumulate until it’s mass slows the cell cycle then it will be selected againstIt will continue to accumulate until it’s mass slows the cell cycle then it will be selected against

Selfish DNA - Scattered Repeats Transposable elementsTransposable elements –Transposition by reverse transcription - disrupts genome and increase number of copies of self –Transposition without reverse transcription - disrupts genome

Selfish DNA - Scattered Repeats Origin - are they ancient and very successful virus?Origin - are they ancient and very successful virus?

The C-factor Paradox Why is there excess DNA?Why is there excess DNA? Selfish DNA provides an attractive hypothesis to solve this question.Selfish DNA provides an attractive hypothesis to solve this question. It would also explain why apparently functionless DNA is functionlessIt would also explain why apparently functionless DNA is functionless