Questions

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Gene Evolution

Pages 293 - 319

Learn…

How did your current genes get here… The role of mutations on genes… Transposons & evolution The role of genes on evolution… Phylogenetic studies

Perspective on the human genome

If we expand the distance between the bases to 1mm

Then the human genome would extend 3200km - stretch across central AfricaEvery 300m = geneEvery gene = 30mCoding region = 1m

More perspectives on the human genome Just about 2% of the entire genome is coding About another 2% has a regulatory role About 95% is ‘junk’ DNA! Most mutations in the ‘junk’ DNA are neutral and

free to accumulate without effect 200 cell divisions from conception to gamete

production 6.4 billion bp of DNA = 100 new differences in the DNA between parent

and child sequences each generation

Genetic variation

Nature has struck a good balance between maintaining the integrity of DNA replication allowing the functioning of the cell & the incorporation of genetic variation for evolution.

In evolution the germ cells are the important factor and not somatic cells

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5 types of genetic change

Gene Mutation Gene Duplication Gene Deletion Exon Shuffling Horizontal Gene Transfer

Gene Mutation

Single base changes (point mutations) Substitutions - swapping of one base for

another - A to G Point deletions - loss of one base Point additions - addition of a base

E.coli maintains about 1 mistake in 10E9, and humans 1 in 10E10

Gene Mutation… Advantageous - gives a selective advantage

and passed on to offspring Selectively Neutral Mutations - no effect and is

kept - also known as Silent Mutations Deleterious - harmful and result in the death of

the cell or individual, normally prior to reproduction

Gene Duplications

Entire genes duplicated - by repetitive sequences on each side of the gene mis-pairing during replication

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Gene Duplications Entire genes duplicated - by repetitive

sequences on each side of the gene mis-pairing during replication

Most important mechanism for generating new genes

Newly duplicated gene is free to diverge through more mutations - duplication and divergence

Gives rise to related gene families - opsin visual pigment genes & globin gene family

Gene Duplications…Globins < 500 million years ago - a single globin gene

was responsible for carrying oxygen 500 million years ago - a duplication took place

in fish followed by divergence - leading to the genes for alpha and beta globin

Then came the alpha2:beta2 hemoglobin In mammals the beta duplicated to give rise to

epsilon chain - used in fetal hemoglobin

Localized duplications In genes such as for immunoglobulin proteins,

each domain is coded for by an exon. Duplication of exons leads to additional

domains in the protein. Since exons are flanked by long introns then

misalignment of introns can introduce exon dulpications.

The 30,000 human genes are proposed to have arisen by duplication and shuffling of just a few thousand distinct exons.

Transposons Transposons are mobile DNA elements akin to

plasmids in bacteria. They are present in large numbers (500,000 Alu-like transposons in human genome)

They are constantly moving around the genome When two Alu-like transposons flank a gene

they sometimes transpose the gene too to the new position.

Horizontal transfer Normal gene transfer through reproduction is

termed VERTICAL GENE TRANSFER Gene transfer across species is termed

HORIZONTAL GENE TRANSFER Very rare in Eukaryotes, but common in

prokaryotes Use of sex pilus to undergo conjugation Important for Antibiotic resistance Theory that the earliest cells exchanged genes via

this method and then differentiated into the three divisions

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Time & relationship Comparison of critical gene sequences

allows the determination of the evolutionary time.

Man and chimp had a common ancestor about 5 million years ago (mya)

Man and Gorilla about 8 mya Man and Orangutan about 13 mya

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