Download Lecture 17 POWERPOINT here

Questions
09_12_Mutation.jpg
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 Africa
Every 300m = gene
Every gene = 30m
Coding 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

09_02_Germ_somatic2.jpg
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 mispairing during replication
09_05_Gene.duplicate.jpg
Gene Duplications




Entire genes duplicated - by repetitive
sequences on each side of the gene mispairing 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
09_13_conjugation.jpg
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

09_15_Phylogen.trees.jpg