Primary Mechanism of Duplication : Unequal Crossing Over

Crossing overBetween Daughter

Strands

Addition (duplication)

Deletion

(tandom duplications)

Crossing overBetween Daughter

Strands

Addition

Deletion

Polyploidization : complete genome duplication

Processed Pseudogene

Creation of a processed pseudogene.

Duplication

Speciation

TD

TS

and are said to be paralogous genes

species1 and species2 are said to be orthologous genes

Species 1 Species 2

Examples of Duplicated Genes

Fate of Duplicated Loci

(1) Both copies retain original functione.g. rRNA genes

(2) Gain new function through mutation and selectione.g. globin genes

(3) Become functionless pseudogenese.g globin genes

All vertebrates but jawless fish have and

Figure 1. Phylogenetic relationship of amphibians (Xenopus and Ambystoma) to other model vertebrate

species and groups of the phylum Chordata.

Million years ago 500 400 300 200 100 0

XenopusAmbystoma

Chicken

PufferfishZebrafish

AmphioxusAgnathans

MouseHumana e

b

c

d

gf

ha = mammalsb = teleostsc = amphibiansd = amniotese = tetrapodsf = gnathostomesg = vertebratesh = chordates

H1: Two independent gene losses

H2: Duplication

Time of duplication may be estimated by observing thephylogenetic distribution of genes

Repeated Duplication Creates Gene Families

Globins are differentially expressed during development

Point mutations generally arise during DNA synthesis or repair

Error rate of DNA polymerasesand the

Efficiency of DNA repair are heritable traits.

Chromosomes, Genes, and Alleles

Traits are determinedto a great extent by genes

Different forms of a gene

are called alleles

Many different genes (loci)per chromosome

Locus A

Locus B

Locus C

Basic Unit of DNA

Nucleic acids are long chains of nucleotides

The Watson-Crick Model of DNA

In humans 35,000?? different proteins are

specified by the simple linear arrangement

of four bases

Nucleotide Bases

Adenine

Guanine

Cytosine

Thymine

}

}

Purines

Pyrimidines

Complimentary

5’ P---dR---P---dR---P---dR---P---dR---OH 3’ | | | | G A A C

3’ OH---dR---P---dR---P---dR---P---dR---P 5’

C T T G | | | |

Schematic Representation of DNA

G A A C

Classes of Gene

• Protein-coding genes : transcribed and translated

• RNA specifying genes : transcribed

• Untranscribed genes

Intron 1 Intron 2

Exon 1 Exon 2 Ex 3 3’ 5’

Initiationcodon

GT AG GT AG

RegulatorySequences

Stopcodon

5’ UTR 3’ UTR

Schematic of Eukaryotic Protein-Coding Locus

How does mRNA specify the amino acid sequence of the protein?

Every three nucleotides forms a codon : each codon codes for an amino acid

Note : different codons specify the same amino acid

Figure 4.3 The Genetic Code (Codons of mRNA)

(i.e. Code is Degenerate)

Example of RNA specifying gene :rDNA array

18S 28SNTS

ETS ITS-1

5.8S

ITS-2

1 array in Tetrahymena19,300 copies in Amphiuma200 copies in Human

ITS NTS ETS 18S ITS 28S NTS

28S

28S

28S

28S

28S

human

chimp

pygmy chimp

gorilla

a BamHIb HindIc BgIIId HpaI

d

d

d

d

a

a

a

a

b

b

b

b

c c

c

c

c

c

c

c

Arnheim, 1983