GENOMEGENOME

GENOME ORGANIZATION ACQUISITION OF NEW GENES ORIGIN OF INTRONS

Ms.ruchi yadavlecturer

amity institute of biotechnologyamity university

lucknow(up)

HUMAN GENOME HUMAN GENOME STRUCTURE STRUCTURE

CONTENTS OF NUCLEAR CONTENTS OF NUCLEAR GENOMEGENOME

MITOCHONDRIAL GENOMEMITOCHONDRIAL GENOMEAt just 16 569 bpContains just 37 genes.13 of these genes code

for proteins involved in the respiratory complex

The other 24 specify non-coding RNA molecules that are required for expression of the mitochondrial genome.

GENOME EVOLUTIONGENOME EVOLUTION

Genome EvolutionGenome Evolution

Acquisition of New Genes

There are two ways in which new genes could be acquired by a genome: ◦By duplicating some or all of the

existing genes in the genome.◦By acquiring genes from other

species

I. Acquisition of new genes by gene duplication

By duplication of the entire genome;

By duplication of a single chromosome or part of a chromosome;

By duplication of a single gene or group of genes.

Whole-genome duplications can result in sudden expansions in gene number

The basis of autopolyploid

 Autopolyploids cannot interbreed successfully with their parents

Hugo de Vries, During his work with evening primrose, Oenothera lamarckiana, de Vries isolated a tetraploid version of this normally diploid plant, which he named Oenothera gigas.

Duplications of individual genes and groups of genes have occurred frequently in the past

Multigene families are common components of all genomes

Examples of multigene families of nonidentical genes are two related families of genes that encode globins

Human Globin GenesHuman Globin Genes

There are several mechanisms by which these gene duplications could have occurred:

Alterations of Chromosome StructureHumans have 23 pairs of

chromosomes, while chimpanzees have 24 pairs

Following the divergence of humans and chimpanzees from a common ancestor, two ancestral chromosomes fused in the human line

Alterations of Chromosome Structure

Genome evolution also involves rearrangement of existing genes

• This is possible because most proteins are made up of structural domains.

• Each domain comprise a segment of the polypeptide chain and hence encoded by a contiguous series of nucleotides.

There are two ways in which rearrangement of domain-encoding gene segments can result in novel protein functions.

Domain shuffling: segments coding for structural domains from completely different genes are joined together to form a new coding sequence that specifies a hybrid or mosaic protein

Domain DuplicationDomain DuplicationThe α2 Type I collagen has repetitive Gly-X-Y It codes for 338 of these repeats, is split into

52 exons, 42 of which cover the part of the gene coding for the glycine-X-Y repeats.

The number of repeats per exon varies but is 5 (5 exons), 6 (23 exons), 11 (5 exons), 12 (8 exons) or 18 (1 exon).

Gene have evolved by duplication of exons leading to repetition of the structural domains.

Domain shuffling: Tissue plasminogen activator (TPA),

Evolution of Genes with Novel Evolution of Genes with Novel FunctionsFunctions

For example the lysozyme gene was duplicated and evolved into the α lactalbumin gene in mammals

Lysozyme is an enzyme that helps protect animals against bacterial infection

α-lactalbumin is a nonenzymatic protein that plays a role in milk production in mammals

II. Acquisition of new genes from other speciesComparisons of bacterial and archaeal genome

sequences suggest that lateral gene transfer has been a major event in the evolution of prokaryotic genomes

Conjugation , for example, enables plasmids to move between bacteria and frequently results in the acquisition of new gene functions by the recipients

A second process for DNA transfer between prokaryotes, transformation , is more likely to have had an influence on genome evolution.

Non-coding DNA and Genome EvolutionTransposable elements and genome

evolution◦ Transposons can initiate recombination

events between chromosomes or between different sites on the same chromosome that lead to genome rearrangements.

Origins of intronsOrigins of intronsSelf-splicing introns evolved in the RNA

world and have survived ever since without undergoing a great deal of change

Origins of GU-AG introns:‘Introns early’ states that introns are

very ancient and are gradually being lost from eukaryotic genomes.

‘Introns late’ states that introns evolved relatively recently and are gradually accumulating in eukaryotic genomes.

‘introns early’ called ‘exon theory of genes’

Minigenes became exons and the DNA sequences between them became introns.

Introns early predictionIntrons early prediction ‘Introns early’ predicts that there should be a

close similarity between the positions of introns in homologous genes from unrelated organisms, because all these genes are descended from an ancestral intron-containing gene

First evidence obtained supported ‘introns early’.• A study of vertebrate globin proteins concluded that each of these comprises four structural domains,

• The first corresponding to exon 1 of the globin gene, the second and third to exon 2, and the fourth to exon 3

• Leghemoglobin gene of soybean was shown to have an intron at exactly the expected position(splits the second and third domains )

Comparing genome sequencesprovides clues to evolution and development