5. Genetic Variation

7/28/2019 5. Genetic Variation

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2. Genetics Variations


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Binary fission in prokaryotic cell

Replication of bacterialchromosome

Asexual reproduction

offspring genetically

identical to parent

where does variation

come from?


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Genetic Variation : in bacteria

Sources of variation spontaneous mutation

transformation

plasmids DNA fragments

transduction

conjugation

transposon

bacteria shedding DNA


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Spontaneous mutation

Spontaneous mutation is asignificant source of variation

in rapidly reproducing species

Example: E. coli

human colon (large intestines)

2 x 1010 (billion) new E. colieach day!

spontaneous mutations

for 1 gene, only ~1 mutation in 10 million replications each day, ~2,000 bacteria develop mutation in that gene

but consider all 4300 genes, then:

4300 x 2000 = 9 million mutations per day per human

host!


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Transformation

Bacteria are opportunists pick up naked foreign DNA wherever it

may be hanging out

have surface transport proteins that arespecialized for the uptake of naked DNA

import bits of chromosomes from other

bacteria

incorporate the DNA bits into their ownchromosome

express new gene

form of recombination


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Conjugation

Direct transfer of DNA between 2 bacterial cellsthat are temporarily joined results from presence of F plasmid with F factor

F for fertility DNA

E. coli male extends sex pilli, attaches to femalebacterium

cytoplasmic bridge allows transfer of DNA


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Transduction

Phage viruses carry

bacterial genes from one

host to another


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Transposons

Insertion oftransposon

sequence in new

position in genomeInsertion sequences

cause mutations

when they happen to

land within the codingsequence of a gene or

within a DNA region

that regulates gene

expression


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Genetic variation : in human

Genetic variability in sexual reproduction independent assortment

homologous chromosomes in meiosis 1

crossing over between homologous chromosomes in prophase 1

random fertilization

random ovum fertilized by a random sperm

metaphase1


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Independent assortment

Meiosis introduces genetic variation gametes of offspring do not have same genes

as gametes from parents

genetic recombination random assortment in humans produces 223

(8,388,608) different combinations

from Mom from Dad new gametesmade by offspring


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Crossing over

During prophase 1 homologous pairs swap

pieces of chromosome

sister chromatids intertwine crossing over

tetrad

synapsis


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Meiosis & Crossing over

Meiosis & crossing over introduce greatgenetic variation to population

drives evolution


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Sexual reproduction creates variability

Sexual reproduction allows us to maintain bothgenetic similarity & differences.

Martin & Charlie Sheen, Emilio Estevez

Michael & Kirk

Douglas

Baldwin brothers


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The end

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5. Genetic Variation