The main properties of dna
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The main properties of DNA. The genetic material must be able to: Store information Replicate (when cells divide) Express information (as proteins) Mutate at a low frequency (less than 1 in a million) DNA is a molecule that is very well suited to doing all 4 of these. Mutation.

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The main properties of dna
The main properties of DNA

  • The genetic material must be able to:

    • Store information

    • Replicate (when cells divide)

    • Express information (as proteins)

    • Mutate at a low frequency (less than 1 in a million)

  • DNA is a molecule that is very well suited to doing all 4 of these


  • Can occur in any cell at any time, cause may be:

    • Internal (e.g. mistakes during replication of DNA)

    • External (e.g. radiation, chemicals)

  • Most mutations have no effect (neutral)

  • A few mutations are harmful

  • A very few mutations are beneficial

  • Only harmful and beneficial mutations are acted on by natural selection

  • Mutations may be non-coding (not in part of gene that codes for protein - have no effect, or affect gene expression) or coding…….

Effects of coding mutations
Effects of coding mutations

  • Synonymous: the cat ate the rat

  • Missense: the fat ate the rat

  • Nonsense: the cat ate the

  • Frameshift: the cax tat eth era t

  • Synonymous has no effect on protein, nonsense makes a smaller protein, missense/frameshift make incorrect protein

Conditional mutations
Conditional mutations

  • The effects of many coding mutations depend on environmental factors

  • Siamese cats have mutation in enzyme for black pigment production, that stops it working at normal body temperature

  • Cooler parts of cat are dark because enzyme OK at lower temperature

Mutation during dna replication
Mutation during DNA replication

  • Replication of DNA is not perfectly accurate, but there are several ways to correct the mistakes



DNA polymerase makes about 1 mistake per 105 bp.

DNA polymerase has a “proof-reading” activity to correct its

own mistakes (99%).

After DNA replication there is a “mismatch repair” system to

correct remaining mistakes (99.9%).

This leaves an overall error rate of about 1 base in 1010.

Mutation due to environmental factors
Mutation due to environmental factors

  • Mutations may be caused by chemicals or radiation

  • Chemicals (“mutagens”) may disrupt hydrogen bonds between bases, by modifying them or getting between them

  • Radiation (including ultra-violet and radioactive emissions) can damage structure of bases

  • These agents may be natural or man-made

Dna excision repair








DNA excision repair

  • Another system to repair mutated or damaged DNA

Mutated DNA

One strand is nicked

DNA removed between nicks

Correct DNA is synthesised

Application mutagen testing
Application - mutagen testing

  • Mutation in somatic (body) cells during the lifetime of an animal can cause cancer

  • It is vital to know if chemicals to which we are exposed are mutagenic

  • Bacteria can be used to test this: the Ames Test

  • Reverse mutation is where the mutant form of an organism mutates again, to go back to the original wild-type state

  • The Ames Test uses a mutant strain of bacterium Salmonella typhimurium that cannot make the amino-acid histidine…..

The ames test
The Ames Test

Culture of

His- bacteria

Petri dishes containing chemical to test,

liver extract, no histidine

The more mutagenic the chemical,

the more His+ colonies are produced

Number of colonies

Amount chemical

Phenotype genotype alleles
Phenotype, Genotype, Alleles

  • The phenotype of an organism is its observable properties

  • The genotype is the set of alleles it has for all of its genes (5,000 in bacteria; 40,000 in humans)

  • The relationship between genotype and phenotype is what genetics is all about

  • New alleles are created by mutation and their effect the phenotype may be dominant or recessive

Significance of genetic variation
Significance of genetic variation

  • Some alleles directly cause specific traits, such as (in humans) rare genetic diseases e.g. Cystic fibrosis, sickle-cell anaemia; (in bacteria) ability to grow on certain sugars

  • Many alleles contribute to many traits of an organism such as size, shape, intelligence, behaviour, and risk of getting diseases e.g. (in humans) cancer, heart disease, asthma

  • Genetic variation is what evolution acts on. Without it there would be no different species.

Multiple genes and quantitative traits
Multiple genes and quantitative traits

  • Many traits like height, IQ show a bell-shaped (normal) distribution in population

  • These are influenced by several genes, so the overall effect depends on the random selection of alleles in an individual

  • e.g. for height genes, you are more likely to have a mixture of tall and short alleles than all tall or all short