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