1. 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

2. Mutation 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…….

3. 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

4. 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

5. Mutation during DNA replication
Replication of DNA is not perfectly accurate, but there are several ways to correct the mistakes
ACGTACGTAACGTG...
TGCATGCATTGAACGGT
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.

6. 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

7. DNA excision repair Another system to repair mutated or damaged DNA a
Mutated DNA g a
One strand is nicked a
DNA removed between nicks a t
Correct DNA is synthesised

8. 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…..

9. 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

10. 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

11. 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.

12. 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
number
height