Presentation on theme: "The main properties of DNA"— Presentation transcript:
1 The main properties of DNA The genetic material must be able to:Store informationReplicate (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 harmfulA very few mutations are beneficialOnly harmful and beneficial mutations are acted on by natural selectionMutations 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 ratMissense: the fat ate the ratNonsense: the cat ate theFrameshift: the cax tat eth era tSynonymous 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 factorsSiamese cats have mutation in enzyme for black pigment production, that stops it working at normal body temperatureCooler 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 mistakesACGTACGTAACGTG...TGCATGCATTGAACGGTDNA polymerase makes about 1 mistake per 105 bp.DNA polymerase has a “proof-reading” activity to correct itsown mistakes (99%).After DNA replication there is a “mismatch repair” system tocorrect 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 radiationChemicals (“mutagens”) may disrupt hydrogen bonds between bases, by modifying them or getting between themRadiation (including ultra-violet and radioactive emissions) can damage structure of basesThese agents may be natural or man-made
7 DNA excision repair Another system to repair mutated or damaged DNA a Mutated DNAgaOne strand is nickedaDNA removed between nicksatCorrect DNA is synthesised
8 Application - mutagen testing Mutation in somatic (body) cells during the lifetime of an animal can cause cancerIt is vital to know if chemicals to which we are exposed are mutagenicBacteria can be used to test this: the Ames TestReverse mutation is where the mutant form of an organism mutates again, to go back to the original wild-type stateThe 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 histidineThe more mutagenic the chemical,the more His+ colonies are producedNumber of coloniesAmount chemical
10 Phenotype, Genotype, Alleles The phenotype of an organism is its observable propertiesThe 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 aboutNew 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 sugarsMany 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, asthmaGenetic 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 populationThese are influenced by several genes, so the overall effect depends on the random selection of alleles in an individuale.g. for height genes, you are more likely to have a mixture of tall and short alleles than all tall or all shortnumberheight