MUTATIONS And their effect

Mutations This is any change in the DNA sequence that is not immediately and properly repaired, or any inherited change in the sequence of DNA. Mutations that occur in the body cells cause cell death or cancer, and are not passed on to the next generation. Mutations are usually recessive and are inherited in a Mendelian way.

Mutations The majority of mutations are disadvantageous. Mutations may result from mistakes in DNA replication, or may be caused by mutagenic agents. Mutations occur spontaneously. (A normal spontaneous mutation rate for a single gene is 1 mutation in 106 – 108 replications, this may be increased by mutagenic agents.)

Mutations There are 2 main types of mutations: Gene Mutations – a change in a nucleotide coding. Chromosome Mutations – these can effect: The number or arrangement of genes on the chromosomes. The number of chromosomes. The number of sets of chromosomes.

DNA Repair There are several complex DNA repair systems in the body. Some mend specific problems, e.g. the damage caused by UV radiation. In rare cases, a human is born without the enzyme system that repairs this damage, these people have to live out of the sun.

DNA Repair Usually enzymes can tell when there is a wrong base pairing, e.g. A with G. The repair enzymes go to work, cutting out a chunk from one side of the DNA containing the mistake. Then new enzymes, using the base pairing code, make a new side from the opposite strand of DNA. The new strand is put into place by another enzyme system.

Mutagenic Agents Ionising Radiation This sometimes causes simple base changes by substituting one base for another, (this may or may not have a bad effect.) It can also cause deletions of large portions of genetic material.

Mutagenic agents cont Ultra-violet light This can cause 2 adjacent thymine bases to bond to each other to form a thymine dimer. An unrepaired dimer inactivates the strand of DNA, so it can not replicate or make mRNA and thus protein.

Mutagenic agents cont X-rays The effect of x-rays is cumulative. An overdose of x-rays causes more harmful mutations. e.g. Muller’s experiments with Drosophila ref pg 136 txtbk

Mutagenic agents cont Chemicals Mustard gas, formaldehyde, nitrous acids, epoxides, acridine dyes and alkylating agents can all cause mutations. Nitrous acid, found in cigarette smoke changes cytosine to uracil. It can also substitute wrong, but similar chemicals for the bases adenine and guanine so when a substituted base DNA tries to replicate all sorts of mistakes are made.

Mutagenic agents cont There is mounting evidence of a correlation between mutagens and carcinogens. Many cancers are caused by a mutation in the body, thus disrupting metabolism and growth causing disease.

Gene Mutations Substitution A single base is changed. This can alter a codon which may or may not have an effect. GAA to GAG would have no effect as it codes for the same amino acid. GAA to GUA changes glutamic acid to valine causing sickle-cell anaemia. A change to the ‘stop’ codon would stop production of a polypeptide at the wrong place. A change in the ‘stop’ codon would cause the polypeptide to go on and on and on.

Substitution

Substitution

Gene Mutations Inversion The positions of 2 nucleotides are inverted. If this happens within a codon it will only affect 1 amino acid.

Gene Mutations Insertion and Deletion Adding or removing a nucleotide causes a shift in the reading frame, this is known as a “frame shift mutation” This results in the wrong amino acids being picked up from the point of mutation onwards. These mutations are usually lethal.

Insertion Deletion

Chromosome Mutations Rearrangement of Genes this is an alteration of the genes either in number or sequence. Deletion a piece of the middle of the chromosome breaks off, and the two ends join up.

Deletion

Chromosome Mutations Inversion a piece of the chromosome falls out, is rotated 180 before it moves back into the chromosome.

Chromosome Mutations Translocation moves a segment of one chromosome to another, non-homologous one. Reciprocal translocation 2 non-homologous chromosomes exchange fragments Non-reciprocal translocation 1 fragment breaks off and joins on to a non-homologous chromosome without receiving a fragment in return.

Translocation

Chromosome Mutations Duplication a segment of the chromosome is repeated. This can have a mild or severe effect depending on how many repeats there are.

Duplication