MUTATIONS: - A source of variation

A change in the nucleotide base sequence of the DNA or a change in

the structure or number of chromosomes

Mutagens

• Mutagens cause mutations which are mistakes in the base sequence of the DNA. (If you change the recipe, you change the cake)

There are two types: Point mutations and chromosomal mutations

•Point Mutations involve changing a single nucleotide base. There are 3 types:

1. Substitution mutations involve replacing one base with another. Like a sports team…

TAC ACC GTA TTC CAT TCC (old DNA) TAC ACC GTG TTC CAT TCC (new DNA)

This may or may not change the product being made, depending on where it occurs. If it is in the last position of a triplet, then not so bad, if it is in the first or second, then it probably will change.

2. Deletion mutations remove a base

TAC ACC GTA TTC CAT TCC (old DNA) TAC ACC GTT TCC ATT CC (new DNA)

3. Insertion mutations add an extra base TAC ACC GTA TTC CAT TCC (old DNA)

TAC ACC GTA TTT CCA TTC (new DNA)

Identify the mutation in the following sequence

• TTC TCC AGA CTT GAT TGA TCT

• Becomes

• TTC TCC AGA CTT TAT TGA TCT

• Or this one…• AAG AGG UCU GAA CUA ACU AGA

becomes

• UAG AGG UCU GAA CUA ACU AGA

Chromosomal mutations

• Having an extra chromosome or one too few• Sometimes, chromosomes fail to separate

during 1st or 2nd stages of meiosis• This means that one gamete (sex cell eg sperm

or egg) may be missing a chromosome OR• The others will have an extra chromosome and,

when fertilised, will have three copies – trisomy• Examples are: downs syndrome, kleinfelters

syndrome

Karyotypes (pictures of chromosome pairs)

• Normal male • Normal female

Examples: Patau’s syndrome

Downs syndrome