2. Mutations

• Gene mutation involve change in DNA base sequence of chromosomes. Types of errors that can occur:

  • Substitution: one base is substituted with another, e.g: ATGCCT becomes ATTCCT (G swapped for T)
  • Deletion: one base is deleted, e.g. ATGCCT becomes ATCCT (G removed)
• Order of DNA bases in gene determine order of amino acids in particular protein. If mutation occurs in gene, sequence of amino acids it codes for (and protein formed) could be altered
• Degenerate nature of genetic code means that some amino acids are coded for more than one DNA triplet (e.g. tyrosine can be coded for TAT or TAC in DNA). This means that not all substitution mutations will results in change to amino acid sequence of protein- some substitution will still code for same amino acid
• Substitution mutation won’t always lead to changes in amino acids sequence, but deletion will- deletion of base will change number of bases present, which will cause shift in all base triplet after it
• Mutations occur spontaneously e.g when DNA is misread during replication. But some things can cause increase in rate of mutations- these are called mutagenic agents. UV radiation, ionising radiation, some chemicals and some viruses are examples of mutagenic agents. Mutagenic agents increase probability of mutation occuring
• Chromosome non-disjunction: failure of chromosome to separate properly
• Sometimes Meiosis goes wrong and cells produced contain variations in number of whole chromosomes or parts of chromosomes. E.g. 2 cells might have 23 whole chromosome, one each from 1-23, but the other 2 might get a bit muddled up, 1 having 2 chromosome 6’s and other no chromosome 6. Called chromosome mutation and is caused because errors are present in gametes
• Down syndrome is caused by a person having an extra copy of chromosome 21 (or part). Non-disjunction means that chromosome 21 fails to separate properly during meiosis, so one cell gets extra copy of 21 and another gets none. When a gamete with extra copy fuses to another gamete at fertilisation, resulting zygote will have 3 copies of chromosome 21