(1) Mutations

• a mutation is a change in the quantity or sequence of base pairs in a DNA molecule
• mutations occur continuously - all the time
• why does mutation have no effect most of the time even though it is continuou ?

  most DNA is introns so does not affect the sequence of amino acids that will be coded
• mutations that occurs during the formation of gametes may be inherited - meiosis
• mutations that occur during the formation of gametes are often what produces sudden and distinct differences between individuals
• any change to one or more nucleotide bases or a change in the sequence of bases is known as gene mutation
• Mutation = a change in the quantity or sequence of DNA bases
• Gene Mutation = a change in the sequence of DNA bases (general definition)
• as the sequence of specific DNA triplet codes determine the sequence of amino acids, mutations in a gene can sometimes lead to a change in the polypeptide that the gene codes for
• Most Mutations :

  • do not alter the polypeptide
  • only alter it slightly so that its structure or function is not changed
• Mutations can arise :

  • simultaneously during DNA replication
  • when cells are exposed to carcinogens or radiation
• carcinogens don't cause cancer but increases the mutation rate (greater chances)
• most mutations do not alter the polypeptide because :

  • mutations occur in the non coding part of DNA (intorns)
  • DNA code is degenerate
• the three different mutations are :

  • substitution (point mutation)
  • deletion
  • insertion
• Substitution Mutation :

  • occurs when base in the DNA sequence is replaced for a different base
  • will only change the amino acid for the triplet in which the mutation occurs
  • will not have a knock-on effect
• the three different forms of substitution mutation are :

  • silent mutation
  • missense mutations
  • nonsense mutations
• Silent Mutations :

  • does not alter the amino acid sequence of the polypeptide
  • because certain codons may code for the same amino acid as the genetic code is degenerate or occurs in introns
• Missense Mutations :

  • alters a single amino acid in the polypeptide chain
  • a single substitution mutation changes a single amino acid in the sequence
  • sickle cell anaemia is an example of missense mutation
• Nonsense Mutations :

  • creates a premature stop codon - leads to a short polypeptide
  • signals for the cell to stop translation of the mRNA molecule into an amino acid sequence
  • causes polypeptide chain that is produced to be incomplete - affects the final protein structure and function
  • cystic fibrosis is an example of nonsense mutation - not always the only cause
• Deletion Mutation :

  • occurs when a nucleotide is randomly deleted from the DNA sequence
  • single deletion will see little change - consequences can be considerable
  • has a knock on effect - sequence of DNA are read in units of three (triplet)
  • if one nucleotide (base) is deleted, this changes the triplet code that follows - frameshift mutation
  • may drastically change the amino acid sequence produced from that gene - changes the ability of the polypeptide to function
• frameshift mutation :

  insertion or deletion shifts the sequence of the bases, changing the following triplet
• in deletion, the triplet shifts - the amino acid that are coded for after the deletion are now changed
• Insertion Mutation :

  • occurs when a nucleotide (with a new base) is randomly inserted/ added into the DNA sequence
  • changes the amino acids that would have been coded for by the original base triplet - created new, different triplet of bases
  • has a knock on effect by changing the triplets further on in the DNA sequence
  • frameshift mutation
  • may drastically change the amino acid sequence produced from that gene - changes the ability of the polypeptide to function
• insertion and deletion mutation is also known as frameshift mutation
• chromosome mutations = changes in the structure or number of whole chromosomes
• chromosome mutations can arise simultaneously
• the two forms of Chromosome Mutations are :

  • changes in whole sets of chromosomes
  • changes in the number of individual chromosomes
• Changes in Whole Sets of Chromosomes :

  • when organisms have 3 or more sets of chromosomes rather than the usual two (diploid - double chromosomes)
  • called polyploidy - multiple sets of chromosomes
  • occurs mostly in plants
• Changes in the Number of Individual Chromosomes :

  • caused when homologous pairs fail to seperate during meiosis
  • mutations often occur during interphase - when DNA is replicated
  • known as non-disfunction
  • results in gametes with an extra chromosomes or one less chromosome
  • if these gametes are fertilised the result could be down syndrome - individual has an extra chromosome 21 (trisomy)
• trisomy is the scientific name of down syndrome
• non disjunction = when homologous pairs fail to separate during meiosis (anaphase)
• down syndrome is when some has an extra chromosome 21
• XY - male
• XX - female