Mutations (College Board AP® Biology): Study Guide

Types of mutation

• Mutations leading to changes in genotype can lead to changes in phenotype

  • this is because if the products of genes (mainly proteins, including enzymes) are disrupted new phenotypes will arise

• The impact of this is that the normal function of the genes and gene products are no longer in operation

  • DNA mutations can be positive, negative, or neutral

  • This is based on the effect they have on the resulting nucleic acid or protein and the phenotypes that are produced by the protein

Gene Mutations

• A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in altered gene function

• Mutations occur continuously and spontaneously

  • Errors in the DNA often occur during DNA replication

• As the DNA base sequence determines the sequence of amino acids that comprise a protein, mutations in a gene can sometimes lead to a change in the polypeptide that the gene codes for, and therefore the phenotype

• Most mutations do not alter the polypeptide or only alter it slightly so that its structure or function is not changed (as the genetic code is degenerate)

• Gene mutations in the DNA base sequence can occur due to the

  • insertion, deletion or substitution of a nucleotide

  • inversion, duplication or translocation of a section of a gene

Insertion of nucleotides 

• A mutation that occurs when a nucleotide is randomly inserted into the DNA sequence is known as an insertion mutation

• An insertion mutation changes the amino acid that would have been coded for by the original triplet, as it creates a new, different triplet of bases

  • This is because every group of three bases (a codon) in a DNA sequence codes for an amino acid

• Insertion mutations also have a downstream effect by changing the triplets further on in the DNA sequence

  • This is sometimes known as a frameshift mutation

• This may fundamentally change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function

Deletion of nucleotides 

• A mutation that occurs when a nucleotide is randomly deleted from the DNA sequence is known as a deletion mutation

• Like an insertion mutation, a deletion mutation changes the amino acid that would have been coded for

• A deletion mutation also has a consequent effect by changing the groups of three bases (codons) further on in the DNA sequence, in the same way as an insertion mutation does

  • This is also known as a frameshift mutation

• This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function

Substitution of nucleotides 

• A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base is known as a substitution mutation

• Unlike an insertion or deletion mutation, a substitution mutation will only change the amino acid for the triplet of bases in which the mutation occurs; it will not have an effect downstream of the mutation site

• Substitution mutations can take three forms:

  • Silent mutations

    • The mutation does not alter the amino acid sequence of the polypeptide

    • This is because certain codons may code for the same amino acid as the genetic code is degenerate)

  • Missense mutations

    • The mutation alters a single amino acid in the polypeptide chain

    • Sickle cell anaemia is an example of a disease caused by a single substitution mutation changing a single amino acid in the sequence

  • Nonsense mutations

    • The mutation creates a premature stop codon (signal for the cell to stop translation of the mRNA molecule into an amino acid sequence)

    • This causes the polypeptide chain produced to be incomplete and therefore affects the final protein structure and function

Inversion within a gene section

• Inversion mutations usually occur during crossing over in meiosis

• The DNA of a single gene is cut in two places

• The cut portion is inverted 180° then rejoined to the same place within the gene

• The result is a large section of the gene is 'backwards' and therefore multiple amino acids are affected

• Inversion mutations most frequently result in a nonfunctional protein

  • In some cases, an entirely different protein is produced

• The mutation is often harmful because the original gene can no longer be expressed from that chromosome

Duplication of a gene

• A whole gene or section of a gene is duplicated so that two copies of the gene/section appear on the same chromosome

• The original version of the gene remains intact and therefore the mutation is not harmful

• Over a period of time, the second copy can undergo mutations which enable it to develop new functions

• Duplication mutations are an important source of evolutionary change

  • E.g., Alpha, beta and gamma hemoglobin genes evolved due to duplication mutations

Translocation of a gene section

• Similarly to inversion, a gene is cut in two places

• The section of the gene that is cut attaches to a separate gene

• This results in the cut gene now being nonfunctional due to having a section missing

• The gene that has gained the translocated section is likely to also be nonfunctional

Chromosome Mutations

• A chromosome mutation is a change in whole chromosomes, rather than just individual genes

  • Gene mutations are much more common than chromosome mutations

• Chromosome mutations tend to have more significant consequences for the phenotype of the organism

• Chromosome mutations can result in changes to chromosome number, such as non-disjunction