8.1 - Alteration of the sequence of bases in DNA can alter the structure of proteins

Question 1

Describe how alterations to tumour suppressor genes can lead to the development of tumours. (3)

Answer 1

• (Increased) methylation (of tumour suppressor genes)
  ‘accept abnormal methylation or hypermethylation’
• Mutation (in tumour suppressor genes)
• Tumour suppressor genes are not transcribed/expressed
  / Amino acid sequence/primary structure altered
  ‘accept mRNA for transcription/transcribed /tertiary structure altered’
• (Results in) rapid/uncontrollable cell division

Question 2

What is a substitution mutation? (1)

Answer 2

• Replacement of a base by a different base (in DNA)

Question 3

Suggest how gel electrophoresis separated the proteins. (2)

Answer 3

• (Depends on) size / mass (of protein)
• (Depends on) charge (of protein)
  ‘accept Smaller / more highly charged move further’

Question 4

Give the meaning of the term mutation. (1)

Answer 4

• A change in the sequence of base pairs in a DNA molecule (that may result in an altered polypeptide)

Question 5

Identify and describe three different types of mutation. (3)

Answer 5

• Deletion mutations involve the removal of one or more (nucleotide) bases
• Insertion mutations involved the addition of one or more (nucleotide) bases
• Inversion mutations occur when a section of a gene is inserted in the opposite order to the original base sequence
• Substitution mutations involve the switching of one base for another
• Translocation mutations involve the relocation of a section of a gene to a different place/gene
• Duplication mutations occur when a section of a gene is copied and inserted more than once / there are two copies of the same section of a gene

Question 6

Explain how a mutation may result in synthesis of a non-functional protein. (2)

Answer 6

• There is a change in the sequence of amino acids / primary structure of the protein
• There is a change in the tertiary structure / 3D structure of the protein (due to the formation of different bonds within the protein)

Question 7

Explain what is meant by the term silent mutation. (2)

Answer 7

• A change in the nucleotide base sequence has no effect on the resulting amino acid that is coded for
• (With no change in the amino acid sequence) the protein synthesized will remain the same / it will have no effect on the protein synthesized

Question 8

List two examples of mutagenic agents. (2)

Answer 8

• Ionising radiation / alpha radiation / beta radiation / gamma radiation / x-rays
• Chemicals OR specific examples (e.g. benzopyrene / tobacco smoke / nitrogen dioxide / alkylating agent)
• UV radiation
• Viruses/bacteria

Question 9

Identify and explain the feature of the genetic code which allows a silent mutation to occur. (2)

Answer 9

• The genetic code is degenerate
• A single amino acid can be coded for by more than one triplet/codon / the same amino acid could be coded for despite the occurrence of a mutation

Question 10

Suggest why a translocation mutation is likely to have a bigger impact than a substitution mutation. (2)

Answer 10

• Translocation mutations (are likely to affect) more than one gene
• A substitution mutation could only alter one nucleotide / will only affect the triplet in which the mutation occurs
• A silent mutation is possible with a substitution but not with a translocation

Question 11

State and explain which stage of the cell cycle mutations are most likely to occur. (2)

Answer 11

• Interphase / the S phase / the synthesis phase
• This is where DNA replication occurs

Question 12

Tumour suppressor genes are a group of genes that can code for proteins that prevent cell division. Explain how a mutation within this type of gene could lead to cancer. (3)

Answer 12

• Mutation leads to a change in the amino acid sequence / primary structure of the coded protein
• The protein has a changed tertiary/3D structure
• The protein expressed is non-functional / no longer complementary to its binding site (e.g. the promoter region at the start of a gene that controls cell division)
• Cell division no longer under control / mitosis occurs rapidly

Question 13

Describe what is meant by an inversion mutation. (2)

Answer 13

• Two breaks in the DNA of a (single) gene occur
• The cut DNA is inverted/rotates 180° then reseals/joins

Question 14

Suggest the effect of an inversion mutation on the functionality of the protein produced. (3)

Answer 14

• Multiple codons/amino acids are affected/changed
• Tertiary structure / 3D shape of the protein changed
• Protein non-functional OR protein has new function