Alteration of the sequence of bases in DNA can alter the structure of proteins (A-level only) Flashcards
Addition
Addition mutations are where one or more bases are added to the DNA sequence.
E.g. ATCGTT → ATCCGTT
In this example, a C is added in the middle of the DNA sequence.
Deletion
Deletion mutations are where one or more bases are removed from the DNA sequence.
E.g. ATCGTT → ATCTT
In this example, the G is deleted from the middle of the DNA sequence.
Substitution
Substitution mutations are where one or more bases are changed in the DNA sequence.
E.g. ATCGTT → ACCGTT
In this example, the first T is substituted for a C.
Inversion
Inversion mutations are where the order of bases in the DNA sequence are inverted (reversed).
E.g. ATCGTT → TTGCTA
In this example, the order is reversed (i.e. the A moves from position 1 to position 6).
Duplication
Duplication mutations are where one or more bases are duplicated (repeated) in the DNA sequence.
E.g. ATCGTT → ATATCGTT
In this example, the A and T found in positions 1 and 2 are duplicated.
Translocation
Translocation mutations are where a section of the DNA sequence is moved to another section of the DNA sequence.
Translocation can be within and across chromosomes.
Encoding amino acids
There are four different bases in DNA (A, T, C, G).
Three bases in a sequence encode an amino acid.
This is called the triplet code or a codon.
Every codon encodes a different amino acid.
Amino acids are joined together to form proteins.
Protein 3D structure
Every amino acid has a side chain (called the R group) that differs slightly.
The bonds that can form between the R groups of different amino acids within a protein can give rise to a complex 3D structure.
The 3D structure of proteins is very important for their function (e.g. the shape of an enzyme active site is determine by its amino acids).
Mutations
Mutations give rise to a change in the base sequence in DNA.
This could alter the amino acids that the DNA encodes.
E.g. CTG encodes leucine.
If this was inverted to GTC, it would encode valine.
If the amino acids in a protein are changed, the 3D structure of the protein may break down.
Effect of mutation
If a protein’s structure is affected by a mutation, this could have harmful effects.
Some genetic disorders are caused by mutations in this way.
E.g. Cystic fibrosis is caused by a mutation that affects proteins in the epithelial cells of the respiratory system.
Triplet codes
Some amino acids are encoded by more than one codon (e.g. tyrosine is encoded by TAT and TAC).
The fact that some amino acids correspond to multiple codons means that DNA is degenerate.
Mutations
The fact that DNA is degenerate means that a mutation in the base sequence does not necessarily affect the protein.
If one base is substituted for another base, (e.g. CAA → CAG), the mutated codon may still encode the same amino acid (e.g. both encode glutamine).
Causes of a frameshift
A frameshift is caused when a base (or multiple bases) is inserted, removed or duplicated into the genome.
This type of mutation has altered the number of bases in the DNA sequence.
This means that all the triplet codes downstream from the mutation will experience a shift.
Effects of a frameshift
A frameshift causes all amino acids that follow the mutation to be affected.
If only one amino acid in a protein is changed in a mutation, the protein may be able to function as normal.
A frameshift impacts many amino acids so it is much less likely that the protein will function as normal.
E.g. Crohn’s Disease
Crohn’s disease is an inflammatory bowel disease that is caused by a frameshift mutation.
When Cytosine is inserted at position 3020 of the NOD2 gene, the subsequent codons are shifted.
This causes the protein to be shortened and it cannot function properly.
