Topic 3 - DNA and Gene Function Flashcards
What is a genetic mutation?
A heritable sequence change in a gene or chromosome; the process by that gives rise to such an alteration. (Note: there is a discussion of this terminology in Topic 3 Part 1 Section 1, ‘Mutations and alleles’.)
What is a null mutation?
Mutations which completely abolishes function, for example by preventing expression of a protein or expression of a protein with no function.
What is a neutral mutation?
Changes in DNA sequence that are not subject to selection; for example a mutation that does not result in a change in the function of the encoded protein (or RNA).
What are codons?
Three consecutive nucleotides along a DNA molecule (or in the mRNA molecule transcribed from it) that specify either a particular amino acid, or the end of the protein synthesis.
In which direction does polynucleotide synthesis occur?
In the 5′ → 3′ direction
What is an open reading frame?
A nucleic acid sequence with series of codons that do not include any stop codons.
What is a nucleotide transversion?
A mutation that involves the substitution of a purine (A or G) for pyrimidine (C or T), or vice versa.
What is a missense mutation?
A nucleotide change in an open reading frame that leads to a change of an amino acid residue in the encoded protein.
What is a nonsense mutation?
A nucleotide change in an open reading frame that changes a codon that encodes an amino acid into a stop codon.
Why might an amino acid change in a protein not affect the protein’s function?
The amino acid may not be crucial for the function or it may be replaced by a chemically similar amino acid.
What is a frame shift mutation?
after the insertion or deletion will be shifted by one nucleotide. As a result, from that point onwards the codons – and hence the amino acid sequence encoded – will be completely changed, and the structure of the polypeptide encoded by the gene will be significantly altered, with probable disruption of its function.
Would the insertion of three nucleotides into an ORF cause a shift in the reading frame?
No. Because the genetic code is translated in triplets, neither insertion nor deletion of three nucleotides (or multiples of three nucleotides) will cause a frameshift.
Explain pleiotropy
A cellular process, signal or gene, that has more than one effect, or more than one phenotypic outcome. At the genetic level, describes a situation where a single mutation affects two or more apparently unrelated phenotypic traits.
Explain polygenic variation.
In contrast to pleiotropy, in which variation at a single locus can lead to more than one phenotypic effect, polygenic variation describes the situation where a phenotype is influenced by more than one gene.
What is epistasis?
Where the phenotype of one gene ‘over-rides’ that of the other, the gene whose phenotype is expressed is said to be epistatic to the other gene.
