Genetic Information and Variation - Mutations Flashcards
What is a mutation?
Any change to the quantity or the base sequence of the DNA of an organism is known as a mutation. Mutations occurring during the formation of gametes may be inherited, often producing sudden and distinct differences between individuals.
What is a gene mutation?
Any change to one or more nucleotide bases, or a change in the sequence of the bases, in DNA is known as a gene mutation.
What can a gene mutation lead to?
Any changes to one or more bases in the DNA triplets could result in a change in the amino acid sequence of the polypeptide. Sometimes this can result in a mutation that is harmful, however, due to the genetic code being degenerate, the amino acid sequence may not always be changed.
How can gene mutations arise?
Gene mutations can arise spontaneously during DNA replication and include base substitution, base insertion and base deletion.
What is base substitution?
The type of gene mutation in which a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base is known as a substitution. A substitution may not always be harmful as the substituted nucleotide may code in that triplet for the same amino acid.
What is the significance of the difference in amino acid?
The polypeptide produced will differ in a single amino acid in certain situations. The significance of this difference will depend upon the shape and precise role of the original amino acid. If it is important in forming bonds that determine the tertiary structure of the final protein, then the replacement amino acid may not form the same bonds. The protein may then be a different shape and therefore not function properly.
Why does the amino acid not change sometimes?
The effect of the mutation is different if the new triplet of bases still codes for the same amino acid as before. This is due to the degenerate nature of the genetic code, in which most amino acids have more than one codon.
What is base deletion? What is base insertion?
A gene mutation by deletion arises when a nucleotide is lost from the normal DNA sequence. The loss of a single nucleotide from the thousands in a typical gene may seem a minor change but the consequences can be considerable. Usually the amino acid sequence of the polypeptide is entirely different and so the polypeptide is unlikely to function correctly. This is because the sequence of bases in DNA is read in units of three bases (triplet). One deleted nucleotide causes all triplets in a sequence to be read differently because each has been shifted to the left by one base (frame shift). This results in completely different amino acids being coded for.
Base insertion is the opposite and happens when a nucleotide is inserted into the chain, and each codon is shifted to the right by one base.
What are chromosome mutations?
Changes in the structure or number of whole chromosomes are called chromosome mutations. Chromosome mutations can arise spontaneously and take two forms.
- Polyploidy: Changes in whole sets of chromosomes.
- Non-disjunction: Changes in the number of individual chromosomes.
What is polyploidy?
Changes in whole sets of chromosomes occur when organisms have three or more sets of chromosomes rather than the usual two. This condition is called polyploidy and occurs mostly in plants, with many modern wheats arising to be polyploidy.
What is non-disjunction?
Changes in the number of individual chromosomes. Sometimes individual homologous pairs of chromosomes fail to separate during meiosis. This is known as non-disjunction and usually results in a gamete having either one more or one fewer chromosome. On fertilisation with a gamete that has the normal complement of chromosomes, the resultant offspring have more of fewer chromosomes than normal in all their body cells.
An example of a non-disjunction in humans is Down’s syndrome, where individuals have an additional chromosome.
What is a mutagenic agent?
A mutagen is a physical or chemical agent that permanently changes genetic material, usually DNA, in an organism and thus increases the frequency of mutations above the natural background level.
