9.1 Mutations Flashcards
What is a gene mutation?
Any change to one or more nucleotide bases, or a change in the sequence of the bases, in DNA
How can gene mutations arise?
Spontaneously during DNA replication and include bases substitution and base deletion
What is substitution of bases?
A nucleotide in a DNA molecule is replaced by another nucleotide that has a different base
What may a substitution result in?
Polypeptide will differ in an amino acid e.g. GTC = histidine GTG = glutamine. Significance depends on the role of the amino acid
Example:
Used in forming bond that determine the tertiary structure, protein may be a different shape and not function properly eg enzyme
Why may a mutation not have an effect?
If the new triplet of bases still codes for the same amino acid as before
Degenerate nature - most amino acids have more than one codon
No change in polypeptide and the mutation will have no effect
What is a deletion of bases?
When a nucleotide is lost from the normal DNA sequence
What effect can deletion of bases have? Why?
Usually the amino acid sequence of the polypeptide is entirely different - unlikely to function correctly
The sequence of bases are read in units of three bases so causes all the triplets in a sequence to be read differently
What is a chromosome mutation?
Changes in the structure or number of whole chromosomes
How do chromosome mutations arise?
Spontaneously
What are the 2 forms in which chromosome mutations can arise?
Changes in whole sets of chromosomes
Changes in the number of individual chromosomes
How do changes in whole set of chromosomes occur?
When organisms have 3 or more set of chromosomes rather than 2
Condition called polyploidy and occurs mostly in plants
How do changes in number of individual chromosomes occur?
Non-disjunction:
Individual homologous pairs of chromosomes fail to separate during meiosis.
Usually results in a gamete having either one more or fewer chromosome. Fertilisation with this gamete, offspring have more or fewer chromosomes than normal in all their body cells. Eg Down’s syndrome (1 extra)
What does meiosis produce?
Produces four daughter cells, each with half the number of chromosomes as the parent cell
Why is meiosis important?
In sexual reproduction two gametes fuse to give rise to new offspring
If each had diploid then the cell produced has double (this would continue)
How is the diploid number of chromosomes restored?
During meiosis, homologous pairs separate, so that only one chromosome from each pair enters a daughter cell - haploid number
Two haploid gametes fuse at fertilisation, the diploid number of chromosomes is restored
