Genetic diversity Flashcards
What is a mutation?
Any change to the quantity or the base sequence of the DNA of any organism.
Mutations occurring during the formation of gametes may be inherited, often producing sudden and distinct differences between individuals.
How can subsitution of bases be significant?
If it is important in forming bonds for tertiary structure, then the replacement amino acid may not form the same bonds and the protein may be a different shape, and not function properly.
If the protein is an enzyme, its active site may no longer fit the substrate and it will no longer catalyse the reaction.
What is substitution of bases?
A nucleotide in a DNA molecule is replaced by another nucleotide that has a different base.
The polypeptide produced will differ in a single amino acid, and the significance will depend upon the precise role of the amino acid.
How can substitution of bases not be significant?
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.
For example, if the third base of GTC is replaced by thymine, it becomes GTT.
As both amino acids code for glutamine, there is no change in the polypeptide produced and so the mutation will have no effect.
What is deletion of bases?
When a nucleotide is lost from the normal DNA sequence.
The loss of a single nucleotide from the thousands in a typical gene can have considerable consequences.
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.
One deleted nucleotide causes all triplets in a sequence to be read differently because each has been shifted to the left by one base.
What are chromosome mutations?
Changes in the structure or number of whole chromosomes.
Two forms: Changes in whole sets of chromosomes.
Changes in the number of individual chromosomes.
What is changes in whole sets of chromosomes?
It occurs when the organisms have three or more sets of chromosomes rather than the usual two.
This condition is called polyploidy and occurs mostly in plants.
What is changes in the number of individual chromosomes?
Sometimes individual homologous pairs of chromosomes fail to separate during meiosis.
This is called 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 or fewer chromosomes than normal in all their body cells.
E.g. in humans is down’s syndrome, where individuals have an additional chromosome 21.
What is meiosis?
It usually produces four daughter cells, each with half the number of chromosomes as the parent cell.
What is the importance of meiosis?
In sexual reproduction two gametes fuse to give new offspring.
If each gamete had a full set of chromosomes (diploid) then the produced cell would have double this number.
In humans, this would be 92, and the doubling would continue at each generation.
To maintain a constant number of chromosomes in adult species, the number must be halved at some stage in the life cycle.
In most animals meiosis occurs in the formation of gametes.
In some plants such as ferns, however, gametes are produced by mitosis, and meiosis occurs in the formation of spores.
How are diploid and haploid related?
Each diploid cell has two complete sets of chromosomes, one provided by each parent.
During meiosis, homologous pairs of chromosomes separate, so that only one chromosome from each pair enters the daughter cell - the haploid number of chromosomes (23 in humans).
When two haploid gametes fuse at fertilisation, the diploid number of chromosomes is restored.
What is the process of meiosis?
In the first division, homologous chromosomes pair up and their chromatids wrap around each other. Equivalent portions of these chromatids may be exchanged by crossing over. By the end of this division the homologous pairs have separated, with one chromosome from each pair going into one of the daughter cells.
In the second meiotic division, the chromatids move apart. At the end, 4 cells have been formed, each containing 23 chromosomes.
How does meiosis produce genetic variation?
It may lead to adaptations that improve survival chances by:
Independent segregation of homologous chromosomes.
New combinations of maternal and paternal alleles crossing over.
What are homologous chromosomes?
A pair of chromosomes, one maternal and one paternal, that have the same gene loci.
What is independent segregation of homologous chromosomes?
During meiosis 1, each chromosome randomly lines up alongside its homologous partner.
In humans this means that there will be 23 homologous chromosomes lying side by side.
One of each pair will pass to each daughter cell, and which one depends on how the pairs are lined up in the parent cell, which is a matter of chance.
