9. Genetic Diversity and Adaptation. Flashcards
What is a mutation?
Any change to the quantity or the base sequence of the DNA of an organism.
what is a gene mutation?
random change to one or more nucleotide bases, or a change in the sequence of the bases, in DNA.
State two different types of gene mutation
- Substitution of bases: when a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base.
- Deletion of bases: when a single nucleotide is lost from the normal DNA sequence.
what is a chromosome mutation?
change in the structure or number of whole chromosomes.
Give the two forms of chromosome mutation
- Changes in whole sets of chromosomes: when organisms have 3 or more sets of chromosomes rather than the usual 2= ‘polyploidy’ occurs mostly in plants.
- Changes in the number of individual chromosomes: when homologous chromosomes fail to separate during meiosis. ‘Non-disjunction’ results in gametes having one more/fewer chromosomes. On fertilisation with a gamete that has a normal complement of chromosomes, the resulting offspring have more or fewer chromosomes than normal in all body cells.
Explain why a deletion gene mutation is more likely to result in a change to an organism than a substitution gene mutation
In deletion, all codons after the deletion are affected. Therefore most amino acids coded for by these codons will be different and the polypeptide will be significantly affected. In a substitution, only a single codon, and therefore a single amino acid, will be affected. The effect on the polypeptide is likely to be less severe.
Explain why mutation that is transcribed onto mRNA may not result in any change to the polypeptide it codes for
The mutation may result from the substitution of one base in the mRNA with another. Although the coding affected will be different, as the genetic code is degenerate, the changed codon may still code for the same amino acid. The polypeptide will be unchanged and therefore there will be no effect.
Errors in transcription occur about 100,000 times more often than errors in DNA replication. Explain why errors in DNA replication can be far more damaging than errors in transcription.
These errors may be inherited and therefore have a permanent effect on the whole organism. Errors in transcription usually affect only specific cells, are temporary and are not inherited. They’re therefore less damaging
define meiosis
A form of cell division- usually produces 4 daughter cells each with half the number of chromosomes as the parent cell.
Produce haploid gametes, which when fused, restore the full diploid number of chromosomes.
Define ‘gene’
A section of DNA that codes for a polypeptide
Define ‘locus’
The position of a gene on a chromosome or DNA molecule.
Define ‘allele’
One of the different forms of the same gene
Define ‘homologous chromosomes’
A pair of chromosomes, one maternal and one paternal, that have the same gene loci.
Briefly outline the 2 stages of meiosis
- First division (meiosis 1) homologous chromosomes pair up and their chromatids wrap around each other. Equivalent portions of these chromatids may be exchanged in a process called crossing over. By the end of this the homologous pairs have separated with one chromosome from each pair going into one of the two daughter cells.
- Second division (meiosis 2) the chromatids move apart. At the end, 4 cells have usually been formed. In humans, each contain 23 chromosomes.
describe how independent segregation of homologous chromosomes leads to genetic variation
During meiosis one, each chromosome lines up alongside its homologous partner. When homologous pairs arranged themselves in this line they do so at random. One of each pair will pass to each daughter cell.
Since the pairs are lined up at random in the parent cell, combination of chromosomes of maternal and paternal origin that going to the daughter cell at meiosis one is a matter of chance.
