3.4.3 genetic diversity due to meoisis/mutation

What is a gene mutation?

Change in the sequence to one or more nucleotide bases, or a change in base sequences.

What is a mutation?

Change in the quantity or structure of DNA.

What is substitution?

When a nucleotide in a DNA molecules is replaced by another nucleotide with a different base.

What is a non-sense mutation?

When a base change results in the formation of a stop codon, which makes the final polypeptide very different.

What is a mis-sense mutation?

When a base change results in a different amino acid being coded for, so the final polypeptide could be different but not necessarily.

What is a silent mutation?

Base change results in the same amino acid being coded for, so there is no change to the polypeptide.

What is base deletion?

When a nucleotide is lost from the normal DNA sequence. The consequences are catastrophic as each codon undergoes a frame shift causing the entire polypeptide to change downstream of the mutation.

Can mutations arise spontaneously?

Yes - they can arise spontaneously during DNA replication, without any external influence.

What are mutatenic agents?

External factors that increase the rate of basic mutation rate.

What are some examples of mutagenic agents, and how do they affect mutations?

Ionising radiation carries enough energy to change DNA.

Carcinogens are chemicals which can change DNA and cause cancer.

What can gene mutations cause proto-oncogenes to mutate to, and what is the effect?

Mutate to oncogenes, which can either mean that the receptor protein on the cell surface is permanently activated, so cell division is activated even in absence of growth factors. Oncogenes may code for the production of excess amounts of growth factor, which also causes excess cell division.

What are tumours, and what are the two main types?

They are mutant cells usually structurally and functionally different from normal cells. Either benign (self contained and less harmful) or malignant (can spread, and more harmful)

What is the difference between a diploid and haploid cell?

Diploid has two sets of chromosomes, but haploid has 1 chromosome set

Are human gametes diploid?

No - they are haploid with a haploid number of 23.

What would happen if human gametes were diploid?

They would form a cell that has 4n chromosome sets which is too many for a human cell. Haploid gametes mean the chromosomes number of 2n is restored during fertilisation.

How many cells are formed from the two nuclear divisions in meiosis?

4

What happens during meoisis?

Homologous pairs of chromosomes separate, so that only one chromosome from each pair enters a daughter cell. It halves the number of chromosomes in the daughter cells, which is essential for gamete formation.

What happens when haploid gametes fuse at fertilisation?

Diploid number of chromosomes is restored

What is genetic variety caused by?

Crossing over, independent segregation, mutations and random fertilisation

What is the process of crossing over?

Homologous chromosomes associate to form bivalents

Chiasma form when chromosomes get twisted and wrap around each other. Tensions are created and portions of the chromatids break off.

These broken portions might rejoin with the chromatids of its homologous partner, and equivalent portions of homologous chromosomes are exchanged.

This produces a new combination of maternal and paternal alleles

What is recombination?

Broken off portions of chromatids recombine with another chromatid in crossing over

What does random fertilisation lead to?

Alleles being randomly distributed across chromosomes and creates genetic variation.

What is independent segregation?

Homologous chromosomes line up randomly, so there are many different ways that pairs could assort themselves, so each gamete gets a different combination of maternal and paternal chromosomes.

What is chromosome non dysjunction?

Failure of homologous chromosomes to separate during cell division so both chromosomes go to one pole of the cell - results in the gamete having too many or too few chromosomes.

What is the rate of non dysjunction like?

High, but often results in early miscarriage