Meiosis and genetic variation Flashcards
What are the 2 types of cell division?
1) Mitosis - produces 2 daughter cells with the same number of chromosomes as the parent cell and as each other.
2) Meiosis - produces 4 daughter cells, each with half the number of chromosomes as the parent cell.
What happens during sexual reproduction?
When 2 gametes fuse to produce new offspring.
What happens to the chromosomes during sexual reproduction and why does this happen?
In order to maintain a constant number of chromosomes- in human that is 46, the number of chromosomes must be halved during the life cycle before it’s passed onto the offspring. This halving happens due to meiosis.
Where does meiosis occur in animals and plants?
Animals - occurs in the formation of gametes.
Some plants e.g. fern - the gametes are produced by mitosis. In the fern life cycle, meiosis occurs in the formation of spores.
What is the haploid number?
Where only one chromosome from each pair of the parent would enter the daughter cell. In humans, it’s 23. When the 2 haploid gametes fuse at fertilisation, the diploid number is restored.
What happens in the 2 nuclear division of meiosis?
First division:
- Homologous chromosomes pair up.
- Their chromatids wrap around each other.
- Equal portions of these chromatids might be exchanged in a process called crossing over.
- By the end of this division, the homologous pairs have separated, with 1 chromosome from each parent to go into the 2 daughter cells.
Second division:
- Chromatids move apart.
- At the end of meiosis 2, 4 daughter cells will be formed, in human, each will contain 23 chromosomes.
Besides halving the number of chromosomes, what else does meiosis do?
It also produces genetic variation among the offspring, which may lead to adaptations to improve survival chances.
What are the 2 ways of which meiosis can produce genetic variation?
1) Independent segregation of homologous chromosomes.
2) New combination of maternal and paternal alleles by crossing over.
What happens during independent segregation?
- During meiosis 1, each of the chromosomes is paired up and lined up alongside its homologous partner.
- When they arrange themselves in line, they are in random order.
- As they are lined up randomly, the combination of chromosomes of maternal and paternal origin that goes into the daughter cell at meiosis 1 depends on chance and this is independent segregation.
How does independent segregation produce new genetic combination?
Each homologous pair of chromosomes has exactly the same gene, therefore, determines the same characteristics e.g. blood groups. But the alleles of these genes may differ e.g. group A/B. Therefore the independent assortment produces new genetic combinations.
What happens after the chromosomes lineup alongside its homologous partner during meiosis 1?
- The chromatids of each pair twist around each other.
- During the twisting process, tensions are created and portions of the chromatids break off.
- The broken portions might then rejoin with the chromatids of its homologous partner.
- Usually, the equivalent portions are exchanged.
- This way, new genetic combinations of maternal and paternal alleles are produced.
The chromatids cross over many times so the process id crossing over.
The broken off portions of chromatid recombine with another chromatid, so this part is called recombination.
This, therefore, increase the genetic variety even further.
What’s the equation to calculate the different possible combinations of chromosomes for each daughter cell during independent segregation?
2n where n is the number of pairs of homologous chromosomes.
e.g. organism with 4 homologous pairs of chromosomes can produce 2^4, 16 different combinations.
What’s the equation to calculate the different possible combinations of chromosomes for each daughter cell after a random pairing of male and female gametes?
(2n)^2 where n is the number of pairs of homologous chromosomes. e.g. 4 homologous chromosomes, there are (2^4)^2, so there are 256 different combinations.
