meiosis Flashcards
- What does meiosis produce?
One diploid cell produces 4 daughter cells, from 2 nuclear divisions (divisions of the nucleus) that are haploid and genetically different from each other.
- Describe and explain the appearance of this chromosome.
Chromosome is formed of two chromatids; Because DNA replication has occurred; Sister chromatids held together by centromere.
- What are homologous chromosomes?
Pairs of chromosomes; With the same genes; At same loci; but maybe different alleles. They could appear with replicated DNA or not.
- Draw a diagram of the nuclear divisions in meiosis and describe what is happening. Use one pair of homologous chromosomes to illustrate your answer.
DNA is replicated and held together by a centromere. During the first nuclear division, homologous pairs of chromosomes line up on the equator and are pulled to the poles of the cell, separating the homologous chromosomes. Giving 2 cells containing 1 of each homologous pair. The cells are now haploid. During the second nuclear division the chromosomes line up on the equator and the chromatids are separated, by being pulled to the poles of cells. There are now 4 cells with only 1 copy of a homologous pair of chromosomes. The cells are haploid.
- Explain how the chromosome number is halved during meiosis.
Homologous chromosomes pair up on the equator; One of each pair goes to each daughter cell. Chromatids are then separated giving daughter cells with just one copy of the chromosome.
- Describe and explain the processes that occur during meiosis that increase genetic variation.
Homologous chromosomes pair up on the equator; forming independent segregation which is when; Maternal and paternal chromosomes are re-shuffled in any combination; Crossing over leads to exchange of parts of non-sister chromatids between homologous chromosomes; Both these processes create new combinations of alleles; This increases genetic variation.
- Describe the process of crossing over and explain how it increases genetic diversity.
Homologous pairs of chromosomes associate; Chiasmata form; Equal lengths of non-sister chromatids are exchanged; Producing new combinations of alleles;
- What are the differences between mitosis and meiosis?
Mitosis maintains the chromosome number, whereas meiosis reduces the chromosome number. In mitosis the homologous chromosomes do not pair, whereas in meiosis the homologous chromosomes associate in pairs. In mitosis there is no crossing-over, whereas in meiosis there is crossing-over. In mitosis there are 2 daughter cells produced, whereas in meiosis 4 daughter cells are produced. In mitosis the cells produced are genetically identical, whereas in meiosis the cells produced are genetically different.
- How can you recognise where mitosis and meiosis occur in a life cycle?
If the chromosome number is reduced (diploid haploid) it is meiosis. If the chromosome number is maintained (diploid diploid OR haploid haploid) it is mitosis.
- What is the formula that can calculate the number of different combinations of chromosomes possible from meiosis?
2n where n is the number of homologous pairs of chromosomes.
- What is the formula to calculate the number of different combinations of chromosomes possible following the random fertilisation of two gametes.
(2n)2 where n is the number of homologous pairs of chromosomes.
- Explain how random fertilisation of haploid gametes further increases genetic variation within a species.
Haploid gametes are genetically unique. When haploid gametes from different parents fuse randomly it produces zygotes with different combinations of chromosomes to both parents and to other offspring.
