Diagram showing the main stages of meiosis. Please note there are two divisions taking place here (meiosis I and II) and four daughter cells are produced all genetically different to the parent cell. The daughter cells all contain the haploid number of chromosomes rather than diploid chromosome number as occurs in the parent cell.
Exam 2: Cell division
Meiosis I + II
It is important that gametes are haploid since at fertilisation half the chromosomes come from the male gamete (e.g. sperm cell) and half from the female gamete (e.g. ovum) in order to produce a diploid zygote. Meiosis produces four genetically different haploid cells during sexual reproduction and involves two consecutive divisions.
Like mitosis, meiosis is a gradual process but for convenience it is divided into the four phases of prophase, metaphase, anaphase and telophase, these phases occurring once in each of the two divisions. You are not required to describe the complete process of meiosis but to describe the significance of the differences between mitosis and meiosis.
The first stage of prophase in meiosis is similar to prophase in mitosis in that the chromosomes shorten and fatten and become visible under a microscope. However, in meiosis the chromosomes associate in their homologous pairs. The pairing of the chromosomes is called synapsis. Each pair of chromosomes, one from the mother and one from the father, is called a bivalent.
Each bivalent consists of four strands made up of two chromosomes each divided into two chromatids. These chromatids wrap around each other and then partially repel each other but remain joined at points called chiasmata. At these points chromatids may break and recombine with a different but equivalent chromatid. This swapping of pieces of chromosomes is called crossing over and is a source of genetic variation.
During the first stage of metaphase of meiosis the pairs of homologous chromosomes arrange themselves randomly on the equator of the spindle. Chance determines how the homologous chromosomes are arranged on the equator and when they separate different combinations are produced. This random distribution and consequent independent assortment of chromosomes produces new genetic combinations.
Comparing mitosis and meiosis
Meiosis I + II
It is important that gametes are haploid since at fertilisation half the chromosomes come from the male gamete (e.g. sperm cell) and half from the female gamete (e.g. ovum) in order to produce a diploid zygote. Meiosis produces four genetically different haploid cells during sexual reproduction and involves two consecutive divisions.
- Meiosis I - where the chromosome number is reduced and crossing over takes place
- Meiosis II - where the two new haploid nuclei divide again in a division identical to that of mitosis.
Like mitosis, meiosis is a gradual process but for convenience it is divided into the four phases of prophase, metaphase, anaphase and telophase, these phases occurring once in each of the two divisions. You are not required to describe the complete process of meiosis but to describe the significance of the differences between mitosis and meiosis.
The first stage of prophase in meiosis is similar to prophase in mitosis in that the chromosomes shorten and fatten and become visible under a microscope. However, in meiosis the chromosomes associate in their homologous pairs. The pairing of the chromosomes is called synapsis. Each pair of chromosomes, one from the mother and one from the father, is called a bivalent.
Each bivalent consists of four strands made up of two chromosomes each divided into two chromatids. These chromatids wrap around each other and then partially repel each other but remain joined at points called chiasmata. At these points chromatids may break and recombine with a different but equivalent chromatid. This swapping of pieces of chromosomes is called crossing over and is a source of genetic variation.
During the first stage of metaphase of meiosis the pairs of homologous chromosomes arrange themselves randomly on the equator of the spindle. Chance determines how the homologous chromosomes are arranged on the equator and when they separate different combinations are produced. This random distribution and consequent independent assortment of chromosomes produces new genetic combinations.
Comparing mitosis and meiosis