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(Maria Gutierrez and Alba Muñoz)
Meiosis
Meiosis
(Maria Gutierrez)
AUDIO: https://drive.google.com/file/d/1kWSRI-OajmCJflCFox8q7LqUJjk9Vxma/view?usp=sharing
Sex cells are the organism's reproductive cells. They are also referred to as gametes. Female gametes are called ova or egg cells, and male gametes are called sperm. Gametes are haploid cells, and each cell carries only one copy of each chromosome. These reproductive cells are produced through a type of cell division called meiosis. It has two parts, meiosis I and meiosis II.
Meiosis I
Meiosis I
Before entering meiosis I, a cell must first go through interphase. As in mitosis, the cell grows during G1 phase, copies all of its chromosomes during S phase, and prepares for division during G2 phase.
In this process the number of chromosomes is reduced by half. Meiosis I is made up of 4 phases: prophase I, metaphase I, anaphase I and telophase I and cytokinesis that occur at the same time.
Prophase I: it is a little bit complex so we divide this process in 5 periods.
Leptotene: chromosomes roll up.
Zygotene: homologous chromosomes are longitudinally and fully paired. The resulting pair is called bivalent.
Pachytene: chromosomes slip and thicken. The groups of four chromatids, or tetrads, of each bivalent are highly visible under the microscope.
Diplotene: homologous chromosomes begin to separate although they are still joined by chiasms. Then the crossing over takes place, which means that DNA fragments are exchanged between homologous chromosomes.
If there are more than one chiasms, the combinations of interlocking fragments are multiple. This way, the resulting chromosomes are completely different from the originals because they contain pieces of the father and mother. This is called genetic recombination.
Diakinesis: homologous chromosomes separate, the nuclear envelope and nucleolus disappear and spindle formation begins.
Metaphase I: each bivalent is placed on the equatorial plate.
Anaphase I: the homologues are pulled apart and move apart to opposite ends of the cell. At each end there will be a full complement of n chromosomes, with two chromatids each. An error in this process can cause alterations in the number of chromosomes of the resulting cells.
Telophase I: two daughter nuclei are formed, the chromosomes despiralize and cytokinesis occurs. The cells can begin a state of rest, during which the DNA is not duplicated, or they can go directly to the second division.
Meiosis II
Meiosis II
It is basically a mitotic division, with the difference that the cells are haploids and, because of it, they only have n chromosomes. The phases are called prophase II, metaphase II, anaphase II, telophase II and cytokinesis. In this division, the two chromatids of each chromosome are separated. As a final result, therefore, four haploid cells remain, with n chromosomes and one chromatid each one. The resultant cells are completely different as the original and as the other ones.
Furthermore, the variability of the offspring has increased because DNA fragments are exchanged between homologous chromosomes (one of which came from the father, and the other from the mother) and because these homologous chromosomes separate at random during metaphase I.
Ovogenesis
Ovogenesis
(Alba Muñoz)
Ovogenesis is the process in which oocytes form in the ovaries. These cells are formed from a diploid meiosis. In this type of meiosis the adult individual has a chromosomal endowment 2n.
Espermatogenesis
Espermatogenesis
(Alba Muñoz)
Spermatogenesis is the process in which sperm is formed. This process occurs in the gonads and while in the woman has an specific lifetime, the man can generate sperm all his life. Sperm are formed by diploid meiosis
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