Section 8-3 Meiosis

1. Section 8-3 Meiosis

2. The Purpose of Meiosis Meiosis is a type of cell division that takes place in gamete cells that results in the haploid (n) chromosome number. This ensures that when the sperm fertilizes the egg the correct diploid (2n) number will occur in the zygote cell for the species.

3. The Purpose of Meiosis Both meiosis and mitosis occur as part of a cell cycle. Meiosis differs because it involves a double round of divisions. The first round reduces the chromosome number by half. The second round changes the chromosomes from the replicated to the unreplicatedform.

4. Meiosis I: Prophase During prophase I the replicated chromosomes organize into homologous pairs during a process called synapsis. The chromatid of one chromosome can exchange a portion of its arm with the arm of part of the other chromatid. This is called crossing over and increases genetic diversity by combining genes in new ways on the paired chromosomes.

5. Meiosis: Metaphase - Telophase I During metaphase I each homologous pair of chromosomes lines up along the midline of the cell. Each whole chromosome move apart during anaphase I and separates into two cells during telophase I. This ensures that each cell contains the haploid number by the end of the division round.

6. Meiosis: Metaphase - Telophase I Two laws govern the movement of chromosomes during meiosis I. The Law of Segregation states that only one of each homologous pair ends up in each cell at the end of telophase I.

7. Meiosis: Metaphase - Telophase I The Law of Independent Assortment allows maternal and paternal chromosomes to each be on the left or right side during metaphase independently of one another. This allows for different trait combinations in the offspring.

8. Meiosis II In some cases meiosis II follows meiosis I immediately and sometimes there is a time delay. The phases of Prophase II – Telophase II are similar to mitosis because the chromosomes line up and separate singly. At the end of this round four haploid cells are produced that contain unreplicatedchromosomes.

10. Gamete Formation Spermatogenesis is the meiosis process that occurs in the testes to create four equal sperm cells. Both rounds of meiosis occur when puberty begins. Sperm cells are a tiny bit of cytoplasm with many mitochondria to supply the energy to propel the flagella.

11. Gamete Formation Oogenesisis the meiosis process that occurs in the ovariesto create one larger egg cell and three smaller polar bodies. This occurs because so much cytoplasm and organelles is required for the developing embryo. Round 1 of meiosis occurs when eggs form at birth and round 2 occurs when puberty begins and happens monthly.

12. Possible Gene Combinations Sexual reproduction allows for a ginormous amount of genetic variability within a species. There are 2(↑23) or 8 million distinct egg or sperm cells that one person can produce! After fertilization this creates 64 trillion diploid combinations!

13. Possible Gene Combinations This does not even take into account additional variation due to crossing over of chromatids or mutations in single nucleotides. There has never been an incidence of identical children born to a couple who were not twins. The odds of this happening are greater than getting struck by lightening and winning the lottery on the same day!

14. Problems During Meiosis Sometimes chromosome abnormalities occur during different stages of meiosis. One in every 200 babies is born with a chromosome disorder. There are five categories that can be detected using techniques like amniocentesis.

15. Problems During Meiosis One type of abnormality is called deletion. It occurs when part of a chromosome breaks off and doesn’t reattach. This is likely during prophase I during crossing over. It can be caused by exposure to radiation, autoimmunity, viruses and other chemical toxins.

16. Problems During Meiosis Another type of abnormality is called inversion. It occurs when part of a chromosome breaks off and reattaches to itself. This can cause varying degrees of problems. If the break is in the middle of a gene then the proper protein will not be made. If the break is in the middle of a regulatory sequence then how other genes are expressed may be altered.

17. Problems During Meiosis Another similar type of abnormality is called translocation. It occurs when part of a chromosome breaks off and reattaches to a non-homologous chromosome. This affects both gene regulation and expression. Many forms of cancer are thought to be caused by translocations involving genes that regulate the cell cycle.

18. Problems During Meiosis Chromosome breakage abnormalities result in disorders like cat cry syndrome, translocation 14-17 and #9 inversion. Organizations have been developed for family support and to promote understanding and acceptance.

19. Problems During Meiosis The last two abnormalities involve entire chromosomes. In monosomy a chromosome is missing from a homologous pair while in trisomy a chromosome is added to the pair to make a triplet. Both of these could occur if the chromosomes fail to separate during either the first or second round of meiosis. This is called nondisjunction.

20. Problems During Meiosis There is only one non-lethal monosomy example and that is females born with only one X chromosome. This is called Turner’s syndrome. Several trisomyexamples include numbers 21, 13, 18, X and Y chromosomes. The most common example is Down’s syndrome.