Meiosis Lesson

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Meiosis Lesson

  1. 1. Meiosis NEREA LÓPEZ PEIRONA 4ºB
  2. 2. Significance of Meiosis (1) Crossing over in bivalent - Produces new combinations of genes in both chromosomes
  3. 3. Chiasma and Crossing Over
  4. 4. Significance of Meiosis (2) Reduction and fusion of gametes - Meiosis produces haploid gametes - In sexual reproduction, a male gamete fertilizes a haploid female gamete to produce a normal diploid zygote
  5. 5. Significance in Mitosis (3) Independent (random assortment) - During metaphase I, homologous pairs of chromosomes align at the equator - It is by chance which “way round” each pair lies, before these homologous pairs of chromosomes separate into two different daughter cells.
  6. 6. e
  7. 7. Stages of Meiosis  Meiosis I Prophase I Metaphase I Anaphase I Telophase I • Meiosis II Prophase II Metaphase II Anaphase II Telophase II
  8. 8. Interphase  Interphase is an important stage preceding meiosis. Without this stage meiosis would not occur.  During this stage, each individual chromatid replicates, similar to mitosis.  B replicates B and b replicates b  At this stage, the chromosomes are long and stringy and are not visible.  **Remember: All somatic cells are diploid in number (2n), therefore for each chromatid there also exists its homolog, which also replicates during interphase.
  9. 9. Early prophase I During early prophase it is the same as in mitosis Chromosomes Nuclear envelope Spindle pole
  10. 10. Prophase I Now, during prophase, also occurs the crossing over, where the homologous chromosomes exchange information. Spindle fiber
  11. 11. Metaphase I The chromosomes align in the middle
  12. 12. Telophase I Each chromosome separates from the other, making two news.
  13. 13. Meiosis I Flowchart
  14. 14. After telophase I the second meiotic division occurs (without DNA duplication before), as a final result we have 4 haploid cells)
  15. 15. Prophase II  The nuclear membranes of the daughter cells disintegrate again. The spindle fibres re- form in each daughter cell
  16. 16. Metaphase II  The chromosomes, each still made up of sister chromatids, are positioned randomly on the metaphase plate with the sister chromatids of each chromosome pointing towards the opposite poles.
  17. 17. Metaphase II  Each sister chromatid is attached to the spindle fibres at the centromere
  18. 18. Anaphase II  The centromeres of the sister chromatids finally separate, and the sister chromatids of each chromosome are now individual chromosomes.  The chromosomes move towards the opposite poles of the cell.
  19. 19. Telophase II  Finally, the nucleoli and nuclear membranes re-form. The spindle fibres break down.  Cytokinesis follows and four haploid daughter cells are formed, each containing half the number of chromosomes and is genetically different from the parent diploid cell.
  20. 20. Telophase II  These haploid cells will develop into gametes.
  21. 21.  Males: all 4 hapoid cells become sperm Females: in oogenesis, only 1 of the haploid cells becomes an egg, and the other 3 are reabsorbed by the body.
  22. 22. Human genetics = 46 chromosome (2n) 23 pairs of chromosome
  23. 23. Mutation  Mutation is a change in structure, arrangement or quantity of the DNA in the chromosome  May be caused by:  Mistakes in the replication of DNA  Damage to the DNA by radioactive and carcinogenic substance  Disruption to the orderly movement of chromosomes during cell division
  24. 24. In Mitosis  If the functions of these genes are disrupted due to mutation, cancers may form.  Somatic mutations are not transmitted to the offspring, but may cause body cells to malfunction  Cancers are caused by somatic mutation
  25. 25. In Meiosis  Meiosis involves an orderly movement and reduction (in meiosis I) of a diploid cell to two haploid cells that subsequently divide (in meiosis II) to form four haploid gametes  Since these are gametes, so any mistakes – caused by disorderly movement of chromosomes during meiosis --- are inherited by the offspring.
  26. 26. Example: non-disjunction or improper segregation (separation) of chromosome  During anaphase I, certain homologous chromosomes fail to segregate, resulting in the production of gametes with either an extra chromosome (n+1) or a missing chromosome (n-1)  If this abnormal gametes unites with a normal gamete, an abnormal zygote will be produced.
  27. 27. Down’s syndrome  3 copies of chromosomes number 21, instead of the normal 2 chromosomes  This means a down syndrome patient has (2n+1 = 47) 47 chromosomes instead of the normal (2n=46) chromosomes
  28. 28. Trisomy 21/ Down Syndrome
  29. 29. References  TRabajo de clase para Biología MEC-BC. IES Pedro de Luna- 2011- Zaragoza SPAIN  by imkaelah on Jul 24, 2013  http://www.slideshare.net/imkaelah/meiosis-lesson?qid=95dae3f0- 5bd5-4b9a-9ffa-9f69c1d878a2&v=default&b=&from_search=1  Rebecca Choong  http://www.slideshare.net/rchoong/mitosis-meiosis-lesson- 3?qid=5905ba13-12a1-4b46-ae5f- 969f78f84434&v=default&b=&from_search=3  by Anesh Jeyakumar on Oct 31, 2011  Slides abducte.d from slideshare.

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