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Meiosis Lesson


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Slides on meiosis.

Slides on meiosis.

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  • 1. Meiosis NEREA LÓPEZ PEIRONA 4ºB
  • 2. Significance of Meiosis (1) Crossing over in bivalent - Produces new combinations of genes in both chromosomes
  • 3. Chiasma and Crossing Over
  • 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. 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. e
  • 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. 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. Early prophase I During early prophase it is the same as in mitosis Chromosomes Nuclear envelope Spindle pole
  • 10. Prophase I Now, during prophase, also occurs the crossing over, where the homologous chromosomes exchange information. Spindle fiber
  • 11. Metaphase I The chromosomes align in the middle
  • 12. Telophase I Each chromosome separates from the other, making two news.
  • 13. Meiosis I Flowchart
  • 14. After telophase I the second meiotic division occurs (without DNA duplication before), as a final result we have 4 haploid cells)
  • 15. Prophase II  The nuclear membranes of the daughter cells disintegrate again. The spindle fibres re- form in each daughter cell
  • 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. Metaphase II  Each sister chromatid is attached to the spindle fibres at the centromere
  • 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. 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. Telophase II  These haploid cells will develop into gametes.
  • 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. Human genetics = 46 chromosome (2n) 23 pairs of chromosome
  • 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. 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. 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. 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. 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. Trisomy 21/ Down Syndrome
  • 29. References  TRabajo de clase para Biología MEC-BC. IES Pedro de Luna- 2011- Zaragoza SPAIN  by imkaelah on Jul 24, 2013  5bd5-4b9a-9ffa-9f69c1d878a2&v=default&b=&from_search=1  Rebecca Choong  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.