1 / 40

4.2 - Meiosis

4.2 - Meiosis. Topic 4 - Genetics. Meiosis - overview. Undergoes cell division twice resulting in gametes. New cells only have half the number of chromosomes e.g. typical human cell has 46 chromosomes (diploid) but a human cell after meiosis only has 23 chromosomes (haploid).

minnie
Télécharger la présentation

4.2 - Meiosis

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 4.2 - Meiosis Topic 4 - Genetics

  2. Meiosis - overview • Undergoes cell division twice resulting in gametes. • New cells only have half the number of chromosomes e.g. typical human cell has 46 chromosomes (diploid) but a human cell after meiosis only has 23 chromosomes (haploid). • Known as reduction division because the number of chromosomes has been reduced.

  3. Meiosis - overview • Chromosomes replicate in the S-phase of interphase to make pairs of sister chromatids. • Meiosis I separates the homologous pairs – this is the reduction division. • Meiosis II separates the sister chromatids. • Meiosis generates four haploid daughter cells and each cell has a unique mix of half of the genetic information of the parent cell.

  4. Homologous Chromosomes • Homo- -logous • Same structure • Somatic cells contain a homologous pair of each chromosome. • One pair is paternal – father • The other is maternal – mother • They are the same size and structure. • Carry the same genes. • Alleles may vary. • 22 human chromosomes are homologous (exception the 23rd pair, sex chromosomes).

  5. Phases of Meiosis Interphase I (G1, S, G2) Prophase I Metaphase I Anaphase I Telophase I Interphase II (no S) Prophase II Metaphase II Anaphase II Telophase II

  6. Phases of meiosis INTERPHASE I Chromosomes replicate. DNA replication occurs in the Synthesis phase of Interphase I Each chromosome becomes a pair of sister chromatids, joined by the centromere at the equator.

  7. Phases of meiosis PROPHASE I Homologous chromosomes associate with each other. Crossing-over between non-sister chromatids. Cross-over results in recombination of alleles and is a source of genetic variation in gametes. Nuclear membrane breaks down and centrioles migrate to the poles.

  8. Phases of meiosis METAPHASE I Homologous pairs (bivalents) line up at equator (M-Metaphase = middle). Random orientation of these homologous pairs (the way they face) leads to genetic variation among gametes. There are well over 8 million possible orientations in humans. Spindle fibers attach to centromere.

  9. Phases of meiosis ANAPHASE I Spindle fibers contract. Homologous pairs are separated and pulled to opposing poles. REDUCTION DIVISION Non-disjunction here will affect the chromosome number of all four gametes.

  10. Non-Disjunction 2n / \ n+1 (n-1) / \ / \ n+1 n+1 n-1 n-1 Chromosomes do not separate in typical fashion during either meiotic division. Result: unequal distribution of chromosomes. Typically two or more homologous chromosomes stick together instead of separating. 2n / \ n n / \ / \ n n n+1 n-1

  11. Phases of meiosis TELOPHASE I Nuclei reform and division by cytokinesis occurs. Haploid cells. Contain one chromosome of each pair. However, each chromatid still has its sister chromatid attached. S phase is not necessary. Cross-over/Recombination produces sister chromatids that will not be exact copies. * Many plants do not have a telophase I stage. **Interphase II occurs before Meiosis II but without the Synthesis phase.

  12. Phases of meiosis PROPHASE II METAPHASE II Reformed nuclei breakdown. No cross-over. New spindle formed. Random orientation of sister chromatids at equator. Spindle fibers attach to centromere.

  13. Phases of meiosis ANAPHASE II Spindle fibers contract causing centromeres to split releasing each chromatid as an individual chromosome. In animal cells the cell membrane pinches; in plant cells a new cell plate forms

  14. Phases of meiosis TELOPHASE II New haploid nuclei form. Cytokinesis. End result is formation of four haploid gamete cells. Fertilization of haploid cells will produce a diploid zygote.

  15. Meiosis vs mitosis MEIOSIS MITOSIS Reduction division 4 haploid daughter cells Daughter cells are genetically unique Sex cell production Cell division 2 diploid daughter cells Daughter cells are genetically identical Cell growth and repair Terminology: Haploid/diploid, somatic/gamete cells,

  16. Outline the process of meiosis

  17. Outline the process of meiosis

  18. Down’s Syndrome • Non-disjunction occurs in the 21st chromosomal pair. • Results is trisomy or receiving 3 chromosomes instead of 2. • Brings about malformations of the digestive system and differing degrees of learning difficulties. • Most common chromosomal anomaly: ~1 birth in 800. • Age of mother increases, risk of non-disjunction increases.

  19. Karyotype of someone with down’s syndrome Generally at risk women are advised to have a karyotype of their fetus analyzed. https://www.youtube.com/watch?v=tDjnNDRP_2o&safe=active

  20. Ethical considerations Karyotyping can give the woman information she needs to decide whether to continue the pregnancy and if she decides to continue, how to prepare. There is some risk to the pregnancy of carrying out the tests. • Who should decide when a karyotype is necessary? • Who should decide whether to keep or to abort the babies which present chromosomal anomalies? • If you found out that your future child had a chromosomal anomaly that would make him or her very different from other children, what would you do? There are many countries where it would be possible to ask that a pregnancy be terminated if parents discover early in the pregnancy that thei child has an atypical number of chromosomes. This is not only a legal question, it is a medical and ethical one, often bringing in philosophical and religious points of view • Should there be laws governing this? • Should expectant mothers over a certain age be obliged to get a karyotype? • Should it be a question of personal freedom? • If one government were to make karyotyping illegal, should it also be illegal to travel to another country to have a karyotype done?

  21. Karyotyping • Extraction of fetal cells by amniocentesis or chorionic villus sampling. • Cells are stained and prepared on a glass slide. • Photomicrograph images are obtained of the chromosomes during mitotic metaphase. • Images are cut out and separated. • The images of each pair of chromosomes are placed in order by size and the position of their centromeres.

  22. Questions?!?!?

More Related