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Meiosis: The Process and Importance in Reproduction and Genetics

This Grade 12 Life Sciences revision covers the process of meiosis, its role in reproduction and genetics, and its importance in introducing genetic variation. Learn about different phases of meiosis, crossing-over, abnormal meiosis, and non-disjunction.

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Meiosis: The Process and Importance in Reproduction and Genetics

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  1. THE PROCESS OF MEIOSIS, MEIOSIS AND REPRODUCTION, MEIOSIS AND GENETICSGrade 12 Life Sciences Revision Refer to the CAPS document and to MEIOSIS, HUMAN REPRODUCTION AND GENETICS webpages

  2. Introduction • Review the structure of a cell (Grade 10) (E.G) • State that: • Chromosomes consists of DNA (which makes up genes) and protein • The number of chromosomes in a cell is a characteristic of an organism (for example humans have 46 chromosomes) • Chromosomes which are single stranded become double stranded (two chromatids joined by a centromere) as a result of DNA replication. You should be able to draw and label a diagram of a chromosome.

  3. Differentiate between: Haploid(n) and diploid(2n) cells in terms of chromosome number and sex cells (gametes) and somatic cells (body cells) • Differentiate between: Sex chromosomes (gonosomes) and autosomes

  4. Meiosis – The process • Define meiosis. (E. G) a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores. • State where meiosis takes place in plants and in animals.

  5. Interphase • State that interphase takes place before meiosis and that although meiosis is a continuous process, the events are divided into different phases for convenience • Describe what happens during interphase as follows: • DNA replication takes place • Single stranded chromosomes become double-stranded • Each chromosome will now consist of two chromatids joined by a centromere

  6. Describe the events of each phase of Meiosis I as follows: Prophase I • Centrosome splits into centrioles which move to opposite poles forming spindle threads. • Nuclear membrane and nucleus start to disappear. • Chromosomes become distinct. • Crossing over takes place as follows: • Homologous chromosomes lie next to each other • Non-sister chromatids of homologous chromosomes overlap • Point of overlap is called the chiasmata • A break occurs at the chiasmata • Chromatid segments (genetic material) have now been exchanged

  7. 2N Chromosomes are visible as doubles eg 2 chromatids joined together by a centromere AND each chromosome pairs up with one of the same size to form a BIVALENT Prophase 1 MEIOSIS 1 – BEGINNING OF PROPHASE 1

  8. PROPHASE 1maternal and paternal chromosomes exchange information

  9. 2N Look at evidence here that crossing over has taken place. 2 chromosome pairs visible Prophase 1 END OF PROPHASE 1

  10. Meiosis 1 continued Metaphase I • Chromosomes align at equator in a random fashion and in homologous pairs (bivalents), attached to the spindle threads on the same fibre, using centromeres. Anaphase I • Spindle threads contract. • Homologous chromosomes are separated and move apart towards the poles. • The cytoplasm starts to divide. Telophase I • 2 new cells have been formed. • Each new cell has half of the chromosome complement of the original cell (it is haploid). • The cells are not identical.

  11. Bivalents line up on the equator and are attached to the spindle fibres by centromeres Metaphase 1 2N METAPHASE 1

  12. Anaphase 1 ANAPHASE 1 Spindle fibres contract towards poles pulling whole chromosomes apart

  13. TELOPHASE 1 TWO cells formed with too much information – still need to halve the amount of DNA

  14. Describe the events of each phase of Meiosis II as follows:

  15. TELOPHASE 1 TWO cells formed with too much information – still need to halve the amount of DNA

  16. 1N Prophase 2 PROPHASE 2 • THIS HAPPENS IN BOTH CELLS AT THE SAME TIME 2 separate chromosomes are visible in each cell

  17. Metaphase 2 METAPHASE 2 Chromosomes align individually on the equator and attach to spindle by centromeres

  18. Anaphase 2 ANAPHASE 2 Spindle fibres contract pulling chromatidsapart

  19. TELOPHASE 2 AND CYTOKINESIS • NOTE EACH DAUGHTER CELL HAS TWO CHROMOSOMES

  20. Importance of Meiosis State the importance of meiosis with regard to each of the following: • Production of gametes (refer to detail of gametogenesis) • Halving of the chromosome number (diploid to haploid) • Mechanism to introduce genetic variation through: • Crossing-over

  21. Crossing over

  22. More on crossing over • Mechanism to introduce genetic variation through: • Crossing-over

  23. The random arrangement of chromosomes at the equator

  24. Abnormal Meiosis • State what is meant by non-disjunction • State the consequences of non-disjunction • Describe how non-disjunction of chromosome pair 21 in humans may lead to the formation of an abnormal gamete with an extra copy of chromosome 21 • Describe how the fusion between the abnormal gamete and a normal gamete may lead to Down syndrome

  25. Normal Meiosis

  26. Non- disjunction Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division

  27. Cell division and no - disjunction • There are three forms of nondisjunction: • failure of a pair of homologous chromosomes to separate in meiosis I, • failure of sister chromatids to separate during meiosis II, • and failure of sister chromatids to separate during mitosis. Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy).

  28. Down syndrome • Down's syndrome is caused by the presence of three copies of the 21st chromosome. • This chromosomal defect is known as Trisomy-21. • Down's syndrome almost always results in mental retardation, though the severity of the retardation varies.

  29. Epicanthic fold Small, oblique eyes Flattened, nasal bridge Open mouth Protruding tongue Broad neck Small underdeveloped ears set low on head Characteristics of Down syndrome

  30. Wide gap between first and second toe Incurved finger Single palmar (“simian”) crease Short broad hands

  31. Comparison of mitosis and meiosis How are mitosis and meiosis similar? • Both mitosis and meiosis are associated with cytokinesis. The end result of both are daughter cells produced from a parent cell. • The fundamental sequence of events in mitosis is the same as in meiosis (in meiosis it happens twice). • Both processes include the breakdown of the nuclear membrane, the separation of genetic material into two groups, followed by cell division and the reformation of the nuclear membrane in each cells

  32. Tabulate differences between mitosis and meiosis

  33. Meiosis and sexual reproduction 1 • Relationship between Gametogenesis and Meiosis • Define each of the following terms: • Gametogenesis • Spermatogenesis • Oogenesis

  34. Meiosis and sexual reproduction 2 • Describe spermatogenesis as follows: Refer to cross section of seminiferous tubule and diagram of a sperm cell • Diploid cells in the seminiferous tubules of the testes undergo meiosis to form haploid sperm cells • Identify and state the functions of the parts of a sperm cell (acrosome, head with haploid nucleus, middle  portion with mitochondria, and a tail)

  35. Meiosis and sexual reproduction 3 • Describe oogenesis as follows: Refer to longitudinal section of the ovary and diagram of an ovum • Diploid cells in the ovary undergo meiosis to form a primary follicle consisting of haploid cells • One cell develops into an ovum contained in a Graafian follicle • Identify and state the functions of the different parts of an ovum (layer of jelly, haploid nucleus, cytoplasm)

  36. Meiosis and genetics 1 • FROM THE CAPS DOCUMENT: • State Mendel’s principle of segregation • ‘Each pair of alleles is sorted into different gametes and • subsequently into different offspring. This is the result • of the way each allele is carried on separate • homologous chromosomes that are separated during • meiosis’. • This is important to remember when doing • monohybrid and dihybrid crosses.

  37. Meiosis and genetics 2 Consequences of abnormal meiosis If meiosis does not proceed according to plan the resulting gametes may have an uneven number of chromosomes. This is usually caused by non-disjunction of chromosomes during Anaphase 1. The most well known example of non-disjunction is Trisomy-21 or Down Syndrome. This is a form of chromosomal aberration rather than a mutation. Refer to the karyotype of this person with Trisomy-21. The person is diploid but has one extra chromosome or three of the number 21 chromosomes.

  38. Sources of information • http://www.macroevolution.net/meiosis-stages.html • http://homework.sdmesa.edu/dancinec/sectional-review/genetics/answ-16.htm • http://humandna.co.in/images/chromosome_large.png • http://www.histology.leeds.ac.uk/female/FRS_ova.php • http://media-1.web.britannica.com/eb-media/88/78588-004-8A1C39DB.jpg • http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-20/20_09.jpg • http://images.tutorvista.com/content/reproduction-in-animals/spermatogenesis-and-spermiogenesis-stages.jpeg • http://www.macroevolution.net/images/crossing-over-300px.jpg • http://image.slidesharecdn.com/crossingover-140716231736-phpapp01/95/crossing-over-7-638.jpg?cb=1405571488 • http://www.ib.bioninja.com.au/_Media/chiasma_med.jpeg • http://facultylounge.whfreeman.com/files/images/figure_07_09.preview.jpg • http://64.40.115.138/file/lu/6/52235/NTIyMzV9K3szNjMyMjA=.jpg?download=1 • http://www.washington.edu/news/files/2012/11/85155_large-down-syndrome-trisomy.jpg • https://abcofbiology.files.wordpress.com/2013/10/table_mitosis_meiosis.jpg • http://highered.mheducation.com/sites/9834092339/student_view0/chapter11/comparison_of_meiosis_and_mitosis.html • Biozone Power Point ‘ Inheritance PP’ Available at: http://intranet.bishops.org.za/Biology/LifeSciences12/GeneticsandInheritanceGr12.htm

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