Chromosomes and meiosis


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A Powerpoint for Grade 12 Life Sciences / Biology students focussing on chromosomes and meiosis. Contains information and diagrams on meiosis, mitosis, the structure of chromosomes, DNA and RNA

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Chromosomes and meiosis

  1. 1. Module Three: Life at a Molecular, Cellular and Tissue Level Life Sciences Matric Syllabus Mind Action Series: Life Sciences Textbook and Workbook
  2. 2. • A bivalent is a pair of homologous chromosomes in close contact with each other.• Chromosomes are long, thread-like structures that form part of the chromatin network in the nuclei of cells, made up of a strand of DNA wound around a histone (protein).• Chiasmata are the points of crossing over where the chromatids break. 3.2) Chromosomes and Meiosis
  3. 3. • Chromosomes are long, thread-like structures that form part of the chromatin network in the nuclei of cells.• They consist of a strand of DNA wound around histones (proteins).• A set of chromosomes in a cell is called a karyotype. It shows the number, size and shape of the chromosomes during the metaphase of mitosis. • They are useful as they can show whether a cell comes from a male or female, as well as any abnormalities in the chromosomes.• Every species has a specific number of chromosomes in its somatic cells. Some organisms have identical chromosome numbers but these need not be related. 3.2) Chromosomes and Meiosis
  4. 4. • In somatic (body) cells of diploid organisms: – The number of chromosomes in each cell is the same. – Chromosomes are made up of two sets: one from the mother, one from the father. They are called diploid cells, or 2n. – A maternal chromosome will have a matching paternal chromosome. Together they will form a homologous pair. The chromosomes forming a pair will have the same size and shape, but may have different alleles for each trait. – The DNA of each chromosome replicates to form two identical threads or chromatids joined by a centromere. This takes place in the interphase of a cell cycle, i.e. between cell divisions. – Replication of DNA is very important to ensure that, as a cell divides, each daughter cell receives a full complement of all the genetic material. 3.2) Chromosomes and Meiosis
  5. 5. 3.2) Chromosomes and Meiosis
  6. 6. • Meiosis is cell division that takes place in the reproductive organs of both plants and animals to produce gametes (sex cells) in animals and spores in plants.• In meiosis, the number of chromosomes is reduced from two sets (2n) to one set (n) in each new daughter cell.• The gametes/spores are called haploid cells because they only have one set of chromosomes, i.e. one chromosome from each homologous pair.• In animals, meiosis takes place in the reproductive organs, the testes (spermatogenesis) and ovaries (oogenesis).• In plants, meiosis takes place in the male anthers to form pollen sacs (microsporangia) and in the female ovaries to form ovules (megasporangia). 3.2) Chromosomes and Meiosis
  7. 7. 3.2) Chromosomes and Meiosis
  8. 8. 3.2) Chromosomes and Meiosis
  9. 9. • The DNA of the parent cells is replicated in interphase preceding both meiosis and mitosis. However, in meiosis, replication is followed by two divisions. – Meiosis 1 is a reduction division which results in two cells being formed, each with half the number of chromosomes of the parent cell, i.e. the haploid number (n). – Meiosis 2 is a copying division which involves the two haploid cells dividing again by mitosis to form 4 haploid cells. 3.2) Chromosomes and Meiosis
  10. 10. • In early prophase, chromosomes become short, fat, and visible.• In late prophase, the chromosomes of homologous pairs lie along side one another, forming a bivalent. The centrioles move to opposite poles. A spindle, made of protein threads, develops across the cell from the two centrioles. At this point, crossing over takes place. Nuclear membrane breaks down. 3.2) Chromosomes and Meiosis
  11. 11. • The bivalents (not the chromosomes) move to the middle of the cell and line up at the equator.• The centromeres become attached to the spindle threads. 3.2) Chromosomes and Meiosis
  12. 12. • The centromeres do not split. The bivalents separate and the chromosomes, not the chromatids, are pulled away from each other by the contracting spindle threads. The chromosomes move to opposite poles of the cell. 3.2) Chromosomes and Meiosis
  13. 13. • The cytoplasm then divides via cytokinesis to form two haploid cells. Both the new cells only have one of each homologous pair of chromosomes. 3.2) Chromosomes and Meiosis
  14. 14. • Each chromosome is made up of two chromatids joined by a centromere. The spindle, made up of protein fibres develops. The nuclear membrane disappears. 3.2) Chromosomes and Meiosis
  15. 15. • Chromosomes move to the middle of the cell where they line up at the equator. The centromeres become linked to the spindle threads. 3.2) Chromosomes and Meiosis
  16. 16. • The centromeres split, allowing each chromosome to separate into two chromatids. Spindle threads contract and pull the chromatids apart. The chromatids, now called daughter chromosomes, move to the poles of the cell. 3.2) Chromosomes and Meiosis
  17. 17. • Daughter chromosomes group together at the poles. A new nuclear membrane starts to form around each set of daughter chromosomes. 3.2) Chromosomes and Meiosis
  18. 18. • The cytoplasm starts to divide forming two new daughter cells, each with the haploid number of chromosomes. A new nucleolus forms. 3.2) Chromosomes and Meiosis
  19. 19. • Meiosis 1 – A reduction division – Early Prophase  Late Prophase  Metaphase  Anaphase  Telophase• Meiosis 2 – A copying division – Late Prophase  Metaphase  Anaphase  Telophase  Cytokinesis• At the end of meiosis four new, non-identical, haploid cells are formed from one parent cell, each with half the original number of chromosomes. The gametes are not identical to the parent cell. 3.2) Chromosomes and Meiosis
  20. 20. 1. First meiotic division Homologous chromosomes come together to form a bivalent; one1 from each pair goes into each daughter cell 2. Two haploid daughter cells2 3. Second meiotic division Each chromosome separates into3 two chromatids, one goes into each daughter cell 4. Four haploid daughter cells4 3.2) Chromosomes and Meiosis
  21. 21. • The number of chromosomes has to be halved – or on fertilization, the zygote would have double the number of chromosomes. The next generation will have double the number of chromosomes, and so on.• Meiosis makes new gene combinations come about, resulting in variation of offspring. 3.2) Chromosomes and Meiosis
  22. 22. • Crossing over is the mutual exchange of pieces of chromosome so that whole groups of genes are swapped between maternal and paternal chromosomes. This takes place in the late prophase of meiosis 1.• The replicated homologous pairs of chromosomes come together in a process called synapsis to form bivalents. They swap pieces of their inner chromatids by breaking and reforming their DNA while they are paired up.• The points of crossing over where the chromatids break are called chiasmata.• In this way, some genes from a maternal chromatid change place with some genes from a paternal chromatid, forming a recombinant chromatid. The outer, unchanged chromatids are called parentals. 3.2) Chromosomes and Meiosis
  23. 23. 3.2) Chromosomes and Meiosis
  24. 24. • The exchange of genetic material produces chromatids with a unique combination of genes. This increases variation among the daughter cells as there will be new combinations of genetic material. This is why offspring will not look the same (except for identical twins) or the same as one parent.• During this exchange, mistakes may occur which lead to mutations. Most mutations are harmful but occasionally may be beneficial. In this way, new genes may be introduced into the genetic make up of a species which can influence evolution. 3.2) Chromosomes and Meiosis
  25. 25. Similarities:• Both are types of cell division.• The DNA of the parent cells is replicated in interphase before cell division starts.• In early prophase, the chromosomes become short and fat, and are visible as two chromatids joined by a centromere. 3.2) Chromosomes and Meiosis
  26. 26. Process Mitosis MeiosisPurpose - Development of an adult organism - Forms gametes or spores (for from a single zygote reproductive purposes) - Growth and repair of tissues - Regeneration of body parts - Asexual reproductionDifferences Involves ONE cell division Involves two cell divisions In prophase, no bivalents are formed and In prophase, bivalents are formed and no crossing over occurs. crossing over occurs In metaphase, the centromeres split In metaphase, centromeres do not split In anaphase, the chromatids of each In anaphase, the chromosomes of chromosome move to opposite poles of each homologous pair move to the cell. opposite poles of the cell. Two daughter cells are formed with the Four daughter cells formed with half same number of chromosomes as the the number of chromosomes as the parent cell (diploid) parent cell (haploid) Somatic cells are formed which are Gametes are formed which are similar genetically to the parent. genetically different to each other and to the parent cell.Location Takes place in growing regions of Takes place in reproductive organs plants/animals
  27. 27. • ‘Genetic variation’ refers to the differences which exist between organisms belonging to the same species• In organisms which reproduce sexually, every one of the offspring (excepting for identical twins) possess a unique combination of genes, therefore there is always variation in sexual reproduction. 3.2) Chromosomes and Meiosis
  28. 28. • This variation of the offspring is because their genotypes differ as a result of: – The crossing over of pieces of chromatids which causes a reshuffling of genes in the gametes formed. – The random movement of maternal and paternal chromosomes to opposite poles of the cell during anaphase of meiosis. This is called the independent assortment of chromosomes and it results in every egg and every sperm formed containing a mixture of maternal and paternal chromosomes, i.e. a huge variety of genetic combinations. – The sheer chance as to which particular sperm fertilises an egg cell during fertilisation. Any two gametes can fuse, resulting in many possible combinations of genes in the zygote. The new individual will have a unique combination of genes, different from either of its parents and siblings. – Mutations. 3.2) Chromosomes and Meiosis
  29. 29. • A mutation is a sudden and unpredictable change in the genetic makeup of an organism. This may be caused by a: – Gene mutation – Chromosome mutation. This type of mutation can only occur during meiosis. It involves a change in the structure and distribution of one or more chromosomes and therefore results in a change in the cell’s karyotype. 3.2) Chromosomes and Meiosis
  30. 30. • Polyploidy is a type of chromosome mutation.• It is a condition of having more than two sets of chromosomes. It is rare in animals but is especially common among ferns and flowering plants. GM’d wheat has strains such as durum wheat that are tetraploid (4n) and bread wheat that are hexaploid (6n).• Polyploidy can occur: – Naturally in certain plant tissues – As a result of abnormal meiosis. If the chromosomes do not split during anaphase 1, the gametes will be diploid and when these fuse a tetraploid zygote will occur. 3.2) Chromosomes and Meiosis
  31. 31. Advantages to Polyploidy:• Creates an instant new species, therefore it plays an important role in evolution.• Polyploid plants will have: – Larger fruits – Larger flowers – Larger storage organs. 3.2) Chromosomes and Meiosis
  32. 32. • Anaploidy is another type of chromosome mutation where cells have extra chromosomes or missing chromosomes.• Affected individuals will have mental and physical characteristics called syndromes.• One such example is Down Syndrome. 3.2) Chromosomes and Meiosis
  33. 33. Down syndrome is an example of anaploidy that occurs in childrenwho are born with an extra copy of chromosome number 21 intheir cells (i.e 2n + 1), a condition known as trisonomy.How Down Syndrome Occurs:• During oogenesis (meiosis in the production of an egg), the two number 21 chromosomes do not separate properly in anaphase one.• Both chromosomes enter one daughter cell, instead of one entering each of the daughter cells formed.• The zygote will have three number 21 chromosomes instead of two, and a total of 47 chromosomes instead of 46.• As the new embryo develops by mitosis, all the cells will have 47 chromosomes. 3.2) Chromosomes and Meiosis
  34. 34. Characteristics of those affected by Down Syndrome:• Varying degrees of mental retardation, differing from person to person.• Distinctive flattened facial features with slightly slanting eyes due to folds of skin at the corner of the eyes.• Other physical features include short stubby fingers and toes with big toes spaced widely apart from the second toe, a large head and abnormal ears.• Heart defects• Happy, loving natureThere is no cure for down syndrome. 3.2) Chromosomes and Meiosis
  35. 35. • Down syndrome is relatively common – 1 in every 900 births are to a Down syndrome child. The older the mother, the more likely Down syndrome will occur.• Down Syndrome children develop slower than others. Some Down syndrome children can attend mainstream schools.• Affected children need support as they are often discriminated against.• A test taken early in pregnancy, involving an ultrasound and a blood test, can more or less pinpoint whether the foetus has Down syndrome or not.• If that test shows that the foetus may be affected by Down Syndrome, the mother could choose to have a risky amniocentesis, which determines the karyotype of the foetus. 3.2) Chromosomes and Meiosis
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