IMMUNITY

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• The Need for Reduction Division during Meiosis
• During fertilization the nuclei of gametes fuse together to form the nucleus of the zygote
• Both gametes must contain the correct number of chromosomes in order for the zygote to be viable
• If a zygote has too many or too few chromosomes it may not survive
• For a diploid zygote

  The gametes must be haploid (n + n = 2n)
• Meiosis produces haploid gametes during sexual reproduction
• First cell division of meiosis

  Reduction division, a nuclear division that reduces the chromosome number of a cell
• In humans, the chromosome number is reduced from 46 (diploid) to 23 (haploid) during meiosis
• The reduction in chromosome number during meiosis ensures the gametes formed are haploid
• The maintenance of chromosome number through reduction division in a mammalian life cycle


  Ensures the gametes formed are haploid
• Homologous Chromosomes


  In diploid cells, there are two complete sets of chromosomes in the nucleus, carry the same genes in the same positions, are the same shape, and help line up alongside each other during meiosis
• During fertilization, a diploid zygote is formed where one chromosome of each homologous pair comes from the female gamete and the other comes from the male gamete
• Chromosomes line up alongside each other during meiosis
• Although homologous pairs of chromosomes contain the same genes in the same order they don’t necessarily carry the same alleles (form) of each gene!
• Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells
• Meiosis produces gametes in plants and animals that are used in sexual reproduction
• Meiosis has two divisions: meiosis I and meiosis II
• Meiosis I


  1. DNA condenses and becomes visible as chromosomes

  2. DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere

  3. Chromosomes are arranged side by side in homologous pairs

  4. Homologous chromosomes are very close together allowing crossing over of non-sister chromatids to occur at the chiasma

  5. Centrioles migrate to opposite poles and the spindle is formed

  6. Nuclear envelope breaks down and nucleolus disintegrates
• Metaphase I


  Bivalents line up along the equator of the spindle with spindle fibers attached to the centromeres
• Anaphase I


  1. Homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle

  2. Centromeres do not divide
• Telophase I


  1. Chromosomes arrive at opposite poles

  2. Spindle fibers start to break down

  3. Nuclear envelopes form around the two groups of chromosomes and nucleoli reform

  4. Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I
• Cytokinesis


  1. Division of the cytoplasm occurs

  2. Cell organelles get distributed between the two developing cells
• Second division of Meiosis: Meiosis II

  1. No interphase between meiosis I and meiosis II so the DNA is not replicated

  2. Almost identical stages to mitosis
• Prophase II


  1. Nuclear envelope breaks down and chromosomes condense

  2. Spindle forms at a right angle to the old one
• Metaphase II


  Chromosomes line up in a single file along the equator of the spindle
• Anaphase II


  1. Centromeres divide and individual chromatids are pulled to opposite poles

  2. Creates four groups of chromosomes with half the number compared to the original parent cell
• Telophase II


  Nuclear membranes form around each group of chromosomes
• Cytokinesis


  1. Cytoplasm divides as new cell surface membranes are formed creating four haploid cells

  2. Cells contain the same number of centromeres as they did at the start of meiosis I but now only have half the number of chromosomes
• The different stages of Meiosis I in an animal cell
• Prophase II, Metaphase II and Anaphase II in Meiosis II of an animal cell
• Understanding the difference between chromosomes and chromatids can be difficult
• We count chromosomes by the number of centromeres present
• During interphase, when the 46 chromosomes duplicate and the amount of DNA in the cell doubles, there are still only 46 chromosomes present because there are still only 46 centromeres present
• However, there are now 92 chromatids, which are strands of replicated chromosomes
• Identifying the Stages of Meiosis
• Cells undergoing meiosis can be observed and photographed using specialised microscopes
• The different stages of meiosis have distinctive characteristics meaning they can be identified from photomicrographs or diagrams
• Homologous chromosomes pair up side by side in meiosis I only
• If there are pairs of chromosomes in a diagram or photomicrograph, meiosis I must be occurring