3.3 Meiosis

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Meiosis may seem very daunting. We can handle mitosis – just about! But meiosis ….? Meiosis is a reductive division and has two distinct phases. Additionally it has those extra and so important twists which result in variation. All of this makes for a challenging little unit.

SYLLABUS

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READING

First, from C.J. Clegg’s Biology for the IB Diploma, second edition (Hodder Education)

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And secondly from Glenn and Susan Toole’s Biology in Context for Cambridge International A Level (Nelson Thornes). This focusses upon the process of meiosis and then, in the second part, 15.2, on the idea of genetic variation.

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SOME VOCABULARY

  • AMNIOCENTESIS – a technique used for the genetic screening of a foetus while still inside the uterus of its mother. Ultrasound is usually used to locate the precise position of the foetus and the placenta within the uterus. A fine needle is then inserted through the abdominal wall of the mother into the amniotic cavity. A sample of the amniotic fluid, containing foetal cells, is withdrawn. The cells can be examined and their chromosomes observed or the DNA studied
  • BIVALENT: a single pair of condensed, homologous chromosomes, lying side by side during meiosis 1. (Each chromosome itself consists of a pair of chromatids.)
  • CHIASMATA (sing. chiasma): X-shaped structures formed at the points of cross-over between homologous chromosomes during prophase 1 (meoisis 1, prophase). (The number of chiasmata per pair of chromosomes depends on the number of cross-overs – maybe 2 or 3. Chiasmata provide the ‘glue’ necessary for the microtubules to pull against in metaphase 1, when separation occurs.)
  • CHORIONIC VILLUS SAMPLING: a method of obtaining embryonic cells for genetic screening. Once the position of the developing embryo has been determined, a fine needle is inserted through the vagina and cervix, and cells are collected from the chorionic villi, which form part of the developing placenta.
  • CHROMATIN v CHROMOSOME v CHROMATID
    • CHROMATIN: the DNA and its associated protein which is present in the nucleus of a non-dividing cell.
    • CHROMOSOME: the DNA and its associated packaging protein in the nucleus of a cell, usually only visible in dividing cells, after it has condensed. (NB Bacterial chromosomes do not have associated protein and they are formed as a loop in the cytoplasm.)
    • CHROMATID: the two strands of genetic material which make up each chromosome, visible in the early stages of mitosis or meiosis, and held together by a centromere. (Chromatids are formed during DNA replication in the S phase of the interphase of the cell cycle.)
  • CONDENSATION (OF CHROMOSOMES): the shortening and fattening of chromosomes during mitosis or meiosis which makes them visible (and no longer referred to as chromatin).
  • CROSSING  OVER: the exchange of sections of chromatid between a pair of homologous chromosomes, during prophase 1 of meiosis, at chiasmata (crossing over points). (The results of crossing-over are recombinants because the chromosomes will have a combination of alleles different to either of the original pair of chromosomes from each parent. This is significant in promoting variation between individuals.)
  • DIPLOID v HAPLOID:
    • DIPLOID: a term which refers to stages in the life cycle of an organism when it has the full complement of pairs of chromosomes. It is abbreviated as ‘2n’. (In humans the diploid number is 46, or 23 pairs, but different organisms have different diploid numbers.)
    • HAPLOID: a term which refers to stages in the life cycle of an organism when the nucleus contains a single copy of each chromosome. It is abbreviated as ‘n’. (Sex cells or gametes are haploid.)
  • GAMETE: the haploid sex cell produced after meiosis.
  • HOMOLOGOUS (CHROMOSOMES): chromosomes which are capable of pairing during meiosis 1. One of each pair is derived from each parent. Homologous chromosomes contain the same sequence of gene loci as each other but may have different alleles.
  • INDEPENDENT ASSORTMENT (OF MATERNAL & PATERNAL, HOMOLOGOUS CHROMOSOMES): Mendel’s law of independent assortment states that the alleles of two (or more) different genes get sorted into gametes independently of one another. (In other words, the allele a gamete receives for one gene does not influence the allele received for another gene.) This results from the random orientation of bivalents on the equator of a cell during Metaphase 1 of meiosis, meaning that not all ‘maternal’ chromosomes get pulled to one pole and ‘paternal’ to the other. (Nevertheless there may be linkage groups of genes on pairs of chromosomes, between which crossing-over is unlikely to occur, and so they remain associated.)
  • NON-DISJUNCTION (OF CHROMOSOMES): the failure of chromosomes to separate properly during either meiosis 1 or meiosis 2, resulting in gametes which contain an incorrect number of chromosomes. (Down’s syndrome in humans results from non-disjunction of chromosome pair number 21 during metaphase 1 of meiosis. After fertilisation an individual will be produced with 47 chromosomes in the nucleus, resulting in Down’s syndrome.)
  • RECOMBINATION: After crossing-over in meiosis 1 recombinant chromosomes are produced which differ from either of their parental chromosomes.
  • REDUCTION DIVISION: Meiosis is a reduction division which results in the formation of haploid gametes, containing half the full number of chromosomes.
  • SYNAPSIS: is the pairing of homologous chromosomes in prophase 1 of meiosis.

COMPARING AND CONTRASTING MITOSIS AND MEIOSIS

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DOWN’S SYNDROME

Down’s syndrome results from the non-disjunction of chromosome #21 at the end of Meiosis 1.scan_20170227-2scan_20170227-3


LAB

There are plenty of labs out there, for meiosis, karyotyping, etc. Here is one I use:

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