Describe the behaviour of chromosomes during meiosis. [8]

  • ref. to nuclear envelope i.e names of stages
  • meiosis I
    •  chromosomes, condense/thicken/ spiralise ;
    •  homologous chromosomes pair/ bivalents form ;
    •  crossing over/ described ;
    • chiasma(ta) ;
    •  spindle fibres/ microtubules, attach to/pull, centromeres/ kinetochores ;
    •  bivalents line up on, equator/ mid-line ; pairs of homologous chromosomes,
    •  independent assortment (of homologous pairs) / ;
    •  chromosomes move to, two ends of cell/ poles ;
  • meiosis II
    • (individual) chromosomes/ pairs of chromatids, line up on, equator/ mid-line ;
    •  at right angles to first equator ;
    • centromeres divide ;
    •  chromatids separate ; chromatids move to (opposite) poles
    •  ref. to haploid/ chromosome number halved/ one set of chromosomes ;

Explain the significance of mitosis[8]

  •  genetic stability
    •  Mitosis produces two cells with the same chromosomal number as the mother cell .
  •  growth
    •  The number of cells in organism increase with mitosis .
  •  cell replacement
    •  Replacement of cell tissue involve mitosis.
  •  regeneration
    •  Some animals are also regenerated some parts of their body by mitosis.
  •  asexual reproduction
    •  Mitosis is the basic of asexual reproduction of new individuals of a spice by one parent organism

Describe how genetic variation in secondary oocytes arises.

  •  During / prophase 1;
  • Crossing over/ chiasmata formation occurs;
  • Leads to new combination of alleles;
  • During metaphase 1;
  • Homologous chromosomes position themselves either way up/ down on equator of spindles
  • Independent assortment
  • Segregation occurs;

Describe the first division of meiosis (meiosis I) in animal cells. [6]

  • reduction division / (to) halve number of chromosomes / diploid to haploid ;
  • homologous chromosomes pair up / bivalents form ;
  • ref. chiasmata / ref. crossing over ;
  • homologous chromosome pairs / bivalents, line up on equator ;
  • independent assortment ;
  • spindle / microtubules, attached to centromeres ;
  • chromosomes of each pair pulled to opposite poles ;
  • by shortening of, spindle / microtubules ;
  • nuclear envelopes re-form ;
  •  cytokinesis

Describe how crossing over and independent assortment can lead to genetic variation.[8]

  •  occur during meiosis I ;
  • crossing over
  • between non-sister chromatids ;
  • of, (a pair of) homologous chromosomes / a bivalent ;
  • in prophase 1 ;
  • at chiasma(ta) ;
  • exchange of genetic material /
  • linkage groups broken ;
  • new combination of alleles (within each chromosome);
  • independent assortment
  • of homologous chromosomes pairs / bivalents ;
  • each pair lines up independently of others ;
  • line up on equator ;
  • (during) metaphase 1 ;
  • results in gametes that are genetically unique ;

Explain how meiosis promotes genetic variation in sexually reproducing organisms[8][z-n2015/2/11(a)]

Explain how meiosis and fertilization can result in genetic variation amongst offspring. [8]

  •  chiasma/ crossing over ;
  •  between non-sister chromatids ;
  •  of, homologous chromosomes/ bivalent ;
  •  in prophase 1 ;
  •  exchange of, genetic material/ DNA ;
  •  linkage groups broken ;
  •  new combination of alleles ;
  •  random/ independent, assortment of, homologous chromosomes/ bivalents (at equator) ;
  •  (during) metaphase 1 ;
  •  random/ independent, assortment (of, sister chromatids/ chromosomes) at metaphase 2 ;
  •  possible chromosome mutation ;
  •  random mating ;
  • random, fusion/fertilisation, of gametes ;

Outline the differences between mitosis and meiosis[8]

mitosis meiosis
Prophase homologous chromosomes remain separate homologous pair up;
no formation of chiasmata; chiasmata;
metaphase pairs of chromatids line up on spindle equator pairs of chromosomes line up on spindle equator
Anaphase centriomeres divide centromeres don’t divide;
chromatids separate chromosomes separate;
chromatids identical chomatids may not be identical
telophase sane no. chromosomes in daughteer as i parent half no. of chromosomes in daughter cells
may occur in haploid, diploid cells only occurs in diploid cells

Outline the factors which increase cancerous growth in an organism[6]

  • uncontrolled mitotic cell division
  • presence of carcinogens
  • e.g.smoke, asbestors
  • ref to oncogenes
  • somatic/gene mutation
  • HIV virus/retrovirus


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