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]

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