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