Describe the behavior 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 ;
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 and fertilization can result in genetic variation amongst offspring. [8] #
Explain how meiosis promotes genetic variation in sexually reproducing organisms [8] [ ZIMSEC November 2015/2/11(a)]
- 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
Multiple-choice questions #
1: During which phase of meiosis does the synaptonemal complex fully form, allowing for crossing over to occur between homologous chromosomes?
A) Prophase I
B) Metaphase I
C) Anaphase I
D) Telophase I
2: In humans, a non-disjunction event during meiosis I can result in gametes with which of the following chromosome combinations?
A) n+1, n-1
B) n+1, n
C) n-1, n-1
D) n, n-1
3: Which of the following statements accurately describes the significance of chiasmata in meiosis?
A) Chiasmata are responsible for the separation of sister chromatids during anaphase I.
B) Chiasmata represent regions of DNA where crossing over has occurred between non-homologous chromosomes.
C) Chiasmata are essential for ensuring accurate alignment of homologous chromosomes during metaphase I.
D) Chiasmata are formed when centromeres divide during anaphase II.
4: In meiosis, Mendel’s law of independent assortment is best explained by which of the following events?
A) Random alignment of chromosomes along the metaphase plate during metaphase I.
B) Synthesis of new genetic combinations through DNA replication.
C) Separation of sister chromatids during anaphase I.
D) Exchange of genetic material between homologous chromosomes.
5: In certain plant species, a phenomenon known as “polyploidization” can occur, resulting in offspring with multiple sets of chromosomes. Which of the following best describes the process leading to polyploidization?
A) Failure of cytokinesis during meiosis II, resulting in the formation of tetraploid cells.
B) Fusion of two diploid gametes during fertilization, resulting in a triploid zygote.
C) Non-disjunction during mitosis, leading to aneuploidy in somatic cells.
D) Simultaneous replication and division of chromosomes during meiosis I.
Solutions #
- A) Prophase I
- B) n+1, n
- C) Chiasmata are essential for ensuring accurate alignment of homologous chromosomes during metaphase I.
- A) Random alignment of chromosomes along the metaphase plate during metaphase I.
- A) Failure of cytokinesis during meiosis II, resulting in the formation of tetraploid cells.
