Cell cycle

Question 1

What happens during interphase of cell cycle

Answer 1

• G1 phase
  
  • cell increases in size and acquires ATP
  • gene expression and synthesis of organelles and proteins
• S phase
  
  • DNA undergoes semi- conservative DNA rep
  • histone proteins synthesised and associate with DNA
  • DNA remains fully extended and uncoiled
• G2 phase
  
  • increase in size and acquire ATP
  • further gene expression and synthesis of organelles and proteins
  • centrioles replicate and mitotic spindles begin to form

Question 2

What happens during prophase of mitosis

Answer 2

1. nuclear envelope and nucleolus disintegrate
2. chromatin becomes more tightly coiled and condense into discrete chromosomes
3. centriole pairs migrate to opp poles of cell
4. spindle fibres continue to develop

Question 3

what happens during metaphase of mitosis

Answer 3

1a. shortening and thickening of chromosomes at maximum
1b. chromosomes migrate and align singly at metaphase plate

2. kinetochore microtubules attach to kinetochores at centromeres of chromosomes

Question 4

what happens during anaphase of mitosis

Answer 4

1a. centromeres divide
1b. sister chromatids separate, now known as daughter chromosomes

2a. daughter chromomses are pulled to opp poles as k microtubules shortening
=> result: at the end of anaphase, the two ends of the cell have equal sets of chromosomes
(allow for production of 2 genetically identical cells eventually)

3a. polar microtubules slide past each other, causing cell to elongate and poles to move further apart

Question 5

what happens during telophase of mitosis

Answer 5

1. nuclear envelope and nucleolus reform
2. chromosomes decondense into chromatin form by uncoiling
3. spindle fibres/microtubules disassemble
4. cell organelles (including the pair of centrioles) and chromosomes become evenly distributed towards 2 poles

Question 6

What happens during prophase I of meiosis

Answer 6

1. nuclear envelope and nucleolus disintegrate
2. chromatin becomes more tightly coiled and condense into discrete chromosomes
3. centriole pairs migrate to opp poles of cell
4. spindle fibres continue to develop

5a. homologous chromosomes pair up to form bivalent in synopsis
b. formation of chiasmata at 1 or more pts between non-sister chromatids
c. crossing over bet non-SC where they undergo exchange of alleles
d. SC now genetically non-identical, known as recombinant chromatids

Question 7

What happens during metaphase I of meiosis

Answer 7

1. k microtubules at each opp pole attach to respective homologue of each bivalent
   2a. HC randomly align at metaphase plate
   b. with independent assortment of HC

Question 8

What happens during anaphase I of meiosis

Answer 8

1a. HC of each bivalent are separated and pulled to opp poles as k microtubules shorten
b. chromosomes move with centromeres leading towards poles

2a. centromeres remain intact,
b. and sister chromatids remain attached to each other

Question 9

What happens during telophase I of meiosis

Answer 9

1. nuclear envelope and nucleolus reform
2. chromosomes decondense into chromatin form by uncoiling
3. spindle fibres/microtubules disassemble
4. cell organelles (including the pair of centrioles) and chromosomes become evenly distributed towards 2 poles

Question 10

What happens during prophase II of meiosis

Answer 10

1. nuclear envelope and nucleolus disintegrate
2. chromatin becomes more tightly coiled and condense into discrete chromosomes
3. centriole pairs migrate to opp poles of cell
4. new spindle fibres appear and are arranged at right angles to spindle fibres of meiosis 1

Question 11

What happens during anaphase II of meiosis

Answer 11

1a. centromeres divide
1b. sister chromatids separate, now known as daughter chromosomes

2a. daughter chromomses are pulled to opp poles as k microtubules shortening

3a. polar microtubules slide past each other, causing cell to elongate and poles to move further apart

Question 12

What happens during telophase II of meiosis

Answer 12

1. nuclear envelope and nucleolus reform
2. chromosomes decondense into chromatin form by uncoiling
3. spindle fibres/microtubules disassemble
4. cell organelles (including the pair of centrioles) and chromosomes become evenly distributed towards 2 poles

Question 13

What happens during metaphase II of meiosis

Answer 13

1a. shortening and thickening of chromosomes at maximum
1b. chromosomes migrate and align singly at metaphase plate
(which is perpendicular to metaphase plate in meiosis I)

2. kinetochore microtubules attach to kinetochores at centromeres of chromosomes

Question 14

what happens during cytokinesis in animals

Answer 14

1. formation of cleavage furrow as shallow groove
2. contractile ring of microfilaments on cytoplasmic side of cleavage furrow contracts
3. cleavage furrow deepens until parent cell is pinched into 2 daughter cells

Question 15

what happens during cytokinesis in plants

Answer 15

1. vesicles which contain materials to construct cell wall and middle lamella,
2. move from GA to middle of cell where they fuse and produce cell plate
3. cell plate enlarges until its surrounding membrane fuses with plasma membrane of parent cell
   => formation of 2 daughter cells w/ own plasma membrane
4. cellulose is laid between the 2 membranes to form cell wall

Question 16

errors in meiosis and their effects on genes

Answer 16

• unequal crossing over during prophase I
  => changes in chromosome structure,
  such as deletion / duplication of several gene loci
• faulty attachment of kinetochore microtubules to centromeres during prophase I / II
• failure of M checkpoint to detect errors and arrest meiosis, in the event of errors
• non-disjunction during anaphase I / II
  => change in chromosome number,
  such as aneuploidy or polyploidy

Question 17

impact of meiosis on evolutionary outcomes of species

Answer 17

microevolution:
- chromosomal aberrations result in variations in phenotypes
- natural selection occurs, in which individuals with chromosomal aberration may have SA or SD under different selection pressures

speciation:
- polyploidy leads to sympatric speciation
- new species may outcompete parental spies and thus replace them
- new species may assume a new niche and so coexist with parental species
- new species may not compete successfully against established species, so it could become extinct

macroevolution:
- adaptive radiation due to evolutionary novelties through morphological innovation

Question 18

significance of M checkpoint

Answer 18

• occurs before metaphase
• checks if there is successful attachment of microtubules to kinetochores of chromosomes
• to ensure successful separation of DNA to daughter cells

Question 19

reason reduction division is necessary during meiosis

Answer 19

• produce gametes that have half the no of chromosomes (i.e. haploid)
• thus restoring diploid condition upon fertilisation
• and therefore preventing doubling of chromosome number

Question 20

slightly different + role follows mitosis process

functions and role of centromeres

Answer 20

• functions:
  (a) holds 2 sister chromatids together (i.e. atas = sister chromatid adhesion)
  (b) allow for formation of kinetochore
  (c) attaches to spindle fibres / microtubules (via kinetochore)
  (d) and thus allows separation of sister chromatids
• role:
  
  • (b) region where kinetochore complex assembles (c) for attachment to kinetochore microtubules
  • allowing proper alignment of chromosomes at metaphase plate (during metaphase)
    and (d) segregation of sister chromatids to the poles (during anaphase)
  • resulting in equal distribution of genetic material

Question 21

similarities between non-sister chromatids of HC

Answer 21

• same chromosome length
• same centromeres position
• same gene loci
• same seq of genes along the sister chromatids

Question 22

differences bet non-sister chromatids of HC

Answer 22

• different alleles at the same gene locus
• nucleotide/base sequences are not identical
  as each homologue is inherited from a different parent

Question 23

how doubling of DNA molecules happen before mitosis

Answer 23

• semi-conservative DNA rep occurs
• during S phase of the cell cycle
• where each DNA strand was used as a template to form a complementary daughter strand

Question 24

why doubling of DNA molecules before mitosis is important

Answer 24

ensures that daughter cells have the correct number of DNA molecules / complete diploid set of chromosomes after cytokinesis

Question 25

how processes during meiosis lead to genetic variation

Answer 25

- prophase 1: - synapsis of HC to form bivalents - formation of chiasmata bet non-sister chromatids of HC - crossing over where non-sister chromatids of HC exchange alleles - resulting in genetically non-identical SC, also known as recombinant chromatids => significance: new combination of maternal and paternal alleles, giving rise to genetic variation in gametes - metaphase 1: - independent assortment of paired HC at metaphase plate - random orientation of each bivalent and is independent of other bivalent => significance: random distribution of maternal and paternal chromosomes in each gamete, increasing genetic variation in gametes

Question 26

role of centrioles in mitosis

Answer 26

- (a) act as microtubule organising centres (MTOCs) (b) for assembly of microtubules / spindle fibres, leading to separation of sister chromatids - (a) replicate and migrate to opposite poles during prophase, (b) thus establishing polarity / the 2 poles of the cell

Question 27

explaining behaviour of nuclear envelope during mitosis

Answer 27

- prophase: - behaviour: nuclear envelop begins to disintegrate - explanation: to allow **attachment** of chromosomes to kinetochore microtubules and **separation** of sister chromatids to the opposite poles - telophase: - behaviour: nuclear envelope begins to reform - explanation 1: resulting in 2 genetically identical **nuclei**, one for each daughter cell - 2: to prevent **degradation / mutation** of DNA in chromosomes that are decondensing into chromatin

Question 28

changes in distance between poles of spindle during mitosis

Answer 28

- metaphase: - decreases - due to pulling action by kinetochore microtubules, when chromosomes align singly at metaphase plate - anaphase: - increases - due to polar microtubules sliding past each other (which results in elongation of cell)