cell & nuclear division

Question 1

what are the 3 subphases in interphase?

Answer 1

G1, S & G2

Question 2

describe what happens during G1 phase of interphase.

Answer 2

G1 phase begins after cytokinesis of the previous cell division. the cell is small in size and low in ATP.
so during G1 phase, cells increase in size and acquire ATP.
intensive cellular gene expression and synthesis of appropriate organelles and proteins occur.

Question 3

describe what happens during S phase of interphase

Answer 3

each DNA molecule undergoes semi-conservative DNA replication, producing 2 identical DNA molecules
histone proteins are synthesised and associate with each DNA molecule
after DNA has replicated, they remain fully extended and uncoiled

Question 4

describe what happens during G2 phase of interphase

last stage before nuclear division

Answer 4

since DNA replication is an energy-consuming process, the cell undergoes a second growth and energy acquisition stage
cells increase in size and acquire ATP
further synthesis of appropriate organelles and proteins occur
centrioles replicate and mitotic spindle begins to form

Question 5

what is mitosis?

Answer 5

mitosis involves the nuclear division of one nucleus into 2 genetically identical nuclei. after cytokinesis, it produces 2 daughter cells that are genetically identical to each other and the parent cell.

mitosis occurs in somatic cells, allowing daughter nuclei to receive precisely the same number of chromosomes as the parental nucleus, maintaining the diploid condition from one generation to the next

Question 6

what are the 4 stages of mitosis?

Answer 6

prophase
metaphase
anaphase
telophase

Question 7

describe what happens during prophase of mitosis.

Answer 7

• nuclear envelope disintegrates
• nucleolus gradually disappears
• chromatin becomes more tightly coiled and condense into discrete chromosomes
• centriole pairs migrate to opposite poles of the cell (only in animal cells, plant cells don’t have centrioles)
• the spindle fibre that began to form in G2 continues to develop

Question 8

describe what happens during metaphase of mitosis

Answer 8

• centriole pairs are positioned at opposite poles of the cell
• shortening and thickening of chromosomes is at its maximum
• 2 sister chromatids are joined at the centromeres of each chromosome. each of the 2 sister chromatids of a replicated chromosome has a kinetochore at the centromere
• kinetochore microtubules attach to the kinetochores at the centromeres of chromosomes
• chromosomes migrate and align singly at the metaphase/equatorial plate. they are pulled to the metaphase plate by kinetochore microtubules

NO pairing of homologous chromosomes

some drugs like colchicine interfere with spindle function and can arrest cells at metaphase

Question 9

describe what happens during anaphase of mitosis

Answer 9

• centromeres divide and sister chromatids are separated
• once the centromeres of sister chromatids are separated, chromatids are known as daughter chromosomes
• daughter chromosomes are pulled to opposite poles of the cell as their kinetochore microtubules shorten
• as kinetochore microtubules are attached at the kinetochore, chromosomes move with the centromere leading towards the poles of the cell, so separated daughter chromosomes are pulled along behind the centromeres, producing a V shape pattern
• poles of the cell move farther apart as polar microtubules slide past each other, elongating the cell
• special motor proteins are involved in the rapid and abrupt movement of chromosomes towards the poles of the cell during anaphase
• at the end of anaphase, the 2 poles of the cell have equal and complete sets of chromosomes

Question 10

describe what happens during telophase of mitosis

Answer 10

• chromosomes decondense
• chromatin forms by uncoiling
• the nucleolus and nuclear envelope reform
• microtubules dissassemble, and a pair of centrioles end up in each daughter cell

Question 11

how is the integrity of genetic information in daughter cells ensured during mitosis?

Answer 11

during interphase, semi-conservative DNA replication occurs, using the parental DNA molecule as the template for making genetically identical copies of daughter DNA molecule. this also occurs before the disintegration of the nuclear membrane and before the distribution of genetic material to the 2 daughter cells.

Question 12

how is entanglement of chromatin & DNA breakage prevented during mitosis?

Answer 12

during prophase, long, thin chromatin is coiled into condensed, thick and discrete chromosomes to prevent entanglement of chromatin and DNA breakage during the separation of genetic material.

this ensures equal distribution of nuclear DNA to daughter cells

Question 13

how is it ensured that each daughter cell has the complete diploid set of DNA during mitosis?

Answer 13

during metaphase, chromosomes align singly at the metaphase/equatorial plate, and no pairing of homologous chromosomes occur
during anaphase, sister chromatids are separated towards opposite poles of the cell by shortening of kinetochore microtubules
during telophase, daughter chromosomes reach the opposite poles of the cell before cytokinesis.

this ensures that each daughter cell will have the complete diploid set of DNA

Question 14

what is cytokinesis?

Answer 14

cytokinesis is the division of the cytoplasm to produce 2 daughter cells

cell organelles become evenly distributed towards the 2 poles of the cell

Question 15

describe cytokinesis in animal cells.

Answer 15

• cytokinesis involves the formation of a cleavage furrow, which pinches the cell in 2
• it first forms as a shallow groove in the cell surface near the metaphase plate
• on the cytoplasmic side of the furrow is a contractile ring of microfilament. the ring of microfilaments contracts, the cleavage furrow deepens until the parent cell pinches into 2 daughter cells, each with a complete nucleus and share of cytosol, organelles and other subcellular structures

Question 16

describe cytokinesis in plant cells

Answer 16

• no cleavage furrow is formed due to the presence of the cell wall
• a cell plate grows across the metaphase plate during telophase
• vesicles derived from the golgi move to the middle where they fuse, forming a cell plate
• the vesicles contain materials to construct both a primary cell wall for each daughter cell and a middle lamella that cements the primary cell walls of adjacent cells together. the materials carried in the vesicles collect in the cell plate as it grow
• the cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter of the parent cell
• 2 daughter cells, each with its own plasma membrane, are formed
• cellulose is laid down between the 2 membrane of the cell plate to form the cell wall

Question 17

⭐️why is mitosis important?

Answer 17

1. genetic stability - no variation in genetic info during mitosis, daughter cells have same no. of chromosomes, semi-conservative DNA replication result in genetically identical daughter DNA = genetically identical daughter cells
2. growth, repair & regeneration - for growth, it’s important that new cells are genetically identical to existing cells to carry out the same functions. damaged cells must be replaced by exact copies of the original. mitosis allows regeneration of missing parts in some organisms
3. asexual reproduction - some animals & plants propagate by asexual reproduction, so offspring are identical to their parents. offspring are genetically identical, ensuring the preservation of favourable traits from generation to generation

Question 18

what is meiosis

Answer 18

meiosis involves 2 successive nuclear divisions, named meiosis I and meiosis II. the 2 nuclear divisions produce 4 haploid, genetically non-identical daughter cells. this ensures that each daughter cell contains only half the original complement of chromosomes of the original parent cell.

meiosis only occurs in specialised cells within the gonads or sex organs of sexually reproducing organisms

Question 19

describe what happens during prophase I of meiosis

either write ‘prophase I of meiosis’ or ‘prophase of meiosis I’. same goes for the other stages. DON’T WRITE ‘prophase I of meiosis I’

Answer 19

• nucleolus disappears
• nuclear membrane disintegrates
• chromatin condenses until the chromosomes become discrete
• homologous chromosomes pair up in a process called synapsis and form a bivalent. the 4 chromatids in each bivalent are called a tetrad
• crossing over, where non-sister chromatids undergo exchange of alleles, occurs. the region where crossing over has occurred is called the chiasma. this results in genetically non-identical sister chromatids which are now known as recombinant chromatids
• crossing over may occur at more than one point
• after synapsis, the 2 homologues remain associated due to sister chromatid adhesion

plural of chiasma - chiasmata

during synapsis, the homologues are bridged by the snptonemal complex consisting of proteins & RNA. this process is precise the brings the genes into precise alignment

Question 20

describe what happens during metaphase I of meiosis

Answer 20

• kinetochore microtubules attach to the kinetochore at the centromere of 1 chromosome of each bivalent, while kinetochore microtubules from the opposite pole attach to the other homologue
• homologous chromosomes randomly align at the metaphase/equatorial plate in a process known as independent assortment.
• independent assortment refers to the fact that when a bivalent lines up on the metaphase plate, the orientation of homologues towards the poles in any bivalent is random and independent of that of any other bivalent

Question 21

describe what happens during anaphase I of meiosis

Answer 21

• the 2 homologous chromosomes separate and move to opposite poles of the cell
• homologous chromosomes are pulled to opposite poles with the centromeres leading, producing a characteristic ‘V’ shape pattern
• the centromeres in anaphase I remain intact and the sister chromatids remain attached to each other
• the physical separation of chromosomes from one another is referred to as disjunction

non-disjunction is the failure of chromosomes to separate, which results in mutations

Question 22

describe what happens during telophase I of meiosis

Answer 22

• chromosomes arrive at opposite poles of the cell
• microtubules usually disassemble
• in animals and some plants, chromatids decondense and a nuclear envelope reforms around each set of chromosomes
• nuclei formed are haploid as the chromosome number and ploidy level have been halved.
• each chromosome still exists as 2 chromatids joined at the centromere, but they may not be genetically identical due to crossing over
• cytokinesis occurs simultaneously with telophase I, forming 2 haploid daughter cells. cytokinesis I doesn’t occur in all species.

Question 23

describe what happens during prophase II of meiosis

Answer 23

• nucleoli disperse and nuclear envelope disintegrates (if formed during telophase I of meiosis)
• chromatin undergoes condensation to reform discrete chromosomes
• in animal cells, centrioles move to opposite poles of the cell at the end of prophase II of meiosis
• new spindle fibres appear and are arranged at right angles to the spindle of meiosis I

Question 24

describe what happens during metaphase II of meiosis

Answer 24

• kinetochore microtubules attach to the kinetochores at the centromeres of chromosomes
• chromosomes migrate and align singly at the metaphase/equatorial plate of the cell
• the metaphase plate of metaphase II is perpendicular to that of metaphase I

Question 25

describe what happens during anaphase II of meiosis

Answer 25

- centromeres **divide** and the 2 chromatids **separate**, and are now called **daughter chromosomes** - daughter chromosomes are **pulled** to opposite poles of the cell, with **centromeres leading** as their kinetochore microtubules shorten - polar microtubules slide past each other, **elongating the cell**

Question 26

describe what happens during telophase II of meiosis

Answer 26

- chromosomes **decondense** into chromatin form by uncoiling - microtubules **disassemble** and a pair of centrioles end up in each daughter cell - **nuclear envelopes reform** around each nucleus - **cytokinesis** occurs, producing 4 haploid genetically non-identical daughter cells from the original single diploid parent cell, each with half the chromosome number and ploidy level of a normal somatic cell

Question 27

⭐️how does meiosis increase genetic variation in gametes?

Answer 27

1. **PROPHASE I** - **crossing over** between non-sister chromatids of **homologous chromosomes** gives rise to **new combinations** of paternal and maternal alleles in each chromatid. homologous chromosome pairs are segregated into **different** daughter cells 2. **METAPHASE I** - **independent assortment** of paired homologous chromosomes at the metaphase plate leads to **random distribution** of paternal & maternal chromosomes in each gamete ## Footnote NOT in meiosis BUT random fertilisation of gametes carrying different combinations of chromosomes adds to genetic variation of the zygote formed

Question 28

how does meiosis distribute half the nuclear DNA to each gamete

Answer 28

1. **INTERPHASE I** - DNA only replicates **once**, which is followed by 2 successive nuclear divisions to produce **haploid** cells 2. **METAPHASE I** - **alignment** of paired homologous chromosomes at the metaphase plate ensures **equal distribution** of chromosomes to each daughter cell in subsequent anaphase I 3. **ANAPHASE I** - **segregation** of homologous chromosomes leads to **equal distribution** of chromosomes to each daughter cell with **halved** chromosome number and **haploid** ploidy level 4. **METAPHASE II** - chromosomes align **singly** at the metaphase plate to ensure **equal distribution** of chromosomes to each gamete. each gamete is **haploid**

Question 29

how does meiosis aid in sexual reproduction?

Answer 29

- meiosis produces gametes with **half** the number of chromosomes in the parent cell in **all organisms** carrying out sexual reproduction. - if this doesn't occur, fusion of gametes during fertilisation leads to **doubling of chromosome number** for each successive generation - meiosis thus stabilises and maintains a constant chromosome number in every generation of a species

Question 30

what are the similarities between mitosis and meiosis

Answer 30

- interphase & semi-conservative DNA replication precedes both processes - microtubules attach to the kinetochore to separate chromosomes in both processes - cytokinesis takes place at the end of both processes