Cell and Nuclear Division

Question 1

what are nuclear and cytoplasmic division in cell division, and what are the two types of nuclear division?

Answer 1

nuclear: separating nuclear DNA to daughter cells
- mitosis, produces two identical cells
- meiosis, produces haploid gametes

cytoplasmic (cytokinesis): separating cytoplasm and organelles into daughter cells

Question 2

what is chromosome number and ploidy level?

Answer 2

chromosome number: number of chromosome per cell of an organism
- 2n, diploid, in homologous pairs (one maternal and one paternal)
- 46 in humans (22 pairs of autosomes, 1 pair of sex chromosomes)
* no correlation between the size of an organism and its number of chromosomes

ploidy level: number of sets of chromosomes within the nucleus of a cell
- haploid cells: n, diploid cells: 2n
- xn, where x>2, is polyploidy

Question 3

define homologous chromosomes (in diploid organisms)

Answer 3

• structurally similar in: size, shape, centromere position, sequence of gene loci
• not genetically identical: different alleles at same gene loci
• allele: alternative form of a gene
  each chromosome of a homologous chromosome pair is called a homologue, and the pair is called a bivalent / tetrad
  one maternal one paternal chromosome

Question 4

what are sister chromatids, and how are they identical?

Answer 4

replicated forms of a single chromosome joined by the centromere
- eventually separated during anaphase of mitosis and anaphase II of meiosis II

identical bc of semi-conservative DNA replication, same alleles at each gene locus
structurally identical: size, shape, centromere position, number of genes and sequence of gene loci
may become genetically non-identical after crossing over in prophase 1 of meiosis 1

Question 5

what are centrioles and centrosome, and their functions?

Answer 5

centrosome = pair of centrioles
centrioles only in animal cells, barrel-shaped with nine triplets of microtubules perpendicular to each other
* at start of mitosis, centriole pairs duplicated and each pair moves to opposite poles, establishing the poles
centrosome is MTOC: microtubule organising centre, where assembly of spindle microtubules begin

Question 6

what are the function of microtubules, and where are the three different types found?

Answer 6

components of cytoskeleton, from globular proteins known as tubulin
organised system of microtubules attaches to centromeres

• astral: from centriole outwards
• kinetochore: attached to centriole and centromere, pulls sister chromatids apart during anaphase
• polar: across the whole cell, elongates it during anaphase

Question 7

what is interphase, and state its three phases

Answer 7

90% of length of cell cycle, longest phase, period of intense metabolic activity
three sub-phases: G1, S, G2

Question 8

what occurs during the three subphases of interphase?

Answer 8

Gap 1 Phase:
- increase in size, acquire ATP
- intensive cellular gene expression and synthesis of appropriate organelles and proteins

Synthesis Phase:
- semi-conservative DNA replication
- histone proteins synthesised to form chromatin

Gap 2 Phase:
- second growth and energy acquisition stage: increase in size, acquire ATP again
- further synthesis of organelles and proteins
- centrioles replicate (haven’t moved to define poles yet)
- mitotic spindle begins to form

Question 9

compare the characteristics of non-dividing cells (in interphase) and dividing cells (in mitosis)

Answer 9

Interphase VS Mitosis
- chromatin (loosely coiled and decondensed) VS chromosomes (highly condensed)
* chromatin can be euchromatin (decondensed, transcriptionally active, lighter on picture) OR heterochromatin (highly condensed, transcriptionally inactive, darker on picture)

• nuclear envelope intact, nucleolus present VS nuclear envelope disintegrated, nucleolus not present
• transcriptionally active VS inactive

Question 10

what is mitosis, where does it occur, what are its main stages, and what does it produce?

Answer 10

nuclear division of one parent nucleus into two genetically identical daughter nuclei (produces two genetically identical daughter cells after cytokinesis)
stages: prophase, metaphase, anaphase, telophase
somatic cells, diploid daughter cells

Question 11

what occurs during prophase of mitosis?

Answer 11

• nucleus:
  nuclear envelope disintegrates
  nucleolus disappears
  chromatin coils and condenses into discrete chromosomes (visible under light microscope)
• cytoplasm:
  centriole pairs migrate to opposite poles of cell (in animal cells, plant cells don’t have)
  spindle fibre from G2 of interphase continues developing

Question 12

what occurs during metaphase of mitosis?

Answer 12

centriole pairs have migrated and defined poles of the cell
chromosomes are at their most condensed and shortened
each chromosome (made up of two sister chromatids joined at the centromere) has kinetochore associated with centromere
kinetochore microtubules attach the centromere to the centrosome
chromosomes align along the metaphase plate
*no pairing of homologous chromosomes in mitosis

Question 13

what occurs during anaphase of mitosis?

Answer 13

shortest stage of mitosis
centromere divides, sister chromatids pulled apart by shortening kinetochore microtubules to opposite poles of the cell (motor proteins involved)
in a characteristic ‘V’ shape
separated sister chromatids known as daughter chromosomes
polar microtubules slide past each other, elongating the cell
two poles of cell have equal and complete sets of chromosomes (unless non-disjunction happens)

Question 14

what occurs during telophase of mitosis?

Answer 14

chromosomes decondense into chromatin
nucleolus and nuclear envelopes reform around DNA at the two poles
microtubules disassemble, a pair of centrioles per daughter cell
mitosis is now complete

Question 15

what is cytokinesis?

Answer 15

division of the cytoplasm to produce two daughter cells
in preparation, cell organelles become evenly distributed towards two poles of parent cell along with chromosomes during telophase
two smaller, genetically identical cells result
begins simultaneously with telophase

Question 16

what occurs during cytokinesis in animals?

Answer 16

cleavage furrow forms, contractile ring of microfilaments on cytoplasmic side (inside)
shallow groove in cell surface near metaphase plate

Question 17

what occurs during cytokinesis in plants?

Answer 17

cell plate across metaphase plate
vesicles from Golgi apparatus move to middle of the cell and fuse
vesicles contain materials for primary cell wall and middle lamella (connects adjacent cells)
cellulose is laid down between two membranes of cell plate to form cell wall

Question 18

what are the three significances of mitosis?

Answer 18

stability:
- two daughter cells are genetically identical due to semi-conservative DNA replication during S phase
- daughter chromosomes equally distributed
- preservation of genetic stability

growth, repair, regeneration:
- more identical cells to increase in size or replace damaged ones

asexual reproduction:
- offspring genetically identical (a clone of) parents
- for unicellular organisms, preserves favourable traits from generation to generation

Question 19

what is meiosis, and what cells are involved/produced?

Answer 19

meiosis is the division of a diploid parent cell (somatic) to form four haploid daughter cells (gamates)
*each daughter cell will only contain half the original complement of chromosomes of the original parent cell

Question 20

what occurs during interphase before meiosis?

Answer 20

semi-conservative DNA replication (once)
replication of centriole pair

Question 21

what occurs during prophase I of meiosis I?

Answer 21

nucleolus disappears, nuclear membrane disintegrates
chromatin condenses into discrete chromosomes

homologous chromosomes pair up to form a bivalent / tetrad during synapsis
- homologues bridged by synaptonemal complex (protein + RNA), brings genes on each homologue into precise alignment

crossing over: exchange of alleles between non-sister chromatids of homologous chromosomes to form recombinant chromatids
- chiasma is an X-shaped microscopic visible region where crossing over has occurred

Question 22

what occurs during metaphase I of meiosis I?

Answer 22

kinetochore microtubules attach to kinetochore at centromere of one homologue (kinetochore microtubule from opp pole attaches to other homologue)

bivalents randomly align along the metaphase / equatorial plate
- independent assortment: orientation of homologues is random and independent of that of any other bivalent

Question 23

what occurs during anaphase I of meiosis I?

Answer 23

two homologous chromosomes of each bivalent separate: disjunction
homologues chromosomes pulled to opposite poles with centromeres leading (characteristic ‘V’ shape)
centromeres intact, sister chromatids remain attached to each other

Question 24

what occurs during both telophase I of meiosis I and telophase II of meiosis II?

Answer 24

microtubules disassemble
chromatids decondense, nuclear envelope reforms (only sometimes in meiosis I)

Question 25

what are the key features of meiosis I, and the events unique to this division?

Answer 25

reductive division: separation of homologous chromosomes
- chromosome number and ploidy level halved, haploid daughter cells, amount of DNA same

unique events:
- synapsis of homologous chromosomes
- crossing over of homologous chromosomes
- independent assortment

Question 26

what occurs during prophase II of meiosis II?

Answer 26

nucleoii disperse, nuclear envelope disintegrates (if formed during telophase I of meiosis I)
chromatin condenses to re-form discrete chromosomes
in animals, centrioles move to define opposite poles of cell (perpendicular to poles in meiosis I), forming centromeres (MTOC: microtubule organising centres)
new spindle fibres appear (also perpendicular to those of meiosis I)

Question 27

what occurs during metaphase II of meiosis II?

Answer 27

kinetochore microtubules attach to kinetochores (protein structures associated to the centromere) of chromosomes
chromosomes migrate and align at the metaphase / equatorial plate (perpendicular to that of meiosis I)

Question 28

what occurs during anaphase II of meiosis II?

Answer 28

centromeres divide, two chromatids separate to form daughter chromosomes
daughter chromosomes pulled to opposite poles, centromeres leading
polar microtubules slide past each other, elongating the cell

Question 29

what are the key features of meiosis II, and the events unique to this division?

Answer 29

equational division: chromosomal number unchanged
haploid gamete cells have halved chromosome number and ploidy level bc of meiosis
*amount of DNA In daughter cell also half that of parent cell (after meiosis I, it was still the same as that of parent cell)

Question 30

what are the events that increase genetic variation in gamates during meiosis, and what is the significance of this?

Answer 30

prophase I: crossing over between non-sister chromatids of homologous chromosomes; creates new combinations of paternal and maternal alleles in recombinant chromatids
metaphase I: independent assortment of paired homologous chromosomes when aligned with metaphase plate; random distribution of paternal and maternal chromosomes
random fertilisation: between gamates with different combinations of chromosomes

genetic variation in genotype and phenotype of offspring produced by fusion of gametes

Question 31

what are the events that lead to the haploid number of chromosomes in daughter cells of meiosis, and what is the significance of this?

Answer 31

interphase I: DNA replicates only once, followed by two successive nuclear divisions (no interphase II)
metaphase I and II, anaphase I and II: alignment of paired homologous chromosomes / chromosomes with two sister chromatids at metaphase plate, equal distribution of chromosomes to each daughter cell, both producing daughter cells with haploid ploidy level

stabilises and maintains constant chromosome number, no doubling of chromosome number in zygote

Question 32

what are the similarities between mitosis and meiosis?

Answer 32

interphase and semi-conservative DNA replication preceeds both
kinetochore microtubules attach to kinetochore associated with centromeres to separate chromosomes
cytokinesis at the end of both processes