Cell Division, Cell Diversity & Cellular Organisation

Question 1

Whats the cell cycle

Answer 1

Question 2

Phases in the cell cycle

Answer 2

Interphase (G1, S, G2 & sometimes G0)
Mitosis (M)
Cytokinesis

Question 3

What occurs in the G1 phase

Answer 3

Cell growth
Biosynthesis (protein/lipid)
Organelles duplicate

Question 4

What goes on in the S phase

Answer 4

DNA rapidly replicates
Short phase (to reduce chance of spontaneous mutations)

Question 5

What happens in the G2 phas

Answer 5

More cell growth
Replicated DNA is checked for mutations / errors OR to check if it has replicated correctly

Question 6

If damaged DNA is detected, the cell cycle stops and cell tries to repair damage, or moves into what phase

Answer 6

G0

Question 7

What happens into G0 phase

Answer 7

Cells undergo senescence / apoptosis or differentiation
-> muscle + neurons remain here for a long time
Senescence-> ages + stops, but won’t die

Question 8

Mitosis vs cytokinesis

Answer 8

Nuclear division by mitosis
Cell division by cytokinesis (division of cytoplasm, by constriction from the edge of the cell)

Question 9

Why is the cell cycle regulated

Answer 9

• prevent uncontrolled cell division (which would lead to tumour formation)
• detect & repair damage to DNA
• ensure cycle only occurs in 1 direction
• ensure DNA is only replicated once per cycle

Question 10

How is the cell cycle regulated

Answer 10

Regulated by checkpoints between the G1 & S phase, and between the G2 and mitosis

Key proteins, cyclin-dependent kinases & cyclins, control the process.

Cyclins are produced at specific times, activating the kinases and allowing progression through the cell cycle

Question 11

What is mitosis

Answer 11

The nuclear division which produces 2 generically identical diploid cells (nuclei)
Maintains the chromosome number

Question 12

Main stages in mitosis

Answer 12

P rophase
M etaphase
A naphase
T elophase

Question 13

What occurs in prophase

Answer 13

-> chromosomes condense + become visible (supercoil)
-> nuclear envelopes breaks down
-> spindle fibres break down

Question 14

What occurs during Metaphase

Answer 14

Chromosomes line up at the equator
Chromosomes attach by their centromeres to spindle fibres

Question 15

What occurs in Anaphase

Answer 15

Centromeres divide: sister chromatids separate at centromere
Motor proteins pull sister chromatids to opposite poles in a V shape -> centromeres travel first

Question 16

What happens in telophase

Answer 16

Nuclear envelope reforms around both sets of chromosomes at opposite poles
Spindle fibres break down
2 genetically identical nuclei formed

Question 17

Significance of mitosis in life cycles

Answer 17

• tissue growth & repair
• production of new stem cells
• maintain chromosome number
• asexual reproduction
• development of body plan
• proliferation of white blood cells (clonal expansion)
• producing gametes from haploid cells

Question 18

Significance of meiosis

Answer 18

Produces haploid gametes which are randomly fertilised
Increases genetic variation -> through independent assortment

Question 19

How is genetic variation produced

Answer 19

-> Independent assortment of homologous chromosomes in metaphase 1 & chromatids in metaphase 2
-> crossing over in prophase 1
- mutation
- non-disjunction
- random fusion of gametes

Question 20

Increased genetic variation in prophase 1?

Answer 20

Crossing over
Alleles are swapped between non-sister chromatids
- chromatids now have a new combination of alleles

Question 21

How is there increased variation in metaphase 1

Answer 21

-> independent assortment of homologous chromosomes
- these line up across the centre of the cell, & each of has different alleles, therefore lots of combos

Question 22

How does increased variation occur through non-disjunction

Answer 22

Homologous chromosomes do not separate in metaphase 1
One more or less chromosome present

Question 23

How does increased variation occur through mutations

Answer 23

Changes nucleotide base sequence
DNA checks don’t recognise the damage
Changes amino acid sequence

Question 24

How does increased variation occur through random fertilisation of gametes

Answer 24

Gametes not genetically identical
Produces large number of allele combinations

Question 25

What are homologous chromosomes

Answer 25

Chromosomes which have the same gene at the same genes loci. They may contain different alleles

Question 26

What is meiosis

Answer 26

Nuclear division which produces 4 haploid cells, which are genetically different to the parent cell
Half the chromosome number

Question 27

What is independent assortment

Answer 27

How the inheritance of 1 chromosome does not affect the inheritance of others

Question 28

What is crossing over

Answer 28

When the 2 chromosomes come together, align, & them recombine & swap parts with each other
This results in a mix of / the creation of new alleles

Question 29

Stages of Meiosis

Answer 29

Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2

Question 30

What happens in Prophase 1 (Meiosis)

Answer 30

Chromosomes condense & supercoil / becoming visible
Chromosomes arrange in homologous pairs
Crossing over of non-sister chromatids occurs
Nuclear envelope breaks down
Spindle fibres form

Question 31

What happens in metaphase 1 (meiosis)

Answer 31

Homologous chromosomes line up across the equator along spindle fibres
Independent assortment of homologous chromosomes occurs
Maternal or paternal chromosome can end up facing either poles

Question 32

What happens in anaphase 1 (meiosis)

Answer 32

Homologous chromosomes separate
Motor proteins pull chromosomes to opposite poles

Question 33

What happens in telophase 1 (meiosis)

Answer 33

Nuclear envelope reforms around each set of chromosomes
Spindle fibres break down
THEN
cytokinesis occurs

Question 34

What happens Prophase 2

Answer 34

Chromosomes condense & supercool
Nuclear envelope breaks down
Spindle fibres form

Question 35

What happened in metaphase 2

Answer 35

Chromosomes line up along the equator
Independent assortment of chromatids

Question 36

What happened in anaphase 2

Answer 36

Chromatids pulled to opposite poles by motor proteins
Centromeres divide

Question 37

What happens in telophase 2

Answer 37

Nuclear envelope forms around each of haploid nuclei
Then, cytokinesis occurs to form 4 haploid cells