Chapter 6: Cell Division

Question 1

Interphase

Answer 1

Growth period of the cell cycle between cell divisions that takes up 90% of cell cycle
- DNA is replicated and checked for errors in the nucleus
- Protein synthesis occurs in the cytoplasm
- Mitochondria (+ chloroplasts in plants) grow and divide, increase in number
- normal cell metabolic processes occur
Consists of 3 stages: G1, S, G2

Question 2

G1

Answer 2

Growth phase 1
Proteins used to produce organelles are synthesised and organelles replicate
- cell increases in size
Cellular contents (except chromosomes) duplicated

G1 Checkpoint 
Before entry to S phase where DNA replication is triggered
Checks for:
- cell size
- nutrients 
- growth factors
- DNA damage

Question 3

S

Answer 3

Synthesis phase
DNA is replicated in the nucleus
- amount of DNA doubles

Question 4

G2

Answer 4

Growth phase 2
Cell continues to increase in size and organelles made
Energy stores are increased
Duplicated DNA is checked for errors

G2 Checkpoint
Before mitotic phases, initiates molecular processes that trigger start of mitosis
Checks for:
- cell size
- DNA replication 
- DNA damage

Question 6

Cancer

Answer 6

Uncontrolled division/growth of cells due to a mutation that forms a tumour
Benign tumours stop growing and do not travel to the rest of the body
Malignant tumours continue to grow and travel to the rest of the body - cancer

Question 7

Chromosomes

Answer 7

Molecule of DNA made of 2 chromatids joined together at a region called the centromere

• 1 chromatid per chromosome before DNA replication
• 2 chromatids per chromosome after DNA replication
• 1 chromatid per chromosome after mitotic division

Question 8

Mitosis

Answer 8

Nuclear division stage in eukaryotic cells
2 daughter cells produced that are genetically identical to the parent cell (same number of chromosomes)
Necessary for:
- cell growth
- replace dead cells
- repair of damaged tissue in multicellular organisms
- asexual reproduction e.g. plants, fungi, some animals
Stages are: prophase, metaphase, anaphase, telophase

Question 9

Microscopy

Answer 9

Light microscope used to view dividing cells
Plant root tips that are rapidly undergoing mitosis in the meristems
- Treated with hydrochloride acid to allow the cells to separate
- squashed to form a single cell layer on the slide
- ethano-orcein stain used to make chromosomes clearly visible

Mitotic index = number of actively dividing cells in field view / number of cells in field view

Question 11

Prophase

Answer 11

Chromatin fibres begin to coil and condense to form chromosomes with 2 sister chromatids, making them visible

• Nucleolus disappears and nuclear envelope breaks down
• Spindle fibres formed by protein microtubules in the centriole attach to centromere - move chromosomes to equator

Question 12

Metaphase

Answer 12

Chromosomes moved by spindle fibres arrange along the equator of the cell in a plane called the metaphase plate
- then held in position for anaphase

Question 13

Anaphase

Answer 13

The pairs of chromatids in each of the chromosomes are separated

• the centromere divides
• sister chromatids migrate to opposite poles of the cell by shortening spindle fibres

Characteristic v shape chromatids being dragged by their centromeres through the liquid cytosol

Question 14

Telophase

Answer 14

Chromatids reach the poles and 2 new sets of chromosomes assemble at each pole

• nuclear envelope reforms around them
• chromosomes uncoil and become long and thin again
• nucleolus reforms and cytokinesis begins

Question 15

Cytokinesis (animals)

Answer 15

Actual division of cells into 2 separate cells
Cleavage furrow around middle of cell
- where cell surface membrane pulled inwards by cytoskeleton until close enough to fuse around the middle
- 2 new daughter cells formed that are identical to the parent cell

Question 16

Cytokinesis (plant cells)

Answer 16

Cell walls prevent cleavage furrow from forming

• vesicles from Golgi apparatus assemble along metaphase plate
• vesicles fuse with each other and cell surface membrane to form new sections of cell wall
• cell wall then forms to form 2 new daughter cells

Question 17

Interphase micrograph

Answer 17

Cell prepares for mitosis - DNA needs to be replicated
Chromosomes are invisible prior to mitosis
- unwound, not condensed
- looks blurry/grainy

Question 19

Mitotic Phase

Answer 19

Period of cell division
Consists of 2 stages:
- Mitosis (nucleus divides)
- Cytokinesis (cytoplasm divides to produce 2 new cells)

Metaphase / Spindle Assembly Checkpoint
Point in mitosis where all chromosomes should be aligned and attached to spindles, mitosis is allowed to proceed
Checks for:
- Chromosomes aligned along the equator of the cell
- chromosome attachment to spindle

Question 20

Meiosis

Answer 20

Cell division where the nucleus divides twice to produce 4 daughter cells- gametes

• Reduction division: each gamete is a haploid (contains half the number of chromosomes in the parent cell)
• Sexual reproduction: 2 gametes (sex cells) fuse to form a zygote (fertilised egg)

Question 21

Homologous chromosomes

Answer 21

Matching pair of chromosomes - one inherited from each parent
Each chromosome in a homologous pair has the same genes at the same loci
- alleles are different versions of the same gene that have the same locus (position on a chromosome)
- homologous chromosomes will be same length/size when visible in prophase and same position of centromeres

Question 22

Meiosis I

Answer 22

Reduction division where pairs of homologous chromosomes are separated into two haploids

Prophase 1 - homologous chromosomes pair up to form bivalents. Chromatids entangle when chromosome moved (crossing over)
Metaphase 1- orientation of each homologous pair is random and independent of other pairs. Independent assortment as maternal/paternal chromosomes can face either pole which allows genetic variation
Anaphase 1- homologous chromosomes moved to opposite poles while chromatids stay attached. Sections of DNA on sister chromatids that were entangled (crossing over) break off and rejoin at points - chiasmata. Genes exchange to form recombinant chromatids with new combinations of alleles which allows genetic variation
Telophase 1- chromosomes uncoil and nuclear membrane reforms

Question 23

Meiosis II

Answer 23

Second division (similar to mitosis) where pairs of chromatids are separated in each daughter cell into 2 more cell - form 4 haploid cells in total

Prophase 2- chromosomes condense and become visible. Nuclear envelope breaks foehn and spindle fibres form
Metaphase 2- individual chromosomes assemble on metaphase plate (like mitosis)l more genetic variation due to independent assortment from crossing over
Anaphase 2- chromatids of individual chromosomes pulled to opposite poles after centromeres divide
Telophase 2- chromosomes uncoil and form chromatin again. Nuclear membrane reforms and nucleolus becomes visible
Cytokinesis forms 4 haploids genetically different from each other and the parent cell