Continuity of Cells

Question 1

Significance of meiosis

Answer 1

•Process of reduction division
oCell division that halves the number of chromosomes in gametes (one from each homologous pair that enters a gamete) i.e. diploid to haploid
Ensures that when male and female gametes fuse, they restore the normal diploid number in zygote.

• Meiosis only take place in diploid (cells that have homologous pairs to start with)

•Produces daughter cells that are genetically different (more genetic diversity within species)
oResult of independent assortment and crossing over
oEnsures organisms of a species can survive in various environments as they are not all equally susceptible to the same disease.

Question 2

Crossing over

Answer 2

• Result of chiasmata formation between chromatids of homologous pairs during late prophase I
• Piece of chromatid from one chromosome swaps places with a piece of chromatid from homologous partner
• 2 chromatids out of the 4/each chromosome has a different combination of alleles (recombinants with new allelic combinations)

Question 3

Independent assortment

Answer 3

•During metaphase I, bivalents are arranged at random on the equator of cell
oOrientation of one chromosome not dependent on the other of the pair.

Question 4

Significance of mitosis

Answer 4

•Mitosis produces genetic constancy
oDaughter cells are genetically identical to parent
Key role in asexual reproduction to produce
genetically identical individuals (clones)
oDaughter cells have the same chromosome
number as each other and parent cell
Mitosis can occur in haploid/diploid cells

Question 5

G1 Phase

Answer 5

• First gap phase.
• Synthesis of macromolecules (proteins and nucleotides) and organelles formed.
• Cell increases in size.

Question 6

G1 Checkpoint

Answer 6

•Checkpoint at the end of the G1 phase

•Before S phase, this checkpoint ensures:
 Cell has reached critical size
 Availability of nutrients for next stages to occur
 Presence of growth (regulatory) factors
 Chemicals that stimulate cell division
 DNA damage

•If cell meets the checkpoint requirements, it proceeds to the S phase

Question 7

G0 Checkpoint (resting phase)

Answer 7

•If cell does not meet the requirements of G1 Checkpoint, enters G0 checkpoint.

•Cells differentiate to specialist functions
 Cells do not enter back into cell cycle

•Checks for cell damage beyond repair/worn out
 Gene is responsible for checking for DNA damage at G1 or G2 checkpoint and if significant damage detected, the gene causes cell to self-destruct

•Some cells can return back into the cell cycle if reasons for entering G0 no longer apply.

•Time variable.
 Neurones remain in G0 permanently
Logical for highly differentiated cells to move out
of the cycle as they will not undergo mitosis in
future.
 Epithelial cells rarely enter G0
 At any one time, most of the cells in the human
body are in G0 and only return to the cell cycle
when stimulated to do so
 Lymphocytes return to cell cycle if triggered by the
presence of specific antigens

•N.B- Resting term is misleading
 Cell is suspended from processes linked to cell division but the cell is still metabolically active

Question 8

S Phase

Answer 8

•DNA synthesis occurs.
 DNA and chromatids formed are identical and
remain attached until separated during mitosis or
meiosis

• Histones (Proteins that bind to support DNA within the chromatids) produced
• Amount of chromatin doubled

Question 9

G2 Phase

Answer 9

•Second Gap phase

•Tubulin synthesised and form microtubules of the
spindle fibres

• Energy stores increased
• Cell continues to increase in size.

Question 10

G2 Checkpoint

Answer 10

•Checkpoint at end of G2 phase

•Before mitosis occurs, this checkpoint ensures:
 DNA replication
 No DNA damage
 Cell size

•Gene causes cell to self-destruct if does not meet these.

Question 11

M (Metaphase) Checkpoint

Answer 11

•Checkpoint at the end of Metaphase during mitosis

•Before Anaphase occurs, this checkpoint ensures:
 Number of chromosomes
 Alignment of chromosomes
 Chromosomes have attached properly to the
spindle fibres

Question 12

Why are checkpoints essential

Answer 12

Checkpoints are essential to ensure that parent cell is able to distribute identical DNA to the daughter cells

Question 13

what is a benign tumour?

Answer 13

Tumour that is encapsulated in a fibrous sheath and does not spread around the body

Question 14

What is a malignant tumour?

Answer 14

Tumour that divides uncontrollably and is the basis of cancer.

Question 15

What is metastases

Answer 15

cancerous cells that break off primary tumour and travel around the body via circulatory system to produce secondary tumours