2.1.6 - Cell division (set A - cell cycle + Mitosis)

Question 1

Outline the two main phases of the cell cycle in eukaryotic cells - give a brief description of them?

Answer 1

• interphase - cells spends most time in this phase, long periods of growth to prepare for cell division (G1,S and G2 phase)
• mitotic phase - period of cell division involving mitosis and cytokinesis

Question 2

Give the three stages of interphase - give a brief description of each?

Answer 2

• G1 - first growth stage, proteins are produced and organelles replicate - cell increases in size
• S - DNA replicated in the nucleus
• G2 - growth stage, cell increases in size with energy stores increasing - duplicated DNA is checked for errors

Question 3

What is the G0 phase?

Answer 3

Phase when the cell leaves the cycle either temporarily or permanently

Can be due to differentiation, DNA being damaged or age

Question 4

Explain two factors which cause the cell to experience G0 phase?

Answer 4

• Differentiation - cell becomes specialised and no longer able to divide - carries out its functions indefinitely
• DNA being damage - damaged cell can no longer divide and enters permanent cell arrest, most normal cells only divide a certain number of times before becoming senescent

Question 5

Give an example of a way the G0 phase can be altered with reference to an example cell?

Answer 5

Lymphocytes (white blood cells) can be stimulated to go back into the cell cycle and start dividing again

Question 6

How is the cell cycle controlled to avoid unsuccessful cell division?

Answer 6

Checkpoints monitor and verify wether the processes have been accurately completed before the cell is allowed to further progress

• G1 checkpoint
• G2 checkpoint
• spindle assembly checkpoint

Question 7

Explain the function of the G1 checkpoint - give examples of what is checks for?

Answer 7

End of the G1 phase before entry into S phase - checks for cell size, nutrients, growth factors and DNA damage

Question 8

Explain the function of the G2 checkpoint - give examples of what is checks for?

Answer 8

Is at the end of the G2 phase (before the mitotic phase) checks for cell size, DNA replication without any damage

• if there’s no damage the cell can enter mitosis

Question 9

Explain the function of the spindle assembly checkpoint - give examples of what is checks for?

Answer 9

Also called metaphase checkpoint, is at the point in mitosis when all the chromosomes should be attached to spindles and have aligned it checks wether this is correct

Question 10

Define mitosis?

Answer 10

Term to describe entire process of cell division in eukaryotic cells which results in two identical daughter cells

• division into two daughter cells that are genetically identical to each other and to the parent cell

Question 11

Explain the role of mitosis in organisms?

Answer 11

The daughter cells being identical is important for growth,replacement and repair of tissues and for asexual reproduction

Question 12

Give all the stages of mitosis in order?

Answer 12

• stage 1 - prophase
• stage 2 - metaphase
• stage 3 - anaphase
• stage 4 - telophase

Question 13

Explain what happens during the first stage of mitosis ‘prophase’?

Answer 13

• Chromosomes condense and thicken (and therefore become visible) Consists of sister chromatids joined at the centromere
• Two centrioles migrate to opposite poles of the cell
• Spindle fibres attach to specific areas on the centromeres and start to move the chromosomes to the centre (equator) of the cell.
• Nuclear envelope disappear

Question 14

Explain what happens during second stage of mitosis ‘metaphase’?

Answer 14

• brief phase
• Individual sister chromatids (chromosomes) are moved by the spindle fibres to align at the metaphase plate/equator at the centre of the cell
• Sister chromatids are attached to the spindle fibres by the centromere

Question 15

Explain what happens during the third phase of mitosis ‘anaphase’?

Answer 15

• Centromeres holding the pairs of chromatids in each chromosome divide - Sister chromatids separate
• Spindle contracts (fibres shorten) - each chromatid is pulled by their centromere to opposite poles of the cell

Question 16

Explain what happens during final stage of mitosis ‘telophase’?

Answer 16

• Chromatids have reached opposite poles of the cell - they uncoil and become long and thin again and are now called chromosomes
• Spindle fibres disappear
• Nuclear envelope reforms and enclose around the chromosomes at each pole

Question 17

Define cytokinesis?

Answer 17

Actual division of cell into two separate genetically identical daughter cells, it begins during telophase

The phase where the cell surface membrane and cytoplasm divide (in animal cells a ‘cleavage furrow forms’)

Question 18

Explain cytokinesis in plant cells?

Answer 18

No cleavage furrow as cell wall prevents it, vesicles from the Golgi apparatus begins to assemble where metaphase plate was formed - vesicle fuse with each other and the cell surface membrane - dividing the cell into two

Question 19

Explain cytokinesis in animal cells?

Answer 19

A cleavage furrow forms around the middle of the cell - cell surface membrane is pulled inwards by the cytoskeleton until its close enough to fuse around the middle and form two cells

Question 20

Explain what happens in the G1 stage of interphase?

Answer 20

G1 - first growth stage, proteins are produced and organelles replicate (eg mitochondria, ribosomes etc) - cell increases in size

Question 21

Explain what happens in the S stage of interphase?

Answer 21

Synthesis stage - replication of each chromosome in the nucleus - now called sister chromatids (which are joined at the centromere)

Question 22

Explain what happens in the G2 stage of interphase?

Answer 22

Second growth stage, cell increases in size with energy stores aslso increasing (ATP molecules increase) - duplicated DNA is checked for errors

• proteins (eg tubulin) are produced to form microtubules for the mitotic spindle

Question 23

Explain 4 functions of the cell cycle?

Answer 23

• Growth of tissue/organism (not of cells)
• Replacement of worn out/damaged cells
• Repair of body tissues (e.g. bone, muscle etc)
• Asexual reproduction/cloning

Question 24

Explain what molecules are involved in the checking process - what do they do?

Answer 24

Specific proof-reading enzymes and repair enzymes are involved

• enzymes when possible will try and repair the error but in some cases the cell will destroy itself to prevent passing on harmful mutations

Question 25

Explain the purpose of the ‘forth’ less major checkpoint during metaphase?

Answer 25

the final check determines whether the chromosomes are correctly attached to the spindle fibres prior to anaphase

Question 26

Explain 5 problems which may arise if the DNA is not checked?

Answer 26

• Mutations in the DNA sequence
• Faulty DNA produced
• Error in copying daughter cells
• Daughter cells will not receive identical genetic information
• Proteins not made or do not function properly

Question 27

Explain the structure of chromosomes?

Answer 27

• Chromosomes only visible during cell division.
• Each chromosome consists of two chromatids joined at the centromere.
• Genetic information (genes/alleles) carried on each chromatid is identical.

Question 28

Explain how the chromosome is prepared for cell division during interphase?

Answer 28

• Before mitosis can occur, each DNA molecule (chromosome) in the nucleus is replicated
during the interphase

• Each chromosome is converted into two identical DNA molecules called chromatids

• The two chromatids are joined together at a region called the centromere

(Condensed chromosomes combined with histone proteins are called chromatids)

Question 29

Briefly summarise all stages of mitosis?

Answer 29

• Prophase – nuclear membrane disintegrates and doubled chromosomes condense, shorten and begin to move to the equator

• Metaphase – the chromosomes line up along the equator attached to spindles by their centromeres

• Anaphase – the chromatids of each replicated double chromosome are ripped apart at the centromere and pulled towards different ‘poles’

• Telophase – chromosomes arrive at the poles and new nuclear membranes form followed by cytokinesis