S1-L10: Cell Cycle & Cytokinesis

Question 1

Outline what each of the following terms refer to:

1-Chromosomes
2-Autosomes (figure 2)
3-Sex chromosomes (figure 2)
4-Histones (figure 1)

Answer 1

• human’s cells contain 23 pairs of chromosomes each (46 total)
• ->22 of these pairs are autosomes-same in males & females (non-sex chromosomes)
• 23rd pair is sex chromosomes- differ between males and females
• histones- highly alkaline proteins found in eukaryotic cell nuclei which package & order DNA into structural units (nucleosomes)

Question 2

Similarly define each of the following:

1-Somatic cells (figure 4)
2-Prokaryotic cells (figure 5)
3-Eukaryotic cells (figure 5)
4-Mitosis (figure 3)
5-Meiosis (figure 3)
6-Cytokinesis (figure 6)
7-Apoptosis 
8-Necrosis

Answer 2

1-all cells of body except sperm & egg cells
2-no nucleus/any other membrane-bound organelles like bacterial cells
3-contain membrane-bound organelles including nucleus like humans/plants/fungi & insects
4-division which results 2 identical daughter cells
5-division which results in 4 sex cells (egg & sperm)
6-cytoplasmic division
7-programmed & targeted cell death-internal signalling
8-death of most/all cells in organ tissue due to disease OR injury OR blood supply failure; caused by external factors

Question 3

What is cell division?

Answer 3

• orderly sequence of events required for duplication of eukaryotic cell into 2 genetically identical daughter cells
• cell undergoes nuclear division (mitosis) AND cytoplasmic division (cytokinesis)

Question 4

Why is cell division essential?

Answer 4

• to replace dead/injured cells from wear, tear, stress or chemical damage
• ->and adds new one in tissue growth
• human being goes from 1 to 75 trillion cells

Question 5

Outline cell division VS cell death

Answer 5

• cell division must be controlled by sequence of events
• cells have finite n. of divisions
• ->controlled by activation of “suicide genes”
• 50-70 billion cells die daily due to apoptosis in average human adult
• delicate balance between cell division and cell death important

Question 6

Define atrophy

Answer 6

-tissue waste away due to degeneration of cells

Question 7

What is dysplasia?

Answer 7

-presence of additional cells of abnormal type within tissue which may signify stage preceding cancer development

Question 8

What is the cell cycle? (figure 7)

Answer 8

-cells must first replicate all their homologous (same relation/relative position OR structure) chromosomes

Question 9

Outline the two main stages of the cell cycle (figure 8)

Answer 9

• interphase- when cell not dividing

- mitotic phase (M)- phase when cell dividing

Question 10

What are the subdivided phases in to which interphase is split in to?

Answer 10

• G1: Gap 1 OR presynthesis stage
• S phase: synthesis phase
• G2 phase: Gap 2 OR postsynthesis stage

Question 11

Give an overview of the interphase of the cell cycle (figure 9)

Answer 11

• phase between two successive mitotic divisions
• busiest & longest time in cell cycle
• important checkpoints here too
• most cells spend small amount of time dividing
• interphase cell in G0 not dividing OR preparing to divide like neurons

Question 12

Outline what happens in the G1 phase of the cell cycle (figure 10)

Answer 12

• cells not pass G1 without growth factors
• strictly controlled-one of important checkpoints
• lasts 8-10 hours of 24 hour cycle
• high metabolism rate
• protein synthesis
• vigorous growth
• duplication of most organelles
• centrosome (just outside nucleus) replication begins

Question 13

What happens in the S phase of the cell cycle and why is it important? (figure 11)

Answer 13

• lasts about 8 hours
• DNA replicates
• precise & accurate DNA replication necessary to prevent genetic abnormalities (which often lead to death/disease)

Question 14

Brief the process of DNA replication (refer to figure 12)

Answer 14

• DNA uncoils due to H bonds breaking between nitrogen bases
• original strands function templates for synthesis of new strand
• each new strand contains bases complementary to original strand
• H bonds form between bases of original & new strand
• ->two daughter molecules created

Question 15

Where does energy for the process of DNA replication come from?

Answer 15

-energy from process comes from ATP hydrolysis

Question 16

Which enzyme catalyses the process of DNA replication?

Answer 16

-DNA polymerase

Question 17

Outline what DNA helicase is

Answer 17

• unwinds helix at locations called replication origins

- ->replication origin forms Y shaped- called replication fork

Question 18

Why is DNA helicase essential? (figure 13)

Answer 18

• essential during DNA replication as separate double-stranded DNA into single strands
• ->allowing each strand to be copied

Question 19

What is the G2 phase of the cell cycle? (figure 14)

Answer 19

• lasts 4-6 hours
• synthesis of enzymes & proteins essential for cell division
• replication of centrioles completed
• ->form spindle apparatus associated with DNA movement

Question 20

Brief the mitotic phase of the cell cycle

Answer 20

• characterised by Karyokinesis (nucleus division) AND
• ->cytokinesis (cytoplasm division)
• continuous process described in four stages
• ->prophase/ metaphase/ anaphase/ telophase

Question 21

Outline the following important structural components part of the cell cycle:

1-Centrosome
2-Kinetrosome
3-Centromere

(refer to figure 15)

Answer 21

1-main microtubule organising centre & regulator of cell-cycle progression
2-protein structure of chromatids where spindle fibres attach during cell division to pull sister chromatids apart
3-part of chromosome which links sister chromatids
–>during mitosis spindle fibres attach to centromere via kinetochore

Question 22

State the 5 different phases of mitosis in order

Answer 22

• prophase
• prometaphase
• metaphase
• anaphase
• telophase and cytokinesis

Question 23

Define the terms:

1-Chromatids

2-Chromatin

Answer 23

1-thread-like structures which contain DNA helix

2-complex of DNA & proteins which form chromosomes

Question 24

Explain what happens in the Pro(first)phase of mitosis

refer to figure 16

Answer 24

• longest sub-phase of mitosis
• chromatin coils & condenses into bar-like chromosomes
• ->condensing chromosome material stops them tangling during mitosis
• each chromosome now made of two chromatids
• kinetochore- needed for spindle attachment
• growing spindle- pushed centrosomes to opposite poles
• needed to separate chromatids
• nucleoli disappear
• cytoskeletal microtubules disassemble–> centriole pairs separate & move towards opposite ends due to new microtubule growth- forms mitotic spindle

Question 25

What happens in the second stage of mitosis-Meta(after)phase? (figure 17)

Answer 25

• chromosomes undergo maximum coiling

- chromosomes arrange themselves along middle of cell with centromere aligned at equator of spindle (metaphase plate)

Question 26

Brief the third stage of mitosis- Ana(between)phase in appropriate detail (figure 18)

Answer 26

• kinetochore of each chromatid splits
• ->separated chromatids now termed-daughter chromosomes
• ->move towards opposite poles due to interactions between kinetochore AND microtubules

Question 27

Explain the Telo(end)phase of mitosis (figure 19)

Answer 27

• begins after chromosomal movement stops AND identical sets of chromosomes are at opposite poles
• ->chromosomes start uncoiling AND transform into chromatin
• ->nuclear envelope reforms around chromatin mass
• ->nucleoli reappear AND mitotic spindle disappears
• ->marks end of karyokinesis

Question 28

Outline what the process of cytokinesis (movement) is (figure 20)

Answer 28

• begins during late anaphase and continues through telophase and beyond
• cell organelles like ribosomes AND mitochondria become evenly distributed around two daughter nuclei
• cytoplasm develops cleavage furrow which deepens finally separating in to two daughter cells
• ->each with same complement of chromosomes as parent cell

Question 29

Define cell density

Answer 29

-relative water content AND composition of it’s dry mass

Question 30

Why is cell density important?

Answer 30

• for G0 to remain alive & functioning without dividing
• to grow and divide
• programmed cell death (apoptosis)

Question 31

What is significant about the signals regulating the process important due to cell density?

Answer 31

-signals regulating these processes are intense area of research

Question 32

Outline the function of Cyclin-dependent Kinases (Cdk’s) (figure 21)

Answer 32

• regulate initiation of cell growth
• cell cycle switched on and off by cyclin
• ->their levels rise and fall in cell cycle

Question 33

Explain what a cell cycle checkpoint is

Answer 33

• they control mechanisms which ensure reliability of cell division
• verify whether process at each phase of cell cycle accurately completed before progression in to next phase
• checkpoints assess DNA damage
• ->when damage found checkpoint uses signal mechanism either to stall cell cycle while repairs made OR
• ->if unable to make repairs to target cell for destruction via apoptosis

Question 34

How does cancer affect the cell cycle?

Answer 34

• it deregulates the cell cycle

- figure 23

Question 35

What is the first cell cycle checkpoint?

Answer 35

• Is phase late in G1 phase called restriction phase- RP/restriction point
• ->cell which should cease division exit cell cycle and enter G0
• ->cells which continually divide in adult human include hematopoietic stem cells & gut epithelial cells

Question 36

In the first cell cycle checkpoint how do cells which need to continually divide do so? (refer to figure 24)

Answer 36

• re-entrant in to cell cycle possible only by covering restriction point
• ->this achieved by growth factor-induced expression of Cyclin D proteins
• -.these overcome G0 barrier AND able to enter cell cycle

Question 37

Outline the second checkpoint in the cell cycle (figure 24)

Answer 37

• second checkpoint located end of G2 phase
• ->triggers start of M phase
• mitotic spindle checkpoint occurs at point in metaphase where all chromosomes should of aligned at mitotic plate AND be under bipolar tension

Question 38

What may happen if there were no cell cycle checkpoints OR they didn’t work?

Answer 38

• gene mutation may occur
• ->DNA repair gene/ tumor suppressor genes inactivated
• ->leads to un-controlled cell growth
• ->formation of cancer