The Cell Cycle

Question 1

What are cells that continue to divide regularly in definite intervals described as?

Answer 1

Cycling cells.

Question 2

What are cyclins?

Answer 2

A family of proteins whose concentration increases and decreases throughout the cell cycle.

Question 3

What are the cell cycle proteins molecules?

Answer 3

Cyclins A,B, and E.

Question 4

What do cyclins activate?

Answer 4

Cyclin dependent kinases (CDKs) which act in conjunction with cyclins.

Question 5

Upon completion of cell cycle, what happens to cyclins and CDKs?

Answer 5

They are degraded: residues of used molecules are taken up by cytoplasmic care taker protein; ubiquitin.

Question 6

What is interphase?

Answer 6

The interval between the end of mitosis and beginning of next. Cell is either resting or performing its specialised work not in mitosis.

Question 7

What happens in S phase?

Answer 7

Relation of DNA to genes is established/ new DNA is synthesized. DNA would have to be duplicated (replication) sometime in the cell cycle to have enough genes for two daughter cells. DNA is duplicated in interphase long before the chromosomes are detected as thread like bodies.

Question 8

What is S phase promoted by?

Answer 8

Cyclin A and CDKs.

Question 9

When does S phase begin and how long does it take?

Answer 9

8 hours after the end of mitosis and takes about 7-8 hours to complete.

Question 10

What happens when crucial steps in the cell cycle, such as DNA replication is not completed?

Answer 10

The cell cycle will be arrested.

Question 11

What is the common axis where the two chains of DNA are coiled around called?

Answer 11

Helical axis.

Question 12

How are the DNA chains paired?

Answer 12

In an antiparallel manner.

Question 13

Which side is the hydrophilic deoxyribose-phosphate backbone of each chain located?

Answer 13

The outside.

Question 14

Which side are the hydrophobic bases stacked?

Answer 14

The inside.

Question 15

What does the spatial relationship between the two strands in the helix create?

Answer 15

A major and minor groove.

Question 16

What does anticancer drugs such as dactinomycin (actinomycin D) do?

Answer 16

Exert their cytotoxic effect by intercalating into the narrow groove of the DNA double helix, thereby interfering with DNA synthesis.

Question 17

What does DNA damage during DNA replication cause?

Answer 17

Deoxynucleotide incorporation error during replication by spontaneous deamination of bases during normal genetic functions. From X-radiation that cause ‘nicks’ in the DNA. From UV irradiation. From various chemicals that interact with DNA.

Question 18

What is Xeroderma Pigmentosum?

Answer 18

DNA repair mechanisms are defective. DNA damage produced by UV irradiation. Results from inborn deficiency of the enzyme “nicking endonuclease”.

Question 19

What are examples of Xeroderma Pigmentosum?

Answer 19

Dry keratosis, hyperpigmentation, and skin atrophy. Increased cutaneous sensitivity to UV light. Ulcers in the cornea could form.

Question 20

What is developed from Xeroderma Pigmentosum?

Answer 20

Squamous cell carcinoma of the skin.

Question 21

What is homologous recombination (HR)?

Answer 21

A major pathway for the repair of DNA double-strand breaks. The defining step of which is homologous directed by the RAD51 protein.

Question 22

What is required for HR?

Answer 22

Hereditary breast cancer suppressors BRCA1 and BRCA2.

Question 23

What is linked to tumour predisposition?

Answer 23

Mutations in a number of HR genes.

Question 24

What is Hereditary Nonpolyposis Colon Cancer (HNPCC)?

Answer 24

Mismatch repair. Chromosome 2 has been shown to contain a genetic error; the affected gene is known as hMSH 2.

Question 25

What does mutations of hMSH 2 account for?

Answer 25

50-60% of HNPCC cases.

Question 26

What is G1 phase?

Answer 26

The period when cells respond to growth factors directing the cell to initiate another cycle; once made, this decision is irreversible. It is also the phase in which most of the molecular machinery required to complete another cell cycle is generated.

Question 27

What is the G1 phase controlled by?

Answer 27

Cyclin E and CDKs.

Question 28

How long does G1 phase last?

Answer 28

Considerable variation, sometimes ranging from less than 2 hrs in rapidly dividing cells, to more than 100 hrs within the same tissue.

Question 29

What is G0 phase?

Answer 29

Cells that retain the capacity for proliferation and in the process of differentiation, but which are no longer dividing, enter this phase. They are described as quiescent and do not enter S phase.

Question 30

What is the role of growth factors in G0 phase?

Answer 30

They can stimulate quiescent cells to leave G0 and re-enter the cell cycle.

Question 31

What is the role of proteins encoded by certain tumour suppressor (eg the gene mutated in retinoblastoma, Rb) genes in G1 phase?

Answer 31

Block the cycle in G1.

Question 32

What is G2 phase?

Answer 32

The cell prepares for division; this period ends with the breakdown of the nuclear membrane and the onset of chromosome condensation.

Question 33

What is the G2 phase controlled by?

Answer 33

Cyclin B and CDKs.

Question 34

How long does G2 phase last?

Answer 34

2-4 hrs to complete.

Question 35

State 4 facts about interphase.

Answer 35

Individual chromosome are not visible. They are uncoiled into very long narrow structures. The nucleus of the cell possess a nuclear envelope and contains a network of chromatin threads or granules and a nucleolus. Replication of DNA takes place in chromosome. Outside the nuclear envelope two paired centrioles are located in cytoplasm.

Question 36

What happens in prophase?

Answer 36

The individual chromosome are first visualized as thin threads and are tightly coiled along the entire length. Each chromosome splits longitudinally into two chromatids except at the centromere. The two paired centriole are separated from each other by elongation of continuous microtubules of the achromatic spindle and occupy the opposite pole of nucleus.

Question 37

What happens in prometaphase?

Answer 37

Subsequently, the nuclear membrane and nucleolus disappear. The chromosomes are now entangled in a meshwork of continuous microtubules.

Question 38

How long does prophase take to complete?

Answer 38

About 1.5hrs.

Question 39

What happens in metaphase?

Answer 39

Centromere region from each chromosome presents a bilateral disc Kinetochores. From kinetochores, a set of chromosomal microtubules are organized and extend bilaterally towards opposite centrioles. The growth of the chromosomal microtubules allows centromere of individual chromosomes with their paired chromatids to occupy equatorial plane of the spindle.

Question 40

How long does metaphase last?

Answer 40

Persists for 20mins.

Question 41

What is the role of colchicine?

Answer 41

Colchicine arrest the cell division at metaphase - prevents the formation of microtubules of the spindle. In colchicine treated metaphase the chromosomes are found to assemble around centriole. Application of colchicine in cell culture is a important tool in karyotyping - this underpins the rationale for many types of cytotoxic drugs used in cancer therapy.

Question 42

What happens in anaphase?

Answer 42

The centromeres split longitudinally and chromatids separate to form two new chromosome. The spindle fibres contract, each pair of newly formed chromosomes separates and migrates to the opposite pole of the spindle. Two identical and complete chromosomes are formed.

Question 43

The separation of the chromatids to form new chromosomes occurs by what?

Answer 43

Contraction of chromosomal microtubules which exert a centrifugal force in centromere region.

Question 44

What is non-disjunction?

Answer 44

Abnormal function of spindle apparatus one or more chromosomes fail to migrate properly in anaphase. One daughter cell receives extra chromosomes and other daughter cell is deficient in this chromosome.

Question 45

What is isochromosome?

Answer 45

The centromere splits transversely instead of longitudinal division. This leads to formation of two daughter chromosome of unequal length.

Question 46

What happens in telophase (in progress)?

Answer 46

A reorganization period - the daughter chromosomes are enveloped by new nuclear membrane and nucleolus appears. The chromosomes become uncoiled and their individual identity is lost. The cytoplasm divides and two complete cells are formed.

Question 47

What happens in telophase (complete)?

Answer 47

In the process of cytokinesis, the cleavage furrow develops around the equator region of achromatic spindle. The contraction of fibrillar component of cytoplasm help in hour glass constriction of cleavage.