WEEK 9 (Cell cycle and cell death)

Question 1

What is the G1 phase?

Answer 1

A period in which cells accumulate the enzymes and nucleotides required for DNA replication

Question 2

What is the S phase?

Answer 2

The period devoted primarily to DNA replication

Question 3

What is the G2 phase?

Answer 3

A short period of preparation for mitosis

Question 4

Which phases are involved in the M phase (Mitosis)?

Answer 4

Prophase
Metaphase
Anaphase
Telophase

Question 5

When does the G0 phase of the cell cycle happen?

Answer 5

Differentiating cells in growing tissues may have very long G1 periods and such cell are often said to be in the G0 phase of the cell cycle

Question 6

What happens in the Start (G1/S) checkpoint?

Answer 6

Prepares for DNA replication and enter S phase

Questions asked are:
- Are cell nutrition, size and environment favourable?
- Is all DNA intact?

Question 7

What happens in the G2/M checkpoint?

Answer 7

Enter Mitosis

Questions asked here are:
- Is DNA completely replicated?

Question 8

What happens in the metaphase/anaphase checkpoint?

Answer 8

Begin chromatid separation and prepare for cytokinesis

Questions asked here are:
- Is all DNA intact?
- Are all chromosomes attached to the mitotic spindle?

Question 9

What are the properties of the Early G1 phase?

Answer 9

ACTIVE CYCLIN-CDK COMPLEX: Cyclin D-CDK4 or 6

EXAMPLES OF TARGET PROTEINS:
- Phosphorylates Rb protein
- Releasing E2F
- A transcription factor that activates genes for many G1 activities and for cyclin A

Question 10

What are the properties of the G2/entry of M phase?

Answer 10

ACTIVE CYCLIN-CDK COMPLEX: Cyclin A-CDK1

EXAMPLES OF TARGET PROTEINS: specific phosphatases and cyclin B

Question 11

Describe what happens in Prophase

Answer 11

CENTROSOMES move to opposite poles, the NUCLEAR ENVELOPE disappears by FRAGMENTATION, and the chromosomes condense and become visible. Having undergone DNA replication, each chromosome consists of two chromatids joined at their centromere regions by KINETOCHORE protein complexes

Question 12

Describe what happens in metaphase

Answer 12

Chromosomes have become aligned at the EQUATORIAL PLATE as a result of their attachments to the dynamic microtubules of the MITOTIC SPINDLE organised by centromeres. Spindle consists of KINETOCHORE MICROTUBULES, POLAR MICROTUBULES which interdigitate near the equatorial plate and shorter ASTRAL MICROTUBULES anchoring the spindle to the cell membrane

Question 13

Describe what happens in Anaphase

Answer 13

KINETOCHORES separate and the chromatids are pulled on their microtubules towards each CENTROSOME

Question 14

Describe what happens in Telophase

Answer 14

Cell pinches itself in two by contraction of the F-actin bundle in the cell cortex, after which the chromosomes DECONDENSE, TRANSCRIPTION resumes, NUCLEI reappear and the NUCLEAR LAMINA and NUCLEAR ENVELOPE reassemble

Question 15

What is distinguishable about Stem cells?

Answer 15

• Divide ASYMMETRICALLY, producing one cell that remains as a stem cell and another that becomes committed to a DIFFERENTIATIVE PATHWAY but divides a few more times at a more rapid rate
• These cells are called PROGENITOR CELLS or “TRANSIT AMPLIFYING CELLS”

Question 16

Describe the stages involved in Mitosis

Answer 16

INTERPHASE
PROPHASE - Chromosomes condense
METAPHASE - Chromosomes line up single file
ANAPHASE/TELOPHASE - Genetically identical daughter cells are produced

Mitosis adds and replaces identical cells

Question 17

Describe the stages involved in Meiosis I & Meiosis II

Answer 17

LATE INTERPHASE - Synapsis and crossing-over begin
PROPHASE I - Crossing over continues & Paired chromosomes condense
METAPHASE I - Homologous chromosomes line up double file
ANAPHASE I & TELOPHASE I - Homologs separate into haploid daughter cells; sister chromatids remain joined
METAPHASE II - Chromosomes line up single file in haploid cells
ANAPHASE II & TELOPHASE II - Sister chromatids separate into nonidentical haploid cells

Meiosis produces haploid cells with new genetic combinations

Question 18

What is Apoptosis?

Answer 18

A rapid, highly regulated cellular activity that shrinks and eliminates defective and unneeded cells.

Question 19

What are the properties of Apoptosis?

Answer 19

• Results in small membrane-enclosed apoptotic Bodies, which quickly undergo phagocytosis by neighbouring cells or cells specialised for debris removal
• Apoptotic cells do not rupture and release none of their contents

Question 20

Apoptosis is controlled by which proteins?

Answer 20

Cytoplasmic proteins in the Bcl-2 family which regulate the release of death-promoting factors from mitochondria

Question 21

Activated by either external signals or irreversible internal damage, specific Bcl-2 proteins induce a process with which features?

Answer 21

• Loss of mitochondrial function and caspase activation resulting in protein degradation throughout the cell
• Fragmentation of DNA
• Shrinkage of nuclear and cell volumes
• Plasma membrane of shrinking cell undergoes dramatic shape changes
• Formation of very small apoptotic bodies and phagocytic removal

Question 22

What is Necrosis/Accidental cell death?

Answer 22

A pathologic process that occurs when cells are exposed to an unfavourable physical or chemical environment that causes acute cellular injury and damage to the plasma membrane

Under physiologic conditions, damage to the plasma membrane may also be initiated by viruses or proteins called performs. Rapid cell swelling and lysis are two characteristic features of this process.