Lecture 6: The cell Cycle

Question 1

What is the period of regular cell growth, metabolic activity and repair called?

Answer 1

Interphase, it includes G1, G2,S

Question 2

In which phase does DNA replication take place?

Answer 2

S phase (synthesis phase)

Question 3

What happens in M phase

Answer 3

The nucleus and cytoplasm divide
Mitosis: nuclear division
Cytokinesis: cytoplasmic division

Question 4

What cells don’t divide in the body?

Answer 4

1. Many mature cells do not divide: like terminally differentiated cells like nerve cells, muscle cells, red blood cells as they become differentiated they loose the ability to divide
2. Some cells only divide when given an appropriate stimulus: like liver cells divide when damages
3. Some divide normally on an ongoing basis : epithelial stem cells

Question 5

Which phase does non-diving cells come under

Answer 5

G0, they don’t divide but are normal and are not dead. But some cells go from G0 to G1 then S-phase -G2 and then M phase

Question 6

What is the cell-cycle control system?

Answer 6

This delays later events until the earlier events are complete and has 3 major check points:
START CHECPOINT or transition (G1-S):
G2-M transition
Metaphase -to-anaphase transition also called spindle assembly check point

Question 7

What is the start transitions

Answer 7

Phase before S phase - G1

Question 8

What do problems in checkpoints cause

Answer 8

Chromosome segregation defects

Question 9

What is cell progression controlled by?

Answer 9

By molecular switches, and is triggered by cyclin-dependent protein kinases.
Cdks-complexes are activated for entry then inactivated

Entry into M-phase is regulated by M-Cdk and phosphorylates other regulatory proteins

Question 10

What is the first phase after interphase and what happens in it ?

Answer 10

It is prophase, this occurs after interphase. During the G1 phase, centrosomes duplication initiated and completed by G2.

S-phase: chromosomes replicates (decondensed)

In prophase the replicates chromosomes condense and nito tic assembly starts

Question 11

What are deposited on the chromatids in S-phase and what happens in end of G2 phase

Answer 11

Cohesins are deposited to hold the two sister chromatids together and at the end of G2, condensions makes the replicated chromosomes compact

Question 12

What are the steps happening in prophase?

Answer 12

1) chromosome condensation and sister chromatid resolution takes place

Cohesins are removed from chromosomes arms, but not from the centromers

Condensing condenses DNA in each sister chromatid

And in order for mitotic spindle assembly to start, it requires disassembly and reassembly of microtubles and needs duplicated centrosomes

Question 13

How many fibrils are required in a cebtriole?

Answer 13

It needs nine fibrils of 3 microtubles pairs

Question 14

How many times are centrosomes duplicated ans when is it initiated and when is it completed by

Answer 14

It is duplicated once per cell cycle, and is initiated in G1, and completed by G2

Question 15

How are centrioles duplicated?

Answer 15

They are semi-conservative

Question 16

When does mitotic spindle assembly start?

Answer 16

In prophase (M phase) this need ms assembly and is assembly of microtubles dynamics

Next is nuclear envelope breakdown

Question 17

When does nuclear envelope breakdown occur

Answer 17

Between prophase and prometaphase

Question 18

How can you break down nuclear envelope?

Answer 18

Phosphorylation of lamins and nuclear pore proteins trigger the disassembly of nuclear envelope into small membrane vesicles

Question 19

What are the steps in prometaphase?

Answer 19

Once the nuclear envelope is disassembled, mitotic spindle assembly can be completed, the KINETOCHORE microtubules in the mitotic spindle attach to the duplicated chromosomes and the chromosome movement begins

Question 20

What does mitotic spindle assembly and function require?

Answer 20

Microtubule dynamics (disassembly and assembly)

Microtubule motor protein activity (kinesins and cytoplasmic dynein)

Question 21

What do astral microtubules do!

Answer 21

They help to position the mitotic spindle

Question 22

What do non-kinetochore microtubules do?

Answer 22

They cross linked microtubules throughout the mitotic spindle like kinesin-5 other microtubules-associated proteins

Question 23

What do kinetochore microtubules do?

Answer 23

They attach duplicated chromosomes to the spindle poles

Question 24

Where are kinetochore microtubles attached and located?

Answer 24

They are located at the centromere of chromosomes. One kinetochore for each sister chromatid in The duplicated chromosome. The kinetochore likes to bind at the plus end of the microtubules.

Question 25

Which sides do connecting protein complexes bind on the microtubules?

Answer 25

Bind to the sides

Question 26

Where are all chromosomes aligned in metaphase

Answer 26

In the metaphase plate

Question 27

What is done to maintain the metaphase spindle

Answer 27

There is addition of tubulin subunits at the plus end and removal of tubulin subunits at the minus end so the length of kinetochore microtubules does not change

Question 28

how are the microtubules removed at the minus end?

Answer 28

With depolymerases

Question 29

What separates the sister chromatids?

Answer 29

Separase

Question 30

What happens to the cohesin in anaphase

Answer 30

Seperase cleaves the cohesin complex

Question 31

What happens in anaphase A?

Answer 31

Kinetochore microtubules are shortened (loss of tubulin at both ends)

And sister chromatids are pulled apart towards opposite ends

Question 32

What happens in anaphase B

Answer 32

Spindle poles move outward (kinesin and cytoplasmic dynein)

Question 33

What happens in telophase

Answer 33

Chromosome seperate into two groups (one at each spindle pole)

Mitotic spindle disassembles, nuclear envelope reassembles, chromosomes decondense and this makes the end of mitosis and contractile ring starts to assemble for cytokinesis

Question 34

How is the nuclear envelope form gain in telophase

Answer 34

Special enzyme called phosphatases, dephosphorylation of nuclear pore proteins and lamins. Nuclear envelope reassembles with nuclear lamina and nuclear pores.

Question 35

How does cytokinesis take place in animal cell?

Answer 35

Cytoplasm is divided in two by a contractile ring of actin and myosin filaments at the cleavage furrow (midway between the spindle poles)

Then interphase microtubules re form in each daughter cells and this marks the end of M phase

Question 36

What filaments does cytokinesis need

Answer 36

Dynamic actin and myosin.

In the beginning of mitosis actin and myosin arrays disassemble and at the end actin and myosin assemble into a contractile ring. The con ration ring becomes smaller and the forces on the cell memebrabe divides the cell into two daughter cells

Question 37

What is Absent in mitosis in plant cell

Answer 37

No contractile ring, vesicles fusing

Question 38

How does telophase take place in cytokinesis in plant cells

Answer 38

Chromosomes seperate into two sets and a PHRAGMOPLAST is formed which is a specific structure to form a cell plate and PHRAGMOPLAST has microtubules, actin filaments and vesicles from Golgi

Question 39

How does cytokinesis take place in a plant cell?

Answer 39

Nuclear envelope reassembles and chromosomes decondense and the cell plate forms which is a transient membrane compartment (Vesicles from Golgi fuse together to divide the cell into two)

Then in G1, cell plate has matured into plasma membranes and cell wall between two daughter cells

Question 40

What is the difference between mitosis and meiosis

Answer 40

Both have one round of DNA replication however, meiosis have 2 rounds of cell division making 4 haploid cells while mitosis is one round of cell division and 2 diploid cells