Chapter 12 - The Cell Cycle & its Regulation

Question 1

Some eukaryotes have cell division that is intermediate between binary fission and mitosis such as what? (Give the two main examples)

Answer 1

1. Dinoflagellates ~2000-4000 species

Single celled organisms with two flagella

2.Diatoms and yeast ~ 100,000 species

Extremely important phytoplankton

Question 2

What happens to dinoflagellates during mitosis?

Answer 2

Nuclear envelope remains intact during mitosis and the chromosomes attach to it

Microtubules pass through the nucleus dividing it, similar to binary fission.

Question 3

What happens to diatoms & yeast during mitosis?

Answer 3

Nuclear envelope remains intact

And, microtubule spindle forms inside the nucleus

Within the nucleus, ANDA separates similar to how it would in eukaryotes.

Question 4

What is a genome?

Answer 4

A genome is an organism’s

Complete set of DNA

including all of its genes. Each genome contains all of the information needed to build and maintain that organism. In humans, a copy of the entire genome—more than 3 billion DNA base pairs—is contained in all cells that have a nucleus.

Prokaryotic cells have a single DNA molecule.

Eukaryotic cells have a # of DNA molecules.

Question 5

What’s the difference between a somatic cell and a gamete cell?

Answer 5

Somatic cell

(non reproductive cells) have two sets of chromosomes. Human somatic cells = 23 pairs of chromosomes.

Gamete cell

(reproductive cells: sperm and eggs) have one set of chromosomes. Human gametes = 23 chromosomes

Question 6

Briefly describe the process of gene replication

Answer 6

(see picture)

Question 7

In the cell cycle, what are the 2 main phases, along with the respective subphases?

Also provide a brief description of each

Answer 7

1. Interphase - 90% of cell cycle

Cell growth and copying of chromosomes in preperation for cell division. Grows during all three processes.

A) G₁ phase

B) S phase

C) G₂ phase

2. Mitotic phase

The actual process of division of one nucleus into genetically identical nuclei.

A) Mitosis

I) Prophase

II) Prometaphase

III) Metaphase

IV) Anaphase

V) Telophase

B) Cytokinesis

Question 8

Describe the process of each subphase during Interphase

Answer 8

1. G1 phase (“first gap”)

The “restriction point” – point of no return in cell cycle

Up until this point (found in G1) all cellular growth has been regulated by growth factors.

After this point, the cell cycle will continue to the next checkpoint at the end of G2.

2. S phase (“synthesis”)

DNA in the nucleus is replicated

Centrosomes are duplicated

Just after mitosis (M-phase) they move to opposite sides of cell.

3. G2 phase (“second gap”)(see picture)

All of machinery needed for mitosis is replicated

The cell continues to grow until it is twice the original size

After this checkpoint, mitosis occurs.

End of phase (going into mitotic phase):

* Nuclear envelope still intact

* Nucleus still intact

* Two centrosomes formed

DNA and chromatin have not yet condensed into chromosomes

Question 9

Describe the process of each subphase (5) during the Mitotic (M) phase (cell division)

Answer 9

1. Prophase

Nuclear envelope still intact

Nucleolus disappears

DNA and chromatin condenses into chromosomes

Mitotic spindle forms (centrosome and microtubules)

Centrosomes push apart (pushed by extending microtubules)

2. Prometaphase

Nuclear membrane fragments

Microtubules from spindles connect to chromosomes

Microtubules connected to kinetochores on chromatids are called kinetochore microtubules

Other microtubules interact with microtubules from opposite side of cell

3. Metaphase

Longest stage in mitosis

Centrosomes are on opposite sides of cell

Mitosis does not proceed until chromosomes have all lined up on the metaphase plate

4. Anaphase

Shortest stage of mitosis

Stage begins when sister chromatids part

The chromosomes begin moving towards each centrosome

Cell elongates as the non-kinetochore microtubules lengthen

5. Telophase

Two daughter nuclei begin to form

Chromosomes become less dense

Daughter cell nuclear envelopes arise from the fragments of the parental cell’s nuclear envelope.

Question 10

Describe the process of prophase (First phase in mitotic phase)

Answer 10

1. Prophase

Nuclear envelope still intact

Nucleolus disappears

DNA and chromatin condenses into chromosomes

Mitotic spindle forms (centrosome and microtubules)

Centrosomes push apart (pushed by extending microtubules)

Question 11

Describe the process of prometaphase (second phase in mitotic phase)

Answer 11

2. Prometaphase

Nuclear membrane fragments

Microtubules from spindles connect to chromosomes

Microtubules connected to kinetochores on chromatids are called kinetochore microtubules

Other microtubules interact with microtubules from opposite side of cell

Question 12

Describe the process of metaphase (third phase in mitotic phase)

Answer 12

Metaphase

Longest stage in mitosis

Centrosomes are on opposite sides of cell

Mitosis does not proceed until chromosomes have all lined up on the metaphase plate

Question 13

Describe the process of Anaphase (fourth phase in mitotic phase)

Answer 13

4. Anaphase

Shortest stage of mitosis

Stage begins when sister chromatids part

The chromosomes begin moving towards each centrosome

Cell elongates as the non-kinetochore microtubules lengthen

Question 14

Describe the process of Telophase (fourth phase in mitotic phase)

Answer 14

5. Telophase

Two daughter nuclei begin to form

Chromosomes become less dense

Daughter cell nuclear envelopes arise from the fragments of the parental cell’s nuclear envelope.

Question 15

Humans have how many chromosomes?

Answer 15

23

Somatic Cells (non reproductive) - 23 pairs of chromosomes

Gametes (reproductive) - 23 chromosomes

Question 16

Describe the process of cytokinesis (after telophase)

Answer 16

1 cell becomes 2

In animal cells, cytokinesis occurs by a process known as cleavage, forming cleavage furrow.

In plant cells, a cell plate forms during cytokinesis

Question 17

Before mitosis can begin, the cell must go through interphase for cell preperation. But, there are several “stop and go checkpoints” that must be passed first. What are these checkpoints? (four of them)

Answer 17

G₁ Checkpoint

Called the “restriction point” , Often viewed as the most important checkpoint

If cell passes through G1 checkpoint, then the cell with proceed through the G1, S and G2 phases of the cycle.

If the cell does not pass through this checkpoint, it can go into G₀.

G₀ Checkpoint

Most cells do not need to actively grow in size and divide and are in a quiescent (“quiet”) state.

Cells such as bone marrow, skin and lining of the guy do not go into G0 as they need to continually divide and grow.

Some cells can be in the G0 for a long time, such as nerve cells which stay like that for over 20 years.

Most cells in your body are G₀.

G₂ Checkpoint

Makes sure the cell is big enough and the environment is favourable. Also checks for damaged DNA.

Two types of regulatory proteins are involved in cell cycle control at the G2 checkpoint:

Cyclin – named for cyclically changing concentration in the cell

Cyclin-dependent kinases (Cdks) - present in cell at relatively constant concentration, but not activated until connected to cyclin.

When cyclin and Cdk bind together, they are called MPF (maturation-promoting factor)

MPF triggers a cell’s passage past the G2 checkpoint into the M phase.

Metaphase (M) Checkpoint

Makes sure the chromosomes are aligned on the metaphase plate.

Question 18

at the G₂ checkpoint, there are two types of regulatory proteins involved in the cell cycle control. What are they? and when bound together, what is this called?

Answer 18

G₂ Checkpoint

Makes sure the cell is big enough and the environment is favourable. Also checks for damaged DNA.

Two types of regulatory proteins are involved in cell cycle control at the G2 checkpoint:

Cyclin

named for cyclically changing concentration in the cell

Cyclin-dependent kinases (Cdks)

present in cell at relatively constant concentration, but not activated until connected to cyclin.

MPF (maturation-promoting factor)

When cyclin and Cdk bind together, this is called the MPF.

MPF triggers a cell’s passage past the G2 checkpoint into the M phase.

Question 19

What are the two signals that stop the cell cycle?

Answer 19

Density-dependent inhibition

crowded cells stop dividing.

Anchorage dependence

cells must be attached to a surface in order to divide.

Question 20

Some external signals are growth factors to the sell, what is an example of an external signal?

Answer 20

Plate-derived growth factor (PDGF)

stimulates the division of human fibroblast cells in culture.

Fibroblasts are loose cells in connective tissue that make up part of the Extra-cellular Matrix (ECM)

Question 21

In terms of cancer terminology, what does transformation mean for the cell? How does this happen?

Answer 21

Transformation

A normal cell converted to a cancerous cell.

How?

Cancer cells may not need growth factors to grow and divide:

* They may make their own growth factor

* They may convey a growth factor

* They may have an abnormal cell cycle control system

Occasionally non-cancerous cells with lose control of the cell cycle.

Normally these cells will undergo apoptosis, also known as “programmed cell death”.

If the cells do not undergo apoptosis then they will continue to grow and can eventually become cancerous

Question 22

What is a benign tumor and a maligant tumor?

Answer 22

Benign tumor

abnormal cells remain at the original site

Malignant tumor

abnormal cells can metastasize (invade surrounding tissues) where they may form secondary tumors

Question 23

What is metastasis?

Answer 23

Metastasis

Cancerous cells can detach from surrounding cells in the tissue/organ and the travel through the blood or lymph vessels and begin to grow elsewhere in the body. If this occurs, the cancerous cells have become “metastasized” making the tumours very difficult to treat.

Question 24

Drugs like Paclitaxel and toxol have been used to treat certain forms of cancer. How does it do this ?

Answer 24

Paclitaxel has been shown to act as a mitotic inhibitor.

How? Microtubules are stabilized

• Prevents the depolymerisation of the microtubules attached to kinetochores.
• Thus, the mitotic spindle is frozen
• Actively dividing cells then cannot proceed beyond Metaphase.