chapter 6 p1

Question 1

What is the cell cycle:

Answer 1

sequence of events that takes place in a cell, resulting in division of the cell, and the formation of two genetically identical daughter cells.

Question 2

Phases of the cell cycle:

Answer 2

In eukaryotic cells the cell cycle has two main phases - interphase and mitotic (division) phase.

Question 3

Interphase:

Answer 3

Cells do not divide continuously - long periods of growth and normal working separate divisions.
These periods are called interphase and a cell spends the majority of its time in this phase.
Interphase is sometimes referred to as the resting phase as cells are not actively dividing.
However, interphase is actually a very active phase of the cell cycle, when the cell is carrying out all of its major functions such as producing enzymes or hormones, while also actively preparing for cell division.

Question 4

During interphase:

Answer 4

• DNA is replicated and checked for errors in the nucleus
• protein synthesis occurs in the cytoplasm
• mitochondria grow and divide, increasing in number in the cytoplasm
• chloroplasts grow and divide in plant and algal cell cytoplasm, increasing in number
• the normal metabolic processes of cells occur (some, including cell respiration, also occur throughout cell division).

Question 5

The three stages of interphase, as shown in Figure 1 are:

Answer 5

• G1 - the first growth phase: proteins from which organelles are synthesised are produced and organelles replicate. The cell increases in size.
• S- synthesis phase: DNA is replicated in the nucleus.
• G2 - the second growth phase: the cell continues to increase in size, energy stores are increased and the duplicated DNA is checked for errors.

Question 6

Mitotic phase

Answer 6

The mitotic phase is the period of cell division.

Question 7

Cell division involves two stages:

Answer 7

• Mitosis - the nucleus divides.
• Cytokinesis - the cytoplasm divides and two cells are produced.

Question 8

G0:

Answer 8

is the name given to the phase when the cell leaves the cycle, either temporarily or permanently. There are a number of reasons for this including:
Differentiation
The DNA of a cell may be damaged
As you age
A few types of cells that enter G, can be stimulated to go back into the cell cycle and start dividing again, for example lymphocytes (white blood cells) in an immune response.

Question 9

Differentiation

Answer 9

A cell that becomes specialised to carry out a particular function (differentiated) is no longer able to divide.
It will carry out this function indefinitely and not enter the cell cycle again

Question 10

The DNA of a cell may be damaged

Answer 10

in which case it is no longer viable. A damaged cell can no longer divide and enters a period of permanent cell arrest (Go).
The majority of normal cells only divide a limited number of times and eventually become senescent.

Question 11

As you age

Answer 11

the number of these cells in your body increases.
Growing numbers of senescent cells have been linked with many age related diseases, such as cancer and arthritis

Question 12

Control of the cell cycle:

Answer 12

It is vital to ensure a cell only divides when it has grown to the right size, the replicated DNA is error-free (or is repaired) and the chromosomes are in their correct positions during mitosis.
This is to ensure the fidelity of cell division - that two identical daughter cells are created from the parent cell.

Question 13

What are Checkpoints:

Answer 13

the control mechanisms of the cell cycle.
They monitor and verify whether the processes at each phase of the cell cycle have been accurately completed before the cell is allowed to progress into the next phase.

Question 14

Checkpoints occur at various stages of the cell cycle:

Answer 14

• G1 checkpoint
• G2 checkpoint
• Spindle assembly checkpoint (also called metaphase checkpoint):

Question 15

• G1 checkpoint

Answer 15

This checkpoint is at the end of the G1 phase, before entry into S phase. If the cell satisfies the requirements of this checkpoint (Figure 2) it is triggered to begin DNA replication. If not, it enters a resting state (Go).

Question 16

• G2 checkpoint

Answer 16

This checkpoint is at the end of G2 phase, before the start of the mitotic phase.
In order for this checkpoint to be passed, the cell has to check a number of factors (Figure 2), including whether the DNA has been replicated without error.
If this checkpoint is passed, the cell initiates the molecular processes that signal the beginning of mitosis

Question 17

• Spindle assembly checkpoint (also called metaphase checkpoint):

Answer 17

This checkpoint is at the point in mitosis where all the chromosomes should be attached to spindles and have aligned.
Mitosis cannot proceed until this checkpoint is passed.

Question 18

Cell cycle regulation and cancer:
Kinase-Mediated Checkpoints:

Answer 18

• The passing of a cell-cycle checkpoint is brought about by kinases: These are a class of enzymes that catalyse the addition of a phosphate group to a protein (phosphorylation).
• Phosphorylation changes the tertiary structure of checkpoint proteins, activating them at certain points in the cell cycle.
• Kinases involved in cell-cycle regulation are activated by binding to a variety of checkpoint proteins called cyclins.
• Binding of the correct cyclin to the appropriate kinase forms a cyclin-dependent kinase (CDK) complex.
• These complexes are activated by enzymes.
• CDK complexes catalyse the activation of key cell-cycle proteins by phosphorylation.
• This ensures a cell progresses through the different phases of its cycle at the appropriate times.
• Different enzymes break down cyclins when they are not needed, signalling a cell to move into the next phase of the cycle.

Question 19

Cell cycle regulation and cancer:
Cell-Cycle Dysregulation in Cancer:

Answer 19

• Cancer is a group of many different diseases caused by the uncontrolled division of cells.
• An abnormal mass of cells is called a tumour.
• Tumours can be benign, meaning that they stop growing and do not travel to other locations in the body.
• If a tumour continues to grow unchecked and uncontrolled, it is termed malignant.
• A malignant tumour is the basis of cancer.
• Tumours are often the result of damage or spontaneous mutation of the genes that encode the proteins that are involved in regulating the cell cycle, including the checkpoint proteins.
• For example, if overexpression of a cyclin gene results from mutation, the abnormally large quantity of cyclins produced would disrupt the regulation of the cell cycle, resulting in uncontrolled cell division, tumour formation, and possibly leading to cancer.

Question 20

Cell cycle regulation and cancer:
CDK Inhibition for Cancer Treatment:

Answer 20

Cyclin-dependent kinases can be used as a possible target for chemical inhibitors in the treatment of cancer.
If the activity of CDKs can be reduced it may reduce or stop cell division and therefore cancer formation.

Question 21

The importance of mitosis:
p1

Answer 21

Mitosis describes eukaryotic cell division, specifically nuclear division (division of the nucleus), an essential stage in cell division.
ensures that both daughter cells produced when a parent cell divides are genetically identical (except in the rare events where mutations occur).
Each new cell will have an exact copy of the DNA present in the parent cell and the same number of chromosomes.
Mitosis is necessary when all the daughter cells have to be identical.

Question 22

The importance of mitosis:
p2

Answer 22

• Necessary for processes like growth, tissue replacement, and repair in multicellular organisms (animals, plants, fungi).
• Mitosis is also necessary for asexual reproduction, which is the production of genetically identical offspring from one parent in multicellular organisms including plants, fungi, and some animals, and also in eukaryotic single-celled organisms such as Amoeba species.
• Prokaryotic organisms, including bacteria, do not have a nucleus and they reproduce asexually by a different process known as binary fission.

Question 23

Chromosomes:

Answer 23

Before mitosis can occur, all of the DNA in the nucleus is replicated during interphase.
Each DNA molecule (chromosome) is converted into two identical DNA molecules, called chromatids.
The two chromatids are joined together at a region called the centromere.
It is necessary to keep the chromatids together during mitosis so that they can be precisely manoeuvred and segregated equally, one each into the two new daughter cells
During interphase DNA combines with proteins called histones to form a dense complex called chromatin.

Question 24

Answer 24

Question 25

The stages of mitosis:

Answer 25

There are four stages of mitosis - prophase, metaphase, anaphase, and telophase - they flow seamlessly from one to another.
Each of these phases can be viewed and identified using a light microscope.
Dividing cells can be easily obtained from growing root tips of plants.
The root tips can be treated with a chemical to allow the cells to be separated - then they can be squashed to form a single layer of cells on a microscope slide.
Stains that bind DNA are used to make the chromosomes clearly visible.

Question 26

Prophase
p1

Answer 26

During prophase, chromatin fibres (complex made up of various proteins, RNA and DNA) begin to coil and condense to form chromosomes that will take up stain to become visible under the light microscope
The nucleolus, a distinct area of the nucleus responsible for RNA synthesis, disappears.
The nuclear membrane begins to break down.

Protein microtubules form spindle-shaped structures linking the poles of the cell.
The fibres forming the spindle are necessary to move the chromosomes into the correct positions before division.

Question 27

Prophase
p2

Answer 27

In animal cells and some plant cells, two centrioles migrate to opposite poles of the cell.
The centrioles are cylindrical bundles of proteins that help in the formation of the spindle.

The spindle fibres attach to specific areas on the centromeres and start to move the chromosomes to the centre of the cell.

By the end of prophase the nuclear envelope has disappeared.