IB Biology: Level 8 - Cell Cycle & Cancer 1.6, 3.2, & 7.1

Question 1

Define the cell cycle.

Answer 1

The cell cycle comprises the events that take place between the formation of the cell and when it divides.

Question 2

Name the stages of the cell cycle.

Answer 2

1. Interphase: G1- phase, S-phase, G2-phase
2. Mitosis
3. Cytokinesis

Question 3

Outline the events that happen in the three phases of interphase.

Answer 3

Interphase is the time between the formation of a cell and when the nucleus divides. It is a period of intense metabolic activity.

Question 4

G1 phase (growth phase 1):

Answer 4

• Cell activities include: Transcription, translation, cell differentiation, cellular respiration, and new organelles are produced.
• Cells that will not divide remain in G1.

Question 5

S phase (synthesis phase):

Answer 5

• Once a cell is instructed to divide it enters S phase. - DNA replication occurs.
• Chromosomes must be replicated to make sure each new cell formed has a full set of chromosomes.

Question 6

G2 phase (growth phase 2):

Answer 6

• The cell prepares further for nuclear division.
• In animal cells, centrioles are replicated.

Question 7

What is Mitosis?

Answer 7

• Mitosis is a type of nuclear division in eukaryotic cells.
• The nucleus divides to produce two daughter nuclei the genetically identical.
• Mitosis is an extremely important process.
• It is crucial that new cells produced for growth and repair are genetically identical to existing cells.
• Asexual reproduction also relies on mitosis.

Question 8

Name the four stages of Mitosis

Answer 8

Mitosis is a continuous process. However, it is split into 4 stages for convenience.
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Question 9

What happens when chromosomes condense?

Answer 9

When chromosomes condense, they coil up to become shorter and thicker.
- In eukaryotic cells, chromosomes are made up of DNA and proteins (histones).
- The proteins help organise the DNA and enable chromosomes to coil.
- Chromosomes are least condensed during G1 of interphase.
- Individual chromosomes are not visible with a light microscope in the nucleus of cells during interphase.
-During mitosis the chromosomes become highly condensed.
-This process is called supercoiling.
- Chromosomes then become visible with a light microscope.
- Fully condensed chromosomes are approx. 250 times thicker that a DNA double helix.

Question 10

Chromosome

Answer 10

• Centromere = location where 2 sister chromatids are joined together.
• Sister chromatids = genetically identical structures produced by DNA replication.
• During mitosis the sister chromatids will separate
• they are then termed chromosomes.
• It is a common misconception that chromosomes are always X-shaped structures. Chromosomes are only X-shaped following DNA replication and before the sister chromatids separate. Otherwise chromosomes in eukaryotes are linear structures.

Question 11

Prophase

Answer 11

• Chromosomes condense by supercoiling (and become visible with a light miroscope).
• Centrioles move towards opposite poles (ends) of the cell and spindle microtubules form.
• Nuclear membrane starts to break down.

Question 12

Metaphase

Answer 12

• Centrioles are at opposite poles of the cell.
• The nuclear membrane is fully broken down.
• Chromosomes are fully condensed.
• Spindle microtubules attach to the centromere of each chromosome
• one spindle microtubule from each centriole attaches to each side of the centromere.
• Chromosomes line up at the equator (middle) of the cell
• due to spindle microtubules pulling on the centromeres.

Question 13

Anaphase

Answer 13

• The centromeres divide and the 2 sister chromatids are separated (each chromatid is now called a chromosome).
• The spindle microtubules shorten, pulling genetically identical chromosomes to each pole.
• At the end of anaphase, each pole has an identical set of chromosomes.

Question 14

Telophase

Answer 14

• The chromosomes at each pole become less condense.
• A nuclear membrane forms around the 2 sets of chromosomes.
• The spindle microtubules break down.

Question 15

Mitotic index.

Answer 15

Indicates the proportion of cells undergoing mitosis in a tissue.More actively growing tissues (such as the root tips of a plant or a tumour) will have a higher mitotic index.

Question 16

How to calculate the mitotic index.

Answer 16

1. View a slide of your chosen tissue under a light microscope.
2. Select a higher magnification so the field of view contains approximately 100 cells.
3. Count all the cells that you can see.
4. Count all the cells in which you can see condensed chromosomes.
5. Use the equation: -Number of cells undergoing mitosis divided by the total number of cells observed.

Question 17

Cytokinesis

Answer 17

• Division of cytoplasm.
• Occurs at the end of mitosis and results in 2 separate daughter cells being produced.

Question 18

How does cytokinesis occur in animal and plant cells?

Answer 18

In animal cells:
- Plasma membrane pinches in all the way around the equator of the cell.
- When the membrane meets the middle of the cell, the cell splits to form 2 identical cells.

In plant cells:
- A line of vesicles form at the equator of the cell. These join together to form plasma membranes and cell walls, separating the 2 daughter cells.

Question 19

Cyclins

Answer 19

• Are proteins that control and coordinate the different phases of the cell cycle.
• They make sure one phase does not start before another phase is complete and stop progression of the cell cycle if the cell should not divide.
• Cyclins pair together with an enzyme cyclin-dependent kinase (CDK) to regulate the activity of other proteins that perform phase-specific roles.
  -Cyclins are like managers that oversee a team of workers. Each phase transition has many workers and one manager.
• Kinases are enzymes that add phosphate to (or phosphorylate) their substrate. Phosphorylation is a common way to regulate the activity of proteins in cells.

Question 20

How were cyclins discovered?

Answer 20

• Cyclins were discovered by accident.
• Their fortunate occurrence is known as serendipity.
• Researchers were studying protein synthesis in sea urchins.
• They noticed the levels of some proteins increased and decreased in regular cycles.
• The researchers realised the importance of their observations.
• These proteins were named cyclins and found to have a key role in the control and coordination of the cell cycle.
• Serendipity has regularly happened throughout the history of science. Perhaps the most well-known is the chance discovery of antibiotics by Alexander Fleming.
• As Louis Pasteur said, “In the fields of observation, fortune favors the prepared mind”.

Question 21

What is the relationship between the concentration of cyclins and the progression of the cell cycle?

Answer 21

• Phase transitions coincide with the high levels of each cyclin.
  e.g. a high concentration of cyclin A coordinates the transition from S phase to G2 phase.
• The cyclins must reach a threshold level for the cell cycle to progress.
• The level of CDK (cyclin-dependent kinase) remains constant.
• The mechanism of controlling the cell cycle by cyclins is highly conserved throughout evolution and is the same in yeast, insects, plants and animals.

Question 22

Mutagen

Answer 22

• An agent that causes changes in the DNA sequence (a mutation).
• Mutagens include chemical substances and ionising radiation.

Question 23

Oncogenes

Answer 23

• Some oncogenes code for proteins that control the cell cycle such as cyclins.
• Mutations in oncogenes may result in their loss of function.
• This causes the cells to keep dividing when they should not.
• A well-known oncogene is BRCA 1. Mutations in this gene greatly increase the risk of developing breast and ovarian cancer. The normal function of the BRCA 1 protein is to repair damage to DNA. People with a faulty BRCA 1 gene are less able to repair damage to their DNA; they are more likely to accumulate mutations in the genes that control the cell cycle.
  Primary tumour
  Formed when a cell that has lost the ability to control the cell cycle keeps dividing to produce a mass of cells.- Many primary tumours are benign (slow growing and their cells do not spread).- Some primary tumours are malignant (fast growing and their cells spread to other parts of the body).- Malignant tumours cause cancer.Some oncogenes code for proteins that control the cell cycle such as cyclins.
• Mutations in oncogenes may result in their loss of function.
• This causes the cells to keep dividing when they should not.

A well-known oncogene is BRCA 1. Mutations in this gene greatly increase the risk of developing breast and ovarian cancer. The normal function of the BRCA 1 protein is to repair damage to DNA. People with a faulty BRCA 1 gene are less able to repair damage to their DNA; they are more likely to accumulate mutations in the genes that control the cell cycle.

Question 24

Primary tumour

Answer 24

• Formed when a cell that has lost the ability to control the cell cycle keeps dividing to produce a mass of cells.
• Many primary tumors are benign (slow growing and their cells do not spread).
• Some primary tumors are malignant (fast growing and their cells spread to other parts of the body).
• Malignant tumors cause cancer.

Question 25

Metastasis

Answer 25

The spread of cells from a malignant tumour to a different part of the body.

Question 26

Secondary tumour

Answer 26

• Formed from cells that have spread (metastasized).
• Unless treated, secondary tumors will lead to death.

Question 27

What is the correlation between the number of cigarettes smoked per day and the number of cancer cases?

Answer 27

Positive correlation:
- More cigarettes smoked per day, the higher the number of cancer cases.
- Lung cancer develops most often but other organs can be affected.
- The positive correlation between the number of cigarettes smoked per day and the number of cancer cases, does not prove smoking causes cancer.
- Supporting the positive correlation is evidence demonstrating that many substances in cigarette smoke are carcinogenic.