Question

1. What is the function of a centromere?
2. Draw the correct order of the steps of mitosis, label them, and state what occurs in each (this should not be a picture from the internet)?
3. Why is apoptosis required to balance mitosis?
4. What is binary fission?
5. What are cell cycle check-points?
6. What are the difference between proto-oncogenes and tumor suppressor genes? How to these impact the cell cycle?
7. What are some life styles choices that can increase your risk of cancer? How about decrease?

Answer 1

1. Function of Centromere :

• Major function of centromere is to join the two copies of a replicated chromosome called sister chromatids. It must be joined together until they are pulled physically into two future daughter cells, ensuring each daughter cell gets exactly one copy of each of chromosome.
• Cohesins are the proteins that is evenly distributed along the chromatids so they are stuck together along whole lengths.
• Another function of the centromere is to attach the microtubules into mitotic spindle. The centromere directs formation of the kinetochore which gets attached to microtubules in the mitotic spindle.

2. Correct order of the steps of mitosis :

1. Prophase is the first step in mitosis. here :

• chromosome condenses and becomes visible
• From centromere, spindle fibers emerges
• Breakdown of nuclear envelop
• Nucleolus disapprears

2. Pro metaphase:

• Condensation of chromosomes
• Appearance of kinetochore in centromere
• Attachment of mitotic spindle microtubules to kinetochore
• Centromere moves to opposite poles.

3. Metaphase:

• Centromeres at opposite poles and full development of mitotic spindle.
• Lining up of chromosomes at the metaphase plate.
• Each sister chromatids attaches to the spindle fiber which is originating from opposite poles.

4. Anaphase:

• Cohesin proteins which are binding to sister chromatids together breaks down.
• Sister chromatids are pulled to opposite poles (called chromosomes)
• Elongation of the cell due to the lengthening of non-kinetochore spindle fibers

5. Telophase:

• Chromosome decondense once reaching the opposite poles
• Nuclear envelope material surrounds each set of chromosomes
• Mitotic spindle breaks down

6. Cytokinesis:

• Separation into two daughter cells.

3. Why is apoptosis required to balance mitosis:

• In all the multicellular organisms, the cell proliferation as well as cell death has to be regulated in order to maintain the tissue homeostasis.
• Apoptosis also called the programmed cell death or cellular suicide, is a cellular self-destruction. Though it is involved in the cell death, apoptosis plays protective role in our bodies.
• Mitosis is involved in cell division, and the uncontrolled mitosis results in tumor growth which has to be regulated thus comes the role of apoptosis to balance mitosis.
• The tumor cell growth results in mutations which then undergoes unscheduled proliferation, chromosomal instability and genomic instability.

4.  What is binary fission?

• Binary Fission, also called asexual reproduction, is the separation of the body into two new bodies. Here the organisms reproduce by duplication of the genetic material( DNA) which then divides into two parts by a process called cytokinesis where each of the new organisms recieive one copy of DNA.

5.  What are cell cycle check-points?

Checkpoint is a point in the cell cycle when the cell examines the internal and the external cues and decides whether the cell has to move forward or not with the cell division.

Three important checkpoints are:

1. G1 checkpoint at the G1/S transition
2. G2 checkpoint at the G2/M transition point
3. Spindle checkpoint, point at transition from metaphase to anaphase

G1 checkpoint at the G1/S transition:

• The major point where the cells choose to divide or not. Once the cells passes through the G1 checkpoint and enters the S phase, the cells become committed to division which is a irreversible process, barring to problems such as DNA damage or the replication errors. Cells crossing through the G1 point will continue with the cell cycle.At the G1 checkpoint, a cell checks whether internal and external conditions are right for division

G2 checkpoint at the G2/M transition point:

• Before M phase, the cells undergo another checkpoint called G2 checkpoint to make sure that the cell division is occurring smoothly and checks whether healthy daughter cells are produced completely, whether the DNA is undamaged etc.
• Any errors or damages noticed, the cell division pauses at the G2 checkpoint and allows for cellular repair to occur. If any defects are found with the DNA, the cell cycle is halted and cells attempt to complete the DNA or repair the damaged DNA.

Spindle checkpoint, point at transition from metaphase to anaphase:

• The next is M checkpoint also known as the Spindle checkpoint, where the cell examines whether all the sister chromatids are attached correctly to the spindle microtubules, this is because separation of the sister chromatids during anaphase is an irreversible step and the cell cycle will not continue further until all the chromosomes are attached to atleast two spindle fibers from the opposite poles of the cell.

6. What are the difference between proto-oncogenes and tumor suppressor genes? ​​​​​​​

One major difference between oncogenes and the tumor suppressor genes is that the oncogens is due to the activation or the turning on of the proto-oncogenes, but tumor suppressor genes results in causing cancer when they are in turned off or in the inactivated form.

Genes which code for the positive cell cycle regulators are known as the Proto-oncoogenes which when mutated becomes oncogenes causing the cells to become cancerous.

Similarly, there are many negative cell cycle regulatory proteins. Tumor suppressor genes which codes for these negative regulator proteins are the type when activated prevents the cells from uncontrolled cell division.

7. What are some life styles choices that can increase your risk of cancer?

• Eat healthy and maintain a balanced diet
• Exercise regularly
• Avoid the usage of tobacco
• Limit your alcohol consumption
• Get cancer screeing test at regular period
• Maintain a healthy body weight.