Cell Cycle & Meiosis Review
1. Identify and briefly explain the five phases of mitosis.
Prophase: Nuclear envelope breaks down, cytoskeleton disassembles, chromosomes condense to become visible, microtubules (formally part of the cytoskeleton) begin to form on centrosomes, creating mitotic spindle.
Prometaphase: Proteins form around centromere to create kinetochores (two per chromosome), mitotic spindle extends and some connect to kinetochores.
Metaphase: Spindles push/pull all chromosomes to the middle of the cell, form metaphase plate.
Anaphase: Sister chromatids are torn apart at the centromere by shortening spindles.
Telophase: Nuclear envelope reforms on either side of cell, chromosomes unwind, mitotic spindle disassembles, cytoskeleton begins to reform.
2. If microtubules were unable to attach to reform after breaking down the cytoskeleton, what effect would this have on mitosis?
The cell would be unable to create the mitotic spindles, hence the chromosomes will be unable to be moved or torn apart.
3. What does 2n or n signify?
2n means the cell is diploid (has two versions of each chromosome, one from each parent), n means the cell is haploid (one version of each chromosome). Cell is likely a gamete.
4. Compare the chromosome number in after mitosis, compared to meiosis.
Chromosome number is still diploid after mitosis (46 in the case of humans), haploid after meiosis (23 in humans).
5. What is a tetrad? How does this make mitosis different than meiosis?
A tetrad is two homologous chromosomes paired up. Synapsis (crossing over) can take place now. This happens in meiosis, but not mitosis.
6. Label which phase of meiosis each cell below should be in, and explain why:
A B C
A
– Anaphase II. Sister chromatids are separating, but each chromatid is unique.
B – Metaphase I – Metaphase plate is made of tetrads.
C – Telophase I – Nuclei are reforming, containing full chromosomes (sister chromatids have not separated).

7. Using a diagram, explain why it is worse to have non-disjunction during meiosis I than meiosis II.
If non-disjunction happens in meiosis I, all four gametes will be aneuploidic (have the wrong number of chromosomes). If it takes place in meiosis II, only 2 of the 4 will be affected.
8. Explain the role of checkpoints in our cells.
Checkpoints ( the term for the collective group of cyclins) ensure that a cell is properly prepared before entering a new phase of the cell cycle.
9. Why don’t tumour cells just die after a certain amount of divisions?
They have an enzyme which is designed to extend their DNA, recovering the amount that was lost during S phase.
10. Why would each of the chromosomes in your sex cells have a piece of your mother’s DNA and your father’s DNA?
Because of the crossing over process that occurs during prophase I.
11. What role do kinetochores play? How do they work?
Kinetochores form on both sides of a centromere (so there are two per chromosome).
They are a group of proteins that act as an attachment site for the growing microtubules of the mitotic spindle.
12. What is the difference between homologous chromosomes and sister chromatids?
Homologous chromosomes are the two different versions of the same chromosome.
Sister chromatids are exact copies of the same chromosome.
13. Why is chemotherapy so hard on the patient?
Antineoplastic drugs target any rapidly dividing cells, not just cancer cells. This includes cells such as those lining hair follicles, epithelial cells, bone marrow (for red and white blood cells), and sperm cell precursors in men.
14. Describe when radiation therapy should be used.
Although it can be desirable to physically remove a tumour as a cancer treatment, the tumours can often be found in areas which are hard to access safely, such as deep within the brain. In these cases, radiation can be a viable alternative.
15. What is the difference between density-dependant inhibition and anchorage dependence?
DDI is a cell control mechanism where healthy cells will not divide while surrounded by other cells. AD is another control mechanism where cells will only divide when attached to a solid surface, known as a substratum.