Prep Guide: Cell Cycle and Cell Division

Key Concepts (Recall & Understanding)

Cell Cycle: The sequence of growth and division of a cell, consisting of Interphase (G1, S, G2) and M Phase (Mitosis/Meiosis).
Interphase: The preparatory phase. G1 involves cell growth, S involves DNA replication, and G2 involves final preparations for division.
Mitosis: A type of cell division resulting in two genetically identical diploid (2n) daughter cells. Its purpose is growth, repair, and asexual reproduction.
Stages of Mitosis:
- Prophase: Chromosomes condense and become visible, the nuclear envelope breaks down.
- Metaphase: Chromosomes align at the metaphase plate (equator).
- Anaphase: Sister chromatids separate and move to opposite poles.
- Telophase: Chromosomes decondense, and two new nuclei form.
Meiosis: A two-stage cell division that results in four genetically unique haploid (n) daughter cells (gametes).
Homologous Chromosomes: Pairs of chromosomes (one from each parent) that carry genes for the same traits.
Crossing Over: The exchange of genetic material between homologous chromosomes during Prophase I of meiosis, leading to genetic variation.

Application Corner

A gardener takes a cutting from a rose plant and it grows into a new plant. Which type of cell division is responsible for this growth, and why?
- Answer: Mitosis. It produces genetically identical cells, allowing the cutting to grow into a complete plant that is a clone of the parent.
If a human skin cell, which has 46 chromosomes, undergoes mitosis, how many chromosomes will each daughter cell have?
- Answer: 46 chromosomes. Mitosis maintains the diploid chromosome number.
Why is it essential for meiosis to be a "reduction division" for sexual reproduction?
- Answer: It halves the chromosome number in gametes (to n). This ensures that when two gametes fuse during fertilization, the resulting zygote has the correct diploid (2n) chromosome number, maintaining the species' chromosome count across generations.

Analytical Thinking

Odd One Out: Prophase, Metaphase, Interphase, Anaphase.
- Odd One: Interphase.
- Category: The rest are stages of mitosis.
Compare and Contrast: List two differences between a cell in the G1 phase and a cell in the G2 phase of the cell cycle.
- Answer:
  1. In G1, the cell has a normal amount of DNA. In G2, the cell has double the amount of DNA because replication occurred in the S phase.
  2. G1 is focused on general growth and protein synthesis, while G2 is focused on synthesizing proteins specifically required for mitosis and checking for errors in the replicated DNA.

Cancer is a disease characterized by the uncontrolled division of abnormal cells. This happens when the normal checkpoints that regulate the cell cycle fail. For example, a cell might have damaged DNA, but a faulty checkpoint in the G2 phase fails to stop the cell from proceeding to mitosis. As a result, the cell divides, passing the damaged DNA to its daughter cells. These cells may then continue to divide uncontrollably, forming a tumor. Many cancer treatments, like chemotherapy, work by targeting and killing rapidly dividing cells, which is a hallmark of cancer.

Assertion-Reason Practice

Assertion (A): Meiosis is essential for sexual reproduction. Reason (R): Meiosis introduces genetic variation through crossing over.

(a) Both (A) and (R) are true and (R) is the correct explanation of (A). (b) Both (A) and (R) are true but (R) is not the correct explanation of (A). (c) (A) is true but (R) is false. (d) (A) is false but (R) is true.

Answer: (b) Both (A) and (R) are true but (R) is not the correct explanation of (A). While genetic variation is a crucial outcome of meiosis, the primary reason it's essential for sexual reproduction is that it reduces the chromosome number by half.

HOTS (Higher-Order Thinking Skills) Question

Colchicine is a chemical that inhibits the formation of spindle fibers during cell division. If a cell is treated with colchicine, at which stage would mitosis be arrested? What would be the consequences for the cell in terms of its chromosome number?

Answer: Mitosis would be arrested at Metaphase. The chromosomes would condense and align at the metaphase plate, but without spindle fibers to pull them apart, the sister chromatids cannot separate and move to opposite poles. The cell would be unable to complete division. If the cell cycle were to eventually bypass this block and re-form a nucleus, it would result in a cell with double the normal number of chromosomes (e.g., tetraploid instead of diploid), a condition known as polyploidy.

Specimen Prep - Cell Cycle and Cell Division