10.1.1 Define Cell Cycle
The cell cycle is the sequence of stages a cell undergoes from its formation to its division into two daughter cells. It includes growth, DNA replication, and division, ensuring cell reproduction and tissue growth or repair. The cycle consists of interphase and cell division (mitosis or meiosis).
10.1.2 Differentiate Between the Major Phases of Cell Cycle: Interphase and Division
Interphase: The preparatory phase where the cell grows, replicates its DNA, and prepares for division. It is the longest phase, typically occupying 90% of the cell cycle. The cell is metabolically active, performing its normal functions.
Division: The phase where the cell divides its duplicated DNA and cytoplasm to form two daughter cells. It includes:
- Mitosis: Division of the nucleus into two identical nuclei (for somatic cells).
- Cytokinesis: Division of the cytoplasm, completing cell division.
In some cases, division involves meiosis (for gamete formation), which reduces chromosome number by half.
Key Differences:
- Duration: Interphase is longer; division is relatively short.
- Activity: Interphase involves growth and DNA replication; division involves physical separation of cellular components.
- Outcome: Interphase prepares the cell; division produces new cells.
10.1.3 Describe Sub-Phases of the Interphase of Cell Cycle
Interphase is divided into three sub-phases:
G1 Phase (First Gap):
- The cell grows, increases in size, and synthesizes proteins and organelles.
- The cell performs its normal functions and prepares for DNA replication.
- A checkpoint ensures the cell is ready to proceed to DNA synthesis.
S Phase (Synthesis):
- DNA replication occurs, resulting in the duplication of the cell’s genetic material.
- Each chromosome is replicated to form two identical sister chromatids joined at the centromere.
G2 Phase (Second Gap):
- The cell continues to grow and produces proteins needed for division.
- The cell checks for DNA replication errors and ensures all components are ready for mitosis.
- A checkpoint confirms readiness for division.
10.1.4 Explain the Importance of Each Sub-Phase of Interphase
G1 Phase:
Allows the cell to grow and synthesize necessary components (e.g., organelles, proteins) for DNA replication and division. The G1 checkpoint ensures the cell is healthy and has sufficient resources, preventing defective cells from progressing.
S Phase:
Ensures accurate DNA replication, doubling the genetic material so each daughter cell receives an identical set of chromosomes. Errors here could lead to mutations or cell dysfunction.
G2 Phase:
Provides time to repair DNA replication errors and synthesize proteins for mitosis (e.g., microtubules for spindle fibers). The G2 checkpoint ensures DNA integrity and readiness for division, preventing defective cell division.
10.1.5 Compare Mitosis and Meiosis
| FEATURE | MITOSIS | MEIOSIS |
| Purpose | Produces two identical daughter cells for growth, repair, or asexual reproduction. | Produces four non-identical gametes (sperm/egg) for sexual reproduction. |
| Occurrence | In somatic (body) cells. | In reproductive cells (germ cells). |
| Number of Divisions | One division (mitosis + cytokinesis). | Two divisions (meiosis I and II). |
| Chromosome Number | Maintains diploid number (2n). | Reduces to haploid number (n). |
| Genetic Variation | Daughter cells are genetically identical to the parent. | Daughter cells are genetically diverse due to crossing over and random assortment. |
| Phases | Prophase, Metaphase, Anaphase, Telophase. | Prophase I, Metaphase I, Anaphase I, Telophase I; then Meiosis II (similar to mitosis). |
| Crossing Over | Does not occur. | Occurs in Prophase I, increasing genetic diversity. |
| Examples | Skin cell division for repair. | Sperm/egg formation in testes/ovaries. |
10.1.6 Differentiate Between Chromosome and Chromatid, and Haploid and Diploid Cells with Examples
Chromosome vs. Chromatid:
Chromosome: A single molecule of DNA, condensed with proteins (histones), carrying genetic information. It exists as a single structure in G1 phase or as a duplicated structure (two chromatids) after S phase. A human cell has 46 chromosomes before DNA replication.
Chromatid: One of the two identical copies of a chromosome formed after DNA replication, joined at the centromere. After mitosis, each chromatid becomes a separate chromosome in the daughter cells. During metaphase of mitosis, a chromosome consists of two sister chromatids.
Key Difference: A chromosome is the whole structure; a chromatid is one half of a duplicated chromosome.
Haploid vs. Diploid Cells:
Haploid Cells: Contain one set of chromosomes (n). In humans, n = 23. These cells are typically gametes. Sperm and egg cells in humans (each with 23 chromosomes).
Diploid Cells: Contain two sets of chromosomes (2n). In humans, 2n = 46. These are somatic cells. Human skin cells or liver cells (each with 46 chromosomes).
Key Difference: Haploid cells have half the chromosome number of diploid cells, enabling genetic combination during fertilization.