Cells divide to produce new cells. This allows organisms to grow, replace damaged cells and reproduce. Before division, a cell must grow and copy its DNA so that genetic material can be passed to the new cells.
Table of Contents
What Is the Cell Cycle?
The cell cycle is the sequence of events in which a cell grows, duplicates its DNA and divides. It has two broad parts:
- Interphase: The cell grows and prepares for division.
- M phase: The nucleus divides, usually followed by division of the cytoplasm.
Phases of Interphase
| Phase | Main event |
|---|---|
| G₁ phase | Cell grows and carries out normal activities |
| S phase | DNA is replicated |
| G₂ phase | Cell continues growing and prepares for division |
Important: During S phase, the amount of DNA doubles, but the number of chromosomes does not double. Each replicated chromosome has two sister chromatids joined at a centromere.
Some cells leave the active cycle and enter G₀, a quiescent stage. They remain metabolically active but are not dividing at that time.
Mitosis
Mitosis divides one nucleus into two nuclei with the same chromosome number. It is followed by cytokinesis, which divides the cytoplasm. The four stages of nuclear division are prophase, metaphase, anaphase and telophase.
Prophase
Chromatin condenses into visible chromosomes. Each replicated chromosome consists of two sister chromatids. The spindle begins to form, and the nuclear envelope breaks down by the end of this stage.
Metaphase
Chromosomes line up at the cell’s equator, called the metaphase plate. Spindle fibres attach to structures called kinetochores on the chromosomes.
Anaphase
The centromeres split. Sister chromatids separate and move towards opposite poles. Once separated, they are called daughter chromosomes.
Telophase
Chromosomes reach the poles and begin to decondense. A nuclear envelope forms around each set, producing two daughter nuclei.
Cytokinesis
Cytokinesis separates the cytoplasm to complete cell division:
- In a typical animal cell, a cleavage furrow pinches the cell inward.
- In a typical plant cell, a cell plate forms near the centre and grows outward.
Result of mitosis: One parent cell produces two daughter cells with the same chromosome number as the parent cell.
Why Is Mitosis Important?
Mitosis supports growth, repair and replacement of cells. It also allows growth in plant meristems. By distributing replicated chromosomes to daughter cells, it usually maintains the chromosome number from one cell generation to the next.
Meiosis
Meiosis is a special division associated with sexual reproduction. It involves one round of DNA replication followed by two divisions, called meiosis I and meiosis II.
Starting with a diploid cell, meiosis normally produces four haploid cells. Their chromosome number is half that of the starting cell.
Meiosis I: Separation of Homologous Chromosomes
In prophase I, homologous chromosomes pair. This pairing is called synapsis, and each paired set is called a bivalent. Exchange of genetic material between non-sister chromatids, called crossing over, can occur.
Prophase I has five stages:
| Stage | Key event |
|---|---|
| Leptotene | Chromosomes become visible |
| Zygotene | Homologous chromosomes begin pairing |
| Pachytene | Crossing over occurs |
| Diplotene | Homologues begin separating but remain joined at chiasmata |
| Diakinesis | Chromosomes condense further before metaphase I |
In metaphase I, bivalents line up at the equator. In anaphase I, homologous chromosomes move to opposite poles while sister chromatids stay together. Telophase I and cytokinesis may then produce two cells.
Remember: The chromosome number is reduced during meiosis I because homologous chromosomes separate.
Meiosis II: Separation of Sister Chromatids
There is no new round of DNA replication between meiosis I and meiosis II. During meiosis II, chromosomes line up again and their sister chromatids separate. Cytokinesis normally leaves four haploid cells.
Mitosis and Meiosis: Main Differences
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Usual number of daughter cells | Two | Four |
| Chromosome number | Maintained | Halved |
| Homologous pairing and crossing over | Usually absent | Occur during prophase I |
| Main role | Growth, repair and cell replacement | Formation of haploid cells for sexual reproduction |
Why Is Meiosis Important?
Meiosis halves the chromosome number so that fertilisation can restore the diploid number. Crossing over and the separation of homologous chromosomes also contribute to genetic variation among offspring.
Check Your Understanding
- In which phase is DNA replicated?
- Does chromosome number double during S phase?
- What separates during anaphase of mitosis?
- What separates during anaphase I of meiosis?
- At which stage of prophase I does crossing over occur?
Answers: (1) S phase. (2) No; DNA amount doubles, but chromosome number stays the same. (3) Sister chromatids. (4) Homologous chromosomes. (5) Pachytene.
For all chapters, visit the CBSE Class 11 Biology – Complete Study Guide.
Frequently Asked Questions
What is the cell cycle?
It is the ordered sequence of cell growth, DNA replication and cell division.
What happens during interphase?
The cell grows, performs normal functions and duplicates its DNA before division.
What do G1, S and G2 stand for?
They are the first growth phase, DNA synthesis phase and second growth phase of interphase.
What happens in the S phase?
DNA is replicated so that each chromosome has two sister chromatids.
What is mitosis?
It is nuclear division that generally produces two genetically similar nuclei with the same chromosome number.
Name the main stages of mitosis.
Prophase, metaphase, anaphase and telophase.
What occurs during metaphase?
Chromosomes align at the cell’s equatorial plane.
What separates during mitotic anaphase?
Sister chromatids separate and move towards opposite poles.
What is cytokinesis?
It is division of the cytoplasm to form separate daughter cells.
Why is mitosis important?
It supports growth, repair and replacement while maintaining chromosome number in the daughter cells.
What is meiosis?
It is a two-division process that produces haploid cells from a diploid cell.
Where does crossing over occur?
It occurs between non-sister chromatids of homologous chromosomes during prophase I of meiosis.
What separates during anaphase I?
Homologous chromosomes separate, while sister chromatids remain together.
What separates during anaphase II?
Sister chromatids separate.
Why is meiosis important?
It forms haploid gametes or spores and generates genetic variation.