Introduction:
Every living organism starts as a single cell. This cell multiplies through cell division, forming millions of cells that make up tissues and organs. The ability of cells to divide is essential for growth, repair, and reproduction. The process is governed by the Cell Cycle, a sequence of events ensuring accurate duplication of DNA and equal distribution of chromosomes.
Phases of the Cell Cycle: The cell cycle is the complete series of events that a cell goes through as it grows and divides. It has two main parts:
Interphase – the preparation stage M Phase (Mitotic phase) – the actual division stage
1. Interphase: Even though it’s sometimes called a “resting phase,” interphase is actually the busiest part of the cycle. The cell is actively growing, copying its DNA, and preparing for division.
2. Interphase has several steps:
G1 Phase (Gap 1):
The cell grows larger, makes proteins, and duplicates its organelles (like mitochondria and ribosomes). This is like a cell “childhood,” where it is getting ready for the future.
S Phase (Synthesis):
This is when the DNA is copied. Each chromosome duplicates so that the cell has two complete sets of genetic material. The number of chromosomes stays the same (2n), but the amount of DNA doubles (2C → 4C).
G2 Phase (Gap 2):
The cell makes the final preparations for division. Proteins needed for spindle fibers (which pull chromosomes apart later) are made, and the cell continues to grow.
G0 Phase (Quiescent Stage):
Sometimes, cells leave the cycle and stop dividing. They still stay alive and active but don’t prepare for further division. For example, nerve cells and heart cells usually stay in this stage permanently.
2. M Phase (Mitosis) – Equational Division
Mitosis is a type of equational cell division where daughter cells receive the same chromosome number as the parent (2n → 2n).
Stages of Mitosis:
1. Prophase
The chromosomes, which were thin and invisible before, now coil up and become short and thick so they can be seen clearly under a microscope. The covering around the nucleus (nuclear envelope) breaks down, and small structures called spindle fibers begin to form, stretching between the two ends of the cell.
2. Metaphase
The chromosomes line up neatly in the middle of the cell, like books arranged on a shelf. Each chromosome is attached to spindle fibers at a special point called the kinetochore. This ensures that they can be pulled apart accurately.
3. Anaphase
At this stage, the “glue” at the center of each chromosome (the centromere) splits. This allows the two identical halves of each chromosome, called sister chromatids, to separate. They are then pulled toward opposite ends of the cell by the spindle fibers, ensuring that each side gets an identical set.
4. Telophase
Once the chromatids reach the opposite ends of the cell, they begin to uncoil and become thread-like again (like they were before mitosis started). A new nuclear envelope forms around each group, so now there are two nuclei in the same cell.
5. Cytokinesis
Finally, the cell itself splits into two. In animal cells, this happens by the cell membrane pinching inward until the cell divides. In plant cells, a new cell wall forms in the middle. As a result, two genetically identical daughter cells are produced.
Significance of Mitosis
1. Growth of multicellular organisms.
2. Replacement of old and damaged cells.
3. Asexual reproduction.
4. Genetic stability across generations.
Meiosis Cell Division – Reduction Division
Meiosis is a special kind of cell division in which chromosome number is reduced to half (2n → n). It occurs during gamete formation.
Meiosis I (Reduction Division)
Prophase I:
Prophase I is the longest and most complex stage of meiosis. It ensures pairing of homologous chromosomes and genetic recombination. It is divided into five substages:
1. Leptotene ("thin threads"): Chromosomes become visible as long, thin threads under a microscope. Chromatin begins to condense. Each chromosome has already duplicated during S phase but chromatids are not yet distinctly visible. The chromosomes start attaching to the nuclear membrane at specific points.
2. Zygotene ("paired threads"): Homologous chromosomes (one from each parent) start pairing process → synapsis. This pairing is precise and gene-to-gene. A protein structure called the synaptonemal complex forms between homologues. Paired homologous chromosomes are called bivalents.
3. Pachytene ("thick threads"):
Chromosomes condense further → become short and thick. Each bivalent is made of four chromatids (two from each homolog), clearly visible as a tetrad. Crossing over occurs during Pachytene stage where exchange of genetic material between non-sister chromatids of homologous chromosomes.
4. Diplotene ("double threads"): Synaptonemal complex dissolves → homologous chromosomes begin to separate. Chromatids remain attached at specific points of crossing over called chiasmata (X-shaped structures). Oocytes of some animals remain arrested in diplotene for months/years until ovulation.
5. Diakinesis (terminalization stage): Final condensation of chromosomes → they are thick and fully visible. Chiasmata move towards chromosome ends (terminalization). Nucleolus disappears. Nuclear envelope breaks down. Spindle fibers form, preparing for Metaphase I.
Metaphase I
At this stage, the paired chromosomes (one from the mother and one from the father) line up together in the middle of the cell, along the equator. Each pair is attached to spindle fibers that come from opposite poles of the cell. The way the pairs line up is completely random, which is important because it helps create genetic variation in the next generation.
Anaphase I
Now the paired chromosomes are pulled apart. Each homologous chromosome from the pair moves to opposite ends of the cell. Unlike mitosis, the sister chromatids of each chromosome stay together at this stage; they are not separated yet. By doing this, the cell reduces the chromosome number by half — preparing to form haploid cells.
Telophase I
Finally, the chromosomes reach the poles of the cell. In some organisms, the nuclear membrane briefly reforms around them. The cell then divides into two through cytokinesis. Each new cell now has only half the original number of chromosomes, but each chromosome is still made up of two sister chromatids. These cells are haploid and will enter the second meiotic division (Meiosis II).
Meiosis II (Equational Division)
What happens in Meiosis II?
Prophase II : Chromosomes condense again. Nuclear envelope (if reformed in Telophase I) breaks down. Spindle fibers start forming.
Metaphase II: Chromosomes line up at the middle of the cell (equator), just like in mitosis. Each chromatid’s centromere attaches to spindle fibers from opposite poles.
Anaphase II: The centromeres finally split. Sister chromatids (which were still joined in Meiosis I) are pulled apart to opposite poles. Now each chromatid becomes an individual chromosome.
Telophase II and Cytokinesis: Chromosomes reach the poles and start de-condensing. Nuclear envelopes form around each set of chromosomes. Cytoplasm divides, resulting in four haploid cells in total (from the original single parent cell).
Significance of Meiosis
1. Maintains constant chromosome number in species.
2. Introduces genetic variation (basis of evolution).
3. Produces gametes for sexual reproduction.
Difference Between Mitosis and Meiosis
Feature | Mitosis (Equational Division) | Meiosis (Reduction Division) |
Occurs in | Somatic cells | Reproductive cells |
No. of divisions | One | Two |
Daughter cells | Two diploid | Four haploid |
Crossing over | Absent | Present in Prophase I |
Genetic variability | No | Yes |
Feature | Mitosis (Equational Division) | Meiosis (Reduction Division) |
FAQs on Cell Cycle and Cell Division
Q1. Why is mitosis important?
A1. Mitosis helps in growth, repair, and replacement of cells while maintaining genetic stability.
Q2. Why is meiosis important in sexual reproduction?
A2. Meiosis produces haploid gametes and increases genetic variability.
Q3. In which phase of cell cycle does DNA replication occur?
A3. DNA replication takes place during the S phase of interphase.
Q4. What is the difference between cytokinesis in plant and animal cells?
A4. Plant cells: Cell plate formation; Animal cells: Cleavage furrow formation.
Q5. What is crossing over?
A5. Exchange of genetic material between homologous chromosomes during Pachytene of Prophase I.
MCQs on Cell Cycle and Cell Division
1. DNA replication occurs in:
a) G1 phase
b) S phase ✅
c) G2 phase
d) M phase
2. The stage best for studying chromosome morphology:
a) Prophase
b) Metaphase ✅
c) Anaphase
d) Telophase
3. Crossing-over occurs during:
a) Metaphase I
b) Prophase I (Pachytene) ✅
c) Anaphase I
d) Telophase I
4. In onion root tip cells (2n=16), number of chromosomes after S phase is:
a) 8
b) 16 ✅
c) 32
d) 4
5. Meiosis produces:
a) 2 diploid cells
b) 4 haploid cells ✅
c) 8 diploid cells
d) None of these
Worksheet for Students
A. Fill in the blanks
1. The phase where DNA replication occurs is called ________.
2. Crossing over occurs during ________ of meiosis.
3. The quiescent stage of the cell cycle is ________.
4. In animals, cytokinesis occurs by ________ formation.
5. Daughter cells of mitosis are ________ (diploid/haploid).
B. Short Answer Questions
1. Write three differences between mitosis and meiosis.
2. Explain the significance of G0 phase.
3. Why is meiosis important for genetic variation?
4. Describe cytokinesis in plant cells.
5. What is the importance of crossing over?
C. Diagram Work
1. Draw the cell cycle stages (G1, S, G2, M).
2. Neat diagrams of mitosis and meiosis I & II.
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