Nuclear Division Prevents Anucleate Cells
- Before a cell divides, its nucleus must divide first to ensure each daughter cell receives a complete set of genetic instructions.
- Without nuclear division, one daughter cell would lack a nucleus (anucleate) and be unable to function.
- Red blood cells are anucleate and survive only about 120 days.
- This is because they cannot produce proteins or repair themselves.
Mitosis Maintains the Chromosome Number and Genome
Mitosis
Mitosis is a type of cell division in which a single parent cell divides to produce two genetically identical daughter cells, each containing the same number of chromosomes as the parent cell.
- Mitosis is crucial for:
- Growth: Multicellular organisms grow by adding new cells through mitosis.
- Repair: Damaged tissues are repaired by replacing lost or injured cells.
- Asexual Reproduction: Eukaryotic organisms such as some protists and plants reproduce by mitosis. (Note: Bacteria use binary fission, not mitosis.)
How Mitosis Works
The diagram shows the process using one homologous pair (pink and blue chromosomes):
- DNA copied: Before mitosis begins, DNA replicates. Each chromosome now consists of two identical sister chromatids joined at the centromere.
- Mitosis: The sister chromatids separate and move to opposite ends of the cell.
- Result: Two daughter cells, each containing one pink chromosome and one blue chromosome, identical to the parent cell.
Mitosis ensures continuity
Why Mitosis Matters
- Mitosis ensures that every cell in a multicellular organism has the same genetic information.
- This uniformity is vital for maintaining the organism's structure and function.
In plants, mitosis in the apical meristems allows roots and shoots to grow, while in animals, it replaces worn-out skin cells.
Meiosis Halves the Chromosome Number and Generates Genetic Diversity
Meiosis
Meiosis is a type of cell division that reduces the chromosome number by half, producing four genetically distinct haploid cells. This process occurs in gamete-producing cells and is essential for sexual reproduction.
- Meiosis is crucial for:
- Sexual Reproduction
- Genetic Diversity through crossing over and independent assortment.
Crossing over and independent assortment will be further explored in later sections.
How Meiosis Works
The diagram shows two consecutive divisions.
- DNA copied: DNA replicates before meiosis begins. Each chromosome consists of two sister chromatids.
- Meiosis I: Homologous chromosomes (pink pair and blue pair) separate. Each daughter cell receives one chromosome from each pair, reducing the chromosome number by half.
- Meiosis II: Sister chromatids separate, similar to mitosis.
- Result: Four haploid cells, each with either a pink or blue chromosome. These cells are genetically distinct.
Why Meiosis Matters
- Prevents Chromosome Doubling: Without meiosis, each generation would have twice the chromosomes of the previous one.
- Generates Variation: Genetic diversity is crucial for evolution and adaptation.
- During crossing over, homologous chromosomes exchange genetic material, creating new combinations of alleles.
- This is why siblings (except identical twins) are genetically unique.
Comparing Mitosis and Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of Divisions | One | Two |
| Number of Daughter Cells | Two | Four |
| Genetic Composition | Identical to parent cell | Genetically diverse |
| Chromosome Number | Maintained (diploid) | Halved (haploid) |
| Role | Growth, repair, asexual reproduction | Gamete production, genetic diversity |
Think of mitosis as a photocopier producing identical copies, while meiosis is like a shuffle that creates unique combinations.
Mitosis and Meiosis are Equally Important
- Mitosis: Ensures stability and continuity within an organism, allowing it to grow and function.
- Meiosis: Introduces diversity, enabling populations to adapt and evolve over time.
- Can you explain the key differences between mitosis and meiosis?
- Try summarizing them in your own words.
