Meiosis - Cell Division - Grade 12

🔬 MITOSIS – “Making Identical Cells”

🔄 What is Mitosis?

Mitosis is a single cell division that creates two identical daughter cells, each with the same number of chromosomes as the parent cell.
Used for growth, repair, and asexual reproduction.

🧪 Mitosis Process:

• Prophase – Chromosomes condense, spindle fibers form.
• Metaphase – Chromosomes line up in the middle.
• Anaphase – Sister chromatids are pulled apart.
• Telophase – Two new nuclei form.
• Cytokinesis – The cell splits.

✅ Product:

• 2 diploid cells (2n)
• Genetically identical to each other and to the parent cell.

 

🔍 What is Meiosis?

Meiosis is a type of cell division that reduces the number of chromosomes by half. It's how sex cells (gametes) — sperm in males and eggs in females — are made.

Think of it like splitting a full pizza (with 46 slices = chromosomes) into two smaller pizzas with only 23 slices each. This is necessary so that when fertilization happens, the full pizza (46 chromosomes) is restored!

🧬 Why is Meiosis Important?

• It produces gametes for sexual reproduction.
• It ensures that offspring have the correct number of chromosomes.
• It introduces genetic variation, which is important for evolution and survival.

🧩 Chromosome Numbers Recap

• Humans have 46 chromosomes in body cells → these are diploid (2n).
• Gametes (sperm/egg) have 23 chromosomes → these are haploid (n).

🔄 Two Main Stages of Meiosis:

Meiosis happens in two parts:

• Meiosis I – Reduces chromosome number.
• Meiosis II – Similar to mitosis, separates chromatids.

We’ll break them down step by step.

🔬 Meiosis I – Reduction Division

➤ Prophase I

• Chromosomes condense (they become visible under a microscope).
• Homologous chromosomes pair up (these are chromosomes with the same genes from mom and dad).
• Crossing over occurs – they exchange pieces of DNA.

🧠 Why it matters: This creates genetic variation – like shuffling a deck of cards.

➤ Metaphase I

• Homologous pairs line up at the equator of the cell (middle).
• Independent assortment – The way they line up is random, creating different combinations.

➤ Anaphase I

• Homologous chromosomes are pulled to opposite poles (ends of the cell).
• Chromatids stay together.

➤ Telophase I

• A nuclear membrane forms around each set of chromosomes.
• The cell splits by cytokinesis.
• We now have 2 haploid cells with half the chromosome number.

🔁 Meiosis II – Like Mitosis

Now we go into a second division. Remember, we started with one cell and now we have 2 haploid cells.

➤ Prophase II

• New spindle forms.
• Nuclear membranes disappear.

➤ Metaphase II

• Chromosomes line up in the middle (equator) again.

➤ Anaphase II

• Sister chromatids are pulled apart to opposite ends.

➤ Telophase II

• Nuclear membranes reform.
• Each cell splits again.

🎉 We end with 4 genetically different haploid cells!

🌟 Key Concepts to Remember

Term	Meaning
Haploid (n)	Half the chromosome number (23 in humans)
Diploid (2n)	Full set of chromosomes (46 in humans)
Crossing over	Exchange of genetic material between homologous chromosomes
Independent assortment	Random alignment of chromosomes during metaphase I
Gametes	Sex cells (sperm/egg) produced through meiosis

📊 Differences Between Meiosis & Mitosis

Feature	Meiosis	Mitosis
Number of divisions	2	1
Cells formed	4 haploid cells	2 diploid cells
Genetic variation	Yes	No (identical cells)
Occurs in	Sex organs (testes, ovaries)	All body cells
Purpose	Sexual reproduction	Growth & repair

🤓 Tips to Understand Meiosis Better

🔁 Meiosis = 2 divisions
🧩 Crossing over + Independent assortment = Variation
🧬 Haploid gametes + Haploid gametes = Diploid zygote

✅ Summary

• Meiosis is how we make gametes.
• It ensures each gamete has half the chromosomes.
• It adds variation, which helps with survival.
• The process goes: Meiosis I (reduction) → Meiosis II (separation) → end up with 4 different cells.