Genetic diversity

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Meosis

Perfect — let’s go through meiosis step-by-step the way AQA A-level Biology expects you to understand it, clearly and simply so it sticks.

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🔹 Overview

Purpose of meiosis:
To produce gametes (sex cells) with half the chromosome number (haploid, n) from a diploid (2n) parent cell.
So humans: 46 → 23 chromosomes.

Two divisions:

1. Meiosis I – reduction division (diploid → haploid)

2. Meiosis II – like mitosis (sister chromatids separate)

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🧬 Meiosis I — Reduction Division 1⃣ Prophase I

Most important stage!

• Chromosomes condense and become visible.

• Homologous chromosomes (same genes, different alleles) pair up → bivalent formation.

• Crossing over occurs:

  • Non-sister chromatids exchange alleles.

  • Produces new combinations of alleles → genetic variation.

• Nuclear envelope breaks down and spindle fibres form.

✅ Key AQA word: “Crossing over between homologous chromosomes leads to genetic recombination.”

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2⃣ Metaphase I

• Homologous pairs line up side by side along the equator of the cell.

• Independent assortment: orientation of each pair is random.

  • This means maternal and paternal chromosomes can go to either pole → variation.

✅ Key idea: “Random assortment of homologous chromosomes” = independent segregation.

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3⃣ Anaphase I

• Homologous chromosomes are pulled to opposite poles by spindle fibres.

• Centromeres don’t split (sister chromatids stay together).

✅ Chromosome number halves — reduction division happens here.

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4⃣ Telophase I

• Nuclear envelope reforms around each set of chromosomes.

• Cytokinesis (cell divides) → two haploid cells (but each chromosome still has two chromatids).

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🧬 Meiosis II — Like Mitosis

No DNA replication before this division.

5⃣ Prophase II

• New spindle apparatus forms in each haploid cell.

• Chromosomes condense again (if they had decondensed).

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6⃣ Metaphase II

• Individual chromosomes line up along the equator.

• Each chromosome’s sister chromatids attach to spindle fibres from opposite poles.

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7⃣ Anaphase II

• Centromeres divide, and sister chromatids separate.

• Chromatids are pulled to opposite poles.

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8⃣ Telophase II

• Nuclear envelopes reform, and cytokinesis occurs.

• Results in four genetically different haploid cells.

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🌟 Summary Table

Stage	Key events	Chromosome state
Prophase I	Crossing over, bivalents form	2n
Metaphase I	Homologous pairs line up randomly	2n
Anaphase I	Homologous chromosomes separate	n
Telophase I	Two haploid cells form	n
Prophase II	Spindle reforms	n
Metaphase II	Chromosomes align singly	n
Anaphase II	Chromatids separate	n
Telophase II	Four haploid gametes	n

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🧠 AQA Exam Tips

• Always say “alleles” when describing variation (not “genes”).

• Write “crossing over → recombination of alleles”.

• Write “independent segregation → different combinations of maternal and paternal chromosomes.”

• Remember: meiosis → haploid and genetically different cells.

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Would you like me to give you a simple labelled diagram of the stages to visualise it (AQA-style, simplified for revision)?