Meiosis: reduction division and variation
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Question
What is meiosis?
Answer
The division that makes **gametes**: one **diploid (2n)** cell divides **twice** into **four haploid (n)** cells that are genetically different.
Question
Why is meiosis called a reduction division?
Answer
Because it **halves** the chromosome number — a **diploid (2n)** cell becomes **haploid (n)** gametes.
Question
Define a diploid cell.
Answer
A cell with **two copies of each chromosome** (one set from each parent); in humans, 46 chromosomes = 23 pairs.
Question
Define a haploid cell.
Answer
A cell with **one copy of each chromosome** — half the diploid number; in humans, 23 chromosomes. Gametes are haploid.
Question
What are homologous chromosomes?
Answer
A **matching pair** of chromosomes — same size, carrying the same genes — one inherited from each parent.
Question
What is separated during meiosis I?
Answer
The **homologous chromosomes** (the matching pairs) — this is where the chromosome number **halves**.
Question
What is separated during meiosis II?
Answer
The **sister chromatids** — finishing the division to give **four** haploid cells.
Question
What is crossing over, and when does it happen?
Answer
In **prophase I**, homologous chromosomes **pair up and swap matching sections**, mixing the alleles on each chromosome.
Question
What is independent assortment, and when does it happen?
Answer
In **metaphase I**, each homologous **pair** is sorted to the poles **at random**, shuffling maternal and paternal chromosomes.
Question
Which two processes make meiosis generate variation?
Answer
**Crossing over** (prophase I) and **independent assortment** (metaphase I).
Question
Why must gametes be haploid?
Answer
So that **fertilisation** (two gametes joining) restores the **diploid** number without doubling it each generation.
Question
Where does meiosis occur in a flowering plant?
Answer
In the **anthers** (making pollen / male gametes) and the **ovules** (making the female gametes / egg cells).
Question
How many cells does one meiosis produce, and how do they compare?
Answer
**Four** haploid cells, all **genetically different** from each other and from the parent cell.
Question
How does meiosis differ from mitosis?
Answer
Mitosis = one division → **two identical** diploid cells; meiosis = two divisions → **four different** haploid gametes.
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Topic 4.4 hub
Cell and nuclear division
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