Week 10 — Meiosis & Sexual Reproduction

Why this matters

A nursing student explaining why two siblings look so different and a breeder choosing which plants or animals to cross both meet the two divisions that make every egg and sperm.

In plain language

Two children of the same parents are never copies of each other, and this week you learn why. You will meet meiosis (the two-step cell division that makes egg and sperm cells, each with half the usual number of chromosomes). You will see how fertilization (an egg and a sperm joining) puts the full number back. And you will find the shuffles inside meiosis that create genetic variation (the differences in DNA among family members and among the members of a species).

• Already covered: Week 1 named evolution by natural selection, Week 3 introduced DNA, and Week 9 taught the chromosome, its sister chromatids, and mitosis.
• This week: diploid and haploid, meiosis I and II in order, crossing over, independent assortment, and the 2ⁿ count.
• How they connect: meiosis divides the same chromosomes as mitosis, but twice, and it supplies the variation natural selection acts on.
• Where we're going next: Week 11 starts the inheritance unit with Mendel's rules, which follow single genes through these gametes.

Focus

How does one cell make four eggs or sperm, each with half the chromosomes and each one different?

By the end of this module, you should be able to:

• [ ] tell diploid from haploid, and homologs from sister chromatids
• [ ] name meiosis I and meiosis II in order and what each separates
• [ ] explain crossing over and independent assortment
• [ ] count gametes with the doubling rule, from 3 pairs to 23

To-do

Plan your week

1. Read Chapter 10 (about 40 minutes), then the lecture and slides.
2. Work the lecture tutorial (60–90 minutes) and the practice set (20 minutes).
3. Run the Biology Lab & Inquiry (2 hours), then the discussion and assignment.
4. Take the quiz.

This week's checkpoint: explain why meiosis I and meiosis II separate different things, and why a human can make 8,388,608 kinds of gametes.

You've got this

Take it in order, and ask your instructor if a step snags.

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