Week 13 — DNA Structure & Replication

Why this matters

A nursing student who will one day read a genetic test result and a crime-lab technician who handles DNA samples both rely on the same twisted ladder and the way it copies itself.

In plain language

Every cell in your body carries the same set of instructions, and this week you learn what those instructions are built like and how a cell copies them. You will picture the double helix (the twisted-ladder shape of a DNA molecule) and learn the one pairing rule that holds its rungs together. You will use that rule to fill in a whole DNA sample from a single number. You will meet the experiment that showed how DNA is copied, where every copy keeps one old strand and gains one new one. And you will see why the copying is so reliable that it rarely makes a mistake.

• Already covered: Week 2 gave the hydrogen bond; Week 3 introduced DNA; Week 9 placed DNA copying in the S phase.
• This week: the double helix and its pairing rule, Chargaff's rules, the history, and semiconservative replication.
• How they connect: hydrogen bonds hold the rungs of Week 3's DNA, and the copying is Week 9's S phase at work.
• Where we're going next: Week 14, Gene Expression, reads a gene into RNA and then into protein.

Focus

What is DNA built like, and how does a cell copy it almost perfectly?

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

• [ ] describe the double helix and pair each base with its partner
• [ ] use Chargaff's rules to find all four base percents from one
• [ ] explain semiconservative replication and the Meselson and Stahl test
• [ ] name helicase, DNA polymerase, and DNA ligase in order with their jobs

To-do

Plan your week

1. Read Chapter 13 (about 45 minutes), then the lecture and slides.
2. Work the lecture tutorial (60–90 minutes) and the practice set (about 20 minutes).
3. Run the Biology Lab & Inquiry (about 2 hours), then the discussion and the assignment.
4. Take the quiz.

This week's checkpoint: given one base percent, fill in the other three with Chargaff's rules.

You've got this

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

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