Why is DNA replication semiconservative?
The question: DNA replication is considered semiconservative because: A) only the non-parent strand acts as a template, B) both strands are made de novo, C) one strand in each new DNA molecule is parental and the other is new, D) the daughter strands are fragmented and dispersive.
C. One strand in each newly formed DNA molecule is parental and the other is newly synthesized.
Why DNA replication is called semiconservative: each new DNA molecule keeps one parental strand and gets one new strand. Each option explained, a two-round strand diagram, and the Meselson and Stahl experiment.
Step-by-step transcript
The question
When a cell copies its DNA, what happens to the old strands? That one idea answers this question.
The options
Here's the question. Four options. Let's work out what really happens first, then come back to them. Option A is wrong. The parent strands are the templates, and both of them are used. B is wrong too. If both strands were made from scratch, nothing would be conserved at all. D describes the dispersive model, with old and new pieces mixed. Experiments ruled that out. So C is right. Each new molecule keeps one parental strand and gets one new strand.
What we know and the plan
We know DNA has two strands, and the bases pair up: A with T, and G with C. We need to find what each new DNA molecule is made of. Semi means half. So we'll follow the old strands through one round of copying, and see what half is kept.
Follow the strands
Start with the parent molecule. Both of its strands are old. Helicase unzips it, and the two strands come apart. Now each old strand is a template, a pattern to copy from. DNA polymerase reads each template, and builds one new partner strand on it, matching A to T and G to C. So each daughter molecule is one old strand, plus one new strand. Half old, half new. That's semiconservative.
Two rounds of copying
Here it is in blue and red. Blue is old, red is new. After one round, both molecules are hybrids: one blue strand, one red. After a second round, the old blue strands are still there, each paired with a new red one.
How we know
In 1958, Meselson and Stahl tested this using heavy and light nitrogen. After one round they saw a single band of medium weight, exactly the hybrid that semiconservative copying predicts.
The answer
So the answer is C. Every new DNA molecule has one parental strand and one newly made strand.
Checking the answer
Quick check. After round two we should see two hybrid molecules, and two all new ones. That matches the picture.
The common mistake
Don't mix it up with the dispersive model, where every strand is a patchwork of old and new pieces. The data ruled that out.
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