Replication of DNA and RNA
Copying DNA
- Before a cell divides, all its DNA must be copied — replication.
- Each new molecule keeps one old strand and gains one new one.
- We call this semi-conservative replication.
Practice
DNA replication is "semi-conservative" because each new molecule has:
Each daughter molecule conserves one parental strand and pairs it with one new strand.
The steps of replication
- the double helix unwinds and the hydrogen bonds break, so the two strands separate.
- each old strand acts as a template; free nucleotides pair with the exposed bases (complementary base pairing).
- the enzyme DNA polymerase joins the new nucleotides into a strand — it can only add them in the 5′→3′ direction.
Practice
During replication, free nucleotides line up because they:
Each template base attracts its complement (A–T, C–G), so the new strand is an exact partner.
Practice
DNA polymerase adds new nucleotides:
DNA polymerase can extend a strand only in the 5′→3′ direction, which is why the two strands are built differently.
Leading and lagging strands
- Because of the 5′→3′ rule, the two new strands are built differently:
- the leading strand is made continuously, following the unwinding.
- the lagging strand is made in short pieces, joined together by DNA ligase.
Practice
The lagging strand is made in short pieces that are then joined by:
The lagging strand is built in fragments (away from the fork); DNA ligase seals them into one strand.
RNA
- RNA is also made of nucleotides, but it is:
- a single strand,
- built with the sugar ribose, and
- uses uracil in place of thymine.
- Messenger RNA (mRNA) carries a copy of a gene's instructions out of the nucleus.
Practice
How does RNA differ from DNA?
RNA is a single strand with ribose sugar and uracil in place of thymine.
You've got it
Key idea
- replication is semi-conservative: each new molecule keeps one old strand
- unwind → strands separate → each is a template → DNA polymerase joins nucleotides 5′→3′
- leading strand continuous; lagging strand in pieces joined by DNA ligase
- RNA: single strand, ribose, uracil (not thymine); mRNA carries the gene copy out of the nucleus