Replication of DNA and RNA
Introduced| English | Meaning |
|---|---|
| replication/ˌreplɪˈkeɪʃn/ | The copying of DNA to produce molecules with the same nucleotide sequence. |
| semi-conservative/ˈsemi kənˈsɜːvətɪv/ | Describing DNA replication in which each resulting double-stranded molecule contains one original strand and one newly synthesised strand. |
| template/ˈtempleɪt/ | An existing nucleic-acid strand whose sequence guides synthesis of a complementary strand. |
| DNA polymerase/ˌdiː en ˈeɪ ˌpɒlɪməˈreɪz/ | An enzyme that synthesises DNA by adding nucleotides to a growing strand using a template. |
| lagging strand/ˈlæɡɪŋ strænd/ | The new DNA strand synthesised discontinuously in short fragments as a replication fork opens. |
| leading strand/ˈliːdɪŋ strænd/ | The new DNA strand synthesised continuously in the same overall direction as a replication fork opens. |
| DNA ligase/ˌdiː en ˈeɪ ˈlɪɡeɪs/ | DNA ligase |
| messenger RNA/ˈmesɪndʒə ˌɑː en ˈeɪ/ | An RNA molecule that carries a gene's coding information to a ribosome for protein synthesis. |
Copying DNA
- Before a cell divides, all its DNA must be copied — replication 复制.
- Each new molecule keeps one old strand and gains one new one.
- We call this semi-conservative 半保留 replication.
DNA replication is "semi-conservative" because each new molecule has:
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Each daughter molecule conserves one parental strand and pairs it with one new strand.
In semi-conservative replication, each new DNA molecule keeps one of the original strands.
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That is exactly what "semi-conservative" means — one old strand is conserved in each copy.
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 模板; free nucleotides pair with the exposed bases (complementary base pairing).
- the enzyme DNA polymerase DNA聚合酶 joins the new nucleotides into a strand — it can only add them in the 5′→3′ direction.
Semi-conservative DNA replication at a fork
Semi-conservative replication
Step through copying DNA. The helix unwinds, each old strand templates a new one, and you end with two identical molecules.
Free nucleotides line up against the exposed template bases by ______ base pairing.
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Each template base attracts its complement (A–T, C–G), so the new strand is an exact partner.
DNA polymerase adds new nucleotides:
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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 后随链
- Because of the 5′→3′ rule, the two new strands are built differently:
- the leading strand 前导链 is made continuously, following the unwinding.
- the lagging strand is made in short pieces, joined together by DNA ligase DNA连接酶.
DNA extracted from cells appears as pale, stringy strands.
Put the steps of DNA replication in order.
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Unwind → expose templates → base-pair → join → two copies, each semi-conservative.
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 信使RNA (mRNA) carries a copy of a gene's instructions out of the nucleus.
DNA: antiparallel strands held by complementary base pairs
How does RNA differ from DNA?
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RNA is a single strand with ribose sugar and uracil in place of thymine.
Match each feature to DNA or RNA.
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DNA: double-stranded, deoxyribose, thymine. RNA: single-stranded, ribose, uracil.
You've got it
- 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