DNA Replication
Introduced| English |
|---|
| replication/ˌreplɪˈkeɪʃn/ |
| template/ˈtempleɪt/ |
| DNA polymerase/ˌdiː en ˈeɪ ˌpɒlɪməˈreɪz/ |
| semi-conservative/ˈsemi kənˈsɜːvətɪv/ |
| helicase/ˈhelɪkeɪs/ |
| topoisomerase/ˈtɒpɔɪsəʊmreɪz/ |
| RNA primer/ˌɑː en ˈeɪ ˈpraɪmə/ |
| leading strand/ˈliːdɪŋ strænd/ |
| lagging strand/ˈlæɡɪŋ strænd/ |
| ligase/ˈlɪɡeɪs/ |
Copy two opposite strands before one cell becomes two
- A dividing cell must copy both strands of its DNA. The strands run in opposite directions, but its copying enzyme can extend a new strand in only one direction.
- Replication 复制 solves this with continuous synthesis on one side and fragments on the other. Follow the enzymes and primers to see why both daughter DNA molecules retain an original strand.
Unzipping the helix
- Replication begins by breaking the base pairs.
- The two strands separate, like a zip opening.
- This exposes the bases along each strand.
- Now each strand can guide the building of a new partner.
Why must a cell copy its DNA before dividing?
Replication ensures both daughter cells receive a complete, identical copy of the DNA.
Each strand is a template 模板
- Each old strand acts as a template.
- Its base sequence decides the new strand by base pairing.
- The enzyme DNA polymerase DNA聚合酶 adds matching bases along it.
- A pairs with T and G with C, so the copy is exact.
How DNA is copied
Step through replication — the helix unzips and each old strand is used as a template for a new one.
During replication, each old strand acts as a…
Each old strand is a template: its base sequence decides the new strand by base pairing.
The enzyme DNA polymerase builds the new strand by adding matching bases.
DNA polymerase reads the template and adds the complementary base at each position.
Semi-conservative 半保留 copying
- Replication makes two DNA molecules from one.
- Each new molecule has one old strand and one brand-new strand.
- Because each keeps half of the original, it is semi-conservative.
- The two copies are identical to the starting DNA.
Because each new DNA keeps one old strand, replication is called ____-conservative.
Replication is semi-conservative: each copy has one original strand and one new strand.
Select all true statements about DNA replication.
Replication keeps the old strands (semi-conservative); it does not discard them. The other three are correct.
Opening a moving replication fork
- Helicase 解旋酶 separates the paired DNA strands. Topoisomerase 拓扑异构酶 reduces twisting ahead of the opening, so the DNA can keep unwinding.
- A short RNA primer RNA引物 supplies a free end. DNA polymerase cannot start a new strand alone: it extends from that primer.
DNA polymerase is present but no RNA primer has formed. What is blocked?
Polymerase requires an existing free end supplied by a primer.
Both new strands grow in one direction
- DNA polymerase adds nucleotides only to the 3′ end: every new strand grows 5′ → 3′. The two template strands run in opposite directions.
- The leading strand 前导链 grows continuously towards the moving fork. The lagging strand 后随链 grows in short pieces away from the fork; each piece needs its own primer.
The lagging strand is synthesized 3′ → 5′.
Each fragment grows 5′ → 3′; discontinuous synthesis solves the opposite template direction.
Joining the pieces
- After RNA primers are removed and replaced with DNA, ligase 连接酶 joins the DNA fragments into a continuous strand. It joins the sugar–phosphate backbone, rather than choosing complementary bases.
- Worked trace: a template runs 3′–A T G C–5′. Its new partner is 5′–T A C G–3′. Both leading and lagging synthesis obey the same 5′ → 3′ rule.
Short DNA fragments have formed but remain unjoined. Which enzyme is most directly missing?
Ligase seals breaks in the backbone between adjacent DNA fragments.
Replication is semi-conservative: it does not build two entirely new molecules and throw the old one away. Each daughter DNA keeps one original strand and pairs it with a new one. Half old, half new — that is the key idea.
Right before a cell divides:
- The cell copies all of its DNA during interphase.
- Each chromosome becomes two identical copies, joined together.
- When the cell divides, each daughter gets one full copy — thanks to replication.
DNA replication copies the genetic code before cell division. The helix unzips, and each old strand serves as a template while DNA polymerase adds matching bases. The result is semi-conservative: each new DNA has one old strand and one new strand, giving two identical copies.