Replication Machinery: Origins, Forks and Telomeres
Replication starts at defined origins and runs in both directions, with one strand made continuously and the other in pieces. Because polymerase cannot start a chain or work backwards, linear chromosomes lose a little end sequence each round unless telomerase restores it.
What a learner can do afterwards
- Explains the lagging strand as a consequence of the direction polymerase can work in.
- Says why primers are needed and what removes them.
- Links the end replication problem to telomere shortening and to what telomerase does about it.
1 · Read
DNA polymerase can only add nucleotides to a free 3 prime end and only builds in the 5 prime to 3 prime direction, and that single constraint shapes the whole fork. The leading strand runs continuously toward the fork, while the lagging strand must be built backward in short Okazaki fragments. The fork is therefore lopsided, with one smooth side and one stop-start side.
Primers exist because polymerase cannot start chains from nothing. Primase lays short RNA starters, polymerase extends them, and the RNA is later removed and replaced with DNA. Ligase then seals the nicks between fragments, finishing the lagging side.
Starts are scattered to match genome size. Bacteria fire a single origin and send two forks around the circle, copying 4.6 million base pairs in about 42 minutes. Eukaryotes scatter many origins, up to 100,000 in humans, so three billion base pairs still finish in reasonable time.
Linear ends create the end replication problem: no primer can sit before the very start, so a little end sequence is lost each round. Telomeres shorten until telomerase intervenes in cells that express it. Bacterial circles never face this, since the two forks meet on the far side with no ends left over.
One-direction building splits the fork, primers start every piece, and ends need telomerase.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.