Quick questions on DNA structure and semi-conservative replication (QCE Biology Unit 4)

DNA is a polymer of nucleotides. Each nucleotide has three parts:

When a cell divides, every DNA molecule must be copied. The copy mechanism is semi-conservative: each daughter double helix contains one parental (original) strand and one newly synthesised strand.

Replication begins at a specific sequence called the origin of replication. Several enzymes act at the replication fork where the helix opens.

DNA polymerase has a built-in proofreading function: it checks each newly added nucleotide and excises mistakes (3 prime to 5 prime exonuclease activity). Combined with mismatch repair after replication, the overall error rate is about one mistake in a billion bases.

Breaks hydrogen bonds between paired bases and unwinds the helix. Single-strand binding proteins keep the separated strands from re-annealing. Topoisomerase relieves the supercoiling tension ahead of the fork.

Synthesises a short RNA primer (about 5 to 10 ribonucleotides) complementary to the template. The primer provides a free 3 prime hydroxyl group, which is what DNA polymerase needs to extend from.

The main copying enzyme in prokaryotes (eukaryotes use a family of polymerases). It adds DNA nucleotides to the 3 prime end of the growing strand, reading the template 3 prime to 5 prime and synthesising the new strand 5 prime to 3 prime. Each new nucleotide arrives as a deoxynucleoside triphosphate (dATP, dTTP, dCTP, dGTP); two phosphates are cleaved off, releasing energy for the bond.

Because the two template strands are antiparallel, only one of them can be copied continuously toward the fork. This is the leading strand. The other, the lagging strand, is copied in short pieces called Okazaki fragments, each starting from a new primer.

Removes the RNA primers and fills the gaps with DNA.

Seals the nicks between Okazaki fragments by forming the final phosphodiester bond, producing one continuous strand.

Genes are sections of DNA. The molecule itself is a polymer of nucleotides.

Synthesis is always 5 prime to 3 prime, never the reverse. The template is therefore read 3 prime to 5 prime.

DNA polymerase cannot start a new strand from scratch. Primase makes an RNA primer first.

Leading strand is continuous and runs toward the fork. Lagging strand is in Okazaki fragments and runs away from the fork.

It is evidence for semi-conservative replication, not for the structure itself (that was Watson, Crick, Wilkins and Franklin).