Click4Biology: 7.2 DNA replication


DNA replication

DNA replication has been outlined in section 3.4.1 . which should be reviewed before tackling this section. This HL section provides more detail on the process of DNA replication which takes places during the S section of the Interphase.

The models of DNA replication are based on some prokaryotic organisms such as E.coli. The diversity of this group however would suggest that we should be cautious in extrapolating the mechanism to the whole group. Eukaryotic organisms have more complex mechanism although they share the same broad mechanism.

Students should pay close attention to the orientation of the nucleotides when the DNA chain is polymerised.

7.2.1 Direction of DNA replication.

7.2.2 Prokaryotic DNA replication.

7.2.3 Eukaryotic DNA replication



7.2.1 State that DNA replication occurs in a 5’→ 3’ direction.(1)







7.2.2 Explain the process of DNA replication in prokaryotes, including the role of enzymes (helicase, DNA polymerase,
RNA primase and DNA ligase), Okazaki fragments and deoxynucleoside triphosphates.(3)





(c) DNA molecule

(d) DNA Helicase enzyme













Retuning to the other parent polynucleotide (red).





The important point to note here is that the DNA polymerase only works by joining 5' nucleotides to 3' nucleotides on the established chain.

The lagging strand is therefore made up of a number of short polynucleotide chains that need joining together.

The short chains are called Okazaki fragments after the Japanese Biochemist Reiji Okazaki.







This diagram is the usual version used to describe the process of leading and lagging strand polymerase activity.

Remember always think about the action of the DNA polymerase III adding the 5' of the free nucleotide to the 3' of the already established new strand. This single fact allows the process to be tracked and alternative diagrams to be interpreted.









7.2.3 State that DNA replication is initiated at many points in eukaryotic chromosomes.(1)

Prokaryotic DNA polymerase can work at around 1000 bases per second which means the whole circular (loop) can replicated between 20 and 40 minutes.

The eukaryotic DNA polymerase works much slower around 50 bases per second. With as many as 80 million bases to replicate the job is achieved in about one hour by having many replication forks