From an article i wrote earlier... its not that confusing... and your not expected to know much at all for edexcel... "DNA Polymerase, Helicase, and DNA ligase" are the only things required.... heres a bit of extra reading if ur interested
DNA ReplicationWhen cells divide, the new cells produced must have a full set of genes in order to function correctly. Genes are made of DNA so it is necessary to replicate the DNA before the cell divides.
Cell division occurs during growth, development (from a fertilised egg to an adult) and replacement of damaged cells. Cell division also occurs during the production of gametes (sex cells).
In all cases, DNA replication has to precede cell division.
During DNA replication, the DNA double helix is copied identically to produce two daughter DNA strands.
The process of DNA replication involves several enzymes and is as follows:
The DNA double helix is unwound by an enzyme called Helicase. It attaches to a region called the promoter and begins breaking the hydrogen bonds between the base pairs. The result is two individual strands of DNA.
Another enzyme called Topoisomerase is also responsible for unwinding of the super-coiled area of the DNA. It does this by releasing tension.
The two parallel strands are held in their place by enzymes called Single Strand Binding proteins.
The point of opening of the two DNA strands into two single strands is called the DNA replication Fork.
The two strands are named according to the orientation of the carbon atoms in the backbone. One strand will run from 3’ to 5’ and the parallel strand runs from 5’ to 3’.
These strands are given names:
Leading Strand - 3 ’ to 5’
Lagging Strand - 5’ to 3’
An enzyme called DNA Polymerase III extends the nucleic acid chain by adding DNA nucleotides to the daughter strands. It is important to note that synthesis of daughter strand takes place only in the 5’ to 3’ direction.
Before DNA Polymerase can attach to the strands, it needs to know where to begin replication, and this is the job of RNA Primase. It adds a small chain of RNA (single stranded DNA called Ribonucleic acid) to the daughter strands so DNA Polymerase III can begin replication.
The leading strand is replicated continuously.
The lagging strand is orientated in a 5’ to 3’ direction so replication takes place opposite direction from the replication fork. This is why the lagging strand cannot be replicated continuously, but is synthesized in tiny fragments called Okazaki fragments.
Towards the end of DNA replication, the RNA Primers need to be removed and the Okazaki fragments need to be joined together. This involves two more enzymes - RNase H (or DNA Polymerase I ) removes the RNA primers by degrading the bonds. The gaps are filled in by DNA polymerase III.
The gaps between Okazaki fragments are filled by DNA polymerase III and then DNA Ligase adds the phosphodiester bonds between the gaps.
