DNA replication is semi conservative in that each DNA molecule produced consists of 1 old and 1 new partner. The reason for this semi conservative nature is that the original DNA strands separate and each functions as a template for a new partner. 

DNA synthesis can not start without an RNA primer since all known DNA polymerases require a primer with a free 3′ OH group for DNA synthesis. A specialized RNA polymerase called DNA Primase synthesizes a short stretch of RNA (~10 nucleotides) which is latter removed by the 5′ to 3′ exonuclease activity of DNA polymerase. It is believed that the reason an RNA polymerase is used to initiate DNA synthesis is that DNA polymerase would have to examine the first base pair to see if it is mismatched whereas RNA polymerases do not need this type of fidelity.  RNA polymerases have no 3′ to 5′ exonuclease acivity. 

DNA polymerase catalyzes the addition of a deoxyribonucleotide to the 3′ OH end of the primer strand that is paired to the template DNA strand. The newly synthesized DNA strand always polymerizes in the 5′ to 3′ direction. Since each incoming deoxyribonucleoside triphosphate must pair with the template strand, the template determines which of the 4 possible deoxyribonucleotides will be added. The reaction is driven by a large free energy change, caused by the release of pyrophosphate. 

In DNA replication, one daughter strand, the leading strand is synthesized continuously whereas the other daughter strand, called the lagging strand, which runs in the 5′ to 3′ direction (meaning that synthesis would have to occur in the 3′ to 5′ direction which can not occur) is synthesized discontinuously in what are called Okazaki fragments. Lagging strand synthesis is always delayed because it must wait for the leading strand to expose the template strand on which each Okazaki fragment is synthesized. For the leading strand, a special primer is needed only at the start of replication since the strand is synthesized continuously. On the lagging strand side of the fork, however, every time the DNA polymerase completes a short DNA Okazaki fragment, it must start synthesizing a completely new fragment at a site further along the template strand. Thus short primers must be made at intervals of 100-200 nucleotides on the lagging strand.The synthesis of each Okazaki fragment ends when DNA polymerase runs into the RNA primer attached to the 5′ end of the previous fragement. This RNA primer is then erased and replaced with DNA and then an enzyme called DNA ligase joins the 3′ end of the new DNA fragment to the 5′ end of the previous one. This process is repeated in order to produce one continuous lagging strand.

For DNA synthesis to proceed, the DNA double helix must be opened up ahead of the replication fork so that incoming deoxyribonucleoside triphosphates can form base pairs with the template strand. Two types of proteins contribute to this process; DNA helicases and single-strand DNA binding proteins. DNA helicasespry apart the helix at rates of up to 1000 nucleotide pairs per second. The unwound portion of DNA is then stablized by a single strand binding protein (SSB). 

Most DNA polymerases will synthesize only a short string of nucleoides before falling off the template. This tendency is counteracted by an accessory protein that fucntions as a regulated sliding clamp. The unwinding of circular DNA at the origin produces + supercoiling which must be relieved by DNA gyrase which introduces supercoils as it hydrolyzes ATP.

As a replication fork moves along double stranded DNA, it creates winding problems which are resolved by DNA topoisomerases. One such topoisomerase, topoisomerase I, produces a transient single strand break in the phosphodiester linkage in one strand. This allows the 2 ends of the DNA double helix to rotate relative to each other, relieving accumulated strain. A second, topoisomerase, topoisomerase II, makes a transient double strand break in the helix which allows a second nearby double strand to pass through the break which also relieves stress.

DNA replication is not performed by a mixture of proteins acting independently. In fact, most of the proteins are held together in a large multienzyme complex that moves rapidly along the DNA. Two DNA polymerase molecules work at the replication fork, one on the leading strand and one on the lagging strand. The DNA helix is opened by a DNA polymerase molecule clamped on the leading strand, acting in concert with one or more DNA helicase molecuels running along the strands in front of it. Helix opening is aided by cooperatively bound molecules of single strand DNA binding protein.

DNA replication has a very high accuracy rate which depends several proof-reading mechanisms. (1) The first proofreading capability is carried out by DNA polymerase itself. The correct nucleotide has a higher affinity for the moving polymerase as compared to an incorrect nucleotide because only the correct nucleotide can correctly base pair with the template. Moreover, an incorrectly bound nucleotide is more likely to dissociate from the polymerase. (2) A second proof-reading mechanism is that DNA polymerase has a 3′ to 5′ exonuclease capability which clips off mismatched nucleotides. (3) A third type of mechanism is sometimes referred to as DNA mistmatch repair. Mismatch DNA repair acts both as a DNA proofreading process and in DNA repair. In bacteria, mismatch repair employs a number of proteins including MutS, MutL, MutH. Human connterparts are hMLH1 and hMSH1. To be effecting proof-reading mechanisms need to be able to distinguish and remove a mismatched nucleotide only on the newly synthesized strand, where the replication error occurs. E. coli accomplish this tasks by adding methyl groups to all A residues in the sequence GATC a certain time after A has been incorporated into a newly synthesized DNA chain. Thus the only GATC sequences that have not yet been methylated are in the new strands just behind a replication fork. Eukaryotes use a different mechanism to distinguish between old and new strands. In eukarytoes, newly synthesized strands are are nicked (single strand breaks). 

Since DNA in eucaryotic chromosomes is composed of both a mixture of DNA and a protein called, chromatin, chromosome duplication requires not only that the DNA be replicated but also that new chromosomal proteins be assembled onto the DNA. 

Eucaryotes have a special problem with their ends on the lagging strand which are replicated. When the replication fork reach the end of a linear chromosomes, there is no place to produce the RNA primer needed to start the last Okazaki fragment at the very tip of the strand. Eucaryotes solve this problem through the use of their special nucleotide sequences (tandom repeats of GGGTTA in humans) at the ends of chromosomes which are incorporated into telomeres. These telomeres attract an enzyme called Telomerase which replicates the end of chromosomes.