How is DNA replicated?

DNA is found in the nucleus of all human cells (except red blood cells). Its double-stranded, helical structure contains all of the information required for the body to produce new proteins- it simply needs to be unwound, read, copied and the information sent out to ribosomes to make proteins. Inside the nucleus, enzymes and free nucleotides wait for their turn. The DNA is first 'unzipped' by an enzyme called DNA helicase. This separates the two strands. Primase attaches to the DNA and moves along, attaching a small number of nucleotides to produce a short strand of RNA attached to the DNA, called a primer. The Primase moves away. DNA polymerase attaches to the primer on the DNA strand, and begins to move along the strand, attaching nucleotides as it goes. This process happens on both separated DNA strands; however, since DNA polymerase can only move in one direction (5'-3'), while one strand is produced continuously, the other is made a small section at a time, producing Okazaki fragments. Exonuclease removes the primers and DNA polymerase fills the gap with nucleotides. DNA ligase catalyses the phosphodiester bond formation, sealing the double strands of DNA.