29) The Process of DNA Replication

Before a cell can divide, its DNA must be replicated exactly, so that identical copies of the cell's genes can be passed on to each of its offspring. Replication begins simultaneously on several chromatin threads and continues until all the DNA has been replicated. DNA replication ensures that all daughter cells have identical genes.
-There are several steps that need to be completed in order for DNA to be replicated:
1) The DNA helices begin unwinding from the nucleus cleosomes.
2) A helicase enzyme untwists the double helix and gradually separates the DNA molecule into two complementary nucleotide chains, exposing the nitrogeneous bases. The site of separation is called the replication bubble and the Y-shaped region at the end of the replication bubble is called the replication fork.
3) Each nucleotide strand then serves as a set of instructions for building a new complementary nucleotide strand from free DNA precursors dissolved in the nucleoplasm.
4) DNA synthesis keep occurring until several different proteins are present in the replisome. The actual initiation of DNA synthesis requires formation of short RNA primers by primase enzymes which are part of the replisome. These primers are eventually replaced by DNA nucleotides.
5) Once the primer is in place, DNA polymerase III positions complementary nucleotides along the template strand and then covalently links them together. DNA polymerase works only in one direction. One leading strand is synthesized continuously following the movement of the replication fork. The other strand called the lagging strand is constructed in segments in the opposite direction and requires a primer initiate replication of each segment.
6) The short segments of DNA are spliced together by DNA ligase. In the end, two DNA molecules are formed from the original DNA helix and are identical to it. Each new molecule consists of one old and one new nucleotide strand.
7) As soon as replication ends, histones associate with the DNA, completing the formation of two new chromatin strands. The chromatin strands condense to form chromatids. The chromatids remain attached, held together by the centromere and a protein complex called cohesin until the cell has entered the anaphase stage of mitotic divison.