Replication is the process by which nucleic acids such as deoxyribonucleic acid (DNA)or ribonucleic acid (RNA) are copied. The current method of replication was first proposed by James Watson and Francis Crick in 1953. Watson and Crick suggested that DNA was replicated by a semiconservative method in which each strand of DNA served as a template for new strands. In the late 1950s, this theory was confirmed through experiments performed by Matthew Meselson and Franklin Stahl. In their experiment, they grew E. colibacteria in a medium, containing a heavy isotope of nitrogen. The bacteria naturally incorporated the heavy nitrogen into their DNA. The bacterial cultures were then placed in a lighter nitrogen, which enabled Meselson and Stahl to follow the production of new DNA and confirm the semiconservative replication model.
DNA replication begins with a double strand of DNA. This structure, or parent molecule, is made up of two complementary strands of DNA that have paired nucleotides. Each nucleotide is paired with a specific partner, adenine (A) with thymine (T), and guanine (G) with cytosine (C). Replication begins at specific sites known as the origin of replication. These sites have a nucleotide sequence that is recognized by certain proteins. When replication is initiated, a short segment of DNA is untwisted by various enzmes called helicases at the origin. Next, the two DNA strands are separated, creating a replication bubble. The replication bubble is stabilized by another set of proteins that bind to DNA. This leaves segments of DNA that have a certain length of unpaired nucleotides. These unpaired nucleotides provide the template needed for the replication of the DNA strands. At each end of the replication bubble, there is a section called the replication fork where the new strands of DNA emerge as they are elongated.
During the replication process, key enzymes known as DNA polymerases, bring new nucleotides onto each strand of DNA following the base pairing rules. DNA polymerases have the limitation of only being able to replicate DNA in one direction, namely 5-3 ft (1.5-0.9 m). A double strand of DNA is antiparallel, however, meaning the nucleotides on one strand are oriented in the opposite way as nucleotides on the other strand. One strand, called the leading strand, can be replicated easily by the polymerase. The other strand, called the lagging strand, employs short strands of replication called Okazaki fragments. These fragments are connected into a single DNA strand by another enzyme called DNA ligase.
Another limitation of DNA polymerase is that it requires a primer to initiate DNA replication. These primers are short stretches of RNA that are joined to the DNA by a primase enzyme. In eukaryotes, the primer is about 10 nucleotides long. With the primer attached to the DNA, the polymerase begins replication from that point. After replication, one DNA molecule has been made into two. Each of these DNA strands is made up of an old strand and a new one. The rate at which replication occurs can be as fast as 500 nucleotides per second in bacteria. In eukaryotes, the rate is more like 50 nucleotides per second.
DNA replication occurs during the S phase of a cell's life cycle. At this time, duplicates of the chromosomes in the cell are visible. Mitosis separates the duplicate chromosomes and DNA is distributed between the two daughter cells. In this way, replication allows the transfer of genetic material from one cell to the next.
This is the complete article, containing 568 words
(approx. 2 pages at 300 words per page).
