DNA Replication Take My Genes Please

In 1868, Swiss biologist Friedrich Miescher carried out chemical studies on the nuclei of white blood cells in pus. His studies led to the discovery of DNA. DNA was not linked to hereditary information until 1943 when work performed by Oswald Avery, Colin MacLeod, and Maclyn McCarty at the Rockefeller Institute revealed that DNA contained genetic information. These studies also revealed that genetic information is passed from "parent cells" to "daughter cells," creating a pathway through which genetic information is passed to the next generation of an organism.

Scientists were baffled about how the exchange of DNA occurred. The answer came in 1953 when American geneticist James Watson and English physicist Francis Crick discovered the double-helical structure of DNA at the University of Cambridge in England. Discovery of the double-helical structure was the key that enabled Watson and Crick to learn how DNA is replicated.

In the late 1950's, Mathew Meselson and Franklin Stahl first described the DNA molecule and how DNA replicates in a process called semiconservative replication. DNA is replicated by taking one parent double-stranded DNA molecule, unzipping it and building two identical daughter molecules. Bases along the two strands of double-helical DNA complement each other. One strand of the pair acts as a template for the other.

DNA is replication requires complex cellular proteins that direct the sequence of replication. Replication begins when the parent double-stranded DNA molecule unwinds; then the two strands separate. The DNA polymerase enzyme uses a strand as a template to make a new strand of DNA. The DNA polymerase enzyme examines the new DNA and removes bases that do not match and then continues DNA synthesis.

The point at which the double-stranded DNA molecule unzips is called the replication fork (Fig. 7-1). The two new strands of DNA each have a base

Parental New New Parental

Fig. 7-1. In semiconservative replication, new strands are synthesized after the replication fork.

Parental New New Parental

Fig. 7-1. In semiconservative replication, new strands are synthesized after the replication fork.

sequence complimentary to the original strand. Each double-stranded DNA molecule contains one original and one new strand. In bacteria, each daughter receives a chromosome that is identical to the parent's chromosome.

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