Chapter 12 Section 2: Replication of DNA Ms. Diana and Ms. Su’ad

Semiconservative Replication When Watson and Crick presented their model of DNA, they also suggested a possible method of replication. Semiconservative Replication – process in which parental strands of DNA separate, serve as templates, and produce DNA molecules that have one strand of parental DNA and one strand of new DNA. The process of semiconservative replication occurs in 3 stages: unwinding, base pairing, and joining.

Semiconservative Replication

Unwinding DNA helicase– an enzyme that is responsible for unwindng and unzipping the double helix. When the double helix is unzipped, the hydrogen bonds between the bases are broken, leaving 1 single strand of DNA. Then, proteins called ‘single-stranded binding proteins’ connect to the DNA to keep the strands separate during replication.

Unwinding

Base Pairing The enzyme DNA polymerase is responsible for adding nucleotides to the new DNA strand. The nucleotides are added to the 3’ end of the new DNA strand. (Remember that each base binds only with its complement : A = T and C = G). This way and identical copy of the parent is made. The 2 new strands are made a slightly different way: ‘Leading’ strand – remains long as the DNA unwinds, built continuously by adding nucleotides to the 3’ end. ‘Lagging’ strand – stretches away from the replication fork, build discontinuously in small segments (parts) called Okazaki fragments by the DNA polymerase in the 3’ to 5’ direction. Okazaki fragments are later connected by an enzyme called DNA ligase. They are about 100-200 nucleotides long.

Base Pairing

Joining During eukaryotic DNA replication there are many areas along the chromosome where replication begins. When the DNA polymerase comes to an RNA primer on the DNA strand, it removes the primer and fills in the place with DNA nucleotides. After the RNA has been replaced, DNA ligase connects the two sections.

Joining

Comparing DNA Replication in Eukaryotes and Prokaryotes Eukaryotic DNA unwinds in multiple areas as DNA is replicated. Each part of a chromosome replicates as a section, which has lengths that vary from 10,000 to 1 million base pairs. In prokaryotes, the circular DNA strand is opened at one place of replication. Prokaryotic DNA is typically shorter and remains in the cytoplasm, not packaged in a nucleus.

Comparing DNA Replication in Eukaryotes and Prokaryotes

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