Dna Replication 2

The model of DNA replication proposed by Watson and Crick is based on the hydrogen-bonded specificity of the base pairs. Complementary strands are shown in different colors. This drawing is a simplified version of the current picture of replication but represents the basic concept suggested by the Watson-Crick structure. The fact that new strands can grow only in the 5 -to-3 direction adds complexities to the detailed mechanism of replication.

Three alternative patterns for DNA replication. The Watson-Crick model would produce the first (semiconservative) pattern. Light blue lines represent the newly synthesized strands

Centrifugation of DNA in a cesium chloride (CsCl) gradient. Cultures grown for many generations in 15N and 14N media provide control positions for heavy and light DNA bands, respectively. When the cells grown in 15N are transferred to a 14N medium, the first generation produces an intermediate DNA band and the second generation produces two bands: one intermediate and one light

Diagrammatic representation of the autoradiography of chromosomes from cells grown for one cell division in the presence of the radioactive hydrogen isotope 3H (tritium) and then grown in a nonradioactive medium for a second mitotic division. Each dot represents the track of a particle of radioactivity.

Light blue lines represent radioactive strands. In the second replication (which takes place in nontritiated solution), both the 3H strand and the nontritiated strand incorporate nonradioactive nucleotides, yielding one hybrid and one nontritiated chromatid.

Left: Autoradiograph of a bacterial chromosome after one replication in tritiated thymidine. According to the semiconservative model of replication, one of the two strands should be radioactive. Right: Interpretation of the autoradiograph. The light blue line represents the tritiated strand

Left: Autoradiograph of a bacterial chromosome in the second round of replication in tritiated thymidine. In this theta (θ) structure, the newly replicated double helix that crosses the circle could consist of two radioactive strands (if the parental strand was the radioactive one). Right: The double thickness of the radioactive tracing on the autoradiogram appears to confirm the interpretation shown here. The light blue helices represent the "hot" strands.

Rolling-circle replication. Newly synthesized DNA is light blue

Pairing between the normal (keto) forms of the bases

Mismatched bases. (a) Mispairs resulting from rare tautomeric forms of the pyrimidines; (b) mispairs resulting from rare tautomeric forms of the purines

DNA-gyrase-catalyzed supercoiling. Replicating DNA generates "positive" supercoils, depicted at the bottom of the diagram, as a result of rapid rotation of the DNA at the replication fork. DNA gyrase can nick and close phosphodiester bonds, relieving the supercoiling, as shown here (relaxed DNA). Gyrase can also generate supercoils twisted in the opposite direction, termed negative supercoils; this arrangement facilitates the unwinding of the he

DNA replication fork.

Chain-elongation reaction catalyzed by DNA polymerase

Chain-elongation reaction catalyzed by DNA poly

Bidirectional replication of a circular DNA molecule

Initiation of DNA synthesis by an RNA primer.

DNA synthesis proceeds by continuous synthesis on the leading strand and discontinuous synthesis on the lagging strand.

The overall structure of a growing fork (top) and steps in the synthesis of the lagging strand.

Telomerase carries a short RNA molecule that acts as a template for the addition of the complementary DNA sequence at the 3 end of the double helix. In the ciliate Tetrahymena, the DNA sequence added is TTGGGG.

The 3 5 exonuclease action of DNA polymerase III