DNA: Carries, Replicates and Recombines Information

DNA is the genetic material:

• Feulgen staining (1923) revealed that DNA resides in the chromosomes.

• Bacterial transformation experiments by Griffith 1928 revealed that a transforming principle changes a rough strain to a smooth strain in Streptococcus pneumonia.

• Avery, MacLeod and McCarty 1944, using biochemical analysis revealed that DNA is the transforming principle in Griffith experiments.

• Using 32P to label DNA and 35S to label protein, Hershey and Chase in 1952 showed that it’s the core DNA and not the protein capsule of T2 phage that is responsible for the lytic function and replication of the phage.

The Watson and Crick Model of DNA:The double helix model of DNA proposed by Watson and Crick depends on the following findings

• Rosalind Franklin and Maurice Wilkins in 1952 reported that the crosswise pattern of X-ray diffraction of DNA fibers indicates a helical structure for DNA

• Erwin Chargaff by analyzing the base composition of DNA extracted from different organisms that A = T and G = C and that (A + G)/(C+T) = 1.

Hence the Watson Crick model is:• DNA is a right handed double helix, with sugar and phosphate being the core and the nitrogenous bases forming the steps of the ladder (G is complementary to C and T complementary to A

• Hydrogen bonds: 3 between G & C and 2 between A & T.

• 3.4 Angstrom between two nucleotides in a single strand and 34 Angstrom every complete turn of the helix

• The two strands of the helix are 20 Angstrom apart

• major groove and a minor groove alternate

RNA differs from DNA in:

• ribose instead of deoxyribose

• uracil instead of thymine

• single strand instead of double strand. The single strand forms double strand areas by twisting upon itself using the same rules of base complementarity (A complementary to U and G complementary to C)

• RNA is transcribed off DNA and functions as a mediator in gene expression protein synthesis

• Some viruses have RNA as their genetic material but during infection the RNA has to revert back to its complementary DNA (cDNA) to cause successful infection.

DNA Replication:

• Semiconservative as proposed by Watson and Crick in their 1953 paper on DNA structure then proved by Meselson and Stahl in 1958

• DNA replication in prokaryotes includes 1) Single origin of replication 2) Initiator protein recognizes and binds at the origin of replication and attracts a DNA helicase to bind 3) DNA helicase catalyzes the unwinding of the double helix 4) Primase synthesize a short RNA primer to initiate polymerization by DNA polymerase III 5) Elongation in the 5’ ---> 3’ direction in both the leading strand (continuous synthesis) and the lagging strand (discontinuous synthesis) at each replication fork 6) DNA polymerase I digests the RNA primer and at the same time fills in the gap with DNA synthesis in the 5’ to 3’ direction.

7) DNA ligase catalyzes the formation of the final phosphoester bond and close the nick in the DNA strand.

8) The primary DNA repair enzyme is DNA polymerase I but DNA polymersae II can serve as an alternate repair polymerase and can replicate DNA under circumstances in which the template is damaged.

9) DNA topoisomerases relax the supercoils formed beyond the replication fork by introducing a cut in the double helix. The cut strands rotate to unwind and then rejoin (Phosphoester bond) by a DNA ligase.

• In eukaryotes, the mechanism of DNA replication is similar except that there are multiple origins of replications and there are five DNA polymerases involved, , ,, and . DNA polymerase is in charge of chromosomal replication (lagging & leading strands), for the lagging strand and for repair of nuclear DNA.