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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.