Topic 3.4 DNA Replication

The discovery of the double helix Beyond the syllabus: The discovery of the double helix Erwin Chargaff (1951): Rule of Base pairing Rosalind Franklin & Maurice Wilkins (1953): X-ray diffraction pattern of DNA James Watson & Francis Crick (1953): Molecular structure of DNA

Chargaff’s Results: Similar amount of A-T and C-G

Franklin’s Results: 2 periodicities for DNA 3.4 Å and 34 Å. Images produced by X-ray crystallography are not actually pictures of molecules. The spots and smudges in FIGURE 16.4b were produced by X-rays that were diffracted (deflected) as they passed through aligned fibers of purified DNA. Crystallographers use mathematical equations to translate such patterns of spots into information about the three-dimensional shapes of molecules, and Watson was familiar with the types of patterns that helical molecules produce. Just a glance at Franklin’s X-ray diffraction photo of DNA not only told him that DNA was helical in shape but also enabled him to deduce the width of the helix and the spacing of the nitrogenous bases along it. The width of the helix suggested that it was made up of two strands, contrary to a three-stranded model that Linus Pauling had recently proposed. The presence of two strands accounts for the now-familiar term double helix. Franklin’s Results: 2 periodicities for DNA 3.4 Å and 34 Å.

The discovery of the double helix: Watson & Crick Model Both Chargaff’s results and the X-ray diffraction pattern could be explained if the model consisted of two polynucleotide chains twisted around each other in the form of a double helix. X-ray diffraction had shown that every complete turn of the helix measured 3.4nm. Watson suggested that if Cytosine paired with Guanine (a pirymidine with a purine) and if Thymine paired with Adenine (a purine with a pirymidine) then 10 bases could fit into one complete turn of the helix (3.4nm) The bases would be held together by hydrogen bonds

The discovery of the double helix: Watson & Crick Model DNA is composed of 2 chains of nucleotides that form a double helix shape. The two strands are antiparallel. Every complete turn of the helix measured 3.4 nm The backbone of the DNA molecule is composed of alternating phosphate groups and sugars. The complimentary nitrogenous bases form hydrogen bonds between the strands. A is complimentary to T and G is complimentary to C.

3.4.1 DNA replication Unwound and separate the DNA helix: Helicase New dNTPS are joined to the template by hydrogen bonds. Complementary Base Pairing ensures identical copies of DNA. The parent strands act as a template for the new (complementary strands) Adenine pairs only with Thymine (A-T) Cytosine pairs only with Guanine (C-G) DNA polymerase enzyme links the phosphate of the new nucleotide to the sugar of the nucleotide before it by covalent bond. 2. Helicase enzyme breaks the hydrogen bonds between complementary base pairs. This unzips the double helix at a position called the replication fork. 3. There is an abundant supply of nucleotides in the nucleus for the formation of the new polynucleotides. 4. Nucleotide bases pair to the bases in the original strands. 5. DNA polymerase joins together the nucleotides together with strong covalent phosphodiester bonds to form a new complementary polynucleotide strand. 6. The double strand reforms a double helix under the influence of an enzyme. 7 Two copies of the DNA molecule form behind the replication fork. These are the new daughter chromosomes.

3.4.2 DNA replication: Complementary Base Pairing Complementary Base Pairing ensures the new DNA molecule is identical to the original – no mistakes are made – so the base-sequence of nucleotides is conserved

3.4.3 State that DNA replication is…

DNA Replication is semiconservative Meselson & Stahl Experiment: DNA Replication is semiconservative