DNA replication Flashcards
Chargaff’s rules:
1]In naturalDNAthe number ofguanineunits equals the number ofcytosineunits and the number ofadenineunits equals the number ofthymineunits. In human DNA, for example, the four bases are present in these percentages: A=30.9% and T=29.4%; G=19.9% and C=19.8%. This strongly hinted towards thebase pairmakeup of the DNA.
2] The composition of DNA varies from one species to another, in particular in the relative amounts of A, G, T, and C bases. Such evidence of molecular diversity, which had been presumed absent from DNA, made DNA a more credible candidate for thegenetic materialthanprotein.
Rosalind Franklin
Franklin is best known for her work on theX-ray diffraction images of DNAwhich led to the discovery of the DNAdouble helix. According toFrancis Crick, her data was key to determining the structureto formulate Crick andWatson’s 1953modelregarding the structure of DNA.
Franklin’s images ofX-raydiffraction confirming the helical structure of DNA were shown to Watson without her approval or knowledge. This image and her accurate interpretation of the data provided valuable insight into the DNA structure, but Franklin’s scientific contributions to the discovery of the double helix are often overlooked.
James Watson & Francis Crick
In 1953,James WatsonandFrancis Cricksuggested what is now accepted as the first correct double-helix model ofDNA structurein the journalNature.Their double-helix, molecular model of DNA was then based on a singleX-ray diffractionimage (labeled as “Photo 51”)taken byRosalind FranklinandRaymond Goslingin May 1952, as well as the information that the DNA bases are paired— also obtained through private communications from Erwin Chargaff in the previous years.Chargaff’s rulesplayed a very important role in establishing double-helix configurations for B-DNA as well as A-DNA.
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.
replication
Helicase unwinds and separates the DNA helix.
New dNTPs (deoxyribonucleoside triphosphate) 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.
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
Conservative replication
would leave the two original template DNAstrands together in a double helix and would produce a copy composed of two new strands containing all of the new DNA base pairs.
Semiconservative replication
would produce two copies that each contained one of the original strands and one new strand
Dispersive replication
would produce two copies of theDNA, both containing distinct regions of DNA composed of either both original strands or both new strands
TheMeselson–Stahl experiment
was an experiment byMatthew MeselsonandFranklin Stahlin 1958 which supported the hypothesis thatDNA replicationwassemiconservative. In semiconservative replication, when the double stranded DNA helix is replicated each of the two new double-strandedDNAhelixes consisted of one strand from the original helix and one newly synthesized. It has been called “the most beautiful experiment in biology.“
