7 main people that contributed to dna
Griffith
Avery, McCarty and MacLeod
Hershey and Chase
Chargaff
Watson and crick
Franklin
Meselson and stahl
how did Griffith contribute to dna
demonstrations of bacterial transformation (feet up, feet down experiment)
explain griffiths’ feet up, feet down experiment
- Streppachoccus types:
Rough – non-pathagenic (does not cause disease)
Smooth – evolved to stop immune systems fighting it (disease-causing) - Injection of live S strain: Mice died, as expected, due to the virulent bacteria.
- Injection of live R strain: Mice survived, as the R strain is non-virulent.
- Injection of heat-killed S strain: Mice survived because the S bacteria were dead and couldn’t cause disease
- Injection of live R strain and heat-killed S strain: Surprisingly, the mice died, and live S bacteria were found in their bodies
- suggested that some “transforming principle” in the dead S strain cells carried genetic information that could change the R strain’s properties, making it harmful
how did Avery, McCarty and MacLeod contribute to dna
found transforming factor is separable
explain how Avery, McCarty and MacLeod found transforming factor is separable
- did some reductionist biology and isolated the nucleic acids, proteins and sugars
- re-ran the Griffith experiment using individual components rather than whole bacteria
- Their conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria
- Many biologists remained skeptical, mainly because little was known about DNA
how did Hershey and Chase contribute to dna
found transforming factor is DNA
explain how Hershey and Chase found transforming factor is DNA
- aimed to find the “transforming principle” in genetics: DNA or protein
- used bacteriophages to investigate, as viruses inject genetic material into host cells
- Two setups: phages with radioactive sulfur (protein) + phages with radioactive phosphorus (DNA)
- After infecting bacteria, only radioactive DNA entered cells
- Conclusion: DNA, not protein, carries genetic information, confirming DNA’s role in heredity
how did Chargaff contribute to dna
- Found proportions of the 4 bases vary between species
- Found amount of A is always equal to T + G is always equal to C (chargaffs rules)
how did Watson and crick contribute to dna
- built a double helix model of DNA based on X-ray data and chemical knowledge
- Franklin’s work showed 2 antiparallel sugar-phosphate backbones with bases inside
- Watson and Crick initially thought bases paired like-with-like, but the width didn’t match X-ray data
- They discovered purine-pyrimidine pairing (A with T, G with C) fit the structure’s uniform width
- This model supported Chargaff’s rules: A = T, G = C.
- The double helix’s structure suggested a copying mechanism for genetic material
how did Franklin contribute to dna
found DNA structure
explain how Franklin found dna structure
- Not awarded nobel prize because she died (only 2 instances where nobel prize was given to dead person)
- using a technique called X-ray crystallography to study molecular structure - produced a picture of the DNA
- enabled Watson to deduce that DNA was helical + to deduce the width of the helix and the spacing of the nitrogenous bases
- The width suggested that the DNA molecule was made up of two strands, forming a double helix
how did Meselson and stahl contribute to dna
found dna used semi-conservative replication
what were the 3 hypothesised dna replication models
- conservative models
- semi-conservative
- dispersive model
what is the correct model on how dna replicates itself. explain it
semi-conservative
- Each strand of a DNA molecule from the parent generation acts as a template for the synthesis of a new strand in the daughter generation
- each new DNA double helix in the daughter cells is a hybrid of 1 old strand + 1 new strand
explain how Meselson and stahl found dna used semi-conservative replication
- grew bacteria in heavy nitrogen (N-15), then moved them to light nitrogen (N-14)
- After DNA replication, they used centrifugation to analyze DNA density
- 1st replication cycle showed DNA of intermediate density - suggest each molecule had one original and one new strand
- 2nd cycle revealed both intermediate and light bands, confirming semi-conservative replication
- This meant each new DNA molecule retained one old strand and incorporated one newly synthesized strand
explain the conservative dna replication
- original DNA double helix acts as a template to create a completely new double helix - makes one daughter molecule containing the original, intact DNA strands + another daughter molecule with entirely new DNA strands
what is helicase
enzyme that unwinds parental double helix at replication forks
what is topoisomerase
relieves the strain of the unwinding done by helicase
what is single-strand binding proteins
binds and stabilises single strands of dna at the bubble
what is dna polymerase I
removes RNA nucleotides of primer from 5’ end and replaces them with DNA nucleotides
what is primase
synthesises RNA primer at 5’ end
what direction does dna polymerase extend in
5’ to 3’ direction
how is dna organised
antiparallel
2 sides of a replication bubble
leading strand - extends continuously
lagging strand - made up of lots of little fragments (Okazaki fragments)
