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DNA - THE GENETIC MATERIAL (structure of DNA (Erwin Chargoffs rules ([A] =…
DNA - THE GENETIC MATERIAL
history of genetics
Walther Fleming (1885) - founder of cytogentics
movement
Friedrich Miescher 1869
the first researcher to isolate and identify nucleic acid
isolated DNA from pus
DNA is the genetic material
Frederick Griffith (1928) - British bacteriologist whose focus was the epidemiology and pathology of bacterial pneumonia
Griffith's Experiment - first widely accepted demonstrations of bacterial transformation
living S cells caused pneumonia in mice, living R cells didn't
heat killed S cells and living heat killed R cells caused pneumonia showing that S cells caused something in R cells
heat killed S cells didn't cause pneumonia
showed that a pathogenic microorganism is capable of ‘transforming’ a non-pathogenic microorganism
Avery / McCarty / MacLoad (1944)
Oswald Avery showed that the transforming activity in S cells is not destroyed by heat
Macyln McCarty showed that the transforming activity in S cells is not destroyed by RNAase or protease so is not protein or RNA
Colin MacLoad showed that the transforming activity is destroyed by DNAase so is most likely to be DNA
Alfred Hershey and Martha Chase (1952)
Hershey–Chase experiment provided additional evidence that DNA, not protein, was the genetic material of life using bacteriophages (a virus which parasitizes a bacterium by infecting it and reproducing inside it)
A: a E. coli cell grown on 32p-containing medium (labels DNA) infected with nonradioactive T2 phage. DNA-labelled progeny phage is used to infect nonradioactive cell. Part of phage remaining attached to cells removed by kitchen blender, showing infected cell. Progeny phage contains some 32p-labelled DNA from the parental phage DNA
B:E. coli cell grown on 35s-containing medium (labels protein) infected with nonradioactive T2 phage. Protein-labelled progeny phage is used to infect nonradioactive cells. Part of phage remaining attached to cell removed by kitchen blender, showing infected cell. Progeny phage contains almost no 35s-labelled protein.
conclusion: DNA from an infecting parental phage is inherited in the progeny phage
viruses
retrovirus
any of a group of RNA viruses which insert a DNA copy of their genome into the host cell in order to replicate, e.g. HIV
rotavirus
most common cause of diarrhoeal disease among infants and young children
genus of double-stranded RNA viruses in the family Reoviridae
an infective agent that typically consists of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is able to multiply only within the living cells of a host
hepatitis C - single stranded RNA virus
biochemical composition of nucleic acids
nucleoside: base and sugar
nucleotide: base, sugar and phosphate
base
purine bases
guanine (G)
adenine (A)
pyrimidine bases
thymine (T)
cytosine (C)
uracil (U)
sugar
deoxyribose in DNA, ribose in RNA
numbering starts at 3 o clock with 1 and finishes at little tail with 5
5 carbon sugar
phosphate
PO4
nucleotides join to form a DNA or RNA chain - 5'-phosphate at one end and 3'OH at other
DNA and light absorbtion
purine structure (and all DNA bases) has an aromatic ring containing delocalised electrons which can be excited with 260nm light hence DNA and RNA absorbs UV
absorption is influenced by the presence or absence of the base-base hydrogen bonds
UV absorbance of single stranded DNA and RNA is higher
structure of DNA
x-ray diffraction and crystallography
determining structure of a crystal - crystalline atoms cause beam of incident X-rays to diffract into many directions - angles and intensities measured - 3D image produced showing electron density - positions, chemical bonds, disorder of atoms can be determined
William Astburys x ray images
Paulings DNA structure
failed attempt - triple helix model with phosphate pointing outwards but this is not possible as negatively charged phosphates repel one another
Watson and Crick were on the same track in 1951-52
Watson and Crick
main competition was Pauling
produced final features
most DNA double helices are right-handed - only Z-DNA is left-handed
double helix is anti-parallel - the 5' phosphate end of one strand is paired with the 3' OH end of its complementary strand (and vice versa).
double-stranded helix, two strands connected by hydrogen bonds between base pairs
C-G base pairs have 3 H bonds
A-T base pairs have 2 H bonds
outer edges of the nitrogen-containing bases are exposed and available for potential hydrogen bonding as well
B form is most common form of DNA
contains major grooves (backbones far apart) and minor grooves (back bones close)
like for like model
implicated that the four bases should occur in any proportion, without dependence on each other.
had the advantage of replication potential but it would distort the sugar-phosphate backbone
Erwin Chargoffs rules
[A] = [T] and [G] = [C] so [A] + [G] = [T] + [C]
a
As a consequence of the above:
([A] + [G]) / ([C] + [T]) = 1
but ([A] + [T]) / ([G] + [C]) = variable
allowed Watson to figure out base pairing rules
other forms of DNA
left handed helix, bases are tilted by -9°, rise/bp = 0.37 nm, one full turn = 4.56 nm, diameter 1.8 nm
A-DNA
bases are tilted by +19°, rise/bp = 0.24 nm, one full turn = 2.46 nm, diameter = 2.3 nm, found in RNA/RNA and DNA/DNA duplexes
G-DNA
four strands, formed in DNA with runs of guanilates - four guanilates hydrogen-bond (Hoogsteen bonding) and form a stack
DNA
diameter = 2nm, one full turn = 3.4nm