The structure of DNA

HideShow resource information
  • Created by: Jenny Le
  • Created on: 14-04-14 14:55

Re-Cap

Miescher (1869) - extracted nuclein from human pus cell nuclei, contains P, protease resistant

Kossel and Neumann (1894) - initial identification of the bases in DNA

Hammersten (1900) - DNA contains a pentose sugar

Levene (1929) - the pentose sugar is 2'deoxyribose

Todd and Levene (1920's) - determined the chemical nature of nucleic acids. Repeating units: nucleotides. Two forms: DNA and RNA.

Todd (1930's) - nucleotide linkages

Caspersson (1934) - DNA is a linear macromolecule

Chargaff (1950's) - constancy in base ratios from different species, A=T and G=C

Franklin and Wilkins (1952) - X-ray studies, DNA is helix and gave measurements for the helix

Watson and Crick (1953) - developed Franklin and Wilkins model - double helix

1 of 10

Nucleotide Building Blocks - Bases

BASES:

  • nitrogenous 
  • planar molecules
  • two sorts; Pyrimidine & Purine

Purines are two ring bases.

  • Adenine
  • Guanine

Pyrimidines are one ring bases.

  • Cytosine
  • Thymine
  • Uracil
2 of 10

Nucleotide Building Blocks - Sugar

SUGAR:

  • Pentose sugar
    • RNA = ribose
    • DNA = deoxyribose
    • (-OH vs. -H at 2'C)
3 of 10

Nucleotides and Nucleosides

Difference between nucleotide and nucleoside:

A nucleotide is a phosphorylated nucleoside.

Sugar + base = nucleoside

Sugar + base + P = nucleotide

In AMP and ATP, both nucleotide, the phosphate group(s) is bonded to the sugar at 5'C by an ester linkage.

Synthesis of DNA requires the triphosphate form.

  • PPi is lost in the reaction
  • Nucleotides are joined together to form a polynucleotide
  • Join via P-sugar-P-sugar

Hence:- phosphodiester bond, sugar phosphate backbone, 3' and 5' ends.

Add dNTP ==> PPi released and base addedd 5' to 3' direction

4 of 10

RNA and DNA

RNA

  • single stranded molecule
  • ribose sugar
  • P groups
  • bases (purines/pyrimidines)
  • phosphodiester links
  • 5' ==> 3'

DNA

  • double stranded molecule - two complementary anti-parallel strands with sugar-P backbone and protruding bases.
  • deoxyribose sugar
  • P groups 
  • bases (purines/pyrimidines)
  • hydrogen bonds
  • 5' ==> 3' for each strand
5 of 10

DNA as genetic material

Most DNA exists as a right handed double helix as predicted by the X-ray crystallgraphy of Wilkins and Franklin. 

It is the B-form found in living cells, other forms can be generated in vitro.

Stable structure:

  • Base stacking/secondary structure and packaging.

Replication:

  • Copy strands using complementary base pairs as the template

Information Store:

  • Uses the sequence of bases as the code. Reads bases in groups of three, 5' to 3'. Gives mechanism for mutations.

Transfer of information:

  • RNA copy, U not T. Transcription transfers information
6 of 10

DNA - Linear or Circular?

Eukaryotic nuclear chromosomes - linear dsDNA, extremely long - H. sapiens, 46 chromosomes, 6.6 x 10^9 bp.

Eukaryotic organelle (mitochondron, chloroplast) - circular dsDNA, intermediate length - H. sapiens mitochondria, 16569 bp.

Bacterial genome - circular dsDNA, extremely long - E.coli, 4.6 x 10^6 bp.

Plasmids - circular dsDNA, relatively short - 2.6 x 10^3 bp.

Viruses - linear dsDNA or circular ssDNA, also RNA genome viruses. 

7 of 10

Heating DNA

When DNA is heated, the double stranded molecule becomes single stranded DNA as the hydrogen bonds break between the base pairs. 

DNA denaturation:

Base pairs separate A=T first, followed by G=C 

Tm: melting temperature, increases with higher GC content.

The reaction is followed by measuring the absorbance at 260nm as ssDNA absorbs more light than dsDNA. 

8 of 10

Cooling DNA

When ssDNA is cooled, the strands will reanneal

  • due to complementary base pairing.

This does not have to be the original DNA strand it was separated from during heating, also referred to as hybridisation

  • add another ssDNA (or RNA) to the reaction.
  • DNA/DNA (RNA:DNA) hybrids formed 
  • relies on the sequences being complementary
  • a sequence of DNA (or RNA) will form complementary base pairs with ANY likely sequence generating stem loop structures, hairpin loops giving secondary structures seen in many RNA molecules e.g. tRNA.
9 of 10

Determining GC content

Determination of GC content of a DNA molecule by...

  • Tm
  • Buoyant Density.

Buoyant Density:

  • a linear function of GC content
  • can be used to purify DNA
    • 8M CsCl has about the same density as DNA 
    • Mix with the DNA sample of interest
    • Centrifuge FAST (125 x g, 125 hours)
    • Set up a density gradient in the centrifuge tube, the DNA will form a band at the point where its density is the same as CsCl.
    • Protein will be found at the top of the gradient, RNA will pellet at the bottom of the tube
    • This process can also be used to purify one DNA sample from another, for example plasmid DNA and bacterial cell genomic DNA will form separate bands.
10 of 10

Comments

No comments have yet been made

Similar Human Biology resources:

See all Human Biology resources »See all The structure of DNA resources »