DNA Structure

DNA Structure
DNA Structure
• DNA – Deoxyribonucleic Acid
• DNA - the thread of life
• DNA – the genetic carrier
Evidence for DNA: Bacterial Transformation
• In 1928, F. Griffith was attempting to
develop a vaccine against Streptoccocus
pneumoniae he noticed 2 strains: one had
a rough surface appearance, but those of
the other strain appeared smooth. He
designated the 2 strains R and S and used
them in 4 experiments:
In the fourth experiment:
• Live R received from dead (heat-killed) S
bacteria a piece of DNA that caused the
formation of the capsule in R bacteria.
New R bacteria gained the gene of the
capsule and became virulent.
• Transformation: transfer a gene from one
bacterium to another
Confirmation of DNA function
• View the animation ‘Hershey and
Chase Experiment” in my Website.
• Bacteriophages are viruses that infect
bacteria.
• The bacteriophage is made of a
protein coat and a DNA core.
Confirmation of DNA function
• Bacteriophages inject their DNA into
the bacterial cell, while the protein
portion remains outside of the cell.
• This experiment confirms that DNA,
not the protein, is the genetic carrier.
View “Steps in the Replication of T4
Phage in E. coli” – animation in my
Website
DNA Structure
• The basic building block of DNA is a nucleotide.
• Each nucleotide is made of 3 sub-units:
* Phosphate – P
* Deoxyribose – R (5-C-compoud)
* One of 4 bases: A, T, C, or G (A-Adenine, TThymine, C-Cytosine, G-Guanine)
* How many different nucleotides are there?
• A DNA molecule is composed of 2
nucleotide chains (strands) oriented in
opposite direction (antiparallel), and are
twisted together (double helix)
• View “DNA Structure” – animation in my
Website.
The first strand (blue) starts with five prime (5’) end,
ends with three prime (3’)end. The second strand
(red) is oriented in opposite direction
Complementary base pairing: A in one strand pairs
with T in the second strand (A=T pairs), and C pairs
with G (C=G pairs)
Example: if a sequence of bases in a strand of a DNA
molecule is: 5’ A T T C G A G C T 3’
What is the sequence of bases in the second strand?
3’ T A A G C T C G A 5’
Because of this complementary base
pairing: A=T, and C=G
• 1. The total amount of A in a DNA
molecule is equal to the total amount of T
• 2. The total amount of C in a DNA
molecule is equal to the total amount of G
• A + T + C + G =100% of a DNA molecule
• Example: If A makes up 23% of a DNA
molecule. C makes up what proportion?
• 27%
• In the complementary base pairing
hydrogen bonds are used
5’
3’
3’
5’
DNA Replication
• View “DNA Replication” and “DNA
Replication Fork” animations in my
Website.