Download The Genetic Code

THE GENETIC CODE
A2 Biology
DNA
• Deoxyribonucleic acid
• Carries the instructions for the
behaviour and construction of cells
• Can make a perfect copy of itself
Structure
• Nucleotide
• Sugar
(deoxyribose)
• Phosphate
• Nitrogenous base
• Complementary
strands
• base pairs
A=T
C=G
Functions of DNA
1. Replication – in dividing cells
2. Expression - Carrying information for
protein synthesis – in all cells (and
future generations)
Replication
• Each daughter cell needs to receive a complete exact
copy of the DNA so it will have all the genes required
to carry our it’s functions and so will future
generations
• Occurs during interphase before cell division
• Genome = sum total of all the genes in an organism
(60,000-80,000 genes in the
human genome)
Semi Conservative Replication
• The DNA double helix unwinds and each
strand acts as a template to make a new
strand
• The result is 2 helices of DNA each made
up of half the original DNA and half new
DNA (semi conservative)
3’ vs 5’
• Anti-parallel nature
• 3’ to 5’ strands
Steps
1. Replication starts at a specific sequence
on the DNA molecule called the
replication origin.
2. The enzyme Helicase unwinds and
unzips DNA using energy, breaking the
hydrogen bonds that join the base pairs,
and forming two separate strands
(replication fork forms).
McGraw Hill animation 1
Replication Fork
The new DNA is built up from the four nucleotides
(A, C, G and T) that are free floating in the
nucleoplasm. The 2 halves of the DNA act as
templates.
4. These nucleotides attach themselves to the bases
on the old strands by complementary base pairing.
5. The enzyme DNA polymerase joins the new
nucleotides to each other by strong covalent
bonds, forming the sugar-phosphate backbone.
3.
Polymerase can
only add
nucleotides to
the 3’ side (free
OH end).
This produces one complete strand (leading
strand) and one strand of fragments (lagging
strand)
RNA Primase creates a short primer to
start copying for the lagging strand.
7. DNA Ligase joins the fragments together.
6.
McGraw Hill Animation
8. A winding enzyme winds the new strands
up to form double helices.
9. The two new molecules are identical to
the old molecule.
Harvard animation
FYI
• DNA replication can take a few hours, and in fact
this limits the speed of cell division. One reason
bacteria can reproduce so fast is that they have a
relatively small amount of DNA.
http://www.wiley.com/college/pratt/0471393
878/student/animations/dna_replication/ind
ex.html
Meselson-Stahl Experiment (1958)
• Originally Watson and Crick proposed the semi-
conservative hypothesis .
• The evidence for the semi-conservative
hypothesis came from an experiment performed in
1958 by Meselson and Stahl.
• They used the bacterium E. coli together with the
technique of density gradient centrifugation/(ultracentrifuge) which separates molecules on the basis
of their density.
• Sumanasinc animation
• Possibilities:
1.
Initial test to see if it will work: Grow bacteria on
medium with normal 14NH4 then centrifuge
Grow bacteria for many generations on medium
with 15NH4 then centrifuge
2. Culture bacteria in 15NH4 . Return heavy DNA
bacteria to 14NH4 medium for 20 minutes (one
generation)
The DNA has replicated just once in 14N medium.
The resulting DNA is not heavy or light, but
exactly half way between the two.
(Thus rules out conservative replication)
3. Grow on 14NH4 medium for 40 mins (two
generations)
After two generations the DNA is either light or
half-and-half.
(This rules out dispersive replication)
The results are all explained by semiconservative replication.
McGraw Hill M-S expt
The Genetic Code
• The sequence of bases on DNA codes for the
sequence of amino acids in proteins.
• There are 20 different amino acids and only 4
different bases, so the bases are read in groups
of 3.
• This gives 43 or 64 combinations, more than
enough to code for 20 amino acids.
• A group of three bases coding for an amino acid
is called a codon, and the meaning of each of the
64 codons is called the genetic code.
i.e.
• If the code consisted of:
• 1 nucleotide – it could code for 1 amino acid
each
• 2 nucleotides - ?
• 3 nucleotides - ?