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DNA
Structure and replication
Nucleotides
3 components
Sugar
Phosphate
Organic base
A bit more about nucleotides
• Nitrogen-containing
base
• Pentose sugar
• Base has H sticking
out
Bases
• 4 different bases:
–
–
–
–
Guanine
Adenine
Thymine
Cytosine
• Purines = A & G
(bigger, 2 rings)
• Pyrimidines = C, T
(smaller, 1 ring)
Joining the nucleotides
• The nucleotides join
together
• Condensation
reaction
• ‘Sugar phosphate
backbone’
• Polynucleotide
strand
Joining the strands
• 2 polynucleotide
strands
• running in opposite
directions
• complimentary base
pairing
• hydrogen bonds
• A with T
• C with G
The Double Helix
• A-T 2 hydrogen
bonds
• G-C 3 hydrogen
bonds
• ‘twisted ladder’
• 10 base pairs for
every complete turn
of the helix
DNA
replication
DNA Replication
DNA Replication
DNA unzips
Nuceotides in the
cytoplasm attach to
the two strands by
base-pairing
DNA polymerase
catalyses the process
Each strand acts as a
template
DNA Replication
Meselson and Stahl
Grew microbes in 15N
growth medium
Then repeatedly on
14N growth medium
DNA was extracted
and separated by
centrifugation
DNA Replication
Making
a
Protein
Genetic Code
• The code is a 3-letter triplet
code.
• Each sequence of 3 bases = 1
amino acid.
• e.g. ATG=Met, TTT=Lys
(called a codon in mRNA)
• 20 different amino-acids used
to make proteins.
• Triplet codes for 43=64
(spares are repeats, stops,
start is always Met)
Protein Synthesis
• The DNA sequence
encodes for the primary
protein sequence.
• Cell functions are
determined by proteins
(enzymes) so DNA
determines cell
activities by determining
protein synthesis.
Click to watch an animation
Stage 1: Transcription
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•
•
•
DNA base sequence determines the amino acid sequence.
Takes place in nucleus
DNA unwinds
Complementary copy (by base pairing) of the coding sequence is
made from RNA (mRNA) using one strand of DNA as template.
Click to watch an
animation
More about Transcription
•
•
•
•
Synthesis is always 5'->3' (extends from 3'OH).
Carried out by RNA polymerase
mRNA leaves nucleus.
[Splicing out of introns occurs in nucleus]
Click to watch
an animation
Stage 2: Translation
• mRNA leaves nucleus and
attaches to ribosome in
CYTOPLASM.
• Ribosome made of rRNA &
protein
• mRNA binds to small subunit
• 1st amino acid is always AUG
(start codon) = Met
• In cytoplasm there are 64 different
molecules of tRNA each with a
specific triplet anticodon.
Click to watch an
animation
More about Translation
• Each tRNA has a specific amino acid
attached by a specific amino-acyl tRNA
synthetase enzyme.
• Process uses ATP & forms activated
molecule to provide energy for peptide
bond
• The anticodon of the correct tRNA then
pairs with the codon of the mRNA.
• This brings two tRNAs together in the
ribosome and allows a peptide bond to be
formed between the two amino acids by
peptidyl transferase.
• Continues until reach one of the three stop
codons (UAA, UAC, UGA).
Click to watch an
animation
Genes & Genomes
• Human genome 3 x 109 bp
• 3% protein coding
• 97% other functions e.g.
telomeres or unknown function
(‘junk DNA’)
• Section of DNA that codes for a
polypeptide is a GENE.
• In humans  100,000 genes (not
all expressed in each cell)
• Genome=total set of information
in one cell.
Click to read more