Transcript The Central Dogma of Genetics

The Central Dogma of
Genetics
The Central Dogma
• Proposed by Francis Crick in
1959
• DNA codes for RNA
• RNA codes for protein
• Protein determines our
physical makeup – phenotype
What are proteins?
• Polymers of amino acids
• Amino acid structure:
Amino
group 
Variable group ↑
Carboxyl
group
What are proteins?
• There are 20 different amino
acids.
• The R group (called a “side
chain”) is different for each
amino acid.
A few A.A. side chains:
• Glycine: H
• Alanine: CH3
• Serine: CH2OH
• Tryptophan:
A few A.A. side chains:
• Only cysteine and methionine
contain sulfur atoms in their side
chains. These atoms can form
“cross bridges” (aka disulfide
bonds)
• In General, side chains can be:
–Polar
-- Non-polar
–Bulky
-- Small
–Positively charged
–Negatively charged
Amino Acids
• Essential A.A.’s must be
supplied by the diet, cannot be
synthesized by organism.
• Non-essential A.A.’s can be
synthesized by organism.
We like to bond
• Links between amino acids are
called peptide bonds.
• Dehydration synthesis (joining
by removal of H2O)
• Carboxyl group of 1 A.A. links
to amino group of another A.A.
• Peptidyl transferase is the
ribosomal enzyme responsible.
Dehydration Synthesis
Levels of structure
• Primary Structure
–The sequence of amino acids
(ex: valine, proline, cysteine…)
• Secondary Structure
–Portions of the polypeptide form
standard shapes:
–Alpha helix
–Beta pleated sheet
Levels of structure
• Tertiary structure
–Unique folds and bends due to
attraction of charges and polar
A.A.s
–Sulfur cross-bridges
• Quaternary structure
–Two or more polypeptides
combine as functional protein
Transcription
• Information encoded in DNA is
converted to mRNA by
transcription.
• RNA is different from DNA:
–Ribose versus Deoxyribose
–Uracil (U) versus Thymine (T)
–Single-stranded versus doublestranded
Transcription
• Occurs on an Open Reading
Frame (ORF).
• An ORF is a sequence of
DNA that gets transcribed:
START codon  many codons
for A.A.’s  Stop Codon
Transcription
• Both strands of DNA contain
genes (ORFs)
• Strand with the same base
sequence as mRNA is the sense
strand (coding strand).
• Complimentary strand is the
antisense strand (anticoding)
• Antisense strand is the template
for mRNA
Transcription
• 3 Stages:
• Initiation
• Elongation
• Termination
Transcription
• Initiation
–RNA Polymerase binds to
promoter region of DNA
• (TATA Box)
–DNA is unzipped around
RNA polymerase
(transcription bubble)
Transcription
• Elongation
–Complementary nucleotides
are added to the mRNA
chain using anticoding DNA
as template.
–New RNA nucleotides are
added to 3’ end (like DNA)
Transcription
• Termination
–RNA polymerase reaches
“terminator sequence” at the
end of gene.
–mRNA floats away, is
processed, then leaves
nucleus through pores in
nuclear envelope.
mRNA Processing
• Introns removed, exons
spliced together.
• Guanine “Cap” added to 5’
end.
• Poly-A “Tail” added to 3’ end.
RNA Processing
Translation
• Information in mRNA used to
produce protein.
• tRNA- “cloverleaf” molecule
–Anticodon
–Amino Acid loading
• Ribosome – RNA and protein
structure, “reads” mRNA
tRNA:
Translation
• mRNA is read from 5’ to 3’
• 3 Bases make up a codon
• Every codon codes for either
an amino acid or STOP
• Ribosome has 3 sites for tRNA
binding: A site (Amino-acyl), P
site (peptidyl), E site (exit).
Process of Translation:
• 1. A “loaded” tRNA pairs with its
codon at the A site.
• 2. A peptide bond forms between
the amino acid and the previous
a.a. chain as the tRNA moves to
the P site.
• 3. The tRNA shifts to the E site
and “exits”, to be reloaded.
Transcription/Translation
are Linked in Bacteria