the central dogma of genetics. the central dogma proposed by francis crick in 1959 dna codes for rna...
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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 Carboxyl
group group
Variable 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.
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 double-
stranded
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/Translationare Linked in Bacteria
