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

http://en.wikipedia.org/wiki/Image:Cdmb.svg

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.

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

http://upload.wikimedia.org/wikipedia/commons/1/19/Betasheet.png

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