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.

Dehydration Synthesis

Levels of structure Primary Structure Secondary 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 Quaternary 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/Translation are Linked in Bacteria