Protein Synthesis

Making proteins from genes One gene-one enzyme

hypothesis (Beadle and Tatum) Later revised to the one

gene-one polypeptide hypothesis

Transcription synthesis of RNA under

the direction of DNA (mRNA)

Translation synthesis of a polypeptide

under the direction of mRNA

DNARNAProtein ‘U’ (uracil) replaces ‘T’ in

RNA The genetic instructions

for a polypeptide chain are ‘written’ in the DNA as a series of 3-nucleotide ‘words’ Triplet code/codons

More than one triplet for each Amino Acid

The Genetic Code

Initiation of Transcription Transcription factors

attach to TATA box Contains sequences of

only adenine and thymine

RNA polymerase attaches to begin transcription

Elongation and Termination Elongation

RNA polymerase continues unwinding DNA and adding nucleotides to the 3’ end

Termination RNA polymerase reaches

terminator sequence Polyadenylation signal--

AAUAAA

Transcription Helpers RNA polymerase: pries DNA apart and hooks RNA

nucleotides together from the DNA code Promoter region on DNA: where RNA polymerase

attaches and where initiation of RNA begins Terminator region: sequence that signals the end of

transcription Transcription unit: stretch of DNA transcribed into an

RNA molecule

mRNA modification 5’ cap: modified guanine molecule

Protection recognition site for ribosomes

3’ tail: poly (A) tail (up to 250 adenine nucleotides) Protection Recognition transport

RNA Splicing Exons (expressed

sequences) kept Introns (intervening

sequences) spliced out Splicing done by snRNPs

which bind together to form a large compound known as a spliceosome

Made of snRNA RNA that acts as an

enzyme= ribozyme

Basics of Translation mRNA moved

through ribosome “code” read in

triplets tRNA transfers

correct amino acid to the polypeptide chain

tRNA Has an anticodon (nucleotide

triplet) Codes for a specific amino

acid

Picking the right Amino Acid Aminoacyl-tRNA synthetase

Joins a specific amino acid to its tRNA molecule

Wobble Relaxation in the base-

pairing rules Third nucleotide in a codon

can be different in many cases and still code for the same amino acid

Ribosomes rRNA (ribosomal RNA)

site of mRNA codon & tRNA anticodon coupling

P site holds the tRNA carrying the

growing polypeptide chain

A site holds the tRNA carrying the

next amino acid to be added to the chain

E site discharged tRNA’s leave the

ribosome

Initiation of Translation Union of mRNA, tRNA, and small ribosomal

subunit Start codon = AUG (codes for methionine)

Large subunit then attaches

Elongation Codon recognition

Anticodon of tRNA binds with the complementary mRNA codon in the A site

Peptide bond formation Large subunit forms a

peptide bond between the amino acid in the A site and the growing polypeptide

Translocation tRNA in A site moves to P

site… the one in the P site moves to the E site

Termination When a stop codon (UAG, UAA, or UGA) is

reached, the A site accepts a “release factor”

Release factor cuts the polypeptide free from the tRNA molecule

Ribosome dissociates

Polyribosomes

Translation of mRNA by many ribosomes many copies of a

polypeptide very quickly

Where does it happen? Protein always begins in the cytosol

Will continue there unless it receives a signal peptide telling it to bind to the ER

SRP’s (signal-recognition particles) Binds to a ribosome and brings it to the membrane of

the ER Attaches to a receptor molecule

Roles of RNA

MutationsPoint mutation- The change

of just one nucleotide Base-pair substitution

One nucleotide substituted for another

Sometimes a “silent mutation” if it still codes for the same amino acid

Missense mutation- still codes for an amino acid, but not the right one

Nonsense mutation- changes to a stop codon and terminates translation

Frame-shift Mutations Insertion

Nucleotide is added Deletion

Nucleotide is lost Causes the codons from that

point on to be incorrectly grouped

Mutagen- anything that causes a mutation (chemicals, X-rays, carcinogens, etc.)