Topic 11- Translation

What is the Ribosome

• what are they made of

• where are they found?

Molecular machine made of 2/3 ribosomal RNA (rRNA) & 1/3 protein that builds proteins by translating genetic information from mRNA.

• Large subunit

• Small subunit

rNA- responsible for structure & function

proteins- help the rRNAs change shape as they catalyze chemical reactions

Ribosomes are found:

• bound to ER (make proteins for secretion, membranes, or organelles)

• floating in cytoplasm (make proteins used inside cell)

What are the 3 parts of the Ribosome?

• what is Translocation

A site- where the new tRNA enters

P site- holds the growing protein chain

E site- where the empty tRNA leaves

mRNA shifts forward by a codon, empties first site and kicks the third out.

tRNA

• function

• composition

Brings specific amino acids to the ribosome during translation. (molecular bridge)

• made of a single strand of RNA

• Has Anticodon (complementary to mRNA codon)

• complex 3D L , cloverleaf shape (due to base pairing)

How does the right amino acid get linked to the right tRNA?

There's a different aminoacyl-tRNA synthetase for each of the 20 amino acids.

The tRNA and amino acid both attach to enzyme, enzyme links them together.

What is Wobble?

• explain significance

Flexible base pairing between tRNA anticodon and mRNA codon that allows one tRNA to recognize multiple codons.

(flexibility at 3rd base of codon)

allows DEGENERACY of the genetic code

multiple codons → same aa → fewer tRNAs needed

How does the ribosome know where to start in bacteria?(initiation)

what is the first amino acid used in bacteria?

The small ribosomal subunit of the ribosome base pairs with the Shine-Dalgarno Consensus Sequence (comes just before start codon) of the rRNA

Formylmethionine (fMet)

P site!

How does the Ribosome know where to start in Eukaryotes? (Initiation)

• which site does Met bind to?

1) tRNA carrying methionine attaches to the small ribosomal subunit

2) with initiation factors, they bind to the 5’ end (5’ UTR) of mRNA (recognizing 5’ cap)

3) They “walk” along the mRNA → 3’ until they reach the first AUG in Kozak sequence

4) Large ribosomal subunit joins

P site!

What are Initiation Factors?

• Specialized proteins that assist translation initiation

• Help ribosome, mRNA, and tRNA find eachother

powered by GTP

What is circles-message amplification

If Poly-A binding protein coats the poly A tail, it interacts with cap binding proteins, circularizing mRNA

= translation of SAME mRNA

What are Stop codons

What are release factors

no tRNA

Recognized by release factor proteins that bind to A site and deconstructs subunit.

UAA

UAG

UGA

indicates termination

What direction are polypeptides formed?

N → C

NH3+ → O=C-O-

Amino Acid Structure

In elongation, the creation of peptide bonds between amino acids is catalyzed by…

rRNA in the Large subunit

peptide bonds = covalent bonds between amino acids

Constitutive expression

Genes continuously expressed under normal cellular conditions.

Not regulated, expressed continually

What is an operon?

Cluster of genes in prokaryotes controlled by a single promoter and transcribed into one mRNA

• Components:

  • Promoter (RNA polymerase binding site)

  • Operator (regulatory switch)

  • Genes (encode proteins)

• Function: Allows genes to be turned ON/OFF together

Allosteric Protein

give example!

Changes shape in response to environment

eg. Repressor protein

Negative Inducible Operon

(give example)

Catabolic pathways (breaks down molecule)

Repressor binds to DNA

Presence of molecule inactivates repressor → comes off DNA

eg Lac Operon

Negative Repressible Operon

(example?)

Anabolic Pathways (makes molecules)

Repressor does NOT bind to DNA

Presence of molecule activates repressor → binds to DNA

eg. tryptophan in e-coli

Define Catabolism and Catabolite Repression

Catabolism: breaking down lactose → glucose

Catabolite Repression: favoring the use of glucose over metabolims of other energy sources

(represses metabolism of other sources in presence of glucose)

cAMP

“Hunger Signal”

Turned on when low Glucose → turns on ALL metabolism

Global regulator that coordinates 100s of genes (pos control mechanism)

What is CAP

The positive effect of cAMP is activated by Catabolite Activator Protein (CAP).

• cAMP binds to CAP'

• together CAP–cAMP complex binds to a site slightly upstream from the lac gene promoter.

• improves binding of polymerase and increases transcription rate 20-fold.

The binding of the cAMP–CAP complex to DNA produces a sharp bend in DNA that activates transcription.