Topic 4 - Gene Expression

Where does transcription occur?

In the nucleus

Where does translation occur?

In the cytoplasm

Where does RNA processing occur?

In the nucleus

Which strand is used as a template for the synthesis of the complementary RNA strand?

The 3’-5’ (the template strand)

Which strand is used as a coding strand?

The 5’-3’

Start codon

AUG (Methionine)

Stop codons

UAA, UAG, AGA

List the characteristics of the genetic code

1. redundant (more than 1 codons encode for a specific amino acid)

2. not ambiguous (each codon can specify only 1 amino acid)

   1. nearly universal

tRNA

Transfer RNA

transfers amino acids to the growing polypeptide chain during translation

mRNA

Messenger RNA

Translated into proteins

rRNA

Ribosomal RNA

Part of ribosome structure

Transcription

DNA → RNA

Stars with the binding of RNA polymerase to the promoter

Promoter

located at the 5’ prime end if the gene

Terminator

Signalizes the end of transcription in bacteria

RNA polymerase

Synthesizes RNA

Transcribes only the 3’-5’ strand

Adds nucleotides only in the 5’-3’ direction

RNA Polymerase I

synthesizes rRNA

RNA Polymerase II

Synthesizes mRNA and snRNA

RNA Polymerase III

Synthesizes tRNA and small RNAs

Important events of Initiation in Transcription

1. the general transcription factors bind to the TATA box

2. RNA polymerase II binds to the promoter

Important events for Elongation of Transcription

1. untwisting of the double helix

2. addition of nucleotides to the 3’ end

Important events of Termination of Transcription

1. In bacteria, mRNA is translated without being processed

2. In eukaryotes, RNA polymerase II, transcribes a polyadenylation signal and the RNA transcript is released

RNA splicing

Removal of introns and binding of exons

It helps in the production of new proteins from the same pre-mRNA transcript

Spliceosomes

Consists on snRNPs and other proteins

They release the introns and join together the exons

Direction of translation

5’-3’ direction

Ribosomes

They read the mRNA in the 5’-3’ direction

Polypeptide synthesis direction

N-terminal to the C-terminal

Prokaryotic Ribosome overall 70S. How many for small and large subunits?

Small : 30S

Large : 50S

Eukaryotic Ribosome overall 80S. How many for small and large subunits?

Small : 40S

Large : 60S

P site

Holds the t-RNA that carried the growing polypeptide chain

A site

Holds the t-RNA that carries the next amino acid

T site

From where discharged tRNAs leave the ribosome

Free ribosomes

In the cytosol → cytosolic proteins

Bound ribosomes

In the RER, secreted and membrane-bound proteins

Start codon in Prokaryotes

fMethionine

Characteristic events of Elongation of Translation

1. the tRNA recognizes the codon

2. Formation of the peptide bond while new amino acids are added to the growing polypeptide chain

3. Translocation (the tRNA, from Asite moves to the Psite and the mRNA moves 3 nucleotides

Rifampicin

What is it used for?

Mode of action?

Which process does it inhibit?

Used for tuberculosis

Mode of action : inhibits bacterial RNA polymerase

Inhibits Transcription

Tetracyclines

Mode of action?

Which process does it inhibit?

Mode of action : Binds the 30S ribosomal subunit, inhibits aminoacyl tRNA attachment

Inhibits Translation

Aminoglycosides

Mode of action?

Which process does it inhibit?

Mode of action : Bind the 30S ribosomal subunit, inhibit initiation complex formation

Inhibits Translation

Chloramphenicol and Clindamycin

Mode of action?

Which process does it inhibit?

Mode of action : Inhibit 50S peptidyl-transferase, inhibit peptide bond formation during elongation

Inhibits Translation

Macrolides

Mode of action?

Which process does it inhibit?

Mode of action : Block the translocation step of elongation

Inhibits Translation

α-amanitin

Inhibits RNA polymerase II

Diphtheria toxin (bacterial toxin)

Inhibits EF-2 → inhibits translocation