Protein Synthesis

What is the central dogma of biology?

Transcription: Initiation

Transcription: Initiation

• RNA polymerase attaches to a “promoter” sequence in front (“upstream”) of a gene– Eukaryotes only: RNA polymerase requires

multiple “Transcription factor” proteins to be able to bind to the promoter

Transcription: Elongation

Transcription: Elongation

• RNA production occurs in a 5’ to 3’ direction• RNA Polymerase catalyzes the synthesis of an

RNA strand from a single DNA strand (Template Strand)

Transcription: Termination

Transcription: Termination

• RNA Polymerase continues to add nucleotides until a terminator sequence is reached

• Once completed, mRNA transcript gets modified in eukaryotes only

Post-Transcriptional mRNA processing (EUKARYOTES ONLY!)

• Exons are the “coding” regions of the DNA and Introns are the “noncoding” regions of the DNA

How is the mRNA transcript modified?

• The introns must be removed and the exons must be spliced together prior to the mRNA leaving the nucleus

• **This is required to produce a functional protein

How is the mRNA transcript modified?

• A “G-cap” (g nucleotide) and “Poly-A tail” (AAAAA) is added to the transcript to facilitate its movement to the cytoplasm

How does alternative splicing contribute to the production of various proteins?

• Having multiple exons in a gene allows eukaryotes to make multiple functioning proteins from one gene

Ribosomes

• Site of Translation (2nd stage of P.S.)• “non-membrane” bound organelle found in all

cells• Composed of two subunits• Structure allows for interaction with mRNA

and tRNA molecules

tRNA• Transfer RNA • Bring specific amino

acids to the ribosome, as dictated by the mRNA sequence

• Anticodon region interacts with mRNA

The Genetic Code

The Genetic Code

• Universal across ALL domains of Life! • Triplet Code: mRNA is read in units of three

adjacent bases (codons)• There are 64 possible codons (for 20 amino

acids)• The code is redundant and unambiguous• The code has “start” and “stop” codons

Translation: Initiation

Translation: Initiation

• mRNA interacts with the ribosome to begin translation at the START codon (AUG) closest to the 5’ end of the mRNA

• tRNA molecule brings methionine (amino acid coded for by AUG codon)

Translation: Elongation

Translation: Elongation

• Subsequent amino acids are brought to the ribosome by tRNA molecules as specified by subsequent, adjacent codons

• Each amino acid is transferred to a growing polypeptide chain

• Peptide bonds formed between chain and new amino acid

• Empty tRNA molecule exits the ribosome and picks up another amino acid

• REPEAT!!!!

Translation: Termination

Translation: Termination

• Once a Stop codon is reached• Release factor protein binds and the

polypeptide is released from the ribosome• Polypeptide moves to golgi body or cytoplasm

for further processing• The ribosome disassembles

How does prokaryotic protein synthesis compare to eukaryotic?

How does prokaryotic compare to eukaryotic protein synthesis?

• Transcription: Prokaryotes can bind DIRECTLY to the promoter region (no transcription factors required)

• No Post-transcription modification of RNA• Prokaryotes have no nucleus so Transcription

and Translation are coupled (occur together) – RNA is transcribed and translated at the same time

What is a polyribosome? How do prokaryotic and eukaryotic organisms benefit from them?

• Many ribosomes translating the same RNA transcript• Enables simultaneous translation of one transcript