The Central Dogma - Protein Synthesis

Protein Synthesis is a central process in cells

the information coded in bases of DNA determines the sequence of amino acids in a polypeptide

The Central Dogma

"Central Dogma": Term coined by Francis Crick to explain how information flows in cells.

Protein Synthesis

Translation

Protein

DNA

RNA

Protein

Allows for Gene Expression

language of amino acids

language of nucleic acids

Transcription

DNA Replication

Translation

polypeptide

RNA

tRNA

The Ribosome

How translation happens:

Translation

Transfer RNA molecules.

Responsible for bringing amino acids to the ribosome

2. Elongation

1. Initiation

tRNA "anti-codon"

The site of protein synthesis.

The only "non-membrane" bound organelle.

All cells have ribosomes.

Composed of two subunits.

Has three "sites":

• The mRNA attaches to the small ribosomal subunit.

• Methionine is brought to the start codon (AUG) by the methionine tRNA.

• The ribosome assembles so that the start codon (AUG) is in the P-site.

• This is called the "translation initiation complex".

The next codon is now available in the A-site for the next incoming charged tRNA

The next codon determines the next amino acid to be brought to the ribosome.

The incoming charged tRNA enters at the A-site.

polypeptide

RNA

• Amino acids are added to tRNA molecules through the action of "amino-acyl tRNA synthase" enzymes.

• A tRNA with the an amino acid attached is said to be "charged"

tRNA binding at the ribosome is mediated by an "anti-codon" loop in the tRNA molecule

tRNA

The ribosome shifts ("Translocates"). The tRNA with the polypeptide is now in the P-site.

The uncharged amino acid is now in the E-site.

• A site: "Aminoacyl"- where amino acids enter the ribosome
• P site: "peptidyl"- where the growing polypeptide is kept.
• E site: "exit"- where empty tRNA molecules leave.

The Ribosome

The growing polypeptide is transfered to the new tRNA molecule. A peptide bond is formed.

How translation happens:

3. Termination

Transfer RNA molecules.

Responsible for bringing amino acids to the ribosome

2. Elongation

1. Initiation

• When a stop codon (UAG, UAA, or UGA) is encountered, a release factor binds to the A-site.

• The polypeptide chain is released.

• The ribosome disassembles.

The Genetic Code:

tRNA "anti-codon"

The site of protein synthesis.

The only "non-membrane" bound organelle.

All cells have ribosomes.

Composed of two subunits.

Has three "sites":

• The mRNA attaches to the small ribosomal subunit.

• Methionine is brought to the start codon (AUG) by the methionine tRNA.

• The ribosome assembles so that the start codon (AUG) is in the P-site.

• This is called the "translation initiation complex".

The next codon is now available in the A-site for the next incoming charged tRNA

The next codon determines the next amino acid to be brought to the ribosome.

The incoming charged tRNA enters at the A-site.

Universal across all domains of life.

Protein Synthesis in Total

• Amino acids are added to tRNA molecules through the action of "amino-acyl tRNA synthase" enzymes.

• A tRNA with the an amino acid attached is said to be "charged"

Prokaryote & Eukaryote

Translation Specifics

tRNA binding at the ribosome is mediated by an "anti-codon" loop in the tRNA molecule

Prokaryotes

Eukaryotes

Since prokaryotes do not have a nucleus, transcription and translation can be coupled.

Eukaryotes can not couple transcription and translation.

Eukaryotes also need to target different polypeptides to different areas of the cell.

The ribosome shifts ("Translocates"). The tRNA with the polypeptide is now in the P-site.

The uncharged amino acid is now in the E-site.

• A site: "Aminoacyl"- where amino acids enter the ribosome
• P site: "peptidyl"- where the growing polypeptide is kept.
• E site: "exit"- where empty tRNA molecules leave.

The growing polypeptide is transfered to the new tRNA molecule. A peptide bond is formed.

3. Termination

Signal Peptide: a small signal peptide sequence on polypeptides that need to be made at the endoplasmic reticulum. Recruits an SRP protein, which modulates "docking" of the ribosome to the rough ER.

Polyribosomes: simultaneous translation of a transcript (even while that transcript is still being made).

• When a stop codon (UAG, UAA, or UGA) is encountered, a release factor binds to the A-site.

• The polypeptide chain is released.

• The ribosome disassembles.

Triplet code: mRNA is read in units of three bases ("codons")

There are 64 possible codons (for 20 possible amino acids).

The code is redundant and unambiguous.

The code has "start" and "stop" punctuation.

The Genetic Code:

Universal across all domains of life.

Protein Synthesis in Total

Prokaryote & Eukaryote

Translation Specifics

Prokaryotes

Eukaryotes

Since prokaryotes do not have a nucleus, transcription and translation can be coupled.

Eukaryotes can not couple transcription and translation.

Eukaryotes also need to target different polypeptides to different areas of the cell.

Signal Peptide: a small signal peptide sequence on polypeptides that need to be made at the endoplasmic reticulum. Recruits an SRP protein, which modulates "docking" of the ribosome to the rough ER.

Polyribosomes: simultaneous translation of a transcript (even while that transcript is still being made).

Triplet code: mRNA is read in units of three bases ("codons")

There are 64 possible codons (for 20 possible amino acids).

The code is redundant and unambiguous.

The code has "start" and "stop" punctuation.

Protein synthesis is accomplished through a process called translation. After DNA is transcribed into a messenger RNA (mRNA) molecule during transcription, the mRNA must be translated to produce a protein. In translation, mRNA along with transfer RNA (tRNA) and ribosomes work together to produce proteins.