Protein Synthesis

Using RNA to make proteins

Going from DNA to Proteins• Let’s review what we’ve done so far:

• We take our DNA and convert it into RNA in a process called ________________.

• This happens in the _____________.

• Our proteins are made from Amino Acids so we need a bridge to go from the RNA we just made to amino acids…

The Genetic Code• The language of mRNA is called the Genetic Code (A, G, U,

C) (contains only 4 letters)

• It is the matching of the RNA sequence to the correct amino acid to make proteins.

• It is based on codons, which are 3 bases together on an mRNA chain.

• Each codon codes for a specific amino acid

• There are 64 codons and 20 amino acids so several codons will code for the same amino acid

• NOTE: The Genetic Code is Universal: Most organisms follow this code to produce protein demonstrating that we are all evolutionary related.

Special Codons: Start/Stop Condons

• Start Codon: AUG• Signals the beginning of protein

synthesis/translation

• Protein synthesis can only begin at this codon.

• Also codes for the Amino Acid: Methionine

• Stop Codons: UAA, UAG, UGA• Signals protein synthesis to stop as soon as they

are reached on the mRNA strand.

• Similar to the period at the end of a sentence or the end of a polypeptide chain.

Genetic Code Square

Genetic Code Circle

What do we have after transcription?

RNA molecules:

•mRNA with the DNA code•tRNA waiting with the amino acid•rRNA making ribosomes for translation

How do we get from RNA to Proteins?

Translation!!

Changing from the language of nucleic acids to the language of proteins in the RIBOSOME

OR

process of converting mRNA to amino acids

All 3 types of RNA are used in protein synthesis

tRNA and Anticodons• Amino acids are free floating in the

cytosol and are transferred to the ribosome by tRNA molecules

• How is tRNA able to do this job? it contains 4 binding sites

• Top part of loop allows for 1 AA to bind

• Bottom part of loop is the anticodon (sequence of 3 nucleotides) that pairs with its complementary matching mRNA

• Ex. If the mRNA: UUU then anticodon : AAA

Note: the pairing of a anticodon with a codon ensures that the Amino Acids are being joined in the correct order

Setting the stage for Translation…

We’ve made our mRNA and its moving into the ribosome

Our tRNA’s are transporting the amino acids over to the ribosomes

OK. We are ready to Translate!

Steps 1-4 of Translation

• Step 1: Ribosomes attach to the start codon (AUG) of the mRNA.

• Step 2: A tRNA with the anticodon of UAC and a methionine binds to the mRNA inside the ribosome.

• Step 3: In the 2nd binding site, the next codon is read and a tRNA is attached

• Step 4: When the second tRNA attaches, the amino acids attached to the 1st tRNA releases from the tRNA and binds to the new amino acid with a peptide bond

Steps 5-6 of Translation• Step 5: Then the

1st tRNA is released and the ribosome moves to another codon (3 nucleotides).

• Step 6: When a stop codon is reached, translation stops and the ribosome releases the mRNA and the created protein.

Protein synthesis analogy

Animationshttp://biology.kenyon.edu/slonc/bio3/ribo/ribo3.html

http://www.youtube.com/watch?v=NJxobgkPEAo