LECTURE                                 PROTEIN SYNTHESIS

In this lecture you will learn:

THE 3 RNAs involved in protein synthesis
1. mRNA is messenger RNA 
- it is the actual "message" that is TRANSLATED into protein
- the base sequence specifies which proteins will be made during translation
2. rRNA is ribosomal RNA
-  the RNA that is the "machinery" that makes proteins
- it is structural, proteins and the rRNA are assembled into a globular structure like an enzyme
- it is composed of 2 subunits
3. tRNA is transfer RNA

- There are 32 different tRNAs and each one binds a SPECIFIC  amino acid
- it holds the amino acid in a physical position so it can be added onto the developing protein polymer chain 
- each tRNA is the product of a separate gene or section of DNA  containing 73-93 nucleotides
each has a "loop and stem" structure
-  the stems are hydrogen bonded to give structure
the paired and H bonded stems can form sections of double helix 
-  the unpaired regions form 3 loops
-  each tRNA carries (at its 3' end) the amino acid specific to that tRNA
- the middle loop has the 3 unpaired bases that form the anticodon base 
- it is the anticodon that pairs with the complementary codon on the mRNA molecule and brings the correct amino acid into alignment to be added to the growing polypeptide chain.


 
DECIPHERING THE CODE: FROM RNA TO PROTEINS

 
 
 
 
 

In order to make proteins it is necessary to faithfully TRANSLATE  from the language of RNA to the language of amino acids. 

Through experimentation scientists discovered that it requires 3 RNA bases to code for a single amino acid. 

Next, scientists worked out the "CODE" for each amino acid.  So 3 bases = 1 CODON 

HERE ARE TWO DICTIONARIES FOR ALL 20 AMINO ACIDS
 
 
 
 
 
 
HOW TO READ THE CODON DICTIONARY
Lets start with the "start" codon, AUG.  Find the A row on the left (first letter). Now look for U in the columns of the second letter and find the A row of the third letter (column on right)

 
 
 
you will notice that most amino acids have multiple codons.  This is called "wobble" and is due to the fact that with 4 different bases and only 3 needed per amino acid, there are a total of 64 different possible combinations (4*3) that only code for 20 different amino acids. 
 

CODON WHEEL

There are 3 letter abbreviations to each amino acid. 
 
 
 
you will notice that most amino acids have multiple codons.  This is called "wobble" and is due to the fact that with 4 different bases and only 3 needed per amino acid, there are a total of 64 different possible combinations (4*3) that only code for 20 different amino acids. 
 

 
tRNA, THE BILINGUAL RNA

 

tRNA (transfer RNA) is THE "bilingual" RNA that "speaks" the base language on one end ----->  (the anticodon) 
 
 
 
 
 
 
 
 
 

and "speaks" the amino acid language on the other end --------->


 
THE STEPS OF TRANSLATION AKA  PROTEIN SYNTHESIS - WATCH MOVIE HERE
The first step was making the RNA required in the nucleus (previous lecture), the mRNA, tRNA, rRNA and processing it for being translated.  The RNAs move out of the nucleus and into the cytoplasm.

STEP 1. INITIATION
a. The small subunit of the ribosome unit finds the leader sequence just upstream from the AUG/start site on the mRNA. 

b. The tRNA carrying the anticodon UAC binds firmly to the codon AUG by hydrogen bonds.  This tRNA carries the amino acid MET (methionine) on the other end. 

c. The large ribosome subunit now joins the small subunit. 

STEP 2. ELONGATION
a. The tRNA carrying the anticodon for the NEXT codon moves into place and binds firmly by hydrogen bonds. 

b. An enzyme now forges the covalent bond between the two amino acids, MET and the 2nd one in line.  The formation of the covalent bond is called "dehydration synthesis".

c. The amino acid MET is released from the tRNA.

d. The tRNA for MET is ejected from the ribosome and floats free to go pick up another MET. 

e. The ribosome moves down the mRNA to the next codon. 

f. Elongation repeats until it runs into a stop codon. 

STEP 3. TERMINATION
a. A "release factor" binds to the stop codon. 

b. An enzyme separates the chain of amino acids (polypeptide/protein) from the last tRNA.

c. The ribosome comes apart and translation ends. 

MOVIE
 
 


 
SUMMARY OF WHOLE SEQUENCE FROM DNA TO PROTEIN
Please understand this picture, will be in quiz
What you are likely to see in exam

REVIEW QUESTIONS:
What is translation?
How many different letters are there in the RNA alphabet?
How many different letters are there in the protein alphabet?
How many of the 4 bases are needed to code for 1 amino acid?
Be able to read the RNA codon to amino acid table.
Where are codons found?
Where are anticodons found?
What is the start codon for all proteins?  Which amino acid does it code for?