Protein Synthesis-Translation

Warm-up #61. How is RNA different than DNA?2. Name the 3 steps of transcription.3. What is the end product of transcription?4. What is the difference between the sense and

antisense DNA strands?5. What is the role of RNA polymerase?6. Describe the initiation step in transcription.7. What are the “extra” things that happens to the

mRNA in eukaryotic cells BEFORE it leaves the nucleus?

mRNA

From nucleus to cytoplasm

DNA

transcription

nucleus cytoplasm

translation

trait

aa

aa

aa

aa

aa

aa

aa

aaaa

aa

protein

Things you will need to know in order to understand translation:

1. mRNA codon chart

2. Redundancy of the code

3. Structure of tRNA

4. Ribosome structure

Redundant but not Ambiguous

• Codons GAA and GAG both specify glutamic acid (redundancy)

• Neither of them specifies any other amino acid (no ambiguity)

mRNA Codons:

·  codon = group of 3 mRNA nucleotides 

**1 codon codes for 1 amino acid

·  During translation, proteins are synthesized according to the genetic message of sequential codons along the mRNA

• For ALL life!– strongest support for a

common origin for all life

• Code has duplicates– several codons for

each amino acid– mutation insurance!

Start codon AUG methionine

Stop codons UGA, UAA, UAG

The mRNA code

shows the “dictionary” for the mRNA codons and their corresponding amino acids (know how to use)

1. DNA: AGC-GTG-CCA2. mRNA: 3. amino acid chain (protein):

______________________

shows the “dictionary” for the mRNA codons and their corresponding amino acids (know how to use)

1. DNA: AGC-GTG-CCA2. mRNA: UCG-CAC-GGU3. amino acid chain (protein) = Serine- Histadine- Glycine

·  Transfer RNA (tRNA) is the interpreter between the 2 forms of information: base sequence in mRNA and amino acid sequence in polypeptides

tRNA Structure

Amino acid location (CCA)

3 exposed bases

-an enzyme links a specific amino acid from the cytosol/cytoplasm to each tRNA molecule on the 3’ end (CCA) using ATP for energy

LE 17-14b

Hydrogenbonds

Amino acidattachment site5

3

3 5

Anticodon

Symbol that is widely used

Anticodon

Three-dimensional structure

LE 17-15Amino acid Aminoacyl-tRNA

synthetase (enzyme)

Pyrophosphate

Phosphates

tRNA

AMP

Aminoacyl tRNA(an “activatedamino acid”)

·  Molecules of tRNA are specific for only 1 amino acid

-one end of tRNA attaches to a specific amino acid

-the other end Hydrogen bonds to mRNA codon by base pairing (anticodon = a sequence of 3 bases on tRNA)

Amino acid

Ribosome Structure• large ribosomal subunit (50s)

• small ribosomal subunit (30s)– each subunit = ribosome/protein complex

• Consists of an mRNA binding site plus

3 tRNA binding sites

E P A

LE 17-16b

P site (Peptidyl-tRNAbinding site)

E site (Exit site)

mRNAbinding site

A site (Aminoacyl-tRNA binding site)

Largesubunit

Smallsubunit

Schematic model showing binding sites

E P A

Process of Translation• Translation includes 4 processes:

initiation, elongation, translocation, and termination

1. Initiation of Translation• mRNA binds to small ribosomal

subunit• Initiator tRNA brings1st amino

acid, Methionine (Met) to the initiation site on mRNA (start codon AUG= And U Go)

• the Large ribosomal subunit binds– Met is in the “P” site– “A” site is available for the

next tRNA

LE 17-17

Met

GTPInitiator tRNA

mRNA

53

mRNA binding site

Smallribosomalsubunit

Start codon (AUG)

P site

5 3

Translation initiation complex

E A

Largeribosomalsubunit

GDP

Met

2. Elongation

• The next tRNA anticodon complementary base pairs with the mRNA codon in the “A” site aligning the appropriate amino acid next to “Met.”

• Ribosome forms a peptide bond between “Met” and the 2nd amino acid and it passes the elongating polypetide chain to the tRNA in the “A” site

3. Translocation• Ribosome moves (translocates-change

location) the tRNA in the “A” site to the “P” site

• The empty tRNA moves to the “E” site where it is released

• mRNA moves along/through the ribosome to expose the next mRNA codon to the “A” site

• the protein chain is growing longer

LE 17-18

Ribosome ready fornext aminoacyl tRNA

mRNA

5

Amino endof polypeptide

E

Psite

Asite

3

2

2 GDP

E

P A

GTP

GTP

GDP

E

P A

E

P A

Fueled by GTP= Guanosine Triphosphate

4. Termination• When a “STOP” codon

(UGA, UAA, UAG) is reached on the mRNA, there isn’t a complementary tRNA so….– UGA = U Go Away– UAA= U Are Awesome– UAG= U Are Great

4. Termination• A “release factor” binds to the “A”

site – Causes the addition of a water

molecule to the polypeptide which hydrolyzes the completed polypeptide from the tRNA

• Ribsomal subunits dissociate (break apart from the mRNA)

• Newly formed protein goes to rough ER for modification, then to golgi for packaging then released from cell (exocytosis)