PROTEIN SYNTHESIS

TRANSLATIONI. INITIATION

The ribosome binds to the mRNA at the start codon(AUG) that is recognized only by the initiator tRNA

II. ELONGATION Complexes, composed of an amino acid linked to tRNA,

sequentially bind to the appropriate codon in mRNA by forming complementary base pairs with the tRNA anticodon

The ribosome moves from codon to codon along the mRNA

Amino acids are added one by one, translated into polypeptidic sequences dictated by DNA and represented by mRNA

TRANSLATION

III. TERMINATION A release factor binds to the stop codon, terminating translation and

releasing the complete polypeptide from the ribosome.

tRNA At the 3' end, a specific amino acid

can be attached to a specific tRNA by means of specific enzymes called aminoacyl-tRNA synthetases. The resulting complex of an amino acid and a tRNA is referred to as an aminoacyl-tRNA

At the bottom loop of the cloverleaf is a series of three unpaired tRNA bases called the anticodon

An anticodon is a series of three tRNA bases complementary to a mRNA codon

RNA Codons for Protein Synthesis

U C A G

UUUU = PheUUC = PheUUA = LeuUUG = Leu

UCU = SerUCC = SerUCA = SerUCG = Ser

UAU = TyrUAC = Tyr

UAA = StopUAG = Stop

UGU = CysUGC = CysUGA = StopUGG = Trp

UCAG

CCUU = LeuCUC = LeuCUA = LeuCUG = Leu

CCU = ProCCC = ProCCA = ProCCG = Pro

CAU = HisCAC = HisCAA = GlnCAG = Gln

CGU = ArgCGC = ArgCGA = ArgCGG = Arg

UCAG

AAUU = IleAUC = IleAUA = Ile

AUG = Met

ACU = ThrACC = ThrACA = ThrACG = Thr

AAU = AsnAAC = AsnAAA = LysAAG = Lys

AGU = SerAGC = SerAGA = ArgAGG = Arg

UCAG

GGUU = ValCUC = ValGUA = ValGUG = Val

GCU = AlaGCC = AlaGCA = AlaGCG = Ala

GAU = AspGAC = AspGAA = GluGAG = Glu

GGU = GlyGCG = GlyGGA = GlyGGG = Gly

UCAG

AUG = start codonUAA, UAG, and UGA = stop (nonsense) codons

QUIZ

DNA sequence : A G C T T A C C G T G G mRNA sequence: tRNA anticodon: Amino Acid Sequence:

Characteristics of the genetic code

1. Specificity specific codons always codes for the same amino acid

2. Universality the specificity of the genetic code has been conserved from the very early stages of evolution

3. Redundancy (degeneracy) a given amino acid may have more than one triplet coding for it

4. Nonoverlapping and commaless

Consequences of altering the nucleotide sequence SILENT MUTATION

The codon containing the changed base may code for the same amino acid Ex.: UCA is for Ser, if the 3rd base is changed (to become UCU), it still codes

for Ser MISSENSE MUTATION

The codon containing the changed base may code for a different amino acid. Ex.: UCA for Ser, changing the 1st base to become CCA, it codes for Pro

Consequences of altering the nucleotide sequence NONSENSE MUTATION

The codon containing the charged base may become a termination codon Ex.: UCA for Ser is given a different base (to become, UAA), the new codon is

a stop codon

Codon recognition by tRNA Antiparallel binding between codon and anticodon

mRNA codon is read 53 by an anticodon pairing in the flipped (35) orientation

Codon is 5-AUG-3; anticodon is 5-CAU-3 Wobble hypothesis

tRNA can recognize more than one codon for a specific amino acid

Ex.: 2 codons for arginine, AGA and AGG, can bind to the same anticodon having a uracil at its 5 end (UCU)

3 codons for glycine GGU, GGC, and GGA can form a base pair from one anticodon, CCI (I is inosine nucleotide, another peculiar base in tRNA molecules)

QUIZ1. New amino acids (other than the initial f-Met) enter at which site?

A) E B) P C) A D) Any of the above 2. At the E site

A) transfer RNA is released B) anticodons match with codons C) peptide bonds are formed between amino acids D) transcription occurs

3. After the ribosome moves to the next codon on the mRNA, the growing peptide chain is found in which position(s)? A) A B) P C) E D) A, P and E

4. The codon on tRNA matches up with the complementary anticodon on mRNA. A) True B) False

5. A stop codon codes for an amino acid as well as the signal to stop. A) True B) False

QUIZ

6-8. Composition of the initiation complex9. The ribosomal subunit that binds with the initiation complex is

_____.10. What was the stop codon shown in the animation?

Translation of mRNA by tRNA: Formation of the Initiation Complex

ribosome binding site

GTP provides energy for the

process

INITIATION In prokaryotes, three initiation factors: IF-1, IF-2, IF-3 In eukaryotes, there are at least ten eIFs Initiation in eukaryotes can be divided into 4 steps

Dissociation of the ribosome into 40s and 60 s subunits Binding of a ternary complex consisting of met-tRNA, GTP,

and eIF-2 to the ribosome to form the preinitiation complex Binding of mRNA to the 40s preinitiation complexto form 43s

initiation complex The combination of the 43s initiation complex with the 60s

ribosomal subunit to form the 80s initiation complex

Translation of mRNA by tRNA: 50S Ribosomal Subunit Attaches to the Initiation Complex

Translation of mRNA by tRNA

Elongation is a Multistep Process

A. Binding of Aminoacyl-tRNA to the A site Elongation factor EF1A forms a ternary complex with GTP and the entering aminoacyl-tRNA

B. Peptide bond formation catalyzed by peptidyltransferaseC. Translocation deacylated tRNA is on the E site

Translation of mRNA by tRNA

Translation of mRNA by tRNA

Translation of mRNA by tRNA

Translation of mRNA by tRNA

Translation of mRNA by tRNA

Posttranslational modification

Trimmings Portions of functionally large, inactive precursor molecules

must be removed by endoproteases to release the active molecule

Covalent alterations Phosphorylation occurs in the hydroxyl groups of serine,

threonine and less frequently in tyrosine Glycosylation Ser or Thr (O-linked) and Asparagine (N-

linked) Hydroxylation Proline and Lysine Other covalent modification Vitamin biotin must be

covalently bound to carboxylase enzyme to be catalytically active

ASSIGNMENT

List down antibiotics that selectively inhibit protein synthesis in bacteria

Antibiotic Mechanism of action1. Tetracycline2. Streptomycin3. Chloramphenicol4. Erythromycin5. Puromycin

PROTEIN SYNTHESISSlide Number 2TRANSLATIONTRANSLATIONSlide Number 5tRNA RNA Codons for Protein Synthesis QUIZSlide Number 9Characteristics of the genetic codeConsequences of altering the nucleotide sequenceConsequences of altering the nucleotide sequenceCodon recognition by tRNAQUIZQUIZTranslation of mRNA by tRNA: Formation of the Initiation Complex INITIATIONTranslation of mRNA by tRNA: 50S Ribosomal Subunit Attaches to the Initiation ComplexTranslation of mRNA by tRNA Elongation is a Multistep ProcessTranslation of mRNA by tRNA Translation of mRNA by tRNA Translation of mRNA by tRNATranslation of mRNA by tRNATranslation of mRNA by tRNAPosttranslational modificationASSIGNMENT