PROTEIN SYNTHESIS

TRANSLATION

I. 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 anticodonThe ribosome moves from codon to codon along the mRNAAmino 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

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

UCAGUUUU = Phe UUC = Phe UUA = Leu UUG = LeuUCU = Ser UCC = Ser UCA = Ser UCG = SerUAU = Tyr UAC = Tyr UAA = Stop UAG = StopUGU = Cys UGC = Cys UGA = Stop UGG = TrpU C A G CCUU = Leu CUC = Leu CUA = Leu CUG = LeuCCU = Pro CCC = Pro CCA = Pro CCG = ProCAU = His CAC = His CAA = Gln CAG = GlnCGU = Arg CGC = Arg CGA = Arg CGG = ArgU C A G AAUU = Ile AUC = Ile AUA = Ile AUG = MetACU = Thr ACC = Thr ACA = Thr ACG = ThrAAU = Asn AAC = Asn AAA = Lys AAG = LysAGU = Ser AGC = Ser AGA = Arg AGG = ArgU C A G GGUU = Val CUC = Val GUA = Val GUG = ValGCU = Ala GCC = Ala GCA = Ala GCG = AlaGAU = Asp GAC = Asp GAA = Glu GAG = GluGGU = Gly GCG = Gly GGA = Gly GGG = GlyU C A G

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

Specificity specific codons always codes for the same amino acid

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

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

Nonoverlapping and commaless

Consequences of altering the nucleotide sequence

SILENT MUTATIONThe codon containing the changed base may code for the same amino acidEx.: UCA is for Ser, if the 3rd base is changed (to become UCU), it still codes for SerMISSENSE MUTATIONThe 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 MUTATIONThe codon containing the charged base may become a termination codonEx.: 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 anticodonmRNA codon is read 53 by an anticodon pairing in the flipped (35) orientationCodon is 5-AUG-3; anticodon is 5-CAU-3Wobble hypothesistRNA can recognize more than one codon for a specific amino acidEx.: 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)

QUIZ

1. 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 complex

9. 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-3In eukaryotes, there are at least ten eIFsInitiation in eukaryotes can be divided into 4 stepsDissociation of the ribosome into 40s and 60 s subunitsBinding of a ternary complex consisting of met-tRNA, GTP, and eIF-2 to the ribosome to form the preinitiation complexBinding of mRNA to the 40s preinitiation complexto form 43s initiation complexThe 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

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

Peptide bond formation catalyzed by peptidyltransferase

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

TrimmingsPortions of functionally large, inactive precursor molecules must be removed by endoproteases to release the active moleculeCovalent alterationsPhosphorylation occurs in the hydroxyl groups of serine, threonine and less frequently in tyrosineGlycosylation Ser or Thr (O-linked) and Asparagine (N-linked)Hydroxylation Proline and LysineOther 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

AntibioticMechanism of action

Tetracycline

Streptomycin

Chloramphenicol

Erythromycin

Puromycin