The genetic code is read sequentially in triplets (codons) without punctuation or spaces between codons. It is degenerate, meaning many codons code for the same amino acid. Inserting or deleting nucleotides can shift the reading frame, changing which codons are read together.

1. Translation

2. • The genetic code is, in fact, a nonoverlapping,
commafree, degenerate, triplet code
• The code is read in a sequential manner starting
from a fixed point in the gene. The insertion or
deletion of a nucleotide shifts the frame
(grouping) in which succeeding nucleotides are
read as codons (insertions or deletions of
nucleotides are therefore also known as
frameshift mutations). Thus the code has no
internal punctuation that indicates the reading
frame; that is, the code is comma free.
• 3. The code is a triplet code.
• 4. All or nearly all of the 64 triplet codons code
for an amino acid; that is, the code is degenerate.

3. • THE BIG RED FOX ATE THE EGG
• The deletion of the fourth letter, which shifts
the reading frame, changes the sentence to
• THE IGR EDF OXA TET HEE GG
• THE IGR EDX FOX ATE THE EGG
• THE BXI GYR EDZ FOX ATE THE EGG

6. Nature of Genetic Code
• The code is highly degenerate.
• The arrangement of the code table is
nonrandom.
• UAG,UAA and UGA are stop codons
• AUG and GUG are chain initiation codons

7. The “Standard” Genetic Code is not universal

8. Transfer RNA and its aminoacylation

12. Two Classes of Aminoacyl–tRNA
Synthetases
Class I
• Arg
• Cys
• Gln
• Glu
• Ile
• Leu
• Met
• Trp
• Tyr
• Val
Class II
• Ala
• Asn
• Asp
• Gly
• His
• Lys
• Pro
• Phe
• Ser
• Thr

13. (b) A
cartoon comparing the positions of the 3. end of tRNA Ile in its
complex with Ile RS in its synthetic mode (left) and in its editing
mode (right).
Note that there is a cleft running between the editing and synthetic sites and that
the 3. end of the tRNA continues its A-form helical path in the editing mode but
assumes a hairpin conformation in the synthetic mode

15. • The overall reaction catalyzed by Glu-AdT
occurs in three stages:
(1) Glutamine is hydrolyzed to glutamate and the
resulting NH3 sequestered;
(2) (2) ATP reacts with the Glu side chain of Glu–
tRNAGln to yield an activated acylphosphate
intermediate and ADP; and
(3) the acylphosphate intermediate reacts with
the NH3 to yield Gln–tRNAGln + Pi

20. The N-formylmethionine residue (fMet) already has an amide bond and can therefore
only be the N-terminal residue of a polypeptide. The tRNA that recognizes the initiation
codon, tRNAf Met, differs from the tRNA that carries internal Met residues, tRNAm
Met, although they both recognize recognize the same codon In E. coli, uncharged
(deacylated) tRNAf Met is first aminoacylated with methionine by the same MetRS that
charges tRNAm Met.The resulting Met–tRNAf Met is specifically N-formylated to yield
fMet–tRNAf Met in an enzymatic reaction that employs N10-formyltetrahydrofolate as
its formyl donor. The formylation enzyme does not recognize Met–tRNAm Met.

29. • The process begins with the binding of eIF3 (which
in mammals consists of 13 different subunits) and
eIF1A (a monomer and homolog of bacterial IF-1) to
the 40S subunit in the inactive 80S ribosome (which
had terminated elongation in its previous elongation
cycle) so that it releases the 60S subunit.
• 2. The ternary complex of eIF2 (a heterotrimer),
GTP, and Met-tRNAimet binds to the 40S ribosomal
subunit accompanied by eIF1 (a monomer) to form
the so-called 43S preinitiation complex. Here the
subscript “i” on tRNAiMet distinguishes this
eukaryotic initiator tRNA, whose appended Met
residue is never N-formylated, from that of
prokaryotes; both species are, nevertheless, readily
interchangeable in vitro.

30. • Eukaryotic mRNAs lack the complementary sequences
to bind to the 18S rRNA in the Shine–Dalgarno manner.
Rather, they have an entirely different mechanism for
recognizing the mRNA’s initiating AUG codon.
Eukaryotic mRNAs, nearly all of which have an m7G
cap and a poly(A) tail, are invariably monocistronic and
almost always initiate translation at their leading AUG.
This AUG, which occurs at the end of a 5’-untranslated
region of 50 to 70 nt, is embedded in the consensus
sequence GCCRCCAUGG, with changes in the purine
(R) 3 nt before the AUG and the G immediately
following it reducing translational efficiency by 10-fold
each and with other changes having much smaller
effects. In addition, secondary structure (stem–loops)
in the mRNA upstream of the initiation site may affect
initiation efficiency.

31. • The recognition of the initiation site begins by
the binding of eIF4F to the m7G cap. eIF4F is
a heterotrimeric complex of eIF4E, eIF4G, and
eIF4A (all monomers), in which eIF4E (cap-binding
protein) recognizes the mRNA’s m7G
cap and eIF4G serves as a scaffold to join
eIF4E with eIF4A.

32. • eIF4B (an RRM-containing homodimer) and
eIF4H (a monomer) join the eIF4F–mRNA
complex where they stimulate the RNA
helicase activity of eIF4A to unwind the
mRNA’s helical segments in an ATP-dependent
process.
• The eIF4F–mRNA–eIF4B–eIF4H complex joins
the 43S preinitiation complex through a
protein–protein interaction between eIF4G
and the 40S subunit-bound eIF3.

33. • eIF5 (a monomer) joins the growing assembly.
The 43S preinitiation complex then translocates
along the mRNA, an ATP-dependent process
called scanning, until it encounters the mRNA’s
AUG initiation codon, which is optimally in the
sequence GCC(A/G)CCAUGG. This yields the 48S
preinitiation complex. The recognition of the
AUG occurs mainly through base pairing with
the CUA anticodon on the bound Met-tRNAiMet ,
as was demonstrated by the observation that
mutating this anticodon results in the
recognition of the new cognate codon instead of
AUG. This explains why the initiator tRNA must
bind to the small subunit before the mRNA.

34. • The formation of the 48S preinitiation complex
induces eIF2 to hydrolyze its bound GTP to GDP +
Pi, which results in the release of all the initiation
factors, thereby leaving the Met-tRNA iMet in the
small subunit’s P site.The hydrolysis reaction is
stimulated by eIF5, acting as a GAP.
• The 60S subunit then joins the mRNA-bound
Met–tRNAiMet 40S subunit complex in a GTPase
reaction mediated by eIF5B (a monomer and
homolog of bacterial IF-2), thereby yielding the
80S ribosomal initiation complex. Thus eukaryotic
translation initiation consumes two GTPs versus
one for prokaryotic initiation

35. • What remains is to recycle the eIF2 GDP
complex by exchanging its GDP for GTP. This
reaction is mediated by eIF2B (a
heteropentamer), which therefore functions
as eIF2’s GEF (guanine nucleotide exchange
factor.