Translation is the process by which proteins are synthesized from messenger RNA (mRNA) in eukaryotes, which are organisms with membrane-bound nuclei. Translation involves mRNA being decoded on ribosomes into a polypeptide chain. It occurs through three main steps - initiation, elongation, and termination. Initiation involves the small ribosomal subunit binding to the 5' end of mRNA and scanning for the start codon. Elongation is the sequential addition of amino acids specified by the mRNA codons. Termination occurs when a stop codon is reached and release factors cause the ribosome to dissociate and release the completed protein.

1. Translation In Eukaryotes
Muhammad Umer Farooq
BAGF13E221

2. What Is Translation??
 Translation is the process by which protein is synthesized from
the information contained in a molecule of messenger RNA
(mRNA).

3. What are Eukaryotes?
 Eukaryotes are organisms with a complex cell or cells, in which
the genetic material is organized into a membrane-bound
nucleus or nuclei and it
also contains cell organelles.

4. Translation: An Overview
 Ribosomes translate the genetic message of mRNA into
proteins.
 The mRNA is translated from 5’ 3’.
 Amino acids bound to tRNAs are inserted in a proper
sequence due to:
- Specific binding of each amino acid to its tRNA.
- Specific base-pairing between the mRNA codon and
tRNA anticodon.

5. Components of Translation
 mRNA:
- Made in the nucleus, transported to cytoplasm.
 tRNA:
- Adaptor molecule that mediate the transfer of information from
nucleic acid to protein.
 Ribosomes:
- Manufacturing units of a cell.
 Enzymes:
- Required for the attachment of amino acids to the correct tRNA
molecule, and for peptide bond formation between amino acids.
 Proteins:
- Soluble factors necessary for proper initiation, elongation and
termination.

6. Enzymes
 catalyze the
attachment of tRNA molecule to its respective amino acid.
- At least one for each tRNA.
- Attachment of amino acid activates/ charges the
tRNA molecule.
 catalyzes the sequential transfer
of amino acid to the growing chain.
- Forms the peptide bond between the amino acids.

7. Ribosomes: Role In Translation
 The smaller subunit is the one that initially binds with the
mRNA.
 The larger subunit provides the enzyme activity:
- Peptidyl transferase
- Catalyzes the formation of peptide bonds joining amino acids.
 The assembled structure of ribosome creates three pockets for
the binding of two molecules of tRNA.
- A site
- P site
- E site

9. Mechanism Of Translation
 Three steps of translation:
- Initiation: Sets the stage for polypeptide synthesis.
- Elongation: Causes the sequential addition of amino
acids to the polypeptide chain as determined by
mRNA.
- Termination: Brings the polypeptide synthesis to the
halt.

10. Initiation
 The initiation codon is an AUG.
- is towards the 5’ end of the mRNA molecule that Is being
translated.

11. Scanning Model of Initiation
• Proposed by M. Kozak
• Small subunit of ribosome (+ initiation factors, GTP and
tRNAiMet) binds to the 5’ Cap, and scans along the mRNA until
the first AUG
• Translation starts at the first AUG

12. Initiation Factors
 eIF-1(and 1A): promotes scanning.
 *eIF-2: binds tRNAiMet to 40S subunit, requires
GTP (which gets hydrolyzed to GDP)
 eIF-2B: catalyzes exchange of GTP to GDP on
eIF-2
 *eIF-3: binds to 40S subunit, prevents 60S subunit
from binding to it
 eIF-5: stimulates 60S subunit binding to the 40S
pre-initiation complex
 *eIF-6: binds to 60S subunit, helps prevent 40S
subunit from binding to it

14. Elongation
 At the start of elongation, the mRNA is bound to the complete two subunit ribosome,
- With the initiating tRNA in the P site,
- and the A site is free for binding to the next tRNA.
 The ribosome moves along the mRNA in a 5’ to 3’ direction, in a step-wise process,
recognizing each subsequent codon.
 The peptidyl transferase enzyme then catalyzes the formation of a peptide bond
between
- the free N terminal of the amino acid at the A site,
- and the Carboxyl end of the amino acid at the P site, which is
actually connected to the tRNA.
 This disconnects the tRNA fMet from the amino acid, and the tRNA at the A site now
carries two amino acids,
- with a free N terminal and the Carboxyl terminal of the second
amino acid connected to its tRNA.

16. Chain Elongation: Translocation
 During translocation the peptidyl-tRNA remains attached to
its codon, but is transferred from the ribosomal A site to the P
site.
 The vacant A site now contains a new codon, and an
aminoacyl-tRNA with the correct anticodon can enter and
bind.
 The process repeats until a stop codon is reached.

17. Chain Elongation: Translocation
 In both prokaryotes and eukaryotes, simultaneous translation
occurs.
- New ribosomes may initiate as soon as the previous
ribosome has moved away from the initiation site,
creating a polyribosome (polysome).
- An average mRNA might have 8–10 ribosomes
attached at a given moment.

20. Termination
 When the ribosome encounters a stop codon,
- there is no tRNA available to bind to the A site of the
ribosome,
- instead a release factor binds to it.
 The details are not very clear, but once the release factor
binds, the ribosome unit falls apart,
- releasing the large and small subunits,
- the tRNA carrying the polypeptide is also released,
freeing up the polypeptide product.