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Chapter 30: Problem 12

How do prokaryotic cells determine whether a particular methionyltRNA \(^{\text {Met }}\) is intended to initiate protein synthesis or to deliver a Met residue for internal incorporation into a polypeptide chain? How do the Met codons for these two different purposes differ? How do eukaryotic cells handle these problems?

Short Answer

Expert verified
In prokaryotic cells, formylation of the methionyltRNA (Met) distinguishes it as the initiator tRNA. The Met codon for initiation and elongation differ with the initiation codon being AUG. In eukaryotic cells, the initiator tRNA^(Met) is distinguishably, not formylated, but has the ability to bind to the initiation complex independent of the elongation factor.

Step by step solution

01

Understanding the Process

During the initiation of protein synthesis in both prokaryotic and eukaryotic cells, a modified form of the specific methionyltRNA (tRNA), known as the initiator tRNA, supplies the first Met to the polypeptide chain. In prokaryotic cells, this initiating tRNA^(Met) is formylated as a distinguishing mark from other tRNA^(Met) used for elongation.
02

Distinguishing between Initiation and Elongation

The Met codons for initiation (start codons) and elongation (internal Met residues) of polypeptide chain differ. The start codon is AUG and codes for the initial methionine (Met) in the protein sequence. Other AUG codons that occur within the mRNA sequence code for internal Met residues in the growing polypeptide. Hence, the same Met codon is read differently which determines whether it initiates the protein synthesis or adds a Met internally.
03

Difference between Prokaryotic and Eukaryotic Processes

While both prokaryotic and eukaryotic cells have methionyl trnas, a major difference lies in the modification of the tRNA^(Met). In prokaryotes, a formyl group is added to the methionine residue of the initiator tRNA^(Met), forming fMet-tRNA^(Met). This distinguishes it from the regular tRNA^(Met) used during elongation. In eukaryotic cells, the initiator tRNA^(Met) is not formylated, rather, it is distinguishable by its ability to bind to the initiation complex independently of the elongation factor.

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Most popular questions from this chapter

How many ATP equivalents are consumed for each amino acid added to an elongating polypeptide chain during the process of protein synthesis?

What is the Shine-Dalgarno sequence? What does it do? The efficiency of protein synthesis initiation may vary by as much as 100 -fold for different mRNAs. How might the Shine-Dalgarno sequence be responsible for this difference?

What ideas can you suggest to explain why ribosomes invariably exist as two- subunit structures, instead of a larger, single-subunit entity?

Why do you suppose eukaryotic protein synthesis is only \(10 \%\) as fast as prokaryotic protein synthesis?

Eukaryotic ribosomes are larger and more complex than prokaryotic ribosomes. What advantages and disadvantages might this greater ribosomal complexity bring to a eukaryotic cell?
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