Transcription, RNA Processing, and Translation: Mechanisms and Molecular Players

Transcription, RNA Processing, and Translation

Overview of Translation

Translation is the process by which ribosomes synthesize proteins using messenger RNA (mRNA) as a template. This process differs between bacteria and eukaryotes in terms of cellular compartmentalization and timing.

• In bacteria: Translation can begin before transcription is complete, allowing multiple ribosomes to translate a single mRNA simultaneously, forming a polyribosome.

• In eukaryotes: Transcription and translation are separated by the nuclear envelope. mRNAs are synthesized and processed in the nucleus, then transported to the cytoplasm for translation.

• Translation takes place on ribosomes, which are complex molecular machines composed of proteins and ribosomal RNA (rRNA).

• Polyribosome: A structure formed when multiple ribosomes attach to an mRNA molecule, increasing protein production efficiency.

Structure and Function of Transfer RNA (tRNA)

Transfer RNA (tRNA) acts as an adapter molecule in translation, bringing amino acids to the ribosome and matching them to the appropriate codons in the mRNA.

• tRNAs are short RNA molecules (75–95 nucleotides) that fold into a stem-and-loop structure.

• Each tRNA has a CCA sequence at its 3' end for amino acid attachment.

• The anticodon loop contains a sequence of three nucleotides that base-pair with the mRNA codon.

• Aminoacyl tRNA: A tRNA linked to its specific amino acid.

How Are Amino Acids Attached to tRNAs?

• ATP is required for the attachment of amino acids to tRNAs.

• Aminoacyl-tRNA synthetases are enzymes that catalyze the addition of amino acids to tRNAs, a process known as "charging" the tRNA.

• There are 20 different aminoacyl-tRNA synthetases, one for each amino acid.

• For each amino acid, there may be one or more corresponding tRNAs.

Wobble Pairing and tRNA Diversity

• Although there are 61 codons for amino acids, most cells have about 40 different tRNAs.

• Wobble pairing allows one tRNA to recognize multiple codons due to nonstandard base pairing at the third position of the anticodon.

Ribosome Structure and Function in Translation

Ribosomes are composed of two subunits and facilitate the synthesis of polypeptides by aligning tRNAs with mRNA codons.

• Small subunit: Holds the mRNA in place.

• Large subunit: Catalyzes peptide bond formation.

• Three tRNA binding sites within the ribosome:

  • A site (aminoacyl): Accepts incoming aminoacyl-tRNA.

  • P site (peptidyl): Holds the tRNA with the growing polypeptide chain.

  • E site (exit): Releases tRNAs that have given up their amino acids.

Steps of Translation

1. Initiation:

   • Begins near the AUG start codon.

   • In bacteria, the small ribosomal subunit binds to the Shine-Dalgarno sequence on the mRNA, aided by initiation factors.

   • The initiator tRNA (carrying f-Met in bacteria) binds to the start codon.

   • The large ribosomal subunit joins, and translation is ready to begin.

2. Elongation:

   • The initiator tRNA is in the P site; E and A sites are empty.

   • An aminoacyl-tRNA enters the A site if its anticodon matches the mRNA codon.

   • Peptide bond formation occurs between the amino acid in the A site and the polypeptide in the P site.

   • The ribosome moves down the mRNA (translocation), shifting tRNAs through the sites and exposing a new codon in the A site.

   • Amino acids are always added to the carboxyl end (C-terminus) of the polypeptide.

3. Termination:

   • Occurs when a stop codon enters the A site.

   • A release factor protein enters the A site, hydrolyzing the bond between the polypeptide and the tRNA in the P site.

   • The completed polypeptide, tRNAs, and ribosomal subunits dissociate from the mRNA.

Is the Ribosome an Enzyme or a Ribozyme?

• The ribosome's active site is composed entirely of rRNA, which catalyzes peptide bond formation.

• Thus, the ribosome is classified as a ribozyme.

• This supports the RNA world hypothesis, which proposes that early life used RNA for both genetic information and catalysis.

Polypeptide Modification after Translation

After translation, most proteins undergo post-translational modifications to become fully functional.

• Post-translational modification: Chemical changes to the polypeptide after synthesis, such as folding, addition of sugars or lipids, and phosphorylation.

• Molecular chaperones: Proteins that assist in the proper folding of other proteins.

• Enzymes may add a phosphate group to regulate protein activity.

Major Steps of Gene Expression in Eukaryotic Cells

Step	Location	Description
Transcription	Nucleus	DNA is transcribed to pre-mRNA
RNA Processing	Nucleus	Pre-mRNA is spliced and modified to form mature mRNA
Translation	Cytoplasm	mRNA is translated into a polypeptide by ribosomes
Post-translational Modification	Various	Polypeptide is folded and chemically modified

Additional info: The above table summarizes the major steps of gene expression in eukaryotic cells, as depicted in Figure 17.17.