Transcription, RNA Processing, and Translation (Part 2): Structure and Function of tRNA and Ribosomes

Transcription, RNA Processing, and Translation (Part 2)

An Overview of Translation

Translation is the process by which the genetic code carried by mRNA is decoded to produce a specific polypeptide. This process occurs on ribosomes and involves several key steps and molecules.

• In bacteria, translation can begin before transcription is complete, allowing for rapid protein synthesis. Multiple ribosomes can attach to a single mRNA, forming a polyribosome (or polysome), which enables the production of many protein copies from one mRNA.

• 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.

Example: In Escherichia coli, polyribosomes

allow for efficient protein synthesis during rapid cell growth.

17.4 The Structure and Function of Transfer RNA (tRNA)

tRNAs are adapter molecules that bring amino acids to the ribosome during translation.

• Transfer RNA (tRNA): Small RNA molecules (75–95 nucleotides) that transfer specific amino acids to the growing polypeptide chain.

• Aminoacyl tRNA: A tRNA molecule linked to its corresponding amino acid.

What is the Structure of tRNAs?

• tRNAs fold into a characteristic stem-and-loop structure due to complementary base pairing.

• The 3' end of the tRNA has a conserved CCA sequence, which is the site of amino acid attachment.

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

Example: The tRNA with the anticodon UAC pairs with the AUG start codon on mRNA and carries methionine.

How Are Amino Acids Attached to tRNAs?

• Attachment of amino acids to tRNAs requires ATP and is catalyzed by enzymes called aminoacyl-tRNA synthetases.

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

• Each synthetase recognizes one amino acid and its corresponding tRNAs.

Equation: $\text{Amino acid} + \text{tRNA} + \text{ATP} \xrightarrow{\text{Aminoacyl-tRNA synthetase}} \text{Aminoacyl-tRNA} + \text{AMP} + \text{PP}_i$

How Many tRNAs Are There?

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

• Wobble pairing: The third position of the tRNA anticodon can form nonstandard base pairs, allowing one tRNA to recognize multiple codons.

Example: The tRNA with anticodon GCI can pair with codons GCU, GCC, and GCA (all coding for alanine).

17.5 Ribosome Structure and Function in Translation

Ribosomes are the molecular machines that synthesize proteins by translating the genetic code from mRNA into a polypeptide chain.

• Ribosomes are composed of proteins and ribosomal RNA (rRNA).

• They consist of two subunits:

  • Small subunit: Holds the mRNA in place during translation.

  • Large subunit: Contains the active site for peptide bond formation.

tRNA Binding Sites in the Ribosome

• A site (Aminoacyl): Binds incoming aminoacyl-tRNA carrying the next amino acid.

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

• E site (Exit): Where tRNAs without amino acids exit the ribosome.

Steps of Translation

• Translation proceeds in three main phases:

  1. Initiation: Assembly of the translation machinery at the start codon.

  2. Elongation: Addition of amino acids to the growing polypeptide chain.

  3. Termination: Release of the completed polypeptide when a stop codon is encountered.

Translation Initiation in Bacteria

• Initiation begins near the AUG start codon.

• The small ribosomal subunit binds to the Shine-Dalgarno sequence on the mRNA, about 6 bases upstream of the start codon.

• Initiation factors help assemble the initiation complex.

• The initiator tRNA carries a modified methionine (f-Met) in bacteria.

• The large ribosomal subunit then binds, positioning the initiator tRNA in the P site.

Elongation: Extending the Polypeptide

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

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

• A peptide bond forms between the amino acid in the A site and the polypeptide in the P site.

• The ribosome moves (translocates) along the mRNA, shifting the tRNAs to the next sites.

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

Is the Ribosome an Enzyme or a Ribozyme?

• The active site of the ribosome is composed entirely of rRNA.

• rRNA catalyzes peptide bond formation, making the ribosome a ribozyme.

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

Translocation and Repetition of Elongation Steps

• Translocation is the movement of the ribosome along the mRNA by one codon.

• Elongation factors assist in this process.

• The three steps (arrival of aminoacyl-tRNA, peptide bond formation, translocation) repeat for each codon.

Termination of Translation

• Termination occurs when a stop codon enters the A site.

• A release factor protein binds to the stop codon, resembling tRNA in shape.

• The release factor hydrolyzes the bond between the polypeptide and the tRNA in the P site, releasing the completed protein.

• The ribosomal subunits, mRNA, and tRNAs dissociate.

Polypeptides Are Modified after Translation

Most proteins undergo additional processing steps after translation, known as post-translational modification.

• Folding of the polypeptide determines its final shape and function. Molecular chaperones assist in proper folding.

• Chemical modifications, such as the addition of sugars, lipids, or phosphate groups, can alter protein activity and localization.

Example: Addition of a phosphate group (phosphorylation) can activate or deactivate enzymes.

Summary Table: Key Components of Translation

Component	Function
mRNA	Carries genetic code from DNA to ribosome
tRNA	Brings amino acids to the ribosome; matches codon with anticodon
Ribosome	Site of protein synthesis; catalyzes peptide bond formation
Aminoacyl-tRNA synthetase	Charges tRNA with the correct amino acid
Release factor	Binds stop codon to terminate translation
Molecular chaperone	Assists in protein folding

Additional info: Post-translational modifications are essential for protein function and regulation. Errors in folding or modification can lead to diseases such as cystic fibrosis or sickle cell anemia.