Biology Lecture 3: Translation Process

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Questions and Answers

What is the role of the ribosome in translation?

It carries amino acids to the growing polypeptide chain.

It decodes mRNA to produce an amino acid chain. (correct)

It initiates transcription from DNA.

It synthesizes nucleic acids.

Which of the following is a structural feature of tRNA?

Codon arm

Signal arm

Peptidyl arm

Amino acid arm (correct)

How many total codons are present in mRNA?

64 (correct)

What does the D arm of tRNA contain?

Dihydrouridine residues Signup and view all the answers

In which direction does translation occur?

5’ to 3’ direction Signup and view all the answers

Which of the following is an initiation codon?

AUG Signup and view all the answers

What is the function of the exit site in the ribosome?

It is where uncharged tRNA leaves the ribosome. Signup and view all the answers

Why is the mRNA composed of codons arranged in a non-overlapping manner?

To ensure each codon codes for a specific amino acid unambiguously. Signup and view all the answers

What is a key characteristic of the genetic code?

It is degenerate and almost universal. Signup and view all the answers

Which stage of translation involves binding of eukaryotic initiation factor 3 with the 40S subunit?

Initiation Signup and view all the answers

Which amino acid is associated with the initiation codon AUG?

Methionine Signup and view all the answers

What is the direction of reading during translation?

5’ to 3’ direction Signup and view all the answers

What does the activation of amino acids (charging tRNAs) entail?

Amino acid activation and tRNA conjugation Signup and view all the answers

What is the role of eIF4F during translation initiation?

To bind to the 5’ end of mRNA Signup and view all the answers

Which one of the following is NOT a stage of translation?

Binding of the cap Signup and view all the answers

During which stage do the 60S and 40S ribosomal subunits bind together?

Initiation Signup and view all the answers

What role does insulin play in protein synthesis?

Stimulates protein synthesis by activating eIF4E Signup and view all the answers

What occurs during the translocation step of elongation?

mRNA and tRNAs move with respect to the ribosome Signup and view all the answers

Which factor is involved in the hydrolysis of GTP during elongation?

eEF2 Signup and view all the answers

What signifies the termination of protein synthesis?

The presence of a stop codon in the A site Signup and view all the answers

Which enzyme catalyzes the peptide bond formation during elongation?

Peptidyl transferase Signup and view all the answers

What is polyribosomes?

Ribosomes that read mRNA simultaneously to produce the same protein Signup and view all the answers

Which part of the amino acid structure does the peptide bond form between?

Carboxylic acid group of the AA in the P site and the amide group of the AA in the A site Signup and view all the answers

What defines the 5’ end of mRNA?

Amino terminus Signup and view all the answers

What is the role of chaperons or heat shock proteins in protein synthesis?

They fold and refold the polypeptide into its 3-dimensional structure. Signup and view all the answers

Which of the following is a function of post-translational modifications?

Imparting functionality by adding functional groups. Signup and view all the answers

What characterizes ribosomopathies?

They result from defects in ribosomal proteins or rRNA genes. Signup and view all the answers

What is the consequence of misfolded proteins in the cell?

They are usually immediately degraded. Signup and view all the answers

Which statement is true regarding Creutzfeldt-Jakob Disease (CJD)?

CJD can be caused by sporadic, genetic, or acquired factors. Signup and view all the answers

How do amyloids relate to diseases like Alzheimer's?

They are insoluble aggregates that accumulate in the body. Signup and view all the answers

What differentiates normal prion proteins (PrPc) from misfolded ones (PrPsc)?

PrPc is harmless when normal, but PrPsc is infectious when misfolded. Signup and view all the answers

What is common regarding the response to protein misfolding in cells?

Cells immediately degrade misfolded proteins. Signup and view all the answers

Study Notes

Lecture 3: Translation

Translation is the process where mRNA is decoded by ribosomes to produce an amino acid chain (polypeptide) that folds into an active protein.
Translation occurs in the 5' to 3' direction.
The process starts with DNA in the chromosome, and utilizes mRNA to pass the genetic information.

Components Needed for Translation

Ribosome: The protein synthesis factory, composed of 40S and 60S subunits.
- Large subunit (60S): contains the enzymatic activity (ribozyme) made of 5S, 5.8S and 28S rRNA and 47 proteins.
- Small subunit (40S): responsible for reading mRNA and monitoring complementarity between codons and anticodons, made of 18S rRNA and 32 proteins.
mRNA: Contains genetic information in the form of codons, arranged in a linear, non-overlapping manner.
- mRNA consists of 64 codons (4³).
- mRNA has a 5' methylated cap and a 3' poly-A tail.
- AUG codon is the initiation codon.
- UAA, UAG, UGA are termination codons.
tRNA: Carriers of amino acids.
- Small RNA molecules (73-93 nucleotides) folded into 3D structures.
- Each tRNA carries a unique amino acid.
- Contains an anticodon that binds to the complementary codon on mRNA.
- Contains:
  - Amino acid arm (3' end) that attaches to specific amino acids (CCA).
  - 5' end rich in poly guanylate (pG).
  - Anticodon arm.
  - D arm has dihydrouridine(D) residues.
  - TψC arm has unusual bases, ribothymidine(T) and pseudouridine(Ψ).

Ribosome Sites

Aminoacyl-site (A site): Where "charged" tRNA resides (carrying an amino acid)
Peptidyl-site (P site): Where the polypeptide chain resides
Exit-site (E site): Where "uncharged" tRNA leaves the ribosome

mRNA: Genetic Information

Composed of numerous codons arranged in a linear, non-overlapping manner.
Each codon codes for a specific amino acid (AA).
There are 64 codons (4³).
AUG specifies methionine (Met), the initiation codon.
UAA, UAG, and UGA are termination codons.
Translation proceeds in the 5' to 3' direction of the mRNA.

Genetic Code Degeneracy

The genetic code is degenerate; multiple codons can code for the same amino acid.

Coding Sequence: Linear & Non-overlapping

The coding sequence of mRNA is linear and non-overlapping. This arrangement creates a direct, one-to-one correspondence between codons and the amino acid sequence they specify.

Translation Stages:

Stage I - Activation of amino acids: Amino acids are activated by attaching to tRNA molecules. A specific aminoacyl-tRNA synthetase is needed to conjugate the correct tRNA with the correct amino acid.
Stage II - Initiation: The ribosome binds to the mRNA, initiator tRNA and the start codon.
- The 40S ribosome subunit binds to eukaryotic initiation factor 3 (eIF3).
- eIF2 and guanosine triphosphate (GTP) bind to methionyl-tRNA.
- The 43S complex then initiates forming the rest of the complex.
- The 60S ribosome subunit joins the 40S subunit.
Stage III - Elongation: New amino acids are added to the growing polypeptide chain.
- Decoding: An aminoacyl-tRNA binds to the A site, elongation factors (eEF1A and GTP) are involved. GTP is hydrolyzed.
- Peptide bond formation: A peptide bond forms between the amino acids in the P and A sites. Peptidyl transferase is part of the 60S ribosome subunit.
- Translocation: The ribosome moves along the mRNA, and the polypeptide chain translocates. eEF2 and GTP are involved. An uncharged tRNA moves from the P site to the E site. The tRNA in the A site now moves to the P site, and the ribosome moves to the next codon.
Stage IV - Termination: A stop codon enters the A site and polypeptide release factors (eRF1 and eRF3) are involved. GTP hydrolysis occurs. Ribosomes dissociates into subunits. tRNA disassociates from the P site.
Stage V - Folding & Post-translational Modifications: Folding of protein into its 3D form, chaperons, heat shock proteins. Disulfide isomerases stabilize structure - disulfide bonds. Post-translational modifications: glycosylation, phosphorylation, acetylation, etc add functional groups.

Polyribosomes/Polyomes:

Multiple ribosomes can read a single mRNA molecule at the same time, leading to multiple identical protein molecules being synthesized from a single mRNA.

Clinical Correlations

Ribosomopathies: Genetic disorders related to defects in ribosomal proteins/rRNA
Diseases related to misfolded proteins: Misfolded proteins can form insoluble aggregates (amyloids). Amyloids accumulate in organs/extracellular spaces, causing pathology. Examples include Alzheimer's disease and Creutzfeldt-Jakob disease (CJD). Proteins misfolding can cause CJD (e.g. Prion protein).

The "Wobble" Hypothesis

Flexibility in the pairing of the third base of a codon with the corresponding anticodon of tRNA can accommodate changes in the base and still yield the correct amino acid. This flexibility in base-pairing allows for more than one tRNA to recognize the same codon.
- This is particularly important for efficiency and for the overall accuracy of the translation process.

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Description

This quiz focuses on the translation process in biology, where mRNA is decoded by ribosomes to synthesize proteins. Key components such as ribosomes and mRNA structures, along with their roles in translation, are covered. Test your understanding of the intricacies involved in producing amino acid chains.