1. Explain the central dogma of molecular biology.
Answer:
The central dogma of molecular biology describes the flow of genetic information within a biological system. It states that:
- DNA is transcribed into RNA (Transcription).
- RNA is translated into proteins (Translation).
DNA stores genetic information, RNA acts as the intermediary, and proteins execute cellular functions. This flow ensures the expression of genes into functional proteins.
2. Describe the role of ribosomes in protein synthesis.
Answer:
Ribosomes are molecular machines that facilitate protein synthesis by decoding mRNA. Ribosomes have two subunits (large and small) that:
- Bind mRNA to initiate translation.
- Provide binding sites (A, P, E) for tRNA.
- Catalyze the formation of peptide bonds between amino acids using peptidyl transferase activity.
3. What are codons, and how are they important in translation?
Answer:
Codons are sequences of three nucleotide bases on mRNA that specify a particular amino acid. They are critical for translation as:
- Each codon corresponds to one amino acid or a stop signal.
- The start codon (AUG) initiates translation.
- Stop codons (UAA, UAG, UGA) terminate translation.
4. Outline the steps involved in the transcription process.
Answer:
- Initiation: RNA polymerase binds to the promoter region of the gene.
- Elongation: RNA polymerase synthesizes the RNA strand complementary to the DNA template.
- Termination: Transcription stops when the polymerase reaches a terminator sequence.
The resulting mRNA carries the genetic code to the ribosome for translation.
5. What is the role of tRNA in protein synthesis?
Answer:
Transfer RNA (tRNA) is an adaptor molecule that:
- Carries specific amino acids to the ribosome.
- Has an anticodon region that base-pairs with the mRNA codon.
- Ensures that amino acids are added in the correct sequence to form a polypeptide.
6. Discuss the importance of the genetic code’s degeneracy.
Answer:
The genetic code is degenerate because multiple codons can encode the same amino acid. This feature:
- Reduces the impact of mutations, as a change in the third base of a codon might not alter the amino acid.
- Provides flexibility and robustness to the protein synthesis process.
7. How does translation terminate, and what happens to the synthesized protein?
Answer:
Translation terminates when a ribosome encounters a stop codon (UAA, UAG, UGA). Release factors bind to the ribosome, causing it to release the polypeptide chain. The synthesized protein then undergoes folding and post-translational modifications to become functional.
8. Compare and contrast transcription in prokaryotes and eukaryotes.
Answer:
- Prokaryotes: Transcription occurs in the cytoplasm, and mRNA does not require processing.
- Eukaryotes: Transcription occurs in the nucleus. The mRNA undergoes splicing, 5′ capping, and 3′ polyadenylation before translation.
9. What are post-translational modifications, and why are they important?
Answer:
Post-translational modifications (PTMs) involve chemical changes to the protein after synthesis. Examples include phosphorylation, glycosylation, and acetylation. PTMs:
- Regulate protein function.
- Enable protein targeting to specific cellular locations.
- Enhance stability and activity.
10. Explain the role of peptidyl transferase in protein synthesis.
Answer:
Peptidyl transferase is an enzymatic activity of the ribosome’s rRNA. It catalyzes peptide bond formation between the amino acid on the tRNA in the P site and the one in the A site, extending the polypeptide chain during elongation.
11. What are polysomes, and what is their significance?
Answer:
Polysomes are complexes of multiple ribosomes attached to a single mRNA strand, simultaneously synthesizing several copies of a protein. They increase the efficiency of protein synthesis.
12. Describe the initiation process of translation.
Answer:
- The small ribosomal subunit binds to the mRNA near the start codon (AUG).
- The initiator tRNA carrying methionine binds to the start codon.
- The large ribosomal subunit joins to form a functional ribosome.
13. How is an amino acid attached to its corresponding tRNA?
Answer:
Aminoacyl-tRNA synthetase catalyzes the attachment of an amino acid to its corresponding tRNA. This process requires ATP and ensures the correct pairing of tRNA and amino acid.
14. Explain the elongation cycle in translation.
Answer:
- A new aminoacyl-tRNA enters the A site of the ribosome.
- Peptidyl transferase forms a peptide bond between the new amino acid and the growing chain.
- The ribosome translocates, moving the tRNA from the A site to the P site, and the empty tRNA exits via the E site.
15. What is the significance of the 5′ cap and 3′ poly-A tail in eukaryotic mRNA?
Answer:
- 5′ Cap: Protects mRNA from degradation and aids in ribosome binding.
- 3′ Poly-A Tail: Stabilizes mRNA and regulates translation efficiency.
16. What is the role of chaperones in protein synthesis?
Answer:
Chaperones are proteins that assist in the proper folding of newly synthesized polypeptides. They prevent misfolding and aggregation, ensuring functional protein formation.
17. Discuss the regulation of protein synthesis.
Answer:
Protein synthesis is regulated at multiple levels:
- Transcriptional control: Determines mRNA production.
- Translational control: Affects mRNA translation efficiency.
- Post-translational control: Modifies protein activity and stability.
18. How do antibiotics target bacterial protein synthesis?
Answer:
Antibiotics such as tetracycline, chloramphenicol, and erythromycin inhibit bacterial protein synthesis by binding to bacterial ribosomes. They exploit differences between prokaryotic and eukaryotic ribosomes.
19. What are nonsense mutations, and how do they affect protein synthesis?
Answer:
Nonsense mutations introduce a premature stop codon in the mRNA sequence, resulting in truncated proteins that are usually nonfunctional.
20. Explain how the genetic code is both universal and specific.
Answer:
- Universal: Most organisms use the same genetic code.
- Specific: Each codon unambiguously codes for one amino acid.
21. Describe the structure of a tRNA molecule.
Answer:
tRNA has a cloverleaf structure with:
- An anticodon loop for mRNA codon binding.
- An acceptor stem for amino acid attachment.
- A 3D L-shaped configuration for interaction with ribosomes.
22. What are the three phases of translation?
Answer:
- Initiation: Assembly of ribosome and mRNA with initiator tRNA.
- Elongation: Addition of amino acids to the growing chain.
- Termination: Release of the completed polypeptide upon encountering a stop codon.
23. Explain the significance of the Shine-Dalgarno sequence in prokaryotes.
Answer:
The Shine-Dalgarno sequence is a ribosome-binding site in prokaryotic mRNA. It aligns the mRNA with the ribosome to ensure correct initiation of translation.
24. How does alternative splicing affect protein synthesis?
Answer:
Alternative splicing generates multiple mRNA variants from a single gene, enabling the production of different proteins and increasing proteomic diversity.
25. What is the difference between free and bound ribosomes?
Answer:
- Free ribosomes: Synthesize proteins for use in the cytoplasm.
- Bound ribosomes: Synthesize proteins destined for secretion or membranes.
26. Discuss the role of GTP in translation.
Answer:
GTP provides energy for translation processes, including ribosome assembly, tRNA translocation, and termination.
27. How do stop codons function in translation?
Answer:
Stop codons (UAA, UAG, UGA) do not code for any amino acid. They signal termination, prompting release factors to disassemble the translation machinery.
28. Explain how protein misfolding can lead to diseases.
Answer:
Misfolded proteins can aggregate and disrupt cellular functions, leading to diseases like Alzheimer’s, Parkinson’s, and prion diseases.
29. What is the role of rRNA in protein synthesis?
Answer:
Ribosomal RNA (rRNA) forms the core of the ribosome and catalyzes peptide bond formation, making it essential for translation.
30. Describe the process of protein targeting.
Answer:
Protein targeting ensures proteins reach their proper cellular locations. Signal peptides direct proteins to specific organelles or membranes, where they perform their functions.
