1. Describe the structure of a ribosome and its significance in protein synthesis.
Answer:
Ribosomes are complex molecular machines made up of ribosomal RNA (rRNA) and proteins. In eukaryotic cells, ribosomes consist of a small 40S subunit and a large 60S subunit, which together form the 80S ribosome. In prokaryotic cells, ribosomes are slightly smaller, with a 30S small subunit and a 50S large subunit, forming the 70S ribosome. The structure of ribosomes allows them to perform their function of protein synthesis effectively. The small subunit binds to the mRNA and reads the genetic code, while the large subunit is responsible for catalyzing the formation of peptide bonds between amino acids.
2. Explain the role of ribosomes in the process of translation.
Answer:
Ribosomes play a crucial role in the process of translation, where genetic information from mRNA is used to synthesize proteins. During translation, ribosomes facilitate the binding of tRNA molecules, which carry specific amino acids. The ribosome reads the mRNA codons and matches them with the correct tRNA anticodons. This ensures that the amino acids are added in the correct sequence to form the polypeptide chain. The ribosome has three main sites (A, P, and E) that allow it to hold tRNA, add amino acids, and release the tRNA after the amino acids are joined.
3. What is the role of the small subunit of the ribosome in protein synthesis?
Answer:
The small subunit of the ribosome plays an essential role in translation by binding to the mRNA molecule. It is responsible for ensuring that the mRNA is read correctly, codon by codon. The small subunit binds to the mRNA at the 5’ end and scans it to find the start codon (AUG), which signals the beginning of translation. Once the start codon is found, the small subunit aligns the mRNA so that the corresponding tRNA can bind to the codon with its anticodon.
4. What is the function of the large subunit of the ribosome in translation?
Answer:
The large subunit of the ribosome is responsible for catalyzing the formation of peptide bonds between amino acids. As the ribosome moves along the mRNA, the tRNA carrying the growing polypeptide chain is located at the P site. The A site binds the incoming tRNA with the corresponding amino acid. When the ribosome moves forward, the tRNA at the A site shifts to the P site, where the amino acid is added to the growing polypeptide chain. The large subunit thus plays a crucial role in the elongation phase of translation.
5. Discuss the process of translation initiation in detail.
Answer:
Translation initiation begins when the small ribosomal subunit binds to the mRNA at the 5’ end. The mRNA contains a special sequence called the ribosome binding site (RBS), where the ribosome attaches. In eukaryotes, the small subunit binds to the 5′ cap of the mRNA, while in prokaryotes, it binds to the Shine-Dalgarno sequence. The initiation factors then bring in the first tRNA, which carries the amino acid methionine (in eukaryotes). The large subunit of the ribosome attaches to form a complete ribosome. The ribosome is now ready to begin elongation.
6. What are the three main sites on the ribosome, and what are their functions?
Answer:
The ribosome contains three main sites: the A site, the P site, and the E site. The A site (aminoacyl site) is where the incoming tRNA carrying an amino acid binds to the mRNA codon. The P site (peptidyl site) holds the tRNA carrying the growing polypeptide chain, where peptide bond formation occurs. The E site (exit site) is where the tRNA that has delivered its amino acid exits the ribosome. These sites work together to facilitate the translation process by ensuring proper tRNA binding, peptide bond formation, and tRNA release.
7. Explain how ribosomes facilitate the elongation phase of translation.
Answer:
During the elongation phase of translation, ribosomes help in the sequential addition of amino acids to the growing polypeptide chain. The process begins when the tRNA carrying the next amino acid enters the A site of the ribosome. The ribosome then catalyzes the formation of a peptide bond between the amino acid in the A site and the polypeptide chain held in the P site. The ribosome then shifts (translocates) along the mRNA by one codon, moving the tRNA in the A site to the P site and the tRNA in the P site to the E site, where it exits. This cycle repeats until a stop codon is reached.
8. How do ribosomes ensure the correct translation of mRNA?
Answer:
Ribosomes ensure correct translation of mRNA by accurately matching mRNA codons with the appropriate tRNA anticodons. The ribosome reads the mRNA in sets of three nucleotides (codons), and each codon specifies an amino acid. The tRNA molecules have anticodons that are complementary to the mRNA codons, ensuring that the correct amino acids are incorporated into the growing polypeptide chain. Additionally, the ribosome’s proofreading ability ensures that incorrect tRNA molecules are not accepted, thus maintaining the fidelity of protein synthesis.
9. What is the role of tRNA in the process of protein synthesis?
Answer:
Transfer RNA (tRNA) plays a crucial role in protein synthesis by carrying amino acids to the ribosome during translation. Each tRNA molecule has an anticodon region that is complementary to the mRNA codons. The tRNA binds to the appropriate codon on the mRNA, ensuring that the correct amino acid is incorporated into the growing polypeptide chain. The amino acids are linked together through peptide bonds catalyzed by the ribosome. After delivering the amino acid, the tRNA is released and can be reused.
10. What are the major differences between prokaryotic and eukaryotic ribosomes?
Answer:
Prokaryotic and eukaryotic ribosomes differ in size and composition. Prokaryotic ribosomes are smaller, with a 70S structure composed of a 30S small subunit and a 50S large subunit. Eukaryotic ribosomes are larger, with an 80S structure, consisting of a 40S small subunit and a 60S large subunit. Despite these differences, both types of ribosomes perform the same fundamental function of translating mRNA into proteins. Additionally, prokaryotic ribosomes are found in the cytoplasm, while eukaryotic ribosomes can be free in the cytoplasm or bound to the rough endoplasmic reticulum.
11. Describe the process of translation termination.
Answer:
Translation termination occurs when the ribosome reaches a stop codon on the mRNA. Stop codons (UAA, UAG, UGA) do not encode for any amino acid, and when the ribosome encounters a stop codon, release factors bind to the ribosome. These release factors catalyze the hydrolysis of the bond between the polypeptide chain and the tRNA in the P site, releasing the newly synthesized protein. The ribosome then dissociates from the mRNA, and the translation machinery is recycled for future use.
12. How do ribosomes interact with the endoplasmic reticulum in eukaryotic cells?
Answer:
In eukaryotic cells, ribosomes may either be free in the cytoplasm or attached to the rough endoplasmic reticulum (ER). When ribosomes are attached to the rough ER, they are involved in the synthesis of membrane-bound or secretory proteins. The ER membrane contains a signal recognition particle (SRP) receptor that recognizes the signal peptide on the nascent protein. As the protein is synthesized, it is translocated into the lumen of the ER, where it can undergo further modifications before being transported to other parts of the cell or secreted.
13. What is the role of ribosomal RNA (rRNA) in protein synthesis?
Answer:
Ribosomal RNA (rRNA) is a crucial component of ribosomes, providing structural support and catalyzing peptide bond formation during protein synthesis. rRNA molecules are integral to the ribosome’s function, as they help to stabilize the interaction between mRNA and tRNA. The rRNA also catalyzes the formation of peptide bonds between amino acids, a process known as peptidyl transferase activity. Without rRNA, the ribosome would not be able to carry out its function of translating genetic information into functional proteins.
14. How does the ribosome ensure the accuracy of protein synthesis?
Answer:
The ribosome ensures the accuracy of protein synthesis through several mechanisms. First, the ribosome’s decoding center selectively binds tRNAs with the correct anticodons corresponding to the mRNA codons. Second, the ribosome’s proofreading ability allows it to detect and reject incorrect tRNA molecules that do not match the mRNA codons. Additionally, elongation factors like GTP hydrolysis help ensure that the correct aminoacyl-tRNA is incorporated into the growing polypeptide chain.
15. Discuss the importance of ribosomes in gene expression.
Answer:
Ribosomes are central to gene expression because they are the molecular machines responsible for translating the genetic code in mRNA into a polypeptide chain, which ultimately folds into a functional protein. The synthesis of proteins from mRNA allows cells to carry out their functions, as proteins are responsible for a wide range of cellular activities. The ribosome reads the mRNA codons and, with the help of tRNA, synthesizes proteins that reflect the gene expression encoded in the DNA.
16. Explain the role of initiation factors in protein synthesis.
Answer:
Initiation factors are proteins that assist in the assembly of the ribosome on the mRNA and the initiation of translation. In eukaryotes, initiation factors help the small ribosomal subunit recognize and bind to the 5′ cap of the mRNA. They also facilitate the binding of the first tRNA carrying methionine. In prokaryotes, initiation factors assist the ribosome in recognizing the Shine-Dalgarno sequence. These factors are critical for the formation of the translation initiation complex, ensuring that translation begins at the correct site on the mRNA.
17. How do antibiotics like tetracycline affect ribosomes and protein synthesis?
Answer:
Antibiotics like tetracycline can interfere with ribosomal function and protein synthesis in bacteria. Tetracycline binds to the small subunit of bacterial ribosomes, preventing the binding of tRNA to the A site. This effectively halts protein synthesis by preventing the addition of new amino acids to the growing polypeptide chain. Since bacterial ribosomes are structurally different from eukaryotic ribosomes, tetracycline selectively targets bacterial ribosomes, making it an effective antibiotic without harming human cells.
18. How is the ribosome involved in the process of protein folding?
Answer:
While the ribosome is not directly involved in protein folding, it plays an indirect role. As the ribosome synthesizes the polypeptide chain, the newly formed protein begins to fold into its three-dimensional structure. Some proteins fold spontaneously as they emerge from the ribosome, while others require molecular chaperones to assist in folding. In some cases, the ribosome itself may help direct proper folding by providing a sheltered environment during translation.
19. How does the ribosome contribute to the efficiency of protein synthesis?
Answer:
The ribosome contributes to the efficiency of protein synthesis by facilitating the simultaneous translation of multiple mRNA molecules. A single mRNA can be translated by several ribosomes at once, forming a structure known as a polysome. This allows the cell to rapidly produce large quantities of protein from a single mRNA transcript. Additionally, the ribosome’s ability to accurately and rapidly match tRNAs to mRNA codons ensures that protein synthesis occurs with high fidelity and speed.
20. How do ribosomes in mitochondria differ from those in the cytoplasm?
Answer:
Mitochondria contain their own ribosomes, which resemble prokaryotic ribosomes in structure and function. Mitochondrial ribosomes are smaller (70S) compared to the larger (80S) ribosomes found in the cytoplasm. This similarity to prokaryotic ribosomes supports the endosymbiotic theory, which proposes that mitochondria originated from ancient prokaryotes. Despite their differences, both mitochondrial and cytoplasmic ribosomes perform the same basic function of translating mRNA into proteins. However, mitochondrial ribosomes synthesize proteins essential for mitochondrial function, while cytoplasmic ribosomes synthesize proteins for other cellular processes.
