1. What is the genetic code, and why is it essential for protein synthesis?
Answer:
The genetic code is the set of rules by which the sequence of nucleotides in mRNA is translated into the sequence of amino acids in proteins. It is essential for protein synthesis because it ensures that the genetic information stored in DNA is accurately and efficiently converted into functional proteins. Each triplet of nucleotides (codon) corresponds to a specific amino acid, facilitating the assembly of proteins required for cellular function.
2. Explain the universal nature of the genetic code with examples.
Answer:
The genetic code is universal, meaning that it is nearly the same in all living organisms. For instance, the codon AUG codes for methionine in humans, bacteria, and plants. This universality suggests that all life shares a common evolutionary origin. However, there are rare exceptions, such as in mitochondrial DNA, where certain codons may specify different amino acids.
3. Describe the degeneracy of the genetic code and its biological significance.
Answer:
Degeneracy refers to the phenomenon where multiple codons encode the same amino acid. For example, the amino acid leucine is encoded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). This feature provides a buffer against mutations; changes in the third base of a codon often do not alter the encoded amino acid, reducing the potential for harmful effects of genetic mutations.
4. How many codons are there in the genetic code, and how are they classified?
Answer:
There are 64 codons in the genetic code, classified as:
- 61 sense codons: Code for amino acids.
- 3 stop codons: UAA, UAG, and UGA, which signal the termination of protein synthesis.
This distribution ensures that all 20 standard amino acids are encoded, with redundancy provided by degeneracy.
5. Why is the genetic code described as non-overlapping and commaless?
Answer:
The genetic code is non-overlapping because each nucleotide is part of only one codon and is read sequentially. It is commaless because there are no gaps or punctuation marks between codons. This ensures a continuous and unambiguous translation of the mRNA sequence into a polypeptide chain.
6. Discuss the role of start and stop codons in the genetic code.
Answer:
- Start codon (AUG): Signals the beginning of translation and codes for methionine. It ensures that the ribosome initiates protein synthesis at the correct position.
- Stop codons (UAA, UAG, UGA): Signal the termination of translation, instructing the ribosome to release the completed polypeptide chain. These codons do not code for any amino acids.
7. What are synonymous codons, and how do they relate to the degeneracy of the genetic code?
Answer:
Synonymous codons are different codons that encode the same amino acid. For example, GGU, GGC, GGA, and GGG all code for glycine. This redundancy, a hallmark of the genetic code’s degeneracy, minimizes the impact of point mutations, particularly in the third position of codons.
8. How does the wobble hypothesis explain codon-anticodon pairing?
Answer:
The wobble hypothesis, proposed by Francis Crick, explains that the third base of a codon can form less stringent pairings with the corresponding base in the anticodon of tRNA. This flexibility allows one tRNA molecule to recognize multiple synonymous codons, contributing to the efficiency and degeneracy of the genetic code.
9. What are the exceptions to the universality of the genetic code? Provide examples.
Answer:
Exceptions to the universality of the genetic code are found in mitochondrial DNA and some protozoans. For example:
- In human mitochondria, UGA codes for tryptophan instead of being a stop codon.
- In ciliated protozoa, UAA and UAG code for glutamine instead of serving as stop codons.
10. Why does the genetic code have stop codons, and how do they function?
Answer:
Stop codons (UAA, UAG, UGA) terminate protein synthesis by signaling the release of the newly formed polypeptide from the ribosome. They do not code for any amino acids, ensuring that translation halts appropriately at the end of the coding sequence.
11. Explain the relationship between codons, anticodons, and amino acids.
Answer:
- Codons: Triplets of nucleotides in mRNA that specify amino acids.
- Anticodons: Triplets of nucleotides in tRNA that are complementary to mRNA codons.
- Amino acids: Building blocks of proteins, brought to the ribosome by tRNA.
The codon-anticodon interaction ensures the correct amino acid is added to the growing polypeptide chain.
12. How does the genetic code protect against mutations?
Answer:
The degeneracy of the genetic code protects against mutations by allowing synonymous codons to code for the same amino acid. For example, a mutation changing GCU to GCA still results in alanine being incorporated, preventing a change in protein function.
13. What is a frameshift mutation, and how does it affect the genetic code?
Answer:
A frameshift mutation involves the insertion or deletion of nucleotides that alters the reading frame of codons. This disrupts the sequence of amino acids, often leading to nonfunctional proteins, as the genetic code is read in triplets.
14. Compare and contrast the universal and degenerate features of the genetic code.
Answer:
- Universal: The same codon specifies the same amino acid across most organisms.
- Degenerate: Multiple codons can encode the same amino acid.
While universality ensures conservation of genetic information, degeneracy provides robustness against mutations.
15. How does the genetic code relate to the central dogma of molecular biology?
Answer:
The genetic code is a critical component of the central dogma, which describes the flow of genetic information from DNA to RNA to proteins. It translates the nucleotide sequence of mRNA into the amino acid sequence of proteins, enabling gene expression.
16. Why are stop codons called nonsense codons?
Answer:
Stop codons are called nonsense codons because they do not specify any amino acids. Instead, they signal the termination of protein synthesis, marking the end of translation.
17. What experimental evidence supports the triplet nature of the genetic code?
Answer:
Experiments using frameshift mutations demonstrated that adding or deleting nucleotides in multiples of three restored the reading frame, proving the triplet nature of the genetic code. Marshall Nirenberg’s experiments also confirmed this.
18. How is the degeneracy of the genetic code an evolutionary advantage?
Answer:
Degeneracy reduces the impact of point mutations by ensuring that many mutations in the third codon position do not alter the encoded amino acid. This feature contributes to genetic stability and evolutionary adaptability.
19. Describe the roles of tRNA and ribosomes in interpreting the genetic code.
Answer:
- tRNA: Matches codons in mRNA with the corresponding amino acids using its anticodon.
- Ribosome: Facilitates the alignment of mRNA and tRNA, catalyzing peptide bond formation to assemble proteins.
20. Why is the start codon AUG crucial in protein synthesis?
Answer:
The start codon AUG initiates translation by coding for methionine, which is the first amino acid in the polypeptide chain. It also establishes the reading frame for subsequent codons.
21. What are the consequences of a mutation in a stop codon?
Answer:
A mutation in a stop codon can result in continued translation, producing an elongated, potentially nonfunctional protein. This can disrupt cellular processes and lead to disease.
22. Discuss the redundancy of the genetic code with examples.
Answer:
Redundancy means that several codons can code for the same amino acid. For example, GCU, GCC, GCA, and GCG all encode alanine. This redundancy helps minimize the impact of certain mutations.
23. How are codon usage biases observed among organisms?
Answer:
Different organisms prefer specific synonymous codons for the same amino acid. For example, E. coli prefers GGC for glycine, while humans may prefer GGG. This bias reflects evolutionary and translational efficiency.
24. How does the genetic code demonstrate co-linearity?
Answer:
The sequence of codons in mRNA corresponds directly to the sequence of amino acids in the protein, showing a one-to-one relationship between nucleotides and amino acids.
25. What is the significance of the wobble position in codons?
Answer:
The wobble position (third base of a codon) allows for flexible base pairing, enabling one tRNA to recognize multiple codons. This increases translational efficiency and reduces the need for multiple tRNA species.
26. How do scientists decode the genetic code in experiments?
Answer:
Scientists use synthetic RNA molecules and in vitro translation systems to match codons with amino acids. For example, poly-U RNA was used to identify phenylalanine as the amino acid encoded by UUU.
27. What role do codons play in genetic disorders?
Answer:
Mutations in codons can result in genetic disorders. For example, a mutation in the codon for glutamic acid to valine in the hemoglobin gene causes sickle cell anemia.
28. Why is the genetic code critical in biotechnology?
Answer:
The genetic code underpins genetic engineering and protein synthesis. For instance, it enables scientists to design recombinant DNA to express proteins in different organisms.
29. Explain the importance of codon-anticodon pairing in translation fidelity.
Answer:
Accurate codon-anticodon pairing ensures the correct amino acid is added to the polypeptide chain, maintaining protein structure and function.
30. Discuss the historical contributions of scientists to decoding the genetic code.
Answer:
Marshall Nirenberg and Har Gobind Khorana played pivotal roles in deciphering the genetic code, using synthetic RNA sequences and experimental systems to assign codons to specific amino acids. Their work earned the Nobel Prize in 1968.
