Molecular Biology

Step 1: Understanding the Concept:
The question asks about the consequence of inhibiting the 5' cap addition to eukaryotic pre-mRNA, specifically its effect on translation. The 5' cap is a modified guanine nucleotide added to the 5' end of the mRNA transcript.
Step 2: Detailed Explanation:
The 5' cap has several crucial functions in eukaryotes:

Protection: It protects the mRNA from degradation by 5' exonucleases.
Nuclear Export: It is essential for the transport of the mRNA from the nucleus to the cytoplasm.
Translation Initiation: It serves as the primary recognition site for the ribosome to bind to the mRNA. The cap is recognized by the cap-binding complex (eIF4F), which then recruits the small ribosomal subunit (40S) to the mRNA. This is a critical step for initiating translation.
Let's analyze the options based on these functions:

(A) Translation will initiate but elongation will be impaired: This is incorrect. The cap is essential for initiation, not elongation. Without the cap, initiation itself is blocked.
(B) Translation will fail to initiate due to improper ribosome binding: This is correct. Without the 5' cap, the ribosome cannot be recruited to the mRNA efficiently, and thus translation cannot begin.
(C) The polyadenylation will not occur: Polyadenylation (addition of a poly-A tail) occurs at the 3' end and is a separate process from 5' capping, although they are often coupled. Inhibiting capping doesn't directly prevent polyadenylation.
(D) Splicing of introns will be unaffected: This is not the most likely or direct effect on translation. While capping can influence the efficiency of splicing for some introns, its most direct and critical role is in translation initiation. The question asks for the effect on {translation}.
Step 3: Final Answer:
Inhibiting 5' capping will prevent the proper binding of the ribosome to the mRNA, thereby causing a failure in the initiation of translation.

Step 1: Understanding the Concept:
The question asks to identify the specific enzyme that creates the short RNA primers needed to initiate DNA synthesis during replication.
Step 2: Detailed Explanation:
Let's review the roles of the key enzymes in DNA replication:

Helicase: This enzyme unwinds the DNA double helix at the replication fork, separating the two strands to make them available as templates.
Primase: This is a type of RNA polymerase. Its specific job is to synthesize short RNA sequences, called primers (about 5-10 nucleotides long), that are complementary to the template DNA strand. DNA polymerase cannot start a new DNA strand from scratch; it can only add nucleotides to an existing 3'-OH group. The RNA primer provides this starting point.
DNA polymerase: This is the main enzyme of replication. It adds deoxynucleotides to the 3' end of the RNA primer, synthesizing the new DNA strand. It also has roles in proofreading and replacing the RNA primers with DNA.
Topoisomerase (or DNA gyrase in bacteria): This enzyme relieves the torsional strain and supercoiling that builds up ahead of the replication fork as the DNA unwinds.
Step 3: Final Answer:
The enzyme specifically responsible for synthesizing the RNA primers is Primase.

Step 1: Understanding the Concept:
The question asks for the correct chronological order of events that occur as a consequence of a point mutation in the DNA, following the principles of the central dogma of molecular biology (DNA RNA Protein).
Step 2: Detailed Explanation:
Let's trace the flow of information from the initial mutation to its final effect:

C. Substitution of a single nucleotide in the DNA sequence: The process must start with the mutation itself. A point mutation is a change in a single nucleotide base in the DNA. This is the root cause.
B. Change in mRNA codon during transcription: During transcription, the mutated DNA sequence serves as a template to create an mRNA molecule. The change in the DNA base will result in a corresponding change in the mRNA codon.
A. Alteration in the amino acid sequence of a protein: During translation, the ribosome reads the mRNA codons. If the changed codon now specifies a different amino acid (a missense mutation), the resulting polypeptide chain will have an altered amino acid sequence.
D. Possible disruption of protein function or structure: The change in the amino acid sequence can alter the protein's three-dimensional structure, which in turn can disrupt or change its biological function. This is the final phenotypic consequence of the mutation.
Step 3: Final Answer:
The correct sequence of events is: DNA mutation (C) mRNA codon change (B) Amino acid sequence change (A) Protein function disruption (D). This corresponds to the order C, B, A, D.

Step 1: Understanding the Concept:
Cancer is a disease of uncontrolled cell proliferation, often caused by mutations in genes that regulate the cell cycle. The two main types of genes involved are proto-oncogenes and tumor suppressor genes.
Step 2: Detailed Explanation:
Let's evaluate how each mutation would affect cancer risk:

(A) Loss-of-function mutation in a tumor suppressor gene: Tumor suppressor genes (e.g., p53, Rb) act as the "brakes" of the cell cycle, stopping division if there is DNA damage. A loss-of-function mutation is like having faulty brakes. This allows cells with mutations to continue dividing, leading to cancer. This is a very common mechanism in cancer development.

(B) Gain-of function in a DNA repair enzyme: DNA repair enzymes fix mutations. A "gain-of-function" would mean the enzyme works better or more efficiently. This would lead to fewer mutations being propagated, which would {decrease} the risk of cancer, not increase it.

(C) Silent mutation in a proto-oncogene: Proto-oncogenes are the "accelerators" of the cell cycle. They need a "gain-of-function" mutation to become oncogenes and promote cancer. A silent mutation, by definition, does not change the amino acid sequence of the protein, so it would not alter the protein's function and would not contribute to cancer.

(D) Deletion of non-coding intronic regions in a tumor suppressor gene: Introns are removed from the mRNA transcript before translation. Deleting an intron would likely have no effect on the final protein, unless the deletion affects critical splice sites or regulatory sequences. This is far less likely to cause a loss of function compared to a mutation directly within a coding region (exon).

Step 3: Final Answer:
The most likely mutation to contribute to cancer from the given options is a loss-of-function mutation in a tumor suppressor gene, as it removes a crucial control point for cell division.

Step 1: Understanding the Concept:
The question asks to identify the correct statements about protein synthesis (translation) specifically in eukaryotic cells. This requires knowledge of the key features that distinguish eukaryotic translation from prokaryotic translation.
Step 2: Detailed Explanation:
Let's evaluate each statement:

A. 3'-Cap of mRNA present. This statement is incorrect. Eukaryotic mRNA undergoes post-transcriptional modification which includes the addition of a 7-methylguanosine cap at the 5' end, not the 3' end. The 3' end typically has a poly-A tail.

B. Ribosomes of 80S type dissociate into 40S and 60S subunits. This statement is correct. Eukaryotic cells have 80S ribosomes, which are composed of a small 40S subunit and a large 60S subunit. (The Svedberg units are not additive).

C. Translation is not simultaneous with transcription. This statement is correct. In eukaryotes, transcription occurs inside the nucleus, while translation occurs in the cytoplasm. Due to this spatial and temporal separation (and the need for mRNA processing), the two processes cannot be coupled, unlike in prokaryotes where they occur simultaneously in the cytoplasm.

D. Initiation codon of mRNA is recognised by anticodon of Met-tRNA. This statement is correct. The start codon for translation in eukaryotes is AUG, which is recognized by the anticodon of a special initiator tRNA carrying the amino acid Methionine (Met).

Step 3: Final Answer:
Statements B, C, and D are correct, while statement A is incorrect. Therefore, the correct option is the one that includes only B, C, and D.