All Free Biology MCQs with Answers

• A. Methionine, Small ribosomal subunit
• B. Methionine, Large ribosomal subunit
• C. Tyrosine, Large ribosomal subunit
• D. Tyrosine, Small ribosomal subunit

Explanation: In prokaryotes polypeptides synthesis begins with the formation of the initiation complex. First, the tRNA molecule carrying a chemically modified methionine (called N-formyl methionine) binds to the small ribosomal subunit. A protein called the initiation factor positions the tRNA on the ribosomal surface at the P side, where the peptide bond is formed.

• A. Aminoacyl site
• B. Exit site
• C. Peptyidyl site
• D. A-site

Explanation: 'E' site is the exit site where empty tRNAs will exit the chromosome. Option B is correct because the site of the ribosome where empty tRNAs (tRNAs that have already donated their amino acids to the growing polypeptide chain) leave the ribosome is called the Exit site (E site).

• A. Elongation factor
• B. Release factor
• C. Initiation factor
• D. tRNA synthetase

Explanation: Initiation factors are proteins that bind to the small subunit of the ribosome during the initiation of translation, a part of protein synthesis. They are divided into three major groups. Prokaryotic initiation factor, Archaeal initiation factor, and Eukaryotic initiation factor. This initiation complex is essential for the start of protein synthesis, as it helps position the ribosome and initiator tRNA at the correct location on the mRNA.

• A. mRNA, Anticodon
• B. mRNA, codon
• C. tRNA, Anticodon
• D. tRNA, codon

Explanation: After the formation of the initiation complex, the large ribosomal subunit binds the tRNA molecule with the appropriate anticodon. The protein called elongation factors assists in binding it to the exposed mRNA codon at the 'A' site. The ribosome, including the large subunit, interacts with the tRNA molecules to ensure proper base pairing between the mRNA codons and the tRNA anticodons, facilitating the translation process. The mRNA is positioned in the ribosome, and the tRNA molecules carry the anticodons that complement the mRNA codons.

• A. mRNA codon, A-site
• B. mRNA codon, P-site
• C. rRNA, A-site
• D. rRNA, P-Site

Explanation: Elongation factors are sets of proteins that are used in protein synthesis in the process of the cell cycle and elongation in some cells. In the ribosome, they facilitate translational elongation, from the formation of the first peptide bond to the formation of the last one. Each time, tRNA is guided to enter on site A.

• A. A
• B. P
• C. U
• D. E

Explanation: The movement of ribosomes along the mRNA molecule in the 5' → 3' direction translocates the initial tRNA to the E site and ejects it from the ribosome. The E site is where the empty tRNA exits the ribosome after it has released its amino acid, and the P site is where the tRNA carrying the growing polypeptide chain is located. The A site is where the incoming aminoacyl-tRNA pairs with the mRNA codon. The U site is not a recognised site in ribosomal function.

• A. Initiation
• B. Start
• C. Non-sense
• D. Anti-sense

Explanation: The nonsense codon, also called stop or termination codons, does not encode any genetic information (amino acid), thus they are not recognised by the anticodon of any tRNA. However, they are recognised by the release factor. A release factor is a protein that allows for the termination of translation by recognising the termination codon or stop codon in an mRNA sequence. UAA, UAG and UGA are stop codons.

• A. Copy, mould
• B. Cast, mould
• C. Model, mould
• D. Model, Copy

Explanation: In semi-conservative replication, each original strand of the DNA duplex serves as a model for the synthesis of a complementary strand, and the newly synthesised strand complements the model, forming a duplex.

• A. Primary structure is conserved
• B. Secondary structure is conserved
• C. Primary structure is changed
• D. Primary structure is disrupted

Explanation: In the process of semi-conservative replication, the primary structure is conserved. The primary structure of DNA refers to the sequence of nucleotides along the DNA strand, and this sequence is faithfully maintained in the newly synthesised DNA strands. Each original DNA strand serves as a template for the synthesis of a complementary strand, preserving the primary structure. The secondary structure, which is the double helical structure, is disrupted temporarily during replication but is reformed as the new strands are synthesised.

• A. Be disrupted
• B. Remain intact
• C. Be opened
• D. Remain partially intact

Explanation: According to conservative replication, the entire parental DNA duplex is conserved, and a completely new duplex is formed.