Question
What describes the structure of proteins?
I. Proteins are made from amino acids linked together by peptide bonds.
II. The sequence of amino acids in all proteins is the same.
III. A protein may consist of more than one polypeptide.
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
▶️Answer/Explanation
Answer: B. I and III only
Explanation:
Proteins are made of smaller units called amino acids, which are connected by peptide bonds to form a polypeptide chain. The sequence of amino acids in each protein is unique and determines how the protein folds and what function it performs. Some proteins are made of only one polypeptide, while others are made of two or more polypeptides working together.
Option Evaluation:
A. Incorrect – Statement I is correct, but Statement II is wrong. Not all proteins have the same amino acid sequence—each protein has its own unique sequence.
B. Correct – Statement I is correct (proteins are made from amino acids linked by peptide bonds), and Statement III is also correct (some proteins consist of more than one polypeptide). This is the accurate answer.
C. Incorrect – Statement II is false, so this option is wrong even though Statement III is correct.
D. Incorrect – Statement II is incorrect, so all three statements cannot be true.
Question
People who suffer from night blindness cannot see well at night because their retinal cells produce an abnormal form of a protein pigment. Which protein is this?
A. Fibrin
B. Myosin
C. Rhodopsin
D. Immunoglobulin
Answer/Explanation
Answer: C. Rhodopsin
Explanation:
Rhodopsin is a light-sensitive protein pigment found in the rod cells of the retina in the eye. Rod cells help us see in low-light (dim) conditions, especially at night. Rhodopsin is made from opsin (a protein) and retinal (a form of vitamin A). When light hits rhodopsin, it changes shape and triggers a nerve signal to the brain, allowing us to see in the dark. If there is a defect in rhodopsin, or if it is not made properly (often due to vitamin A deficiency), it can cause night blindness.
Option Evaluation:
A. Incorrect – Fibrin is a protein involved in blood clotting, not vision.
B. Incorrect – Myosin is a motor protein involved in muscle contraction, not related to the retina or vision.
C. Correct – Rhodopsin is the correct protein. It is essential for night vision, and abnormal forms of rhodopsin lead to night blindness.
D. Incorrect – Immunoglobulin is another name for antibodies, which are part of the immune system, not involved in sight.
Question
What is the minimum number of nucleotides needed to code for a polypeptide composed of 210 amino acids?
A. 70
B. 210
C. 420
D. 630
Answer/Explanation
Answer: D. 630
Explanation:
Proteins are made of amino acids, and the instructions for building them are stored in DNA and carried out by RNA. In the genetic code, each amino acid is coded by a group of 3 nucleotides, called a codon. So, to figure out how many nucleotides are needed, you multiply the number of amino acids by 3.
We’re asked to find the minimum number of nucleotides needed to code for 210 amino acids, so we focus only on what’s needed to make the amino acids—not the start or stop signals.
210 amino acids × 3 nucleotides per amino acid = 630 nucleotides
Option Evaluation:
A. Incorrect – 70 nucleotides would only code for about 23 amino acids (70 ÷ 3), which is far too few.
B. Incorrect – 210 nucleotides would only code for 70 amino acids (210 ÷ 3), not enough to build a 210-amino-acid chain.
C. Incorrect – 420 nucleotides would code for 140 amino acids (420 ÷ 3), still not enough.
D. Correct – 630 nucleotides are exactly what’s needed. 210 amino acids × 3 = 630.
Question
Which reaction occurs when a dipeptide is formed from amino acids?
A. Hydrolysis
B. Condensation
C. Transcription
D. Oxidation
Answer/Explanation
Answer: B. Condensation
Explanation:
A dipeptide is formed when two amino acids join together. This happens through a condensation reaction (also called dehydration synthesis), where a molecule of water (H₂O) is removed as the carboxyl group (–COOH) of one amino acid bonds with the amino group (–NH₂) of another. The bond formed between them is called a peptide bond.
This is the same type of reaction used to build long chains of amino acids (polypeptides or proteins).
Option Evaluation:
A. Incorrect – Hydrolysis is the opposite process. It breaks a peptide bond by adding water, not forming one.
B. Correct – Condensation is the correct term for forming a dipeptide. It removes water to create a peptide bond between amino acids.
C. Incorrect – Transcription is a process in the nucleus where mRNA is made from DNA. It’s not related to building proteins directly from amino acids.
D. Incorrect – Oxidation involves losing electrons, usually in energy-related reactions. It’s not the reaction that forms peptide bonds.