Question
What are components of DNA and RNA?
▶️Answer/Explanation
Answer: D
Explanation:
Key differences and similarities between DNA and RNA:
| Feature | DNA | RNA | Both DNA & RNA |
|---|---|---|---|
| Sugar | Deoxyribose | Ribose | Incorrect |
| Bases | A, T, G, C | A, U, G, C | A, G, C (shared); T only in DNA, U only in RNA |
| Uracil (U) | Not present | Present | Incorrect |
| Adenine (A) | Present | Present | Present in both |
Reason:
- DNA contains deoxyribose sugar.
- RNA contains ribose sugar.
- Adenine is present in both DNA and RNA.
- Uracil is only in RNA, not in DNA so options with uracil under “Both DNA and RNA” are wrong.
Question
Watson and Crick worked out the three-dimensional structure of DNA.
What did the model they built show for the first time?
A. DNA is a polynucleotide.
B. DNA contains equal quantities of adenine and thymine.
C. DNA molecules have a helical shape.
D. DNA is a double helix with antiparallel sugar–phosphate backbones.
▶️Answer/Explanation
Answer: D. DNA is a double helix with antiparallel sugar–phosphate backbones.
Explanation:
Watson and Crick proposed the first accurate three-dimensional structure of DNA. Their model revealed important details about DNA’s shape and how its strands are arranged, which explained how genetic information is stored and copied.
Evaluating the options:
A. Incorrect – It was already known that DNA is a polynucleotide (a chain of nucleotides) before their model.
B. Incorrect – The idea that adenine and thymine are present in equal amounts (Chargaff’s rules) was known before Watson and Crick’s work.
C. Incorrect – While DNA’s helical shape was suggested by earlier X-ray diffraction studies, Watson and Crick’s model went beyond just showing a helix.
D. Correct – Watson and Crick’s model was the first to show that DNA is a double helix with two antiparallel sugar–phosphate backbones, and that the bases pair specifically (A with T, G with C), explaining DNA’s structure and function.
Question
This micrograph is from a dividing cell.
What is shown in this image?
A. Two condensed chromosomes consisting of four chromatids
B. A chromosome from a cell in telophase of mitosis
C. A chromosome from a cell in G1 of interphase
D. A chromosome from a cell in metaphase
▶️Answer/Explanation
Answer: D. A chromosome from a cell in metaphase
Explanation:
In dividing cells, chromosomes become visible as distinct structures when they condense. During metaphase, chromosomes line up in the middle of the cell, and each one consists of two sister chromatids joined at a centromere. A micrograph from this stage often shows X-shaped chromosomes, which are duplicated chromosomes prepared for separation.
Evaluating the options:
A. Incorrect – Two chromosomes with four chromatids would suggest two duplicated chromosomes, but metaphase typically shows individual chromosomes aligned, not grouped in this way. This phrasing is confusing and not a standard description.
B. Incorrect – In telophase, chromosomes de-condense and become less visible as the nuclear envelope reforms. The micrograph would not show clearly condensed chromosomes like the ones seen in metaphase.
C. Incorrect – During G1 of interphase, chromosomes are not visible under a light microscope because they are in a loose, uncoiled form called chromatin.
D. Correct – This image likely shows a chromosome in metaphase, where it is fully condensed and visible, typically with two sister chromatids joined at the centromere, forming an X shape.
Question
Which feature is common to both mRNA and DNA?
A Covalent bonds between adjacent nucleotides
B Hydrogen bonds between guanine and cytosine
C Ribose sugar attached to phosphate
D Antiparallel arrangement of polynucleotide strands
Answer/Explanation
Answer: A. Covalent bonds between adjacent nucleotides
Explanation:
DNA and mRNA are both nucleic acids made of chains of nucleotides. Each nucleotide contains a sugar, a phosphate group, and a nitrogenous base. While they have some differences (like the sugar used and whether they are single- or double-stranded), they also share important structural features.
Evaluating the options:
A. Correct – Both DNA and mRNA have covalent bonds (called phosphodiester bonds) linking one nucleotide to the next along their backbone. This is a basic feature of all nucleic acid chains.
B. Incorrect – Hydrogen bonds between guanine and cytosine occur only in double-stranded DNA, not in single-stranded mRNA.
C. Incorrect – Only mRNA contains ribose sugar. DNA contains deoxyribose.
D. Incorrect – Antiparallel arrangement refers to the two strands in double-stranded DNA, not to mRNA, which is single-stranded.
Which structure found in eukaryotes has a single membrane?
A. Nucleus
B. Lysosome
C. Chloroplast
D. Mitochondrion
▶️Answer/Explanation
Answer: B. Lysosome
Explanation:
In eukaryotic cells, organelles are often surrounded by membranes. Some have a single membrane, while others (like mitochondria and chloroplasts) have double membranes. The lysosome, which breaks down waste materials, is an example of a single-membrane-bound organelle.
Evaluating the options:
A. Incorrect – The nucleus has a double membrane called the nuclear envelope, which surrounds the genetic material.
B. Correct – The lysosome is a membrane-bound organelle with a single membrane. It contains digestive enzymes to break down waste and damaged cell parts.
C. Incorrect – Chloroplasts (found in plant cells) have a double membrane and internal structures like thylakoids.
D. Incorrect – Mitochondria also have a double membrane, with an outer membrane and a folded inner membrane (cristae).
Question
What is the arrangement of subunits in a DNA nucleotide?
A. sugar – base – phosphate
B. sugar – phosphate – base
C. phosphate – sugar – base
D. sugar – phosphate – base – base – phosphate – sugar
▶️Answer/Explanation
Answer: C. phosphate – sugar – base
Explanation:
A DNA nucleotide is made up of three basic parts:
- A phosphate group
- A deoxyribose sugar (a 5-carbon sugar)
- A nitrogenous base (adenine, thymine, guanine, or cytosine)
These parts are joined in a specific order: the phosphate is attached to the sugar, and the base is also attached to the sugar, but on a different carbon. This forms the building block of DNA.
Evaluating the options:
A. Incorrect – This places the base before the phosphate, which doesn’t match the actual structure.
B. Incorrect – While the sugar is central, the order here is misleading. Phosphate comes before sugar when forming the DNA backbone.
C. Correct – This is the correct order: phosphate – sugar – base. The phosphate attaches to the sugar’s 5′ carbon, and the base attaches to the sugar’s 1′ carbon.
D. Incorrect – This is a random, overly long and incorrect chain of parts, not the basic structure of a single DNA nucleotide.
Question
What is the arrangement of the components of nucleotides in a single DNA strand?
▶️Answer/Explanation
Answer: A
Explanation:
A single DNA strand consists of repeating nucleotides, each composed of:
- A sugar (S) (deoxyribose),
- A phosphate group (P),
- An organic base (B) (A, T, C, or G).
These nucleotides are connected in the following manner:
- The phosphate group links the sugar molecules of adjacent nucleotides, forming the sugar-phosphate backbone.
- The organic base is attached to the sugar.
So, the repeating unit along the DNA strand is:
sugar–phosphate–sugar–phosphate, with bases branching off each sugar.
This matches Option A, where:
- The backbone is S–P–S–P,
- And each S has a base (B) attached.
Incorrect Options:
B: Starts with a phosphate and alternates incorrectly (P–S–P).
C: Bases are part of the backbone and sugars follow bases, which is not correct.
D: The sequence B–S–P is not the correct structural order for the DNA backbone.
Question
What is the correct arrangement for the components of one strand in a DNA molecule?
▶️Answer/Explanation
Answer: C
Explanation:
In a DNA strand, the basic unit is a nucleotide, and each nucleotide is composed of:
- A phosphate group (attached to the 5′ carbon of the sugar),
- A deoxyribose sugar,
- A nitrogenous base (attached to the 1′ carbon of the sugar).
These nucleotides are joined in a sugar-phosphate backbone, where the phosphate of one nucleotide is bonded to the sugar of the next. The bases stick out from the sugar.
So the correct linear sequence is:
phosphate — deoxyribose — base — phosphate — deoxyribose — base — …
This structure is clearly represented in Option C, where:
- The phosphate-deoxyribose-phosphate sequence forms the backbone.
- The bases are attached to the deoxyribose sugars, pointing outward.
Why Others Are Incorrect:
A: Puts bases inline with the backbone, disrupting the sugar-phosphate link.
B: Starts with a base and places phosphate incorrectly under sugar.
D: Bases precede sugars and phosphate alternates incorrectly.
Question
What is bonded to phosphates in a strand of RNA?
A. Only carbohydrates
B. Adenine, guanine, cytosine and thymine
C. Adenine, guanine, cytosine and uracil
D. Pentoses and bases
▶️Answer/Explanation
Answer: D. Pentoses and bases
Explanation:
In RNA, each nucleotide is made of three parts: a phosphate group, a ribose sugar (a pentose), and a nitrogenous base (A, U, G, or C). The phosphate connects to the sugar of the next nucleotide, forming the sugar-phosphate backbone. The base is attached to the sugar, not directly to the phosphate, but it’s part of the overall nucleotide structure.
Evaluating the options:
A. Incorrect – Saying “only carbohydrates” (sugars) is incomplete. Bases are also part of the structure bonded through the sugar.
B. Incorrect – This lists thymine, which is found in DNA, not RNA. RNA uses uracil instead.
C. Incorrect – While the listed bases are correct for RNA (A, U, G, C), they are not directly bonded to the phosphate — they are bonded to the sugar.
D. Correct – This is the best answer. In RNA, phosphates are bonded to pentose sugars, which are in turn bonded to bases. So, the full structure involves pentoses and bases.
Question
What are linked by hydrogen bonds?
A. Hydrogen and oxygen within a water molecule
B. Phosphate and sugar within a DNA molecule
C. Base and sugar between DNA nucleotides
D. Hydrogen and oxygen in different water molecules
▶️Answer/Explanation
Answer: D. Hydrogen and oxygen in different water molecules
Explanation:
Hydrogen bonds are weak attractions that occur when a hydrogen atom, which is covalently bonded to a more electronegative atom (like oxygen or nitrogen), is attracted to another electronegative atom nearby. These bonds are important in biology for holding molecules together without being as strong as covalent bonds.
Evaluating the options:
A. Incorrect – Within a water molecule, hydrogen and oxygen are connected by covalent bonds, which are strong bonds sharing electrons. Hydrogen bonds form between different molecules, not within one molecule.
B. Incorrect – Phosphate and sugar in DNA are linked by covalent bonds (phosphodiester bonds), not hydrogen bonds.
C. Incorrect – The base is attached to the sugar by a covalent bond. Hydrogen bonds occur between bases on opposite DNA strands, not between bases and sugars.
D. Correct – Hydrogen bonds form between the hydrogen atom of one water molecule and the oxygen atom of another water molecule. This explains many of water’s unique properties.