Structure of DNA

Structure of DNA MDCAT MCQs with answers

23 past paper MCQs on Structure of DNA, from the Biological Molecules unit of MDCAT Biology. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.

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1.DNA stores biological information in discrete units termed: (MDCAT 2022)

  1. genes
  2. phenotypes
  3. karyotypes
  4. cells
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Correct answer: (a) genes

A gene is the length of nucleotides that codes for one polypeptide.

2.The complementary base pairing in DNA is important because it: (UHS MDCAT 2025)

  1. Maintains the tertiary structure of enzymes
  2. Provides energy for all cell metabolism
  3. Enables protein to fold properly
  4. Allows DNA act as a genetic blueprint during replication
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Correct answer: (d) Allows DNA act as a genetic blueprint during replication

Each strand specifies the other, so the base sequence can be copied faithfully and read as a blueprint.

3.In Watson and Crick's DNA model, which of the following pairs with Cytosine? (KMU MDCAT 2025)

  1. Adenine
  2. Guanine
  3. Thymine
  4. Uracil
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Correct answer: (b) Guanine

Cytosine pairs with guanine through three hydrogen bonds.

4.Guanine - cytosine pairs enhance DNA stability due to: (SIBA MDCAT 2025)

  1. Larger size
  2. Closer pairing
  3. More hydrogen bonds
  4. Double ring structure in both
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Correct answer: (c) More hydrogen bonds

Guanine and cytosine are held by three hydrogen bonds against the two of adenine and thymine.

5.Which of the following is NOT related to DNA? (SIBA MDCAT 2025)

  1. Adenine
  2. Thymine
  3. Uracil
  4. Guanine
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Correct answer: (c) Uracil

Uracil belongs to RNA; DNA carries thymine in its place.

6.Which of the following base pairing maintains the constant diameter of the DNA double helix as per Watson and Crick's model? (SZABMU MDCAT 2025)

  1. Purine - Purine
  2. Pyrimidine - Pyrimidine
  3. Purine - Pyrimidine
  4. Sugar-Phosphate
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Correct answer: (c) Purine - Pyrimidine

A large purine always pairs with a small pyrimidine, so every rung of the ladder is the same width.

7.DNA stores biological information in discrete units termed as: (UHS MDCAT 2023)

  1. Genes
  2. Phenotypes
  3. Karyotypes
  4. Cells
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Correct answer: (a) Genes

A gene is the length of DNA that codes for one polypeptide.

8.It is the basic unit of biological information: (PMC MDCAT 2022)

  1. Trait
  2. Gene
  3. Genotype
  4. Phenotype
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Correct answer: (b) Gene

A gene is the length of DNA carrying the information for one polypeptide.

9.The genetic Information for a specific character Is determined by (PMC MDCAT 2022)

  1. Genome
  2. Transcription
  3. Gene
  4. Trait
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Correct answer: (c) Gene

A gene is the length of DNA carrying the information for one character.

10.Which one of the following is found in both messenger RNA and DNA of a mammalian cell? (PMC MDCAT 2020)

  1. Double helical structure
  2. Ribose sugar
  3. Thymine
  4. Sugar – phosphate backbone
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Correct answer: (d) Sugar – phosphate backbone

Both are built on a backbone of alternating sugar and phosphate.

11.What is the effect of enzyme DNA ligase? (PMC MDCAT 2020)

  1. DNA is broken up at specific sites
  2. DNA fragments are joined together
  3. DNA replication occurs
  4. DNA transcription occurs
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Correct answer: (b) DNA fragments are joined together

DNA ligase seals the sugar phosphate backbone between two fragments.

12.The nitrogen-containing bases in nucleotide are of two types; Purines and Pyrimidines. The purines bases are: (UHS MDCAT 2019)

  1. Adenine, Guanine, and Cytosine
  2. Guanine and Cytosine
  3. Adenine and Guanine
  4. Adenine and Thymine
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Correct answer: (c) Adenine and Guanine

Adenine and guanine have the double ring of a purine; cytosine and thymine are pyrimidines.

13.As a result of replication, parental DNA would become completely dispersed and that each strand of all the daughter molecules would be a mixture of old and new DNA. This is called as: (UHS MDCAT 2019)

  1. Dispersive idea
  2. Conservative idea
  3. Disruptive idea
  4. Semi-conservative idea
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Correct answer: (a) Dispersive idea

The dispersive idea would have each daughter molecule a patchwork of old and new; replication is in fact semi-conservative.

14.Meselson and Stahl transferred few bacteria grown in N15 medium to N14 medium for replicating their DNA. What would be the result after two rounds of replication? (UHS MDCAT 2019)

  1. 100% heavy duplex
  2. 100% hybrid duplex
  3. 50% hybrid duplex and 50% heavy duplex
  4. 50% hybrid duplex and 50% light duplex
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Correct answer: (d) 50% hybrid duplex and 50% light duplex

Each strand of the heavy duplex serves as a template, so after two rounds half is hybrid and half is fully light.

15.Ester linkages occur in: (UHS MDCAT 2019)

  1. Nucleic acids
  2. Nucleus
  3. Carbohydrates
  4. Proteins
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Correct answer: (a) Nucleic acids

The sugar and phosphate of the backbone are joined by phosphodiester, that is ester, linkages.

16.A complete turn of the double helix of DNA comprises of: (UHS MDCAT 2018)

  1. 34 nm
  2. 3.4 Angstrom
  3. 3.4 nm
  4. 34 micrometers
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Correct answer: (c) 3.4 nm

One complete turn of the helix is ten base pairs, that is $3.4\,\mathrm{nm}$.

17.Single ringed pyrimidines are: (UHS MDCAT 2018)

  1. Uracil, Cytosine and Thymine
  2. Cytosine, Guanine and Uracil
  3. Adenine and Guanine
  4. Cytosine, Adenine and Thymine
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Correct answer: (a) Uracil, Cytosine and Thymine

The pyrimidines are cytosine, thymine and uracil, each with a single ring.

18.The region of the chromosome or, more specifically, the length of the DNA molecule, which has a particular function is called _______. (UHS MDCAT 2018)

  1. Kinetochore
  2. Locus
  3. Allele
  4. Gene
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Correct answer: (d) Gene

A gene is the length of DNA that carries the information for one function.

19.What is the subunit of DNA (UHS MDCAT 2017)

  1. Phosphoric Acid
  2. Base
  3. Nucleotide
  4. Sugar
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Correct answer: (c) Nucleotide

The repeating unit of DNA is the nucleotide: a base, a sugar and a phosphate.

20.The process of DNA replication in which each strand acts as a template for new double helix, composed of one parental strand and one newly polymerized strand refers: (UHS MDCAT 2017)

  1. PCR
  2. Semi conservative model
  3. Conservative mode
  4. Both PCR & Semi conseervative model
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Correct answer: (b) Semi conservative model

Each daughter molecule keeps one old strand and one new, which is semi-conservative replication.

21.DNA polymerase III adds nucleotides to DNA strand, replication always proceeds from: (UHS MDCAT 2017)

  1. 3’ → 5’
  2. 5’ → 3
  3. Sometime “3’ → 5’ & “5’ → 3’ ”
  4. None of the given options
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Correct answer: (b) 5’ → 3

A polymerase can only add to a free 3 prime end, so the new strand grows five to three.

22.Which one of the following nitrogen base is NOT present in DNA? (UHS MDCAT 2016)

  1. Adenine
  2. Guanine
  3. Uracil
  4. Cytosine
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Correct answer: (c) Uracil

Uracil belongs to RNA; DNA carries thymine.

23.Phosphodiester bond is: (UHS MDCAT 2016)

  1. P—O—C—P—O—C
  2. C—O—P
  3. C—O—P—O—C
  4. C—C—O—P
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Correct answer: (c) C—O—P—O—C

A phosphate joins the third carbon of one sugar to the fifth carbon of the next.

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