DNA STRUCTURE

DNA STRUCTURE – Biochemical composition of DNA

• Discover – Miescher- Pus-cell -Name Nuclein 
• Altman -Nucleic Acid 
• Each nucleotide unit consists of three types of compounds which are as follows :

(1) A pentose sugar -the 5 carbon deoxyribose sugar

(2)  2. Phosphoric Acid (H₃PO₄)

(3)  Nitrogenous base-a heterocyclic ring containing nitrogen

1. Pentose sugar molecule

• Pentose sugar was identified by Lewine (1909). 
• It is 5 Carbon Sugar 

These are Two type –

1. Deoxyribose Sugar  – In DNA always deoxyribose sugar is found which is lacking in one oxygen atom at 2’ carbon 
2. Ribose sugar – It is Found in RNA but in this oxygen is present at 2’ carbon

• The deoxyribose sugar has a pentagonal ring with 5 carbon atoms (1″, 2′, 3′, 4′ and 5 carbons). 
• Among which 1 carbon is attached to the nitrogenous base and 3′ and 5′ carbons are attached to phosphoric acid. 
• At carbon 1′ always the OH group is present which is the point of attachment of the base. 
• This is linked to the 1-nitrogen (1-Nitrogenation) atom of the pyrimidine bases and to the 9-nitrogen atom (9-Nitrogenation) of purine bases.

(2)  2. Phosphoric Acid (H₃PO₄)

• It found in the form of H₃PO in DNA
• It joins sugar at 3’ and 5’ carbon by the condensation reaction.
• DNA has an acidic nature due to the presence of phosphoric acid because it have the negatively charged oxygen molecule.

3. Nitrogenous bases

• It has nitrogen in their ring so these are basic nature.
• These are of two types : 

(a) Purines and (b) Pyrimidines.

(a) Purines : 

• Purines are double ringed nitrogen compounds,
• in which two benzene rings (5 and 6 membered) are fused. 
• One ring is called Pyrimidine Ring (2N +4C ) and another is called Imidazole Ring (2N +3C)
• In DNA two types of purines : Adenine (A) and Guanine (G) are found. 

(b) Pyrimidines : 

• Pyrimidines are 6 membered single benzene ring compounds. 
• They are Thymine (T) and Cytosine (C). But in RNA instead of thymine, Uracil (U) is present. 

Nucleosides

• A molecule of deoxyribose sugar attached with a molecule of nitrogenous base without phosphate group is known as nucleoside. 
• In a molecule of DNA four types of nucleosides are found, which are :

(a) Adenosine : Adenine + Deoxyribose

(b) Guanosine : Guanine + Deoxyribose

(c) Cytidine : Cytosine + Deoxyribose

(d) Thymidine : Thymine + Deoxyribose

Nucleotides

• A nucleotide consists of one molecule of deoxyribose sugar, one molecule of phosphoric acid and one of the four nitrogenous bases. 
• In DNA molecule following

four types of nucleotides are present :

(a) De-oxyadenylic acid : Adenine+Deoxyribose +Phosphoric acid (DAMP)

(b) De-oxyguanylic acid : Guanine+Deoxyribose +Phosphoric acid (dGMP)

(c) De-oxycytidylic acid : Cytɔsine+Deoxyribose +Phosphoric acid (DCMP)

(d) De-oxythymidylic acid : Thymine+Deoxyribose +Phosphoric acid (dTMP)

Chargaff’s Rule

• According to this rule the amount of Purine is equals to that of Pyrimidine in DNA 
•  According to this law A=T and GC are always equal to one  except for o x 174 Coliphage here A:T and G:C ratio is not one. 
• Chargaff’s rules are also called as “Base rule” or ‘base ratios’.

Watson and Crick model of DNA

This model of DNA proposed by Watson and Crick and supported by Wilkins et. al., by x-ray crystallography of DNA.

According to this model.

• DNA consists of two, polynucleotide chains which are twisted around each other and  forming a right-handed double helix around the same central axis.
• These chains are antiparallel, means one chain (3! 5) appears as ascending and another one (5, 3′) as descending. 
• In both the chains orientation of the sugar molecule is in the opposite direction.
• In this the phosphate groups of lies on the outside of the deoxyribose and the nitrogenous bases are stacked inside the helix..
• The nitrogenous bases of the two chains are linked through hydrogen bonds formed between oxygen and nitrogen atoms of the adjacent bases.
• Size of one turn of helix measures about 34 Å. 

• One helix contains 10 base paired nucleotides.
• The distance between two nucleotide is 3.4 Å. 
• Each successive base pair in the stack turns 36° in a clock-wise direction. 
• The double helix, therefore, makes a complete turn´of 360° every ten base pairs like circular staircase.
• The adenine (A) base always pairs with thymine (T) by two H-bonds.
• (C) pairs with guanine (G) by three H-bonds . 
• Due to this type of pairing, the two chains or strands are complementary to each other.
•  for example :
• I strand 5′ A-T-G-A-C-T-A-G 3
• II strand 3 T-А-С-Т-G-A-T-C 51
• Due to this property, during DNA replication the two dissociated chains serve as templates for the synthesis of new complementary chains.
• The diameter of the DNA molecule is about 20 A or 2 nanometers (1 nanometer = 1 billionth of a meter).
• The twisting of the strands results in the formation of narrow (minor) and wide (major) grooves.

Multiple-Choice Questions (MCQs) on DNA Structure

1. Who discovered DNA and named it ‘Nuclein’?

A) Watson and Crick
B) Chargaff
C) Miescher
D) Altman

Answer: C) Miescher

2. Who renamed ‘Nuclein’ as ‘Nucleic Acid’?

A) Franklin
B) Altman
C) Griffith
D) Wilkins

Answer: B) Altman

3. What are the three main components of a nucleotide?

A) Pentose sugar, Phosphate group, Nitrogenous base
B) Glucose, Phosphate group, Amino acid
C) Fatty acid, Phosphate group, Nitrogenous base
D) Ribose sugar, Carbon dioxide, Nitrogenous base

Answer: A) Pentose sugar, Phosphate group, Nitrogenous base

4. Which sugar is present in DNA?

A) Ribose
B) Glucose
C) Fructose
D) Deoxyribose

Answer: D) Deoxyribose

5. Who identified pentose sugar in DNA?

A) Chargaff
B) Miescher
C) Levine
D) Watson

Answer: C) Levine

6. In DNA, deoxyribose sugar lacks oxygen at which carbon position?

A) 1’
B) 2’
C) 3’
D) 5’

Answer: B) 2’

7. What is the function of phosphoric acid in DNA?

A) To store genetic information
B) To form a backbone by linking sugars at 3’ and 5’ carbons
C) To encode proteins
D) To act as an enzyme

Answer: B) To form a backbone by linking sugars at 3’ and 5’ carbons

8. Why is DNA acidic in nature?

A) Presence of deoxyribose sugar
B) Presence of phosphate group
C) Presence of nitrogenous bases
D) Presence of hydrogen bonds

Answer: B) Presence of phosphate group

9. Which of the following are purines?

A) Adenine and Cytosine
B) Guanine and Thymine
C) Adenine and Guanine
D) Thymine and Uracil

Answer: C) Adenine and Guanine

10. Which of the following are pyrimidines?

A) Adenine and Guanine
B) Guanine and Cytosine
C) Cytosine and Thymine
D) Adenine and Thymine

Answer: C) Cytosine and Thymine

11. Which nitrogenous base is found in RNA but not in DNA?

A) Adenine
B) Thymine
C) Uracil
D) Guanine

Answer: C) Uracil

12. A nucleoside consists of which components?

A) Sugar + Nitrogenous base
B) Sugar + Phosphate
C) Nitrogenous base + Phosphate
D) Sugar + Phosphate + Nitrogenous base

Answer: A) Sugar + Nitrogenous base

13. What is the full form of dGMP?

A) Deoxyribose Guanine Monophosphate
B) Deoxyguanosine Monophosphate
C) Deoxyribose Glycine Monophosphate
D) Deoxyguanine Monophosphate

Answer: B) Deoxyguanosine Monophosphate

14. Which scientist proposed the base pairing rule?

A) Watson and Crick
B) Chargaff
C) Franklin
D) Miescher

Answer: B) Chargaff

15. According to Chargaff’s rule, which base pairs with thymine in DNA?

A) Cytosine
B) Guanine
C) Uracil
D) Adenine

Answer: D) Adenine

16. Who proposed the double helix model of DNA?

A) Watson and Crick
B) Franklin and Wilkins
C) Griffith
D) Altman

Answer: A) Watson and Crick

17. What is the nature of DNA strands?

A) Parallel
B) Antiparallel
C) Single-stranded
D) Zigzag

Answer: B) Antiparallel

18. The double helix of DNA is twisted in which direction?

A) Left-handed
B) Right-handed
C) Straight
D) Random

Answer: B) Right-handed

19. How many base pairs are present in one complete turn of the DNA helix?

A) 5
B) 10
C) 15
D) 20

Answer: B) 10

20. The distance between two nucleotides in DNA is

A) 3.4 Å
B) 34 Å
C) 20 Å
D) 10 Å

Answer: A) 3.4 Å

21. The length of one complete turn of the DNA double helix is

A) 3.4 Å
B) 10 Å
C) 34 Å
D) 20 Å

Answer: C) 34 Å

22. The diameter of the DNA double helix is

A) 3.4 Å
B) 10 Å
C) 20 Å
D) 34 Å

Answer: C) 20 Å

23. How many hydrogen bonds are formed between Adenine and Thymine?

A) One
B) Two
C) Three
D) Four

Answer: B) Two

24. How many hydrogen bonds are formed between Guanine and Cytosine?

A) One
B) Two
C) Three
D) Four

Answer: C) Three

25. What is the function of hydrogen bonds in DNA?

A) To store genetic information
B) To hold the two strands together
C) To join sugar and phosphate
D) To break the DNA helix

Answer: B) To hold the two strands together

26. What is the complementary sequence of 5’-A-T-G-C-3’?

A) 5’-T-A-C-G-3’
B) 3’-T-A-C-G-5’
C) 5’-G-C-A-T-3’
D) 3’-G-C-A-T-5’

Answer: B) 3’-T-A-C-G-5’

27. The major and minor grooves in DNA are formed due to

A) Phosphate groups
B) Twisting of the DNA strands
C) Hydrogen bonding
D) Base pair stacking

Answer: B) Twisting of the DNA strands

28. What is the function of DNA in a cell?

A) Energy production
B) Protein synthesis
C) Storage and transmission of genetic information
D) Structural support

Answer: C) Storage and transmission of genetic information

29. Which enzyme is responsible for DNA replication?

A) RNA Polymerase
B) DNA Helicase
C) DNA Ligase
D) DNA Polymerase

Answer: D) DNA Polymerase

30. Which enzyme unwinds the DNA double helix during replication?

A) DNA Polymerase
B) Helicase
C) Ligase
D) Topoisomerase

Answer: B) Helicase

31. What is the role of DNA ligase in replication?

A) Synthesizes RNA primers
B) Joins Okazaki fragments
C) Unwinds the DNA strands
D) Adds nucleotides

Answer: B) Joins Okazaki fragments

32. What is the complementary base pairing rule?

A) A pairs with C, G pairs with T
B) A pairs with T, G pairs with C
C) A pairs with G, T pairs with C
D) A pairs with U, G pairs with C

Answer: B) A pairs with T, G pairs with C

33. DNA replication is

A) Conservative
B) Semi-conservative
C) Dispersive
D) Random

Answer: B) Semi-conservative

34. Who experimentally proved that DNA replication is semi-conservative?

A) Griffith
B) Meselson and Stahl
C) Hershey and Chase
D) Watson and Crick

Answer: B) Meselson and Stahl

35. What is the function of topoisomerase in DNA replication?

A) Unwinds the DNA helix
B) Relieves supercoiling stress
C) Synthesizes RNA primers
D) Seals the Okazaki fragments

Answer: B) Relieves supercoiling stress

36. What is the function of RNA primers in DNA replication?

A) Act as a starting point for DNA synthesis
B) Unwind the DNA strands
C) Repair DNA mutations
D) Join Okazaki fragments

Answer: A) Act as a starting point for DNA synthesis

37. DNA polymerase adds nucleotides in which direction?

A) 5’ → 3’
B) 3’ → 5’
C) Randomly
D) Both directions equally

Answer: A) 5’ → 3’

38. Which strand is synthesized continuously during DNA replication?

A) Lagging strand
B) Leading strand
C) Both strands
D) Neither strand

Answer: B) Leading strand

39. Short DNA fragments formed on the lagging strand are called

A) Okazaki fragments
B) DNA primers
C) RNA primers
D) Exons

Answer: A) Okazaki fragments

40. In which phase of the cell cycle does DNA replication occur?

A) G1 phase
B) S phase
C) G2 phase
D) M phase

Answer: B) S phase

41. Which scientist discovered transformation in bacteria using DNA?

A) Watson
B) Griffith
C) Meselson
D) Chargaff

Answer: B) Griffith

42. Which scientists confirmed that DNA is the genetic material using bacteriophages?

A) Meselson and Stahl
B) Hershey and Chase
C) Watson and Crick
D) Griffith

Answer: B) Hershey and Chase

43. In DNA, where are the nitrogenous bases located?

A) Outside the helix
B) Inside the helix
C) Between phosphate groups
D) Randomly scattered

Answer: B) Inside the helix

44. Which part of DNA is responsible for its acidic nature?

A) Sugar
B) Phosphate group
C) Nitrogenous bases
D) Hydrogen bonds

Answer: B) Phosphate group

45. Which type of bond links nucleotides together in a DNA strand?

A) Hydrogen bonds
B) Phosphodiester bonds
C) Covalent bonds
D) Ionic bonds

Answer: B) Phosphodiester bonds

46. DNA is mostly found in which part of a eukaryotic cell?

A) Cytoplasm
B) Ribosome
C) Nucleus
D) Mitochondria

Answer: C) Nucleus

47. The sugar-phosphate backbone in DNA is linked by

A) Hydrogen bonds
B) Covalent bonds
C) Phosphodiester bonds
D) Peptide bonds

Answer: C) Phosphodiester bonds

48. What is the function of the hydrogen bonds in DNA structure?

A) Provides energy
B) Holds nitrogenous bases together
C) Acts as an enzyme
D) Forms peptide bonds

Answer: B) Holds nitrogenous bases together

49. The two strands of DNA run

A) In the same direction
B) In opposite directions
C) Randomly
D) Parallel but not complementary

Answer: B) In opposite directions

50. DNA replication ensures

A) Energy production
B) Protein breakdown
C) Genetic continuity
D) Cell death

Answer: C) Genetic continuity