Describe DNA Structure and Components - Biology

1. Identify the correct base pairing for the DNA sequence $\text{C-G-G-A}$ on the complementary strand.

   $\text{G-C-C-T}$. Apply base-pairing rules: C pairs with G, A pairs with T.

2. What type of bond forms between complementary bases across the two DNA strands?

   Hydrogen bonds. Weak bonds that hold complementary base pairs together.

3. Which base pair has two hydrogen bonds: A–T or C–G?

   A–T. A-T forms two hydrogen bonds for weaker pairing.

4. What is the overall 3D shape of a DNA molecule called?

   Double helix. Two antiparallel strands twisted into this characteristic spiral structure.

5. What pentose sugar is found in DNA nucleotides?

   Deoxyribose. The five-carbon sugar component of DNA nucleotides.

6. What is a DNA polynucleotide?

   A chain of nucleotides linked by phosphodiester bonds. Multiple nucleotides joined together form a DNA strand.

7. Which pairing keeps the DNA helix a consistent diameter: purine-purine, pyrimidine-pyrimidine, or purine-pyrimidine?

   Purine-pyrimidine. This pairing maintains uniform helix diameter and stability.

8. If a DNA molecule has $30\%$ adenine, what percentage is thymine?

   $30\%$. Base-pairing rules mean A = T, so both are $30\%$.

9. What base-pairing rule states which DNA bases pair together?

   A pairs with T; C pairs with G. Watson-Crick base pairing ensures complementary strand formation.

10. Which DNA bases are classified as pyrimidines?

    Cytosine and thymine. Single-ring structures that are smaller than purines.

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

    Thymine. DNA contains thymine while RNA contains uracil instead.

12. What structural feature of DNA helps explain why A pairs with T and C pairs with G?

    Purine-pyrimidine pairing maintains uniform helix width. Large purine paired with small pyrimidine maintains constant width.

13. Where are the nitrogenous bases positioned in the DNA double helix?

    On the interior, paired across the two strands. Bases stack in the center and pair across the double helix.

14. What is the relationship between the number of A bases and T bases in double-stranded DNA?

    The amount of A equals the amount of T. Chargaff's rule: A always equals T in double-stranded DNA.

15. What term describes the specific order of bases along a DNA strand?

    Base sequence (nucleotide sequence). The linear arrangement of bases that encodes genetic information.

16. What is the key difference between deoxyribose and ribose at the $2'$ carbon?

    Deoxyribose has H; ribose has OH at the $2'$ carbon. DNA's sugar lacks the hydroxyl group that RNA's sugar has.

17. What term describes the weak interactions that allow DNA strands to separate during replication?

    Hydrogen bonding between bases. Weak bonds allow strand separation during DNA processes.

18. How many hydrogen bonds typically form between C and G in DNA?

    3 hydrogen bonds. G-C pairs have more hydrogen bonds than A-T pairs.

19. What type of bond attaches a nitrogenous base to the deoxyribose sugar?

    Glycosidic bond. Links the base to the $1'$ carbon of the sugar.

20. Which base is complementary to adenine (A) in DNA?

    Thymine (T). A purine pairs with a pyrimidine following base-pairing rules.

21. What term describes the two DNA strands running in opposite directions?

    Antiparallel. Strands run in opposite directions for proper base pairing.

22. Which DNA bases are classified as purines?

    Adenine and guanine. Double-ring structures that are larger than pyrimidines.

23. What is the relationship between the number of C bases and G bases in double-stranded DNA?

    The amount of C equals the amount of G. Chargaff's rule: C always equals G in double-stranded DNA.

24. Which end of a DNA strand has a free phosphate group?

    The $5'$ end. This end has an unattached phosphate group on the sugar.

25. Which DNA component varies and encodes genetic information: the backbone or the base sequence?

    The base sequence. Variable base sequence carries genetic information, not backbone.

26. Which four nitrogenous bases are found in DNA?

    Adenine, thymine, cytosine, guanine. The four nitrogen-containing bases that encode genetic information.

27. What is meant by the statement that DNA strands are complementary?

    Each base sequence determines the other by pairing rules. Base-pairing rules ensure one strand determines the other's sequence.

28. If a DNA molecule has $18\%$ cytosine, what percentage is guanine?

    $18\%$. Base-pairing rules mean C = G, so both are $18\%$.

29. What is the name for a DNA nucleotide without its phosphate group?

    Nucleoside. Base attached to sugar without the phosphate group.

30. What three components make up one DNA nucleotide?

    Phosphate group, deoxyribose sugar, nitrogenous base. Three essential parts that together form each DNA building block.

31. How many hydrogen bonds typically form between A and T in DNA?

    2 hydrogen bonds. A-T pairs have fewer hydrogen bonds than G-C pairs.

32. Which carbon atoms are connected by a phosphodiester bond in DNA?

    Between the $3'$ OH of one sugar and the $5'$ phosphate of the next. Links the $3'$ OH of one sugar to the $5'$ phosphate of another.

33. Which base is complementary to cytosine (C) in DNA?

    Guanine (G). G forms three hydrogen bonds with C in Watson-Crick pairing.

34. Identify the correct base pairing for the DNA triplet $\text{A-C-G}$ on the complementary strand.

    $\text{T-G-C}$. Apply base-pairing rules: A pairs with T, C pairs with G.

35. Which base is complementary to thymine (T) in DNA?

    Adenine (A). T forms two hydrogen bonds with A in Watson-Crick pairing.

36. If a DNA molecule has $40\%$ guanine, what percentage is cytosine?

    $40\%$. Base-pairing rules mean C = G, so both are $40\%$.

37. Identify the correct statement about strand direction: Are the two strands $5'\to 3'$ in the same direction or opposite directions?

    Opposite directions (antiparallel). Antiparallel orientation is essential for proper base pairing.

38. If a DNA molecule has $12\%$ thymine, what percentage is cytosine?

    $38\%$. T = A = $12\%$, so C = G = $(100\% - 24\%)/2 = 38\%$.

39. What is meant by the statement that DNA strands are antiparallel?

    One strand runs $5'\to 3'$ and the other runs $3'\to 5'$. Strands have opposite orientations for proper base pairing geometry.

40. Identify the correct base pairing for the DNA sequence $\text{T-T-A-C}$ on the complementary strand.

    $\text{A-A-T-G}$. Apply base-pairing rules: T pairs with A, C pairs with G.

41. What does the abbreviation DNA stand for in biology?

    Deoxyribonucleic acid. The full name of the molecule that stores genetic information.

42. Which end of a DNA strand has a free hydroxyl group on the sugar?

    The $3'$ end. This end has an unattached OH group on the sugar.

43. What are the two ends of a DNA strand called based on carbon numbering?

    $5'$ end and $3'$ end. Named for the carbon atoms in the deoxyribose sugar ring.

44. If a DNA molecule has $20\%$ adenine, what percentage is guanine?

    $30\%$. A = T = $20\%$, so C = G = $(100\% - 40\%)/2 = 30\%$.

45. Which base is complementary to guanine (G) in DNA?

    Cytosine (C). C forms three hydrogen bonds with G in Watson-Crick pairing.

46. Which base pair has three hydrogen bonds: A–T or C–G?

    C–G. C-G forms three hydrogen bonds for stronger pairing.

47. What type of bond links the sugar and phosphate to form the DNA backbone?

    Phosphodiester bond. Covalent bond connecting nucleotides to form the DNA backbone.

48. What is the basic repeating subunit (monomer) of DNA?

    Nucleotide. The building block that polymerizes to form the DNA chain.

49. What is the name of the sugar-phosphate portion of DNA that forms the outside of the helix?

    Sugar-phosphate backbone. Forms the structural framework on the outside of the helix.

50. What term describes the covalent linkage that makes the DNA backbone strong and stable?

    Phosphodiester linkage. Strong covalent bonds provide structural stability to DNA.

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