1.5 Structure of DNA and RNA

Question 1

Describe the structure of a nucleotide

Answer 1

A nucleotide includes a phosphate group, pentose sugar and a nitrogen-containing base

Question 2

Name the pentose sugars in DNA and RNA

Answer 2

DNA : deoxyribose
RNA: ribose

Question 3

State the role of DNA and RNA in living cells

Answer 3

• Both DNA and RNA carry information
• DNA hold genetic information
• RNA transfers this genetic information from DNA to ribosomes for protein synthesis

Question 4

How do polynucleotides form?

Answer 4

Condensation reactions between nucleotides form strong phosphodiester bonds (sugar-phosphate backbone)

Question 5

Describe the structure of DNA

Answer 5

Double helix of 2 polynucleotide strands (deoxyribose)
H-bonds between complementary purine and pyrimidine base pairs on opposite strands:
adenine (A) + thymine (T)
guanine (G) + cytosine (C)

Question 6

Which bases are purine and which are pyrimidine?

Answer 6

A + G = 2-ring purine bases
T + C + U = 1-ring pyrimidine bases

Question 7

Name the complementary base pairs in DNA

Answer 7

2 H-bonds between adenine (A) + thymine (T)
3 H-bonds between guanine (G) + cytosine (C)

Question 8

Name the complementary base pairs in RNA

Answer 8

2 H-bonds between adenine (A) + uracil (U)
3 H-bonds between guanine (G) + cytosine (C)

Question 9

Relate the structure of DNA to its functions

Answer 9

• Sugar-phosphate backbone and many H-bonds provide stability
• Long molecule stores lots of information
• Helix is compact for storage in nucleus
• Base sequence of triplets codes for amino acids
• Double-stranded for semi-conservative replication
• Complementary base pairing for accurate replication
• Weak H-bonds break so strands separate for replication

Question 10

Contrast DNA and RNA

Answer 10

• DNA is double-stranded, whereas RNA is single-stranded
• DNA sugar is deoxyribose, whereas RNA sugar is ribose
• DNA has bases: adenine, thymine, cytosine and gunanine whereas RNA bases are: adenine, uracil, cytosine and guanine