3.1.5 Structure of DNA and RNA

Question 1

Describe the structure of a nucleotide

Answer 1

Nucleotides contain a phosphate, pentose sugar and nitrogenous base

Question 2

What are phosphodiester bonds, and where do they form

Answer 2

• Phosphodiester bonds are strong covalent bonds
• Phosphodiester bonds form between C5 of phosphate group and C3 of an adjacent deoxyribose sugar

Question 3

How would you describe the 2 polynucleotide strands, and why

Answer 3

• Anti-parallel because the two strands run in opposite directions
• In the first strand, C5 is pointing upwards, whereas in the second strand, C3 is pointing upwards

Question 4

What are the two polynucleotide strands held together by

Answer 4

• Hydrogen bonds between the two complimentary bases

Question 5

State the 4 bases, and describe which ones are complimentary

Answer 5

• Adenine
• Cytosine
• Guanine
• Thymine
• A+T
• C+G

Question 6

What are the 2 types of bases

Answer 6

• Prymidine
• Purine

Question 7

Describe the structure of prymidine bases

Answer 7

• A single 6 sided ring shape
• Cytosine and Thymine (Both have a Y)
  (CUT the pyramid)

Question 8

Describe the structure of purine bases

Answer 8

• 2 rings: one is 6 sided, and one is 5 sided
• larger molecule, yet shorter name

Question 9

How many hydrogen bonds form between A and T

Answer 9

2

Question 10

How many hydrogen bonds form between C and G

Answer 10

3

Question 11

How many bases code for 1 amino acid

Answer 11

3

Question 12

Explain how the structure of DNA relates to its function
(6)

Answer 12

• large molecule, therefore can carry a lot of information
• double stranded, so 1 strand can act as a template to replicate DNA accurately
• coiled, so it’s compact, therefore lots can be stored in a small space
• specific sequence of bases, which codes for the order of amino acids, used to synthesise proteins
• weak hydrogen bonds between complimentary bases, that can be easily overcome to separate the strands for DNA replication
• strong phosphodiester bonds within the sugar-phosphate backbone, which are strong and unreactive, and so they protect the more chemically reactive bases inside the double helix

Question 13

Whatdoes RNA atand for, and describe its function

Answer 13

• Ribonucleic acid
• RNA is a single stranded polynucleotide, which is involved in protein synthesis

Question 14

Describe the similarities between the structure of DNA and RNA

Answer 14

• Both polymers
• Both are made up of nucleotides containing a phosphate group, nitrogenous base and pentose sugar
• Both contain adenine, guanine and cytosine
• Both contain phosphodiester bonds
• Both contain C,H,N,O,P

Question 15

Describe the difference in structure between DNA and RNA

Answer 15

• DNA is double stranded, whereas RNA is single stranded
• DNA is longer and larger than RNA
• DNA nucleotide contains deoxyribose sugar, whereas RNA nucleotide contains ribose sugar
• DNA nucleotides contain thymine, whereas RNA nucleotides contain uracil

Question 16

A virus has the following percentages of bases in its
DNA: A 24.7%, G 24.1%, C 18.5%, T 32.7%

Does this virus have double or single stranded DNA?
Explain your answer

Answer 16

Single stranded because amounts of A and T / C and G / complementary bases are different; therefore no base-pairing;

Question 17

DNA and RNA can be found in bacteria.

Give two ways in which thenucleotides in DNA are different
from the nucleotides in RNA.

Answer 17

• DNA contains thymine and RNA contains uracil
• DNA contains deoxyribose and RNA contains ribose