3.1.5.1 - Structure of DNA and RNA

Question 1

Describe the basic functions of DNA and RNA in all living cells

Answer 1

• DNA Holds genetic information which codes for polypeptides (proteins)
• RNA Transfers genetic information from DNA to ribosomes

Question 2

Name the two types of molecule from which a ribosome is made

Answer 2

RNA and proteins

Question 3

Describe the differences between a DNA nucleotide and an RNA nucleotide

Answer 3

• The pentose sugar in DNA nucelotides is deoxyribose, wherease the pentose sugar in RNA nucelotides is ribose
• In a DNA nuceotide there is a thymine base whereas in RNA nucleotides this base is Uracil

Question 4

Describe how nucleotides join together to form polynucleotides

Answer 4

● Condensation reactions, removing water molecules

● Between phosphate group of one nucleotide and deoxyribose / ribose of another

● Forming phosphodiester bonds

Question 5

Why did many scientists initially doubt that DNA carried the genetic code?

Answer 5

The relative simplicity of DNA - chemically simple molecule with few components

Question 6

Describe the structure of DNA

Answer 6

● Polymer of nucleotides (polynucleotide)
● Each nucleotide formed from deoxyribose, a phosphate group and a
nitrogen-containing organic base
● Phosphodiester bonds join adjacent
nucleotides
● 2 polynucleotide chains held together by
hydrogen bonds
● Between specific complementary base
pairs - adenine / thymine and cytosine / guanine
● Double helix stucture

Question 7

Describe the structure of (messenger) RNA

Answer 7

● Polymer of nucleotides (polynucleotide)
● Each nucleotide formed from ribose, a phosphate
group and a nitrogen-containing organic base
● Bases - uracil, adenine, cytosine, guanine
● Phosphodiester bonds join adjacent nucleotides
● Single helix

Question 8

Compare and contrast the structure of DNA and (messenger) RNA

Answer 8

• The pentose sugar in DNA is deoxyribose whereas the pentose sugar in RNA is ribose
• DNA has the thymine base whereas RNA has the uracil base
• DNA is double stranded and so firms a double helix structure whereas RNA is single stranded and so forms a single helix
• DNA is long and consists of many nuceolotides whereas RNA is shorter and consists of fewer nucleotides
• DNA has hydrogen bonds and base pairing whereas RNA does not have hydrogen bonds and base pairing

Question 9

Suggest how the structure of DNA relates to its functions

Answer 9

● Two strands → both can act as templates for semi-conservative replication

● Hydrogen bonds between bases are weak → strands can be separated for replication

● Complementary base pairing → accurate replication

● Many hydrogen bonds between bases → stable / strong molecule

● Double helix with sugar phosphate backbone → protects bases / hydrogen bonds

● Long molecule → store lots of genetic information (that codes for polypeptides)

● Double helix (coiled) → compact

Question 10

Suggest how you can use incomplete information about the frequency of
bases on DNA strands to find the frequency of other bases

Answer 10

1. % of adenine in strand 1 = % of thymine in strand 2 (and vice versa)
2. % of guanine in strand 1 = % of cytosine in strand 2 (and vice versa)
   Because of specific complementary base pairing between 2 strands