3.1.5 Structure of DNA & RNA

Question 1

What are the functions of DNA and RNA in living cells?

Answer 1

1. Deoxyribonucleic acid (DNA) holds genetic information.
2. Ribonucleic acid (RNA) transfers genetic information from DNA to ribosomes.

Question 2

DNA structure:

Answer 2

1. Pentose sugar (deoxyribose)
2. Nitrogen-containing base (adenine, thymine, guanine and cytosine)
3. Phosphate group

Question 3

RNA Structure:

Answer 3

1. Pentose sugar (ribose)
2. Nitrogen-containing base (adenine, uracil, guanine and cytosine)
3. Phosphate group

Question 4

What are the differences between DNA and RNA structure?

Answer 4

1. DNA nucleotides have pentose sugar deoxyribose whereas RNA nucleotides have ribose.
2. DNA nucleotides can have thymine whereas RNA nucleotides have uracil instead.
3. DNA molecules are double stranded whereas RNA molecules are single stranded.
4. DNA is longer whereas RNA is shorter.

Question 5

How do nucleotides join together to form polynucleotides?

Answer 5

1. DNA: 2 strands joined in anti-parallel held together by hydrogen bonds between specific complementary base pairs – AT (2 H bonds) and CG (3 H bonds).
2. A condensation reaction between 2 nucleotides forms a phosphodiester bond.

Question 6

Describe how the structure of DNA is related to its function.

Answer 6

1. Large number of (individually) weak hydrogen bonds between complementary bases on different strands → stable / strong molecule and can be unzipped for replication.
2. Double helix (with sugar phosphate backbone) → protects bases / hydrogen bonds.
3. Long molecule → store lots of (genetic) information (can code for sequence of amino acids in the primary structure of a protein).
4. Double stranded → semi-conservative replication can occur as both strands can act as templates.
5. Complementary base pairing (A-T, C-G) → accurate replication / identical copies can be made
6. Double helix (coiled) → compact.

Question 7

Structure of mRNA:

Answer 7

1. Polynucleotide.
2. Each nucleotide formed from ribose, a phosphate group and a nitrogen containing organic base.
3. Bases: uracil, adenine, cytosine and guanine.
4. Phosphodiester bonds join adjacent molecules.
5. Single helix.