Module 1. Nucleic acids

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

rRNA and proteins

Question 3

Describe the differences between a DNA nucleotide and an RNA nucleotide

Answer 3

• DNA nucleotide
  →Pentose sugar is deoxyribose
  →Base can be thymine
• RNA nucleotide
  →Pentose sugar is ribose
  →Base can be uracil

Question 4

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

Answer 4

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

Question 5

Describe the structure of DNA
(5)

Answer 5

1. Polymer of nucleotides
   → (Polynucleotide)
2. Each nucleotide formed from deoxyribose, a phosphate group and an** organic, nitrogenous base**
3. Phosphodiester bonds between/join adjacent nucleotides
4. 2 polynucleotide strands held by hydrogen bonds
5. Hydrogen bonds between complementary base pairs - adenine, thymine and cytosine, guanine

• double helix

Question 6

Describe the structure of (messenger) RNA

Answer 6

● 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 7

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

Answer 7

• DNA nucleotide
  → Pentose sugar is **deoxyribose **
  → Has the base thymine
  → Double stranded / double helix
  → Long (many nucleotides)
  → Has **hydrogen bonds / base pairing **
• RNA nucleotide
  → Pentose sugar is ribose
  → Has the base uracil
  → Single stranded / single helix
  → Shorter (fewer nucleotides)
  → Does not

Question 8

Suggest how the structure of DNA relates to its functions

Answer 8

● 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 9

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

Answer 9

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

Question 10

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule.
[2 marks]

[2.1 - JUN20] [AS]

Answer 10

1. Condensation reaction/loss of water
2. Between phosphate and deoxyribose
3. Catalysed by DNA polymerase

Question 11

Why is semi-conservative replication important?

Answer 11

Ensures genetic continuity between generations of cells.

Question 12

Describe the process of semi-conservative replication of DNA
(5)

Answer 12

1. DNA helicase unwinds the double helix by breaking hydrogen bonds;
2. Both strands act as templates;
3. Free** DNA nucleotides** (attracted to exposed bases) and attach by specific complementary base pairs
   → Hydrogen bonds form between adenine-thymine and guanine-cytosine
4. DNA polymerase joins adjacent nucleotides (on new strand by condensation reactions)
5. Forming phosphodiester bonds;
6. Each new DNA molecule consists of one original/template strand and one new strand;
   → = semi-conservative

Question 13

Use your knowledge of enzyme action to suggest why DNA polymerase moves in opposite directions along DNA strands

Answer 13

● DNA has antiparallel strands
● So shapes / arrangements of nucleotides on two ends are different
● DNA polymerase is an enzyme with a specific shaped active site
● So can only bind to substrate with complementary shape (phosphate end of developing strand)

Question 14

Describe the role of two named enzymes in the process of semi-conservative replication of DNA.
[3 marks]

[9.1 - JUN18]

Answer 14

1. DNA helicase causes breaking of hydrogen bonds (between DNA strands)
   Reject ‘helicase hydrolyses hydrogen bonds’.
2. DNA polymerase joins the DNA nucleotides
3. Forming phosphodiester bonds

Question 15

Use your knowledge of enzyme action and DNA replication to explain why new nucleotides can only be added in a 5’ to 3’ direction.
[4 marks]

[11c PMT]

Answer 15

1. Reference to DNA polymerase;
2. (Which is) specific;
3. Only complementary with / binds to 5’ end of strand
4. Shapes of 5’ end and 3’ end are **different **/ description of how different.

Question 16

Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication

Answer 16

Watson and Crick

Question 17

Describe the work of Meselson and Stahl in validating the Watson-Crick model of semi-conservative DNA replication

Answer 17

1. Bacteria grown in medium containing heavy nitrogen (¹⁵N), and nitrogen is incorporated into DNA bases
   ● DNA extracted and centrifuged → settles near the bottom, as all DNA molecules contain two “heavy” strands.
2. Bacteria transferred to medium containing **light nitrogen (¹⁴N) **and allowed to divide once
   ● DNA extracted and centrifuged → settles in the middle, as all DNA molecules contain one original “heavy” strand and one new “light” strand.
3. Bacteria in light nitrogen (¹⁴N) allowed to divide again
   ● DNA extracted and centrifuged → half settles in the middle, as it contains one original “heavy” strand and one new “light” strand; half settles near the top, as it contains two “light” strands.