Nucleic acids

Question 1

What is the difference between pyrimidines and purines?

Answer 1

• Pyrimidines - smaller bases which contain single carbon ring (C and T)
• Purines - larger bases with double carbon ring (A and G)

Question 2

Which bonds form the sugar-phosphate backbone?

Answer 2

Phosphodiester bonds

Question 3

How many hydrogen bonds do each pair of bases have?

Answer 3

(T, A) = 2

(C, G) = 3

Question 4

Why are the base pairings only between a pyrimidine and a purine?

Answer 4

Maintains a constant distance between DNA backbones

Question 5

Differences between DNA and RNA? (4)

Answer 5

• DNA is double helix while RNA is single helix
• DNA has deoxyribose while RNA has ribose
• DNA has thymine while RNA has uracil
• DNA is the whole genetic code of the organism while RNA is only a section of genes

Question 6

What is the method for DNA extraction from a plant and why are these steps needed? (5)

Answer 6

1) Grind sample in mortar and pestle (breaks down cell wall)
2) Mix sample with detergent (breaks down cell membrane)
3) Add salt (breaks hydrogen bonds between DNA and water)
4) Add protease enzyme (break down proteins associated with the DNA)
5) Add a layer of ethanol on top of sample (Causes DNA ppt. out of solution)

Question 7

What is the semi-conservative model for DNA replication? (4)

Answer 7

1) DNA Helicase unzips and unwinds DNA
2) Free nucleotides bind to their complementary bases
3) DNA Polymerase forms phosphodiester bonds to make two new sugar backbones

• Each new molecule is contains one original strand and one new strand. = definition

Question 8

Explain discontinuous replication. (6)

Answer 8

• Strands are antiparallel (one 3’ to 5’ and one 5’ to 3’)
• DNA Polymerase only binds from 3’ to 5’ direction
• Strand unzipped from 3’ end can continuously replicate (leading strand)
• Strand unzipped from 5’ end so DNA Polymerase needs to wait until a section of strand has unzipped to start (lagging strand)
• The DNA Polymerase then has to keep going back to polymerise the new sections that are being unzipped
• DNA is produced in sections (Okazaki fragments)

Question 9

What is a mutation?

Answer 9

A random and spontaneous replication error

Question 10

How many possible combinations of bases are there?

Answer 10

64 (4x4x4x4)

Question 11

How are there more codon combinations than amino acids? (3)

Answer 11

• Multiple codons can code for a single amino acid
• There are stop codons and a start codon
• It’s a degenerate code

Question 12

What is a nucleotide made out of?

Answer 12

• Nitrogenous base
• pentose sugar
• phosphate group

Question 13

Why is there a start codon?

Answer 13

So that there is no overlapping and the bases are read in frame