2.1.3 - Nucleotides and Nucleic Acids

Question 1

Describe the structure of a nucleotide.

Answer 1

Pentose sugar is bound to a phosphate group and a nitrogenous base

Question 2

Name the pentose sugars in DNA & RNA

Answer 2

DNA: deoxyribose
RNA: ribose

Question 3

Describe how polynucleotide strands are formed and broken down

Answer 3

Condensation reactions between nucleotides form strong phosphodiester bonds (sugar-phosphate backbone). Hydrolysis reactions use a molecule
of water to break these bonds.
Enzymes catalyse these reactions.

Question 4

Describe how polynucleotide strands are formed and broken down.

Answer 4

Condensation reactions between nucleotides form strong phosphodiester bonds (sugar-phosphate backbone). Hydrolysis reactions use a molecule of water to break these bonds. Enzymes catalyse these reactions.

Question 5

Describe the structure of DNA.

Answer 5

Molecule twists to form double helix of 2 deoxyribose polynucleotide strands (so there are 2 sugar-phosphate backbones). H-bonds form between complementary base
pairs (AT & GC) on strands that run antiparallel.

Question 6

Name the purine bases and describe their structure.

Answer 6

adenine C5H5N5
guanine C5H5N5O

Both are two ring molecules

**Trick to remember:
Purine = pure
AnGels are pure and A and G refer to the bases
Angels have TWO wings = two ring molecules

Question 7

Name the pyrimidine bases and describe
their structure.

Answer 7

cytosine C4H5N3O
uracil C4H4N2O
thymine C5H6N2O2

These are all one-ring molecules

**Trick to remember:
You can CUT a PYRAMID from the single pointy top
CUT = Cytosine, Uracil, Thymine
Pyramid = pyrimidine
SINGLE pointy top = ONE-ring molecule

Question 8

Name the complementary base pairs in DNA and RNA.

Answer 8

DNA: 2 H-bonds between adenine (A) + thymine (T)
RNA: 2 H-bonds between adenine (A) + uracil (U)

Both have 3 H-bonds between guanine (G) + cytosine (C)

Question 9

Why is DNA replication described as semiconservative?

Answer 9

Strands from original DNA molecule act as
templates. New DNA molecule contains 1 old strand & 1
new strand (specific base pairing enables genetic material to be conserved accurately).

Question 10

Explain the role of DNA helicase in semiconservative
replication.

Answer 10

Breaks H-bonds between base pairs to form 2 single strands, each of which can act as a template.

Question 11

How is a new strand formed during semiconservative
replication?

Answer 11

1. Free nucleotides from nuclear sap attach to exposed bases by complementary base pairing.
2. DNA polymerase joins adjacent nucleotides on new strand in a 5’ → 3’ direction via condensation reactions to form phosphodiester bonds.
3. H-bonds reform.

Question 12

Identify features of the genetic code.

Answer 12

● Non-overlapping: each triplet is only read once.
● Degenerate: more than one triplet codes for the same amino acid (64 possible triplets for 20 amino acids).
● Universal: same bases and sequences used by all species.

Question 13

How does a gene determine the sequence of amino
acids in a protein?

Answer 13

Consists of base triplets that code for a
specific amino acids.

Question 14

Describe how DNA can be purified by precipitation.

Answer 14

Add ethanol & a salt to aqueous solution.
Nucleic acids precipitate out of solution.
Centrifuge to obtain pellet of nucleic acid.
Wash pellet with ethanol & centrifuge again.

Question 15

What does transcription produce and where does it
occur?

Answer 15

produces mRNA
occurs in nucleus

Question 16

Outline the process of transcription.

Answer 16

1. RNA polymerase binds to promoter region on a gene.
2. Section of DNA uncoils into 2 strands with exposed bases. Antisense strand acts as template.
3. Free RNA nucleotides are attracted to their complementary bases.
4. RNA polymerase joins adjacent nucleotides to form phosphodiester bonds to produce mRNA

Question 17

What happens after a strand of mRNA is transcribed?

Answer 17

● RNA polymerase detaches at terminator region.
● H-bonds reform & DNA rewinds.
● splicing removes introns from pre-mRNA in eukaryotic cells.
● mRNA moves out of nucleus via nuclear pore & attaches to ribosome

Question 18

What does translation produce and where does it occur?

Answer 18

Produces proteins
Occurs in cytoplasm on ribosomes (which are made of protein + rRNA)

Question 19

Outline the process of translation.

Answer 19

1. Ribosome moves along mRNA until ‘start’ codon.
2. tRNA anticodon attaches to complementary codon on mRNA.
3. Condensation reactions between amino acids on tRNA form peptide bonds. Requires energy from ATP hydrolysis.
4. Process continues to form polypeptide chain until ‘stop’ codon is reached.

Question 20

Describe the structure of adenosine triphosphate
(ATP) and adenosine diphosphate (ADP).

Answer 20

nucleotide derivative of adenine
ATP has 3 inorganic phosphate groups
ADP has 2

Question 21

What is a mutation?

Answer 21

An alteration to the DNA base sequence.
Mutations often arise spontaneously during DNA replication