2.1.3: Nucleotides and nucleic acids

Question 1

Draw the structure of a nucleotide.

Answer 1

circle- phosphate group
pentagon- 5 carbon sugar
rectangle- 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 the structure of DNA.

Answer 4

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 5

Name the purine bases and describe their structure

Answer 5

adenine C5H5N5
guanine C5H5N5O
two-ring molecules

Question 6

Name the pyrimidine bases and describe their

structure.

Answer 6

one-ring molecules

thymine C5H6N2O2 cytosine C4H5N3O uracil C4H4N2O2

Question 7

Name the complementary base pairs in DNA and

RNA.

Answer 7

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 8

Why is DNA replication described as semiconservative?

Answer 8

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 9

Explain the role of DNA helicase in semiconservative

replication.

Answer 9

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

Question 10

How is a new strand formed during semiconservative

replication?

Answer 10

• Enzyme helicase unzips the double helix and the hydrogen bonds between bases break
• Free nucleotides line up with their complimentary bases and hydrogen bonds form
• DNA polymerase allows the free nucleotides to attach to their complimentary bases
• A complimentary strand has been formed for either template strand
• Any fragments in the double helix are joined by ligase
• There are now two identical DNA molecules formed – each with a daughter and parent strand

Question 11

Identify features of the genetic code.

Answer 11

● 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 12

How does a gene determine the sequence of amino

acids in a protein?

Answer 12

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

Question 13

Describe how DNA can be purified by precipitation.

Answer 13

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 14

What does transcription produce and where does it

occur?

Answer 14

produces mRNA

occurs in nucleus

Question 15

Outline the process of transcription.

Answer 15

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 nucleotides are attracted to their complementary bases.
4. RNA polymerase joins adjacent nucleotides to form phosphodiester bonds.

Question 16

What happens after a strand of mRNA is transcribed?

Answer 16

● 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 17

What does translation produce and where does it

occur?

Answer 17

Produces proteins

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

Question 18

Outline the process of translation.

Answer 18

1. Ribosome moves along mRNA until ‘start’ codon.
2. tRNA anticodon attaches to complementary bases 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 19

Describe the structure of adenosine triphosphate

ATP) and adenosine diphosphate (ADP

Answer 19

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

Question 20

What is a mutation?

Answer 20

An alteration to the DNA base sequence.

Mutations often arise spontaneously during DNA replication.