2.3 Nucleotides and nucleic acids

Question 1

Name the pentose sugars in DNA & RNA

Answer 1

DNA: deoxyriboseRNA: ribose

Question 2

Describe how polynucleotide strands are formed and broken down

Answer 2

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 3

Describe the structure of DNA.

Answer 3

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

Question 4

Name the purine bases and describe their structure.

Answer 4

Name the purine bases and describe their structure.
adenine C5H5N5
guanine C5H5N5O

Question 5

Name the pyrimidine bases and describe their structure.

Answer 5

thymine C5H6N2O2
cytosine C4H5N3O
uracil C4H4N2O RNA

Question 6

Name the complementary base pairs in DNA and RNA

Answer 6

DNA: 2 H-bonds betweenadenine (A) + thymine (T)

RNA: 2 H-bonds betweenadenine (A) + uracil (U)

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

Question 7

Why is DNA replication described as semiconservative?

Answer 7

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 8

Explain the role of DNA helicase in semiconservative replication.

Answer 8

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

Question 9

How is a new strand formed during semiconservative replication?

Answer 9

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 10

Identify features of the genetic code

Answer 10

● 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 11

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

Answer 11

Consists of base triplets that code for a specific amino acids

Question 12

Describe how DNA can be purified by precipitation.

Answer 12

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 13

What does transcription produce and where does it occur?

Answer 13

produces mRNA

occurs in nucleus

Question 14

Outline the process of transcription.

Answer 14

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 15

What happens after a strand of mRNA is transcribed?

Answer 15

● 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 16

What does translation produce and where does it occur?

Answer 16

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

Question 17

Outline the process of translation.

Answer 17

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 18

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

Answer 18

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

Question 19

What is a mutation?

Answer 19

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