Nucleic acids + ATP

Question 1

Name the pentose sugars in DNA & RNA.

Answer 1

DNA: deoxyribose
RNA: ribose

Question 2

State the role of DNA in living cells.

Answer 2

Base sequence of genes codes for functional RNA + amino acid sequence of polypeptides.
Carries genetic information + determines inherited characteristics = influences structure + function of organisms.

Question 3

State the role of RNA in living cells.

Answer 3

mRNA: Complementary sequence to 1 gene from DNA with introns spliced out. Codons can be translated into a polypeptide by ribosomes.
rRNA: component of ribosomes (along with proteins)
tRNA: supplies complementary amino acid to mRNA codons during translation

Question 4

How do polynucleotides form?

Answer 4

Condensation reactions between nucleotides form strong phosphodiester
bonds (sugar-phosphate backbone).

Question 5

Describe the structure of DNA.

Answer 5

double helix of 2 deoxyribose strands
H-bonds between complementary purine &
pyrimidine base pairs on opposite strands:
adenine (A) + thymine (T)
guanine (G) + cytosine (C)

Question 6

Which bases are purine and which are pyrimidine?

Answer 6

A & G = 2-ring purine bases

T & C & U = 1-ring pyrimidine bases

Question 7

Name the complementary base pairs in DNA.

Answer 7

2 H-bonds between
adenine (A) + thymine (T)
3 H-bonds between
guanine (G) + cytosine (C)

Question 8

Name the complementary base pairs in RNA.

Answer 8

2 H-bonds between
adenine (A) + uracil (U)
3 H-bonds between
guanine (G) + cytosine (C)

Question 9

Relate the structure of DNA to its functions.

Answer 9

● sugar-phosphate backbone & many H-bonds provide stability
● long molecule stores lots of information
● helix is compact for storage in nucleus
● base sequence of triplets codes for amino acids
● double-stranded for semi-conservative replication
● complementary base pairing for accurate replication
● weak H-bonds break so strands separate for replication

Question 10

Describe the structure of messenger RNA (mRNA).

Answer 10

● Long ribose polynucleotide (but shorter than DNA).
● Contains uracil instead of thymine.
● Single-stranded & linear (no complementary base pairing).
● Codon sequence is complementary to exons of 1 gene from 1 DNA strand.

Question 11

Relate the structure of messenger RNA (mRNA) to

its functions

Answer 11

● Breaks down quickly so no excess polypeptide forms.
● Ribosome can move along strand & tRNA can bind to exposed bases.
● Can be translated into a specific polypeptide by ribosomes.

Question 12

Describe the structure of transfer RNA (tRNA).

Answer 12

● Single strand of about 80 nucleotides.
● Folded into clover shape (some paired bases).
● Anticodon on one end, amino acid binding site on the other:
a) anticodon binds to complementary mRNA codon
b) amino acid corresponds to anticodon

Question 13

Order DNA, mRNA and tRNA according to increasing length.

Answer 13

tRNA
mRNA
DNA

Question 14

Why did scientists initially doubt that DNA carried the

genetic code?

Answer 14

Chemically simple molecule with few components.

Question 15

Why is DNA replication described as

‘semiconservative’?

Answer 15

● Strands from original DNA molecule act as a template.

● New DNA molecule contains 1 old strand & 1 new strand.

Question 16

Outline the process of semiconservative

DNA replication.

Answer 16

1. DNA helicase breaks H-bonds between base pairs.
2. Each strand acts as a template.
3. Free nucleotides from nuclear sap attach to exposed bases by complementary base pairing.
4. DNA polymerase catalyses condensation reactions that join adjacent nucleotides on new strand.
5. H-bonds reform.

Question 17

Describe the Meselson-Stahl experiment.

Answer 17

1. Bacteria were grown in a medium containing heavy isotope 15N for many generations.
2. Some bacteria were moved to a medium containing light isotope 14N.
3. Samples were extracted after 1 & 2 cycles of DNA replication.
4. Centrifugation formed a pellet. Heavier DNA (bases made from 15N) settled closer to bottom of tube.

Question 18

Describe the structure of adenosine triphosphate

Answer 18

nucleotide derivative of adenine with 3 phosphate groups

Question 19

Explain the role of ATP in cells.

Answer 19

ATP hydrolase catalyses ATP → ADP + Pi
● Energy released is coupled to metabolic reactions.
● Phosphate group phosphorylates compounds to make them more reactive.

Question 20

How is ATP resynthesised in cells?

Answer 20

● ATP synthase catalyses condensation reaction between ADP & Pi
● during photosynthesis & respiration

Question 21

Explain why ATP is suitable as the ‘energy currency’ of cells.

Answer 21

● High energy bonds between phosphate groups.
● Small amounts of energy released at a time =less energy wasted as heat.
● Single-step hydrolysis = energy available quickly.
● Readily resynthesised.