3.1.5 Nucleic Acids

Question 1

Name the Pentose sugars in DNA and RNA

Answer 1

DNA = deoxyribose
RNA = Ribose

Question 2

What does DNA stand for?

Answer 2

Deoxyribonucleic acid

Question 3

What does RNA stand for?

Answer 3

Ribonucleic acid

Question 4

State the role of DNA in living cells

Answer 4

• Base sequence of genes codes for functional RNA and amino acid sequence of polypeptides
• Genetic info determines inherited characteristics which influences the structure and function of organisms

Question 5

How do Polynucleotides form?

Answer 5

Condensation reaction between nucleotides forms strong phosphodiester bonds

Question 6

Describe the structure of DNA

Answer 6

Double helix of 2 polynucleotide strands

Question 7

Name the complementary base pairs in DNA

Answer 7

Adenine and Thymine (2 H-bonds)
Guanine and Cytosine (3 H-bonds)

Question 8

Name the complementary base pairs in RNA

Answer 8

Adenine and Uracil (2 H-bonds)
Guanine and Cytosine (3 H-bonds)

Question 9

Why do scientists initially doubt that DNA carried the genetic code?

Answer 9

It’s a chemically simple molecule with few components

Question 10

Why is DNA replication described as ‘semiconservative’?

Answer 10

• Strands from original DNA molecule act as a template
• New DNA molecule contains 1 old and 1 new strand

Question 11

Describe the Meson-stahl experiment

Answer 11

1) Bacteria was grown in a medium containing heavy isotope 15N for many generations
2) Some bacteria was moved to a medium containing light isotope 14N and samples were extracted after 1 and 2 cycles of DNA replication
3) Centrifugation formed a pellet. Heavier DNA settled closer to the bottom of the tube

Question 12

Relate the structure of DNA to its function

Answer 12

• Sugar phosphate backbone and many H-bonds provide stability
• Long molecule stores lots of info
• Helix is compact for storage in the nucleus
• Base sequence of triplets codes for amino acids
• Double stranded for semi conservative replication
• Complementary base paring for accurate replication
• Weak H-bonds break so stands can separate for replication

Question 13

Outline the process of Semiconservative DNA replication

Answer 13

• DNA helicase breaks the H-bonds between base pairs
• Each strand acts as a template
• Free nucleotides from nuclear sap attach to exposed bases by complementary base pairing
• DNA polymerase catalyses condensation reactions between adjacent nucleotides on the new strand
• H-bonds reform

Question 14

Why does each new DNA strand produced point in opposite directions?

Answer 14

DNA has antiparallel strands and the shape of the nucleotides is different due to enzymes having active sites with specific shape and so this means only substrates with complementary shapes can bind

Question 15

State the role of RNA in living cells

Answer 15

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
tRNA: supplies complementary amino acid to mRNA codons during translation

Question 16

Describe the structure of messenger RNA (mRNA)

Answer 16

• Long ribose polynucleotide (shorter than DNA)
• Contains uracil instead of thymine
• Single stranded and linear
• Codon sequence is complementary to exons of 1 gene from 1 DNA strand

Question 17

Describe the structure of transfer RNA (tRNA)

Answer 17

• Single stranded of about 80 nucleotides
• Folded over into a clover shape
• Anticodon on one end and amino acid binding site on the other
  Anticodon binds to complementary mRNA codon
  Amino acid corresponds to anticodon

Question 18

Relate the structure of messenger RNA (mRNA) to it’s functions

Answer 18

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

Question 19

Order DNA , mRNA and tRNA according to increasing length

Answer 19

tRNA
mRNA
DNA