1.5 Nucleic acids are important information-carrying molecules

Question 1

What is the function of DNA and RNA?

Answer 1

DNA: deoxyribonucleic acid holds genetic information
RNA: ribonucleic acid transfer genetic information from DNA to ribosomes
- Ribosomes are formed from RNA and proteins

Question 2

What is the structure of DNA?

Answer 2

• 2 strands joined in anti-paralell, held together by hydrogen bonds between specific complementary base pairs (A-T and C-G)
• Twisting into a double helix
• Phosphate group, nitrogenous base, deoxyribose

Question 3

What is the structure of RNA?

Answer 3

• Single stranded
• Phosphate group, nitrogen-containing base, ribose

Question 4

What are the differences between DNA and RNA molecules?

Answer 4

• DNA molecules are double stranded (double helix), whereas RNA molecules are single stranded
• DNA is longer whereas RNA is shorter

Question 5

What are the differences between DNA and RNA nucleotides?

Answer 5

• DNA nucleotides have the pentose sugar deoxyribose, whereas RNA nucleotides have the pentose sugar ribose
• DNA nucleotides can have the base thymine, whereas RNA nucleotides have uracil instead

Question 6

Structure of DNA related to its functions:

Answer 6

• Double stranded = both strands can act as templates for semi-conservative replication
• Weak hydrogen bonds between bases = can be unzipped for replication
• Complementary bonds between bases = stable/strong molecule
• Double helix with sugar phosphate backbone = protects bases/H bonds
• Long molecule = store lots of (genetic) info (that codes for polypeptides)
• Double helix (coiled) = compact

Question 7

What is the process of DNA replication?

Answer 7

1. DNA helicase breaks hydrogen bonds between bases, unwinds double helix
2. = two strands which both act as templates
3. Free floating DNA nucleotides attracted to exposed bases via specific complementary base pairing, hydrogen bonds form
4. DNA polymerase joins adjacent nucleotides on new strand by condensation, forming phosphodiester bonds (= sugar phosphate backbone)
5. Replication is semi-conservative - each new strand formed contains one original/template strand and one new strand
6. Ensures genetic continuity between generations of cells

Question 8

Why does DNA polymerase move in opposite directions along the DNA strand?

Answer 8

• Antiparrallel
• DNA polymerase has an enzyme with a specific shaped active site which can only bind to substrate with a complementary shape
• Can only bind to and add nucleotides to the phosphate (3’) end of the developing strand (so works in a 5’ to 3’ direction)

Question 9

What is evidence for semi-conservative replication?

Answer 9

• Bacteria grown in a nutrient solution containing heavy nitrogen (15 N) for several generations
• Nitrogen incorporated into bacterial DNA bases
• Bacteria then transferred to a nutrient solution containing lighting nitrogen (14 N) and allowed to grow and divide twice
• During this process, DNA from different samples of bacteria was extracted, suspended in a solution in separate tubes and spun in a centrifuge