Nucleic Acids

Question 1

How is a DNA nucleotide different to an RNA nucleotide?

Answer 1

DNA contains thymine and RNA contains uracil;

DNA contains deoxyribose and RNA contains ribose;

Question 2

How is the structure of mRNA different to

the structure of tRNA

Answer 2

mRNA is longer / has more nucleotides than tRNA;

mRNA is a straight molecule but tRNA is a folded molecule/clover-leaf shaped molecule;

mRNA contains no paired bases/hydrogen bonds but tRNA has some paired bases/hydrogen bonds;

Question 3

Contrast the structures of DNA and mRNA

Answer 3

DNA double stranded/double helix and mRNA single-stranded;

DNA (very) long and RNA short;

Thymine/T in DNA and uracil/U in RNA;

Deoxyribose in DNA and ribose in RNA;

DNA has base pairing and mRNA doesn’t/ DNA has hydrogen bonding and mRNA doesn’t;

DNA has introns/non-coding sequences and mRNA doesn’t;

Question 4

Explain how the structure of DNA is related to its functions.

Answer 4

1. Sugar-phosphate (backbone) / double stranded / helix so provides strength / stability /protects bases / protects hydrogen bonds;
2. Long / large molecule so can store lots of information;
3. Helix / coiled so compact;
4. Base sequence allows information to be stored / base sequence codes for amino acids / protein;
5. Double stranded so replication can occur semi-conservatively / strands can act as templates/ complementary base pairing / A-T and G-C so accurate replication / identical copies can be made;
6. (Weak) hydrogen bonds for replication / unzipping / strand separation / many hydrogen bonds so stable / strong;

Question 5

Describe how DNA is replicated. [6]

Answer 5

1. Strands separate / H-bonds break;
2. DNA helicase (involved);
3. Both strands / each strand act(s) as (a) template(s);
4. (Free) nucleotides attach;
5. Complementary / specific base pairing / AT and GC;
6. DNA polymerase joins nucleotides (on new strand);
7. H-bonds reform;
8. Semi-conservative replication / new DNA molecules contain one old strand and one new strand;

Question 6

Describe and explain how the structure of DNA results in accurate replication. [6]

Answer 6

1. two strands therefore semi-conservative replication (possible);

2 base pairing/hydrogen bonds holds strands together

3 hydrogen bonds weak/easily broken, allow strands to separate;

4 bases (sequence) (exposed so) act as template /can be copied;

5 A with T, C with G / complementary copy;

6 DNA one parent and one new strand;

Question 7

explain why the DNA polymerase act in opposite directions.

Answer 7

DNA has antiparallel strands/described;

shape of the nucleotides is
different/nucleotides aligned differently;

Enzymes have active sites with specific shape;

Only substrates with complementary shape/only the 3’ end can bind with active
site of enzyme/active site of DNA polymerase;

Question 8

explain why new nucleotides can only be added in a 5’ to 3’ direction.

Answer 8

DNA polymerase;

(Which is) specific;

Only complementary with/binds to 5’ end (of strand);

Shapes of 5’ end and 3’ end are different;