DNA - Important

Question 1

Outline semi-conservative replication.

Answer 1

1. DNA helicase breaks the hydrogen bonds between bases on the two DNA strands. This makes the helix unwind to form two single strands.
2. Each original strand acts as a template for a new strand. Complementary base paring means that free floating DNA nucleotides are attracted to their complementary exposed bass on each template strand.
3. Condensation reaction join the nucleotides of the new strands together - catalysed by DNA polymerase.
4. Hydrogen bonds form between the bases in the original and new strands.
5. Now, each new DNA molecule contains one strand from the original molecule and one new strand.

Question 2

Outline what happens in ‘translation’

Answer 2

1. The mRNA attaches itself to a ribosome and tRNA molecules carry amino acids to it.
2. A tRNA molecule with an anticodon that’s complementary to the first codon in the mRNA attaches itself to the mRNA by specific base pairing.
3. A second tRNA molecule attaches itself to the next codon in the same way.
4. The two amino acids attached to the tRNA molecules are joined by peptide bonds. The first tRNA molecule moves away, leaving its amino acid behind.
5. A third tRNA molecule binds to the next codon on the mRNA molecule. Its amino acid binds to the frets two and the second tRNA molecule moves away.
6. The process continues forming a polypeptide chain until there’s a stop signal on the mRNA molecule.
7. The polypeptide chain knives away from the ribosome and translation is complete.