y1 key sequences

Question 1

Outline the process of semiconservative DNA replication.

Answer 1

• DNA helicase breaks H-bonds between base pairs the double helix unwinds
• each strand acts as a template
• free nucleotides from nuclear sap attach to exposed bases by complementary base pairing
• DNA polymerase catalyses condensation reactions that join adjacent nucleotides on new strand with phosphodiester bonds
• H-bonds reform

Question 2

Describe the Benedict’s test for reducing sugars.

Answer 2

1. Add an equal volume of Benedict’s reagent to a sample.
2. Heat the mixture in an electric water bath at 100°C for 5 mins.
3. Positive result: colour change from blue to orange and brick-red precipitate forms.

(monosaccharides and maltose)

Question 3

Describe the Benedict’s test for non-reducing sugars.

Answer 3

1. Negative result: Benedict’s reagent remains blue
2. Hydrolyse non-reducing sugars e.g. sucrose into their monomers by adding 1cm³ of HCL. Heat in a boiling water bath for 5 mins. (warm acid)
3. Neutralise the mixture using sodium carbonate solution. (alkali)
4. Proceed with the Benedict’s test as usual.

( dipeptidases and polysaccharides not maltose)