- Define and explain DNA Structure and Replication in your own words
- Use key terms such as semi-conservative accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on DNA Structure and Replication: describe the double helix and the semi-conservative replication mechanism.
Describe the double helix and the semi-conservative replication mechanism.
Key ideas
Enzymes and DNA replication
Enzymes are specific because each active site has a shape complementary to one substrate — the induced-fit model adds that the active site moulds slightly around the substrate. Rate rises with temperature and substrate concentration until the enzyme denatures or becomes saturated; each enzyme has an optimum pH. DNA replicates semi-conservatively: the double helix unwinds, each strand acts as a template, and free nucleotides pair by complementary base pairing (A–T, C–G) as DNA polymerase builds the new strands.
The molecules of life
Carbohydrates (sugars and starch) provide energy and structure; lipids store energy and form membranes; proteins do the work of the cell as enzymes, carriers and structural fibres; nucleic acids (DNA and RNA) store and transmit genetic information. Each can be identified by a food test: Benedict's reagent turns orange-red with reducing sugars, iodine turns blue-black with starch, Biuret reagent turns purple with protein, and the emulsion test gives a milky layer with lipids.
Key term — semi-conservative: Describes DNA replication in which each new DNA molecule contains one original strand and one newly synthesised strand.
Outline the three key stages of semi-conservative DNA replication.
The double helix unwinds and the strands separate; each strand acts as a template with free nucleotides pairing by complementary base pairing (A–T, C–G); DNA polymerase joins the nucleotides to form two molecules, each with one old and one new strand.
Answer: The double helix unwinds and the strands separate; each strand acts as a template with free nucleotides pairing by complementary base pairing (A–T, C–G); DNA polymerase joins the nucleotides to form two molecules, each with one old and one new strand.
- Claiming both DNA strands are brand new after replication Replication is semi-conservative: each daughter molecule keeps one original strand and gains one new strand.
- Saying high temperature kills enzymes Enzymes are not alive, so they cannot be killed. High temperatures denature them by disrupting the bonds that hold their 3D shape.
Practice
Carbohydrates – energy source (e.g. glucose) or structure (cellulose); lipids – energy storage and membranes; proteins – enzymes, transport and structure; nucleic acids – storing and transmitting genetic information.
The solution turns purple (from blue), which indicates the presence of protein.
Primary structure is the sequence of amino acids in the polypeptide chain. Tertiary structure is the overall three-dimensional folding of that chain, held by hydrogen, ionic and disulfide bonds.
35 °C is 10 °C higher, so the rate doubles: 6 × 2 = 12 units.
Quick check
Which of these best defines "semi-conservative"?
Why does changing pH reduce the rate of an enzyme-controlled reaction?
- DNA Structure and Replication: describe the double helix and the semi-conservative replication mechanism.
- Enzymes and DNA replication: Enzymes are specific because each active site has a shape complementary to one substrate — the induced-fit model adds that the active site moulds slightly around the substrate.
- monomer: A small molecule that joins with others to form a polymer; for example, amino acids are the monomers of proteins.
- Watch out for: claiming both DNA strands are brand new after replication