DNA Molecule Model

DNA stands for deoxyribonucleic acid. In OCR Biology, the structure you need here is a molecule made from two DNA polynucleotide strands. A polynucleotide is a chain of nucleotides; you have already met nucleotide structure and phosphodiester-linked polynucleotides in the previous lesson, so this lesson focuses on how two strands fit together to make DNA.

Each DNA strand has a sugar-phosphate backbone. In the double-stranded molecule, the two backbones run along the outside and the bases point inwards. The bases are the part that can pair with a base on the opposite strand.

Complementary base pairing means the base on one strand determines the base opposite it. If one strand has A, the opposite base is T. If one strand has G, the opposite base is C. This is not a random attraction; it is a specific structure-function relationship in the DNA molecule.

The diagram shows the core model. Use it to keep three ideas together: two backbones outside, bases inside, and the strands running in opposite directions.

[DIAGRAM: dna_double_helix_model: Lesson 32: DNA structure and DNA precipitation - diagram 01; asset_slug: 032_m02_1_3_dna_structure_and_dna_precipitation__diagram_01; recommended_method: drawn_biology; description: A clean 16:9 drawn biology diagram of a DNA double helix showing two antiparallel sugar-phosphate backbones, base pairs inside, A-T and G-C complementary pairs, hydrogen bonds and twisting into a double-helix shape.]

Antiparallel Strands

The two DNA strands are antiparallel. This means they run in opposite directions. You may see the ends labelled 5' and 3', but for this row the important idea is simpler: one strand runs one way, while the other strand runs the opposite way.

Antiparallel orientation matters because the bases on the two strands are correctly positioned to form complementary pairs. The strands are not two identical chains sitting side by side. They are two chains with opposite direction and matching base partners.

Hydrogen bonds form between the complementary bases. These bonds hold the two strands together across the middle of the molecule. A-T pairs and G-C pairs fit because their bases can form the correct hydrogen-bonding pattern.

Double Helix Shape

The base pairs make the two strands into a paired structure, rather like a ladder at first: the sugar-phosphate backbones are the sides and the hydrogen-bonded base pairs are the rungs. DNA is not a flat ladder, though. The paired strands twist, producing the double-helix shape.

This shape is important because it links the molecule's stability with its information-carrying role. The sugar-phosphate backbones protect the base pairs on the inside, while the base sequence remains available as stored genetic information.

Do not turn this into a detailed chemistry answer. OCR needs the structure named in the row: hydrogen bonding, complementary base pairs, antiparallel DNA polynucleotides and twisting into a double helix. Details such as major and minor grooves, exact helix dimensions and chromatin packing are outside this lesson boundary.

Precipitating DNA Practical

The practical investigation in this row is purification of DNA by precipitation. Precipitation means making a dissolved substance come out of solution as a solid. In a school DNA extraction, the visible solid is best described as a DNA-rich precipitate, because it may still contain some RNA, protein or salts.

The method works because DNA is inside cells and is associated with other cell material. A good method must release the DNA, separate it from some unwanted material, and then make it come out of solution.

[DIAGRAM: dna_precipitation_workflow: Lesson 32: DNA structure and DNA precipitation - diagram 02; asset_slug: 032_m02_1_3_dna_structure_and_dna_precipitation__diagram_02; recommended_method: drawn_biology; description: A clean 16:9 drawn biology workflow diagram showing DNA precipitation steps: breaking open cells with detergent and salt, filtering/removing debris and proteins with protease, adding cold ethanol, and spooling a white DNA-rich precipitate.]

Cold ethanol is used because it improves visible precipitation and helps reduce enzyme activity that could damage DNA. Ethanol is flammable, so it must be kept away from flames, and the practical needs eye protection, hygienic handling of biological material, and the risk controls set by the teacher or local guidance.

After the detergent step, protease has a different purpose. It does not release DNA by breaking membranes. It digests proteins, including DNA-associated proteins, so the DNA-rich material is less contaminated by protein.

Judging The Method

A practical answer should connect each observation to the method. Seeing white stringy material after adding alcohol supports that DNA-rich material has precipitated, but it does not prove the sample is pure DNA. A careful conclusion says what the method supports and what its limits are.

Here is the reasoning sequence:

1. Cells must be opened so DNA can enter solution.
2. Membranes and proteins must be dealt with because DNA is inside nuclei and associated with proteins.
3. Cold ethanol must be added because DNA is insoluble in alcohol in the presence of salt.
4. The visible white/stringy material is recorded as DNA-rich precipitate, not automatically pure DNA.

For example, if a method uses salt and ethanol but no detergent, the problem is not just "detergent is missing". The consequence is that cell-surface and nuclear membranes may not be disrupted properly, so less DNA may be released into the solution.