DNA and Genetic Material: the double helix of nucleotides, semi-conservative replication, how genes code for proteins, plasmids and vectors, and extracting DNA from cells

What this module is about

DNA is the instruction set of life and the central molecule of biotechnology. This module builds it up from its chemical structure, shows how it copies itself, explains how its sequence becomes a protein, introduces the plasmids that carry genes between cells, and ends with a practical: getting DNA out of cells so you can see and use it. Every later technique, from PCR to genetic profiling, rests on the ideas here.

The full set of dot points for this module is at /sg-o-level/biotechnology/syllabus/dna-and-genetic-material.

The structure of DNA

The dot point on the structure of DNA describes DNA as a double helix of nucleotides. Each nucleotide has a sugar, a phosphate and one of four bases: adenine, thymine, cytosine and guanine. The two strands are held together by complementary base pairing: A pairs with T, and C pairs with G. This single rule is the foundation of the whole module, because it means the sequence of one strand fixes the sequence of the other, which makes both copying and reading possible.

Copying the message: replication

The dot point on DNA replication explains how a cell makes a perfect copy before it divides. The two strands separate, and each acts as a template: free nucleotides pair with the exposed bases following the A-T and C-G rule, and are joined into a new strand. Because each new molecule keeps one old strand and gains one new strand, replication is semi-conservative. The accuracy of the process comes directly from complementary base pairing.

Reading the message: genes to proteins

The dot point on genes, proteins and the genetic code shows how a base sequence becomes a protein. A gene is a length of DNA coding for a protein. In transcription the gene is copied into messenger RNA, and in translation the mRNA is read at a ribosome in three-base groups called codons, each specifying an amino acid. The order of bases therefore sets the order of amino acids, and hence the protein. This is why moving a gene into a new organism (as in recombinant insulin) makes that organism produce the corresponding protein.

Carrying the message: plasmids and vectors

The dot point on plasmids and vectors introduces the delivery system. A plasmid is a small ring of DNA in bacteria, separate from the main chromosome. It makes an ideal vector because it is small, can be cut open and joined with a foreign gene, is readily taken up by bacteria, and copies itself when the cell divides, carrying the inserted gene into every daughter cell. The plasmid you met as a cell structure in the previous module now becomes the practical tool of genetic engineering.

Getting the message out: extracting DNA

The dot point on extracting DNA from cells is the module's hands-on finale. Cells are broken open to release their contents, detergent dissolves the cell and nuclear membranes, salt helps the DNA clump together, and cold alcohol is added so the DNA, which does not dissolve in alcohol, precipitates out as visible white threads. Each step has a clear purpose, and being able to explain why each reagent is used is what earns the marks.

How the module fits together

• One rule, many uses. Complementary base pairing explains structure, replication and reading alike.
• Copy then read. Replication preserves the message; transcription and translation express it.
• Carry then extract. Plasmids deliver genes in; extraction takes DNA out for study.
• Purpose per step. In the practical, naming the reason for each reagent beats describing the action alone.

Check your knowledge

A mix of recall and application across the five dot points. Try them timed, then check the solutions.

1. Name the four bases of DNA and state which pairs with which. (2 marks)
2. Explain why DNA replication is described as semi-conservative. (2 marks)
3. Write the complementary strand for the sequence TAGGCAT. (1 mark)
4. State two reasons why a plasmid makes a good vector. (2 marks)
5. Explain why cold alcohol is added during DNA extraction. (2 marks)