Molecular genetics, at a glance

A chromosome contains a long DNA molecule carrying many genes. A gene is a sequence of nucleotides with instructions for a polypeptide and is a unit of inheritance.

• Genes are sections of DNA, not separate objects stored alongside DNA.
• A chromosome carries many genes along its DNA.
• At this syllabus level, a gene codes for one polypeptide and can be passed from parent to offspring.

Keep in mind: A gene is a DNA nucleotide sequence. Its information is used to make a polypeptide; the gene and its protein product are different kinds of molecule.

DNA has two nucleotide strands twisted into a double helix. Each nucleotide contains sugar, phosphate and a base; A pairs with T, and C pairs with G.

• A nucleotide contains a sugar, a phosphate group and one base.
• Sugar and phosphate form each strand's backbone; bases face inwards.
• Complementary pairing is A with T and C with G. Complementary does not mean identical.

Keep in mind: They are complementary: each A is opposite T, and each C is opposite G. The paired letters are not identical.

The sequence in a gene carries information used to assemble a particular amino acid sequence. The polypeptide can fold into a protein with a specific role.

• The genetic code is carried by the sequence of DNA bases.
• A gene specifies a polypeptide; its amino acid sequence contributes to its shape and function.
• DNA is used as information. It is not converted into the protein as a raw material.

Keep in mind: The DNA sequence supplies information for synthesis. The protein is built from amino acids; it is not made by converting DNA material into protein.

A gene can be transferred into cells of another organism. If the introduced gene is maintained and used, it can give the recipient a new ability to make a particular product.

• The transferred material is genetic information, not the finished protein itself.
• A recipient remains its own type of organism; it does not acquire every feature of the donor.
• The gene must be delivered and function appropriately; simple contact between organisms does not transfer a working trait.

Keep in mind: A selected sequence provides particular information. The recipient retains its other genes and structures; it does not become the donor organism.

A suitable human insulin-coding sequence can be inserted into a plasmid and introduced into bacteria. Selected bacteria are grown to produce the protein, which is recovered and purified.

• The insulin gene provides instructions; a plasmid carries those instructions into bacteria.
• Restriction enzymes cut DNA and ligase joins DNA. They have different roles.
• Production is followed by recovery, purification and quality checks; a bacterial culture is not the finished medicine.

Keep in mind: The product must be recovered and purified, with any necessary processing and quality checks. Bacterial cells and other culture materials are not the finished insulin medicine.

Evaluate the particular application: what benefit is expected, who gains or carries risk, what evidence is needed, and how harm or unfair access could be reduced.

• Medicine: reliable protein production can help patients; quality and fair access matter.
• Plants: useful traits may protect yield or improve composition; ecological effects and farmer access require evaluation.
• Animals: useful traits may benefit production or research; welfare, containment and necessity matter.

Keep in mind: Explain who benefits or could be harmed, how the effect could occur, and what evidence or conditions would support a decision. Different applications require different judgements.