Genes, enzymes & metabolism

One gene → one enzyme → one step. ℹ

Excellence chain

"Allele x codes a non-working enzyme 2, so B cannot become C; B accumulates and the flower is white." Pathway + gene + enzyme + phenotype — all four parts.

Two ways a pathway can be blocked, and how to tell them apart

GENETIC block: a mutation produces a non-working enzyme — the gene itself is different.
ENVIRONMENTAL block: the gene and enzyme are normal, but a missing cofactor or wrong temperature stops the enzyme from working.

Pathway term	Meaning
Precursor / substrate	The starting molecule entering the pathway
Intermediate	A partly-converted molecule between steps
Enzyme	The protein catalysing one specific step, coded for by one gene
Final product	The molecule produced once all steps are complete

Substitution vs frameshift is a severity question, not just a naming one

Papers ask WHY a frameshift (insertion/deletion) is more severe than a substitution — not just that it is. A substitution can only ever change ONE codon, so at most ONE amino acid. A frameshift changes the READING FRAME, so every codon downstream is different, which usually wrecks the whole protein and often creates a premature stop codon. Say both halves: what changes, and why that makes the difference in scale.

Practice for this standard covers the whole topic — flashcards and quiz.