Cell division & heredity · Topic 15 of 17

Protein Synthesis & Mutations

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Biology · gene expression

Transcription copies a gene into mRNA; ribosomes translate its three-base codons into protein. A mutation permanently changes the DNA sequence.

01 · Step 1Transcription: DNA → mRNA (nucleus)

02 · Step 2Translation: mRNA → protein (ribosome)

03 · Inherited?Only germ-line mutations are inherited

DNA → mRNA → protein (transcription, then translation) must know

Codon = 3 mRNA bases = 1 amino acid (or a stop signal) must know

Mutation = a permanent change in the DNA sequence must know

Point mutation: 1 base swapped → same, different or stop amino acid

Frameshift: a base added or deleted → every later codon misread

Spontaneous mutations happen naturally, at low rates (most errors are fixed)

Induced mutations: caused by mutagens — chemicals, UV light, X-rays

Transcription: DNA → mRNA, in the nucleus (RNA polymerase)

Translation: mRNA → protein at a ribosome (tRNA brings amino acids)

Start codon = AUG (methionine) · stop codons = UAA, UAG, UGA

64 codons for 20 amino acids — the same code in nearly all life

mRNA pairs with the DNA template: A→U, T→A, C→G, G→C

Only germ-line (egg/sperm) mutations are inherited; somatic ones are not

DNA TACGGCATT → mRNA AUG·CCG·UAA (start, 1 amino acid, stop)

THE BIG RED BUS − G → THE BIR EDB US: every later "codon" changes

mRNA = message · tRNA = amino-acid carrier · rRNA = ribosome

Commonly confused

	Point (substitution)	Frameshift
What changes	One base swapped for another	Base(s) inserted or deleted
Codons affected	Just one	Every codon after the change
Protein	Same, one amino acid changed, or early stop	Usually nonfunctional
Usual harm	Often smaller	Usually greater

Frameshift is usually worse than a point mutation (whole message garbled)

Show 3 more facts

Mutations are the source of new alleles — effects from none to harmful

Eukaryotic mRNA is processed (cap and tail added) before leaving the nucleus

Prokaryotes: transcription and translation happen together in the cytoplasm

Pearl

Transcription writes the message; translation reads it three letters at a time.

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