SME
Grade 9 Science · Biology
DNA structure, replication and protein synthesis
Use this for “An mRNA has 12 bases ending in a stop codon. How many amino acids and peptide bonds does the chain have?”
Diagram
12
bases
3
=
4
codons.
c
=
4.
The last is the stop codon, so the chain has 4 − 1 = 3 amino acids: Met–Ala–Lys.
Peptide bonds: 2
Water molecules released: 2
This is an example. Type your own values to start a new problem.
a
=
b
÷
3
−
1
c
Codons
worked out (not typed)
p
Peptide bonds
worked out (not typed)
w
Water molecules released
worked out (not typed)
Formulas
c
=
b
3
4
=
12
3
a
=
c
−
1
3 = 4 − 1
p
=
a
−
1
2 = 3 − 1
w
=
p
2 = 2
Assumptions
•
Three mRNA bases make a codon; AUG starts the chain and codes Met, and a stop codon adds no amino acid.
•
So a coding mRNA of b bases, ending in its stop codon, codes b ÷ 3 − 1 amino acids.
•
Each peptide bond joining two amino acids releases one water molecule.
•
The picture draws a gene of up to 15 bases whole; a longer one, its first 12 bases, “…” and its stop codon.
Step-by-step
WE KNOW
b = 12
FIND
codons (c), amino acids in the chain (a), peptide bonds (p), water molecules released (w)
Step 1 · Find codons (c)
Use c = b ÷ 3
Every three bases are one codon.
c
=
12
3
c
=
4
Step 2 · Find amino acids in the chain (a)
Use a = c − 1
Every codon but the last codes an amino acid; the last is the stop codon.
a
=
4
−
1
a
=
3
Step 3 · Find peptide bonds (p)
Use p = a − 1
A peptide bond joins each amino acid to the next: one fewer bond than amino acids.
p
=
3
−
1
p
=
2
Step 4 · Find water molecules released (w)
Use w = p
Each peptide bond releases one water molecule.
w
=
2
Check
Put the numbers back in. Both sides should match.
4
=
12
3
✓
3 = 4 − 1 ✓
2 = 3 − 1 ✓
2 = 2 ✓
Topic check
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From gene to protein
Use this for “Put the steps of protein synthesis in order, from the gene in the nucleus.”