SME
Grade 9 Science · Biology
Mutations, gene expression and biotechnology
Use this for “Base 7 of the template strand TACCGGTTCATT changes from T to A. Which codon holds it, and is the mutation silent, missense or nonsense?”
Diagram
A substitution at base 7, codon 3.
Nonsense: the codon becomes a stop, so the protein is cut short.
Now Met–Ala–Stop.
This is an example. Type your own values to start a new problem.
p
Base changed
example
b
New base
example
k
Codon holding it
worked out (not typed)
j
Its place in the codon
worked out (not typed)
a
Amino acid before
worked out (not typed)
n
Amino acid after
worked out (not typed)
e
Kind of mutation
worked out (not typed)
Formulas
k
=
⌈
p
3
⌉
3
=
⌈
7
3
⌉
j
=
p
−
3
×
(
k
−
1)
1 = 7 − 3 × (3 − 1)
a
=
amino
acid
of
codon
k
Lys = amino acid of codon 3
n
=
amino
acid
of
codon
k
with
base
p
changed
to
b
Stop = amino acid of codon 3 with base 7 changed to A
e
=
kind
of
change
from
a
to
n
nonsense = kind of change from Lys to Stop
Assumptions
•
The template strand is TACCGGTTCATT; its mRNA AUG GCC AAG UAA codes Met–Ala–Lys, then stop.
•
Silent: the new codon codes for the same amino acid. Missense: a different amino acid. Nonsense: the codon becomes a stop, so the protein is cut short.
•
Start lost: AUG (bases 1–3) changes, so no protein starts here. Stop lost: UAA (bases 10–12) codes an amino acid, so the ribosome reads on.
•
Most amino acids have more than one codon, often differing only in the third base, so many substitutions there are silent.
Step-by-step
WE KNOW
p = 7, b: A
FIND
codon holding it (k), its place in the codon (j), amino acid before (a), amino acid after (n), kind of mutation (e)
Step 1 · Find codon holding it (k)
Use k = ⌈p ÷ 3⌉
Bases 1 to 3 are codon 1, bases 4 to 6 codon 2, and so on: divide by 3 and round up.
k
=
⌈
7
3
⌉
k
=
3
Step 2 · Find its place in the codon (j)
Use j = p − 3 × (k − 1)
The
codons
before
it
hold
3
×
(
k
−
1)
bases;
the
rest
is
its
place
in
its
own
codon.
j
=
7
−
3
×
(3
−
1)
j
=
7
−
3
×
2
j
=
7
−
6
j
=
1
Step 3 · Find amino acid before (a)
Use a = amino acid of codon k
Pair each template base with its mRNA base (A–U, T–A, G–C, C–G), then look the codon up in the codon table.
a
=
amino
acid
of
codon
3
Template TTC → mRNA AAG
AAG codes for Lys
a
=
Lys
Step 4 · Find amino acid after (n)
Use n = amino acid of codon k with base p changed to b
Swap the base in the template, pair it again, and look the new codon up.
n
=
amino
acid
of
codon
3
with
base
7
changed
to
A
Template ATC → mRNA UAG
UAG codes for Stop
n
=
Stop
Step 5 · Find kind of mutation (e)
Use e = kind of change from a to n
The same amino acid is silent, a different one missense and a new stop nonsense; Met changed loses the start, and the stop changed loses the stop.
e
=
kind
of
change
from
Lys
to
Stop
e
=
nonsense
→
the
protein
stops
after
2
amino
acids
Check
Put the numbers back in. Both sides should match.
3
=
⌈
7
3
⌉
✓
1 = 7 − 3 × (3 − 1) ✓
Lys = amino acid of mRNA codon AAG ✓
Stop = amino acid of mRNA codon UAG ✓
nonsense = kind of change from Lys to Stop ✓
Topic check
See what you know: 5 questions on this topic
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