SME
Principles of Biology I
Cell division
Use this for “A human cell has 2n = 46. How many chromatids does it hold after S, how much DNA is in a gamete, and how many kinds of gamete are there?”
Diagram
G₁: 2n = 46 chromosomes of one chromatid each, 2c.
S copies each one: 46 chromosomes, 2 × 46 = 92 chromatids, 4c.
Meiosis
I
parts
the
pairs:
n
=
46
2
=
23
chromosomes,
still
2
chromatids
each,
2c.
Meiosis II parts the sisters: 23 chromatids, 1c.
Each pair goes either way: 2²³ = 8,388,608 kinds of gamete.
This is an example. Type your own values to start a new problem.
2n
Chromosomes in a body cell
example
n
Chromosomes in a gamete
worked out (not typed)
X
Chromatids after S
worked out (not typed)
C
Kinds of gamete
worked out (not typed)
Formulas
n
=
2
n
2
23
=
46
2
X
=
2
×
(
2
n
)
92 = 2 × 46
C
=
2
n
8,388,608
=
2
23
Assumptions
•
A G₁ cell holds 2n chromosomes of one chromatid each; its DNA content is called 2c.
•
S copies every chromosome into two sister chromatids, doubling the DNA to 4c; meiosis I parts the pairs (2c) and meiosis II the sisters, leaving a gamete 1c.
•
Each pair lines up either way round at metaphase I; crossing over is left out of the 2ⁿ kinds of gamete.
Step-by-step
WE KNOW
2n = 46
FIND
chromosomes in a gamete (n), chromatids after S (X), kinds of gamete (C)
Step 1 · Find chromosomes in a gamete (n)
Use n = (2n) ÷ 2
A gamete keeps one of each pair.
n
=
46
2
n
=
23
Step 2 · Find chromatids after S (X)
Use X = 2 × (2n)
S copies each chromosome into two sister chromatids.
X
=
2
×
46
X
=
92
Step 3 · Find kinds of gamete (C)
Use C = 2ⁿ
Each pair lines up either way round, so every pair doubles the kinds of gamete.
C
=
2
23
C
=
8,388,608
Check
Put the numbers back in. Both sides should match.
23
=
46
2
✓
92 = 2 × 46 ✓
8,388,608
=
2
23
✓
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