Chromosome number counts the centromeres in the cell. DNA amount counts all the genetic material, including copies still joined together.
The two quantities change at different moments.
This lesson is part of cell division and builds on both sequencing mitosis and sequencing meiosis.
What does the two-row table look like?
Use a cell with 2n = 4 for mitosis. Record the chromosome number and the DNA amount, where the starting DNA is 1 unit.
| Point in the cycle | Chromosomes | DNA amount |
|---|---|---|
| Start of interphase | 4 | 1 unit |
| After replication | 4 | 2 units |
| Metaphase | 4 | 2 units |
| Anaphase | 8 | 2 units |
| Each daughter cell | 4 | 1 unit |
The chromosome count rises only in anaphase, because that is when chromatids separate and each one is counted as its own chromosome.
Worked example: read a graph of DNA amount
Question. A graph shows the DNA amount per cell over time.
The line starts at 10 units, rises to 20 units, stays at 20, falls to 10, and then falls again to 5. Identify the process and explain each change. (4 marks)
Step 1. Rising from 10 to 20 is DNA replication in interphase.
Step 2. Falling from 20 to 10 is the end of the first division, when the cell splits into two.
Step 3. Falling from 10 to 5 is the second division.
Step 4. Two falls mean two divisions, so the process is meiosis. Mitosis would show only one fall, from 20 to 10.
Answer: meiosis, with replication then two halvings giving cells with one quarter of the peak amount.
The mistake that costs marks
The slip is to say the chromosome number doubles in interphase because the DNA doubled. It does not. The two copies are joined at one centromere and still count as one chromosome.
Before you fill in the table, ask: “Am I counting centromeres or measuring DNA?”
Check yourself
A cell has 12 chromosomes and 24 units of DNA at metaphase of mitosis. State the chromosome number and DNA amount in each daughter cell.
Answer
Each daughter cell has 12 chromosomes and 12 units of DNA.
The DNA is shared equally between the two cells, and each receives one chromatid from every chromosome.
What to study next
Apply the same table in explaining variation from meiosis, then test yourself with the cell division practice set. If you would like a teacher to check your tables, see online one-to-one Biology tuition.