Question

It is possible to measure the DNA content of a single nucleus using a microspectrophotometer. These measurements were made on a large number of nuclei in cells of a growing embryo. The measured DNA levels in individuals cells ranged between 2.8 0g and 5.6 og per nucleus, one nucleus, however, had 4.2. What stage of the cell cycle was it?

Can someone explain how I would get to the answer? What are the ranges per cycle?

Answer 1

Amount of DNA in different stages of mitosis is as follows -

Stage of cell cycle	amount of DNA
G1	C (parental cell)
S	C to 2C
G2	2C
PROPHASE	2C
METAPHASE	2C
ANAPHASE	2C
TELOPHASE	2C
CYTOKINESIS	C (daughter cell)

Amount of DNA in various stages of meiosis -

Stage of meiosis	amount of DNA
G1	C
S	C to 2C
G2	2C
Prophase 1	2C
Metaphase 1	2C
Anaphase 1	2C
Telophase 1	2C
Cytokinesis 1	C
Prophase 2	C
Metaphase 2	C
Anaphase 2	C
Telophase 2	C
Cytokinesis 2	C/2

So, in this question, you are given with 3 numbers, 4.2, 5.6 and 2.8.

The range of DNA in different nuclei is 2.8 to 5.6

5.6 = 2 × 2.8

It means C = 2.8 and 2C = 5.6

So, the cell before entering mitosis, in G1 phase and after completing mitosis, after telophase or after cytokonesis, will have 2.8 og DNA.

In S phase, the DNA will divide actively, so the amount of DNA in this stage will vary. It will be from 2.8 to 5.6, because DNA is doubling, it has not been doubled. At the end of S phase, the cell will have 2C or 5.6 og DNA. So, the cell having 4.2 og DNA will also be present at this stage, 4.2 is in between 2.8 to 5.6.

Then from G2 to telophase, cell will have 2C or 5.6 og DNA.

I have given you amount of DNA in different mitosis and meiosis stages. You only need to look up on mitosis stages for this question. For more information you can refer to stages of meiosis.

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