# Unsure about definition of “n” and “C” values in mitosis

I am preparing for a Biology exam and I'm reviewing the "n" and "C" notation used in mitosis.

My professor said that when the cell replicates its DNA in S phase of mitosis, we get twice the "amount" (C) of DNA, but we still have the same "number" of chromosomes.

Does this mean that if the replicated chromosomes were to "split apart" from their centromeres, we would have a 4n cell (so a cell with 4 sets of chromosomes)?

At the same time, my intuitive response sort of says, "well, they're replicated, so they're exactly the same, so you'd technically have 2 sets, not 4 right?".

I don’t know what the professor inferred exactly in class. Our somatic cell consists of two set of chromosomes that one set is maternal, and the other is paternal. In other words, human somatic cell has diploid number of chromosomes (2n). However, mature ovum and sperm are haploid number of chromosomes (n). The fertilization combines these two haploids into one diploid.

Somatic cell is always diploid throughout mitosis, but the amount of DNA is duplicated during interphase, especially in S phase. The amount of DNA may be represented as “C” for better understanding. The number of chromosome and the amount of DNA are 2n/2c in G1 phase during interphase. The number of chromosome and the amount of DNA are changed into 2n/4c in S phase. The amount of DNA is separated exactly half into two daughter cells in mitosis. The number of chromosome and the amount of DNA come back to 2n/2c after mitosis.

While I've not heard of the "C" notation previously, "n" is common notation that indicates the ploidy of a cell. The "n" refers to the number of unique chromosomes within a cell. Eukaryotic cells are generally diploid, or 2n. The multiplying number - "2" in the diploid case - refers to the number of versions of each unique chromosome. For example, humans are diploids (2n). We have 23 unique chromosomes (22 regular chromosomes and 1 sex chromosome), but we have 2 variants of each chromosome to give us a total of 2*23 = 46 chromosomes.

Prior to S phase, diploid cells contain 2n content (maybe "content" refers to "C" notation?) of DNA. During S phase the cell duplicates its DNA content generating 2 sets of 2n chromosomes (2 copies of 46 chromosomes for a total of 92 chromosomes in the human case). You are correct in that it is 2 sets of 2n rather than 4n, since the convention would indicate 4n as a tetraploid (many types of plants are in fact tetraploid, https://en.wikipedia.org/wiki/Polyploid).

At the end of the mitotic cell cycle, cell division functions to separate the 2 sets of 2n chromosomes into two distinct cells, each containing 2n chromosomes. If all 4 chromosomes separated from each other and the cell was able to divide into quarters rather than halves, that would yield 4 1n cells. This latter idea is what occurs in meiosis.

• For human male, XY are unique chromosomes, that doesn't imply n=24 instead oh 23. Edit your definition please. – Macrophage Dec 30 '17 at 4:10
• I appreciate the point regarding X and Y chromosomes being different relative to each other. They do, however, fall under the same generalized heading of "sex chromosomes", which are a unique subgroup of chromosomes that manifest themselves in different ways in different species (mammalian sex chromosomes versus Drosophila sex chromosomes versus C. elegans sex chromosomes versus ZW system of sex chromosomes). For this reason a more generalized reference to sex chromosomes is appropriate, though I will make edits given the explicit example of humans being used. – khyndswitte Dec 30 '17 at 18:53
• See en.wikipedia.org/wiki/Sex-determination_system for an overview of sex determination – khyndswitte Dec 30 '17 at 18:54