Chromosomes and Chromatids
These are two terms which often confuse a lot of people. What's a chromosome and what is a chromatid? Are they identical (they can be, but not always). What do these terms have to do with ploidy or the chromosomal number? I wrote this short piece as an addendum to this outline of cell division.
In a normal eukaryotic cell which is not actively dividing, chromosomes are not visible. The DNA is loosely coiled, dispersed through the nucleoplasm. However, during cell division, the DNA condenses into ordered structures called chromosomes, which are visible under the microscope. An entire chromosome is actually a single DNA molecule, which has been coiled several times to pack it into a small volume.
Because chromosomes are only visible during cell division, techniques to view or count chromosomes (such as karyotyping) typically arrest the cell during some phase of division when chromosomes are clearly visible, most often during metaphase. However, there is a problem with this procedure. During metaphase, the cell's DNA has already been duplicated (it was duplicated during the S period of interphase, before the cell even started to divide). Therefore, such pictures are misleading, because they show the cell with twice the usual amount of DNA that it normally contains.
The pictures above are typical microphotographs of chromosomes. The one on the left is through a light microscope, stained with Chromomycin A3. The middle and right photographs were taken through a scanning electron microscope. All images were taken with the cell arrested during metaphase. While the chromosomes are clearly visible in metaphase, these chromosomes contain twice the usual complement of DNA. This is why they appear bifurcate, with two "sister chromatids" each.
So a chromatid in these pictures is half a chromosome, but since the chromosome contains twice the normal DNA, each chromatid is actually a full chromosome in the cell's resting state. Each chromatid contains the full DNA of a chromosome in the cell's normal resting state.
So a chromatid is really just a word used to refer to one of the two identical copies of DNA, at those stages of the cell cycle when two copies of DNA exist. When there is only a single copy of the DNA (when the cell is not dividing), there are no chromatids, there are only chromosomes. However, because chromosomes are not visible during the non-dividing period of the cell, because the only photographs of chromosomes come from cell division, when DNA has already been duplicated, the "chromatid" terminology was introduced.
Often, cells are referred to using the C and N terminology. Thus a human, non-dividing somatic cell is referred to as "2c/2n". The "n" represents ploidy. Ploidy refers to the number of sets of chromosomes contained in an organism or a cell. A human somatic cell contains 46 chromosomes, in 23 pairs. We think of these chromosomes as existing in pairs because each chromosome has a homologous chromosome - homologous meaning similar but not identical.
We inherit our DNA from our parents. Each one of us has roughly half of our nuclear DNA from our father, and the other half from our mother. These paternal and maternal contributions are homologous - meaning that for each gene, we have two alleles, one from the father and one from the mother. In other words, out of our 46 chromosomes, 23 came from our fathers, and 23 from our mothers. So we have 23 pairs of chromosomes. For each pair, one chromosome came from our father, and the other from our mother. Each pair of chromosomes codes for approximately the same features. Any gene which is present on one chromosome must be present on the other chromosome of the pair as well, in the same locus, but it may occur as a different allele. There are some exceptions to this, specially with the 23rd pair, the sex chromosomes. Since the X and Y chromosomes are very different, there are genes present on both X and Y which have no counterpart in the other. But by and large, genes on the autosomes (the first 22 pairs), and even many genes on the X and Y chromosomes, have homologues on the other chromosome of the pair.
Because our chromosomes exist in pairs (and consequently we have 2 alleles of each gene), we are a diploid species. This is why our somatic cells are represented as "2n". Our gametes (sperm and ova), on the other hand, are haploid, and are represented as "n". Other species may have different ploidy, for example:
- triploid (3n): seedless watermelons
- tetaploid (4n): salmonidae fish
- pentaploid (5n): Kenai birch
- hexaploid (6n): some types of wheat, kiwi fruit
- octaploid (8n): acipenser (a genus of sturgeon fish)
- decaploid (10n): some strawberries
- dodecaploid (12n): some types of amphibians, e.g. Xenopus ruwenzoriensis
The other part of the terminology is the "c" part. This is more difficult to define. Various sources list "c" as meaning "number of chromosomes" or "number of chromatids" or "DNA content" or "constant". Of all these, "number of chromosomes" makes the most sense to me, since each chromosome is one separate DNA molecule, and it is always present (unlike chromatids, which represent duplicated DNA, and therefore only exist during cell division when DNA is duplicated).
Human somatic cells are 2c because they contain a paired number of chromosomes. Why is this necessary to state, isn't this the reason they were 2n? There is a small but important difference. The "n" designation refers only to ploidy, and not chromosome number. So, for example, after the S period of interphase, when the cell has doubled its DNA, it goes from 2c to 4c. However, its ploidy hasn't changed, it's still diploid. Humans will never have 3 different alleles of a gene like a triploid species might have.
[Note: Different alleles is the key here, not number of copies. Since humans have 2 alleles of each gene (say alleles A and B of gene X), when the DNA is duplicated, they now have 4 alleles of gene X - two A's and two B's. However, a triploid species can have 3 different alleles - A, B and C, which humans don't have. However, note that there are disease conditions where this is possible, for example trisomy, where some specific chromosome exists as a triplet rather than a pair.]
The easy way to remember is that ploidy for a species is constant for somatic cells. It will only vary for gametes, which usually have half the ploidy of somatic cells. On the other hand "c" represents the amount of DNA. We call the amount of DNA in a somatic cell "2c". We could have called it "c" if we wanted to, but then when we were talking about gametes (which only have half the DNA of a somatic cell), we would have to call their DNA content "0.5c". Whole numbers are easier to deal with, so we simply start with the kind of human cell which has some DNA, but the least possible DNA (we specify "some" DNA because we also have cells with no DNA at all, such as red blood cells), and we call that 1c/1n. Then other cells can be expressed in multiples of c and n, thus a resting somatic cell is 2c/2n, a cell during mitosis is 4c/2n, etc.