What is a chromosome?

Question

I read the wikipedia article and am confused if a chromosome is the pair of two chromotoids or if each chromatoid is considered a chromosome.

I've heard someone say we have 23 pairs of chromosomes for a total of 46 chromosomes.

I am trying to be precise with my words here.

Even more confusing they always seem to pick this certain state of the chromosoes lifepsan to describe them.

The point at which it replicates to create a pair.

Why not just describe the chromosome before it replicates for simplicity sake.

Would it be correct to say that before the chromosome divides, there are simply 23 of them?

Answer 1

Would it be correct to say that before the chromosome divides, there are simply 23 of them?

No, in a normal, non-dividing cell of the human body there are 46 chromosomes in the cell's nucleus. The 46 chromosomes are separate, but are often considered as 23 pairs of chromosomes. The two chromosomes of a pair are called homologous chromosomes because they contain very similar, but not quite identical, information. One member of each pair comes from the mother and one from the father.

Each of these non-dividing chromosomes effectively consists of a single chromatid, but because there is only one, indistinguishable from the chromosome as a whole, it is not talked about.

Why not just describe the chromosome before it replicates for simplicity sake.

It is often described in that form (as one very long molecule of double-helix DNA). However, a chromosome is also very commonly illustrated in the mid-replication form where it has two chromatids and takes an X-like shape. One reason for this is that chromosomes can be seen in a light microscope only in this mid-replication form, and that is the form in which they were first observed and first shown to be the basis of heredity (long before they were known to be composed of DNA).

Answer 2

Firstly, it's important to understand that this is indeed a very common point of confusion. Tip: don't be thrown off by the color coding, because there are different things you can color-code for. I think what I show below is the minimum picture necessary to understand the difference between a chromosome, pairs of chromosomes, chromatids, and sister chromatids.

Humans have 23 pairs of chromosomes. One originally from the mother, one originally from the father. Chromosomes can be homologous as a single chromatid, or as a paid of chromatids. Let's assume we are looking at chromosome 1 below. It's important to understand that chromosome 1 is BOTH in purple AND in green. We can have a single chromatid chromosome, we can have a pair of single chromatids, and we can also have a pair of chromosomes where each is a pair of chromatids:

As you can see above, we can have 4 chromatids of chromosome 1, or even two copies of two pairs of chromosome 1 (that would be 4 chromosomes; not pictured). I'll explain further.

Crucially, the replication process depends on whether it is mitosis (two diploid daughter cells from one diploid mother cell) or meiosis (four haploid daughter cells from one diploid mother cell) we are talking about. Remember, almost all cells arise from mitosis, except the sex cells, which arise from meiosis.

Best to draw it out and just take a moment to look at it, and compare it side-by-side for a while. It's not that complicated when you start with the knowledge that the imaginary cell below has 3 CHROMOSOMES, OR 3 PAIRS OF CHROMOSOMES (just as we have 23 pairs of chromosomes):

Remember, in this image, 2n=6 (diploid) which means that n=3 (haploid).

In humans, 2n=46, n=23. In other words, we have 23 chromosomes that we keep in pairs (46). To make a mitotic cell division, we have to duplicate what we have (92). Then each daughter takes half of those, and so now each daughter cell is still diploid (46).

In meiosis, it's slightly different. You start with 92 chromatids, of pairs of 23 chromosomes, and first divide them into daughter cells (46 chromatids, but only 23 chromosomes!). In a second division, you remain with 23 chromosomes but all 4 daughter cells have a single chromatid each! Like so:

And then we have fertilization: when sex cells combine their genetic material, the mom chromatid 1 (in egg) and the dad chromatid 1 (in sperm) pair up to create a new chromosome, chromosome 1. n+n=2n! We've made a diploid baby!

Congratulations.