Chromosome, Chromatin, Chromatid - What is the difference!!!
To be honest with you guys, I use to get really confused between these three terms. So one day I
decided no matter what I am going to clear them up. And guess what? When I understood the
difference I realized that I already knew these things but didn’t actually knew how to differentiate
them. Funny ha!! So I thought why not share it with you all. Hope it will be helpful.
These terms sound very similar and that is why it can be confusing sometimes. So let’s start from
chromatin and then will move to chromosome and then to chromatid. Why this particular order!
You will know once you read it. It becomes very to understand in this order.
We know that DNA is our genetic material. But DNA is too long (about 2m) to be fit inside the
small nucleus (5μm - 10μm in diameter). So in order to fit inside the nucleus, DNA has a packaging
strategy. And this strategy involves wrapping of DNA around the histone proteins. So our double
stranded DNA would wrap around histone proteins and it looks like show in figure a. This
assembly where we have DNA and histone (DNA + Histone) looks like thread is called
CHROMATIN. And this DNA in chromatin phase is not condensed yet.
Chromatin = DNA Wrap around Histone Proteins OR DNA + Histone Proteins
Now when cell undergoes cell division, the DNA will start getting condensed (remember in
chromatin phase as we just saw DNA is not condensed). So this condensed form of DNA would
appear like in figure b and this form is called CHROMOSOME.
Chromosome = Condensed form of DNA
Figure a. Chromatin
Figure b. Chromosome
Figure c. Chromatid
Now we know that when the cell is dividing, each chromosome needs to replicate and make its
identical copy. Because the newly divided cell should have the complete set of chromosomes. To
achieve this each chromosome duplicates itself and they will be attached at the point of centromere
which looks like shown in figure c. This assembly where we are observing DUPLICATED
chromosome initially attached at the point of centromere, each strand in that or copy in that is
called CHROMATID. We sometimes refer to it as sister chromatids when we talk about
recombination right!! So the chromatid is nothing but duplicated chromosome which has not yet
separated. Now when cell will divide each of the new cell will get one copy of chromatid and this
chromatid will act as chromosome itself in that new cell. (The same chromatid would decondenses
into a chromatin strand)
Chromatid = Duplicated Chromosome Attached at the Point of Centromere
Time for a true story now. For a really long time while I was studying, in mind chromosome was
like shown in figure c whereas actually it’s the duplicated chromosome or two chromatids but that
is how I thought it looked like. Reason was, that is how we use to observe in karyotyping
metaphase stage where the chromosomes have duplicated but yet not separated. My lack of
understanding of course 
To view a video on this topic from my YouTube channel, click on http://youtu.be/Jd2uMAsDfPg