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Chromosome, Chromatin, Chromatid – What is the Difference?

Chromosome, Chromatin, Chromatid – What is the Difference?

To be honest with you guys, I use to get really confused between the terms – chromosome, chromatin and chromatid. So one day I decided no matter what, I am going to clear them up. And guess what? When I understood the difference, I realised 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 easy 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

Chromatin

a. Chromatin

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 

 

                                          b. Chromosome                                                             

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)

  • Chromatids = Duplicated Chromosome Attached at the Point of Centromere

 c. Chromatids

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. Lack of my understanding of course  🙁

Watch this video for better understanding.