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I understand that this is a single chromatid, but would this be considered a chromosome? Also before mitosis, the chromosomes appear as single chromatids but during interphase they replicate to form sister chromatids which are attached by a centromere and are also collectively known as a chromosome. I do not understand the difference between the two of them. Are they both to be termed chromosomes?
Here is the thing, chromosome refers to a lot of different things ( however, all are coiled up DNA).
Let me explain step by step. You have DNA, it has a diameter of 2nm, it is coiled with histone proteins and make 10-11nm fiber. This is what we refer to as the chromatin fiber(Beads on a string). At this point we are in interphase where too much coiling is not required because a lot of transcription, replication going on. At the S phase of the interphase, DNA replication takes place and sister chromatids are formed or do they? The answer is yes and no. The thing is even though DNA has replicated and coiled upto certain extent, we dont call the replicated dna as sister chromatids because we only consider them to be sister chromatids when we can clearly see them as separated threads connected at centromere during the M Phase. So, to put it in simple words, you have the sister chromatids formed (during the S phase) long before you are actually calling them as sister chromatids, you just wait for them to condense further and when you can finally see them as distinguishable threads you call them as sister chromatids.
Now, your real question can be answered. You have posted two chromosomes, one green and the other is red, red has two chromatids connected at centromere. Lets start with the red chromosome. The red one is the metaphase chromosome. Try to remember cell cycle. You have your DNA replicated by the S phase of interphase, then comes G2 and then M Phase starts with prophase, at this point your chromatin fiber gradually starts getting condensed and by the end of prophase, your 10nm coiled DNA (Chromatin fiber) ends up coiled to 700nm fiber but remember, you have two 10nm fibers connected at centromere after the S phase but they were so thin that you could not distinguish the two threads separately, now you can because they coiled in a very organized manner and now you have two 700nm fibers attached at the centromere, this is the maximum condensation achieved and as this final condensed version is observed at the metaphase it is named metaphase chromosome.
So, the red one is definitely a chromosome. What about the green one? That too is a chromosome. the green one is formed at the next step of cell cycle that is anaphase. Here the two sister chromatids split at the centromere and move towards the opposite poles of the cell as new chromosome. but wait, haven't I called them sister chromatids throughout the answer and suddenly calling them chromosomes? Yes, the previously called sister chromatids are called as individual chromosomes the moment they are separated from each other at anaphase.
So in summary, the thread like structure that you see in interphase are coiled DNA(Not called chromosomes), chromosomes are also coiled dna but the difference is that these are way more coiled. The red one is metaphase chromosome (1400nm) with two sister chromatids(700+700=1400). The green one is also chromosome but these are actually sister chromatids that got separated at anaphase(only 700nm), you can call them as daughter chromosomes
I hope this helps
Before S-phase the nucleus has two copies of each chromosomal DNA, e.g. a pair of Chr-1 DNA molecule.
During S-phase each DNA molecule replicates and after condensation ends up forming a chromosome(dyad) with two chromatids (sister chromatids). At metaphase a cell has 46 dyads (considering mitosis/Meiosis-I).
This dyad then separates at the centromere in Anaphase and the two chromatids move towards two opposite poles. On reaching the poles they are called chromosomes.
The word for such chromosome is dyad, but it is relatively less in use I would say.
