Thanks for commenting, but…I hate to give bad news: the backward writing is just a videography trick. After filming, the video is flipped across the vertical axis so that the writing looks forward (but I'm reversed). In other words: I'm not left-handed.
@@JosephRoss oooooh.... I see. Came to the comment section to speak out kudos for mirrored writing skills. Now I'd change that into kudos for videography skills. 😅
I'm not familiar with this problem, but a quick web search suggests that some Windows operating systems can have this issue when playing videos (e.g. www.thewindowsclub.com/green-screen-playing-videos-windows)…so hopefully (I assume) this is not a problem specific to this video. But I'm glad you were able to get something out of the video anyway!
hi. so i think i get what u mean about the 92 chromosomes part but i need clarification because this stuff is pretty confusing. So before the cell goes through S phase, there are 46 chromosomes (more specifically 46 chromatin) in the nucleus of the cell. I believe in the video, you are trying to say that in S phase, the 46 chromatin in the nucleus replicates to a total of 92 chromatin (but formally speaking, there is now 92 chromatid). even though there is technically 92 chromatin in the nucleus (after s phase), there are actually 46 chromatin. this is because each chromatin plus its replicate (total of 92) is joined by a centromere, which makes 1 chromatin ....so technically the 92 total copies of chromatin, is formally referred to as 92 chromatid. is my interpretation correct??? please someone help bio killing me
It would help me if we made distinction between: - a cell being produced from cells of my parents - a cell being produced from another stable cell of my body. - an autosome vs a sex chromosome XY It wasn't clear to me what stage of the cell you assumed at different points in this video. How does the DNA replicate in case1 given it has two different sources? Could you take some chromosome-pair as example, say chromosome#5 and the sex chromosome#23 and tell how each pair divide & replicate the DNA in normal body scenario; and how each pair divide & replicate/transfer DNA in sexual reproduction of another cell. I would be grateful for any guidance or material.
You appear to be confusing ploidy with c-value. A chromosome that consists of two chromatids is still considered to be ONE chromosome. The number of chromosomes does not change following S-phase, the c-value does. I see this mistake made commonly by instructors and even some text books and it confuses the hell out of students.
Good point to clarify. I had a good conversation with one of my students about this question last semester. Technically, "ploidy" is defined as the DNA content during interphase before replication. Therefore, any diploid (2n) organism is always diploid. However, I'll respond to your question with a question: if you have a diploid (2n) cell and it is going to divide, then what has to happen first? It has to replicate its DNA. So, S phase does result (although only temporarily) in a cell that has four copies of every chromosome (called sister chromatids at the time, though…). Then, as soon as cell division occurs, each of the two resulting daughter cells is 2n (has two copies of every chromosome).
@@itsdharun That means it temporarily has 4 copies of each chromosome…but that's not 4n. It is still a cell from a diploid (2n) organism - it just temporarily has twice as much DNA as normal.
You are correct - good catch! The point I was trying to make is that the number of double-helices in the cell doubles from 2c to 4c. Indeed, ploidy is defined during interphase.
Hi - thanks for the comment. In response: it depends on what you (and I) mean by "chromosome number." If, for example, in humans, you would argue that we have 23 chromosomes (autosomes 1-22, and then the sex chromosome), then I would agree that the chromosome number remains the same (we have 23 different nuclear chromosomes). However, the point I'm making here is about how many copies of each of those chromosomes exist in a nucleus before and after synthesis. Then we can continue to discuss the difference between a chromosome and a sister chromatid, if we want to be really semantically accurate. In general, the main idea is that the number of chromosomes (which I define operationally as double-stranded DNA molecules) doubles (say, in humans) from 46 to 92 in S phase, and then the completion of mitosis produces two diploid (2n=46) daughter cells.
@@JosephRoss During S phase, chromosome number (designated as "n") does not change but actually DNA is replicated or doubled (designated as "N"). Chromosome number before and after S phase or DNA replication will remain be the same, however DNA number will be changed to double after S phase. We will say that before DNA replication the chromosomes had only 1 chromatid and after replication the chromosomes have 2 chromatids (called sister chromatids). Now, sister chromatids of every chromosome will go separately into each of the 2 daughter cells in case of mitosis and return to their 1 chromatid condition as before cell division. Sir, with due respect you have got wrong definition of the chromosome. We can never say that chromosome number is doubled after S phase. A sister chromatid is not called a chromosome but for a sister chromatid to be called a chromosome, it has to be detached at the centromere from its sister chromatid counterpart (as what happens in anaphase of mitosis).
@@muhammaduzair3659 I understand your point, which I think is valid. Since you're responding to my response to a similar comment, I also think that you already understand the key point I am trying to make: DNA content (regardless of whether you call a DNA double helix either a chromosome or a sister chromatid) doubles during mitosis. My goal with this video is to make sure that my students understand this aspect of genetics.
@@geetachauhan5329 My interpretation of your comment is that you are agreeing with my original point, that DNA content doubles in a cell after mitosis. This is irrefutably true. However, part of the point that the others have made earlier in this thread is that it is inaccurate to refer to post-synthesis cells as "tetraploid" (4n). Formally, n refers to the number of homologous chromosomes in a nucleus. Many species, like humans, are diploid: they have two homologs (e.g. one version of chromosome 1 inherited from the father, another version from the mother). This is designated by the terminology "2n." After synthesis, there are still only two versions (homologs) of each chromosome in the nucleus - hence still 2n. The way that geneticists have traditionally designated DNA content is with "C." So, before synthesis, a human nucleus is 2n (diploid) and also 2C (2 copies of each chromosome, for example the 2 homologs of chromosome 1). After synthesis, the nucleus is still 2n (as explained above), but DNA content has doubled from 2C to 4C. To summarize, most geneticists would formally state that, before DNA synthesis, a nucleus is 2n, 2C; afterwards, it is 2n, 4C.
hi. so i think i get what u mean about the 92 chromosomes part but i need clarification because this stuff is pretty confusing. So before the cell goes through S phase, there are 46 chromosomes (more specifically 46 chromatin) in the nucleus of the cell. I believe in the video, you are trying to say that in S phase, the 46 chromatin in the nucleus replicates to a total of 92 chromatin (but formally speaking, there is now 92 chromatid). even though there is technically 92 chromatin in the nucleus (after s phase), there are actually 46 chromatin. this is because each chromatin plus its replicate (total of 92) is joined by a centromere, which makes 1 chromatin ....so technically the 92 total copies of chromatin, is formally referred to as 92 chromatid. is my interpretation correct??? please someone help bio killing me
Yes, during S phase, each of the 46 chromosomes is duplicated (totaling 92 chromosomes - although after S phase they are referred to as sister chromatids, which do eventually pair at their centromeres during mitosis). These concepts seem like they should be simple, but there are a lot of details here that make truly understanding all of this very complicated.
