Homologous chromosomes, sister chromatids, bivalents etc. explained
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- Опубліковано 27 січ 2021
- What's the difference between a pair of homologous chromosomes and sister chromatids? What is chromatin and what is a chromosome? The answers to these questions is in this video.
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I suddenly realised that I did not fully understand what a chromosome is. I read 3 different books on the matter and watched a bunch of videos on YT, but I was feeling something was missing. This guy explained it in 30 seconds (3:30).
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This is very clear. Just one question though, we have a total of 46 chromosomes in somatic cells (23 maternal and 23 paternal). Does this mean that we also have 46 individual strands of DNA in the nucleus and this DNA is not a long continuous strand that breaks into 46 parts to form the 46 chromosomes ? thanks!
Thank you! Yes, exactly that, you're absolutely right. Only during mitosis do these 46 molecules become visible and can be distinguished, otherwise they all appear to be in one 'lump' during interphase. But actually, as you say, they are 46 separate molecules of DNA.
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Thank you very much! I'm glad it was useful.
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Hope the exam went well!
Awesome! Would of loved meiosis covered as well in this to display the differences
Thank you! We'll hopefully do a separate video on meiosis in the future.
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This video is suggested by our Biology teacher
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11:50 how come they be 46 pairs of duplicated chromosomes, when two sister chromatids form 1 duplicated chromosome, so 92 chromatids means 46 duplicated chromosomes and so 23 pairs of duplicated chromosomes
Sir in S phase if the chromosomes are
duplicated does it mean the chromosome number of a human would be 92?
No, so after S phase, there are 92 chromatids (not chromosomes). The number of chromosomes is 46 as it was before replication. The number of chromosomes never changes. This is purely because 'duplicated chromosomes' and 'chromosomes' are still both confusingly called 'chromosomes'. It's just a terminology nuance really.
Thanks
But why do x and y chromosomes have different numbers of genes?
You can think of the Y chromosome as bearing the 'extra' genes that are needed by males. Everyone needs at least one X chromosome; males then need a Y chromosome in order for them to be male; females don't need it so just have another X chromosome instead.
Everything I understood greatly sir but one doubt, at last after replication, there are 92 chromatids ✅46 duplicated chromosomes(chromosomes as of expressed) or 23 pairs of duplicated chromosomes . You said it before as before replication 1chromatid and 1chromosomes . After replication 1chromosome and 2 chromatid so as of this only the number of chromatids change but not the no.of chromosomes am I right sir. Please reply and get me clear sir.
That confused me as well. I think after replication, there should be 46 chromosomes, 23 pairs of chromosomes, and 96 chromatids. As he said the chromosome number shouldn’t change after replication. Correct me if I am wrong please, but that’s how I get from the video. 😢
My apologies for the late reply. Yes, this is absolutely right. The chromosome number never changes. In every diploid cell of the body, there are always 46 chromosomes. There might be 46 chromatids, which means the cell hasn't replicated its DNA yet; or there might be 92, in which case the cell is about to divide. But the number of chromosomes is still 46.
Spot on!
Thankyou sir..... 😊🔥
11:35 actually, you still have 46 "chromosomes" but each sister chromatid has a twin = 92 "chromatids". Typically, chromosomes are counted by the number of centromeres and this stays constant upon DNA replication in S phase. After replication, the diploid cell can be said to have 46 "double-stranded" chromosomes.
I have a question also sir: does it mean that before replication the single/straight line chromosomes (not X shape) are not attached by a centromere? [as they exist in pairs]
And do chromosomes initially exist as single chromatids or as chromatin which later stains to form single chromosomes?
Initially, before replication, chromosomes exist as chromatids made of chromatin. Chromatin is just the name for the material. But the chromatids are not condensed, in fact they are all entangled with each other, forming a ball that resembles a bowl of spaghetti. But each chromatid, each molecule of DNA, is still separate and is not attached to any other ones. Think of it this way: take 46 pieces of string and clump them together into a ball - this is the state of the nucleus in interphase before replication. All the 46 pieces of string are separate - these are the chromatids. Whatever the string is made of - cotton, for example - is the material, the chromatin. I hope that makes sense.
Homologous, non-duplicated chromosomes (straight line chromatids, as you say) are not attached to each other in any way. Neither before replication, nor during, nor after. Only sister chromatids are attached to each other by a centromere.
Okay..might be stupid qsn but....sister chromatids are the replicated chromosome that is attached at the centromere and are genetically same but homologous chromosomes are a pair of chromosomes...(where 1 chromosome is maternal and has 2 sister chromatids and the other is paternal and also has 2 sister chromatids)..and homologous chromosomes have same set of genes but are not entirely genetically similar?)
Yes, this is all absolutely correct, except that homologous chromosomes don't necessarily have to be duplicated (i.e., contain two sister chromatids) - two single, non-duplicated chromosomes can still be called homologous.
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@11:41 YOU ARE INCORRECT
DNA REPLICATION TAKES PLACE IN S PHASE AFTER G1
CHROMOSOME NUMBER DOES NOT INCREASE BRO
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I have a question. I understood that after duplication 1 chromatid and 1 chromosome becomes 2 chromatid and 1 chromosome. In the video in prophase you say there are 92 chromosomes and 92 chromatids. Shouldn't it be 92 chromatids AND 46 chromosomes? I did not quite understand this so I could be wrong.
yes there should be correction. 23 pair, 46 chromosome & 92 chromatid.
I've had SO MUCH CONFUSION over this!!! Whether "chromosome" and "chromatid" should be used interchangeably and everywhere I go, every textbook I check, "chromosome" = "chromatid". I was so baffled cuz they'd say humans have 46 "CHROMOSOMES" and in my head ik damn well that a chromosome = 2 chromatids and ik we have 23 PAIRS of *chromatids* (or 23 chromosomes) so meaning we technically have 46 *CHROMATIDS*
So pretty much after DNA replication we have *92 chromatids* or *46 CHROMOSOMES* but the confusion didn't stop there. Now I was puzzled as to whether, on one chromosome, the sister chromatids were both of maternal or paternal origin...or whether one chromosome (2 chromatids) comprised of mum's and dad's chromosomes!!!
Yes, precisely!
I have a question... First you said that after duplication the chromosomes, it is called 1 .but I'm prophase how they become 92 ?
I'm afraid I don't quite understand your question. Before replication, 1 chromosome = 1 chromatid. Therefore, there are 46 chromosomes (or 46 chromatids) in all diploid cells in the G1 phase, so before duplication of DNA. In prophase of mitosis, so this is after all the DNA has been replicated, 1 chromosome = 2 chromatids. So one chromosome at this stage is made up of two identical sister chromatids. There are still 46 chromosomes, but this time there are 92 chromatids. 46 chromatids go to one daughter cell, and the other 46 to the other daughter cell. Is this what you're asking?
@@chembio3384 I think you made a mistake on the video, @11:51 it says 92 chromosomes for meiosis prophase. Otherwise, brilliant video.
I counted,,,,,,,
Dear Sir,it is true that homologus chromosomes are exactly identical in all respects but how you will differentiate them as which chromosome is maternal or paternal.
As far as I know, there isn't any technology (yet) that can identify a particular chromosome as paternal or maternal in a cell from a tissue specimen, if that's what you mean? But what you could do is the following: suppose that height is under the control of one single gene. An individual is tall and heterozygous. If the individual's father is short, then the paternal chromosome of this individual will contain the recessive allele. The maternal chromosome will contain the dominant allele, and the mother is therefore tall. So you could determine which chromosome is maternal and which is paternal in a homologous pair by applying the principles of genetics, though you would need some way of identifying the dominant/recessive alleles. Is this what you're asking?
Sir, i absolutely agree with you that their is no technique to find out which chromosome in a homologous is maternal or paternal, though i think it ia possible in case of y chromosome of male in xx and xy system.
@@surindernath1969 Yes, you're absolutely right, the X and Y chromosomes can be distinguished visually even on a micrograph.
But it cannot have 92 chromosomes after replication but 92 chromatids or still 46 chromosomes, isn't it? Because
1 chromosome = 2 chromatids also
Yes, you're absolutely right. After DNA replication, there are 92 chromatids, or 46 (duplicated) chromosomes. If I said otherwise in the video, it's a mistake.
5:05 shouldn’t it be 92 chromatids in 46 chromosomes not 46 chromatids
No, before S phase, there are 46 chromatids in a diploid cell. 46 separate molecules of DNA. 46 chromosomes. After replication, there will be 92 chromatids, but still 46 chromosomes.
'WHY DONT WE CALL SINGLE CHROMOSOMES " CROMOSOMES" AND DUPLICATED CHROMOSOMES "CHROMATID" ...DOES THAT MAKE SENCE TOO YOU? BECAUSE THAT SOUNDS LESS CONFUSING TOO ME .
In fact, it would make more sense to call single chromosomes "chromatids" and duplicated chromosomes "chromosomes". However, we don't get to decide what terminology the scientific community uses, I'm afraid.
@@chembio3384 OH I THINK WE DO , LESS CONFUSING MORE COMON CENCES
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what the fuck why does no source mention how chromosomes are still counted as a single chromosome even after being doubled
I know, it's surprising that most A level textbooks seem to miss this point. I guess that before DNA replication, you can't see chromosomes under the microscope; the genetic material exists as a 'bowl of spaghetti'. There are still 46 molecules of DNA though, and some people call one such molecule a chromosome. Once duplicated, it still has the same name... but now it's condensed and becomes visible. This is how chromosomes were first discovered, and the same name has just been used ever since to mean both things.
Thank you so much!! I've been agonising on how to draw the difference of chromosomes and chromatids 😭 now I know that tge X shape is still considered to be 1 chromosome. 8th grade science is confusing..🥲
Ty for explaining it well!!! :))