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does one DNA molecule = one chromosome?


paul
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In eukaryotes the chromatids consist of a single DNA molecule. So if it is unreplicated it corresponds to the number of chromosomes, but after the S phase the the chromsome consists of two sister-chromatids, so the number of chromsomes remain the same, yet the number of DNA molecule doubles (one for each chromatid).

 

Edit: In prokaryotes one chromosome always consists of a single DNA molecule. Mostly circular, but there are exceptions.

Edited by CharonY
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In eukaryotes the chromatids consist of a single DNA molecule. So if it is unreplicated it corresponds to the number of chromosomes, but after the S phase the the chromsome consists of two sister-chromatids, so the number of chromsomes remain the same, yet the number of DNA molecule doubles (one for each chromatid).

 

Edit: In prokaryotes one chromosome always consists of a single DNA molecule. Mostly circular, but there are exceptions.

 

.......to clarify further, there are 23 pairs of chromosomes in each (normal) human cell.

Plus, we have mitachodrial DNA, which is much like the prokaryotic DNA Charon described above.

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  • 2 months later...

Also,

A bacterium may have a chromosome and plasmids.

So there is some distinction based on size of the molecule.

In some cases the distinction between chromosome and plasmid gets a little fizzy, so like many other concepts it depends on what you mean.

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or does one DNA molecule = all the chromosomes, ie, all the genetic material in our cells?

 

 

Hello. First time here. This is interesting because I just started studying this and was just learning about this particular thing. My understanding is that a single (1.8 meter long?) dna molecule is crumpled up to make one chromosome. This would mean that each human nucleus has 46 dna molecules in all.

 

What I'm confused about at the moment is, does this mean that before duplication, the nucleus only has 23 chromosomes? I haven't gotten that much straightened out yet, still reading...

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So there is some distinction based on size of the molecule.

In some cases the distinction between chromosome and plasmid gets a little fizzy, so like many other concepts it depends on what you mean.

 

Actually the distinction is not based on size, but on the existence of essential functions. In other words, if e.g. non-redundant housekeeping genes are located on a plasmid, it would be considered a chromosome.

 

Vosh, only the gametes have 23 chromomsomes after fertilization (fusion of two gametes) you got 46. However, each chromosomes consists of one (before S phase) or two sister chromatids.

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Actually the distinction is not based on size, but on the existence of essential functions. In other words, if e.g. non-redundant housekeeping genes are located on a plasmid, it would be considered a chromosome.

 

Vosh, only the gametes have 23 chromomsomes after fertilization (fusion of two gametes) you got 46. However, each chromosomes consists of one (before S phase) or two sister chromatids.

 

You mean that before that phase (which I'll have to look up, never heard of S phase, seems like everywhere I look there is either a new thing, or the same thing is referred to by a different term...), a single dna molucule, which coiled up is a chromosome, is by itself, but after the S phase it gets joined to another via a centromere? Many thanks.


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or does one DNA molecule = all the chromosomes, ie, all the genetic material in our cells?

 

Well, I googled it, and apparently the term "chromatid" is used to refer to duplicated chromosomes that are joined by a centromere (forming that x shape). So, a dna molecule coiled up is a chromosome (and I think it was called that before it was known to be dna, but someone can sharply correct me on that), when the chromosome duplicates, the two copies are referred to as chromatids, or sister chromatids. When the two are separated, they are then called daughter chromosomes. They're always

"dna" and "chromosomes", but there are these different terms for referring to those different situations... I guess is one way to look at it.

 

So, in answer to the question (again - I'm learning by thinking out loud about this), a chromosome is one dna molecule coiled up (so we can see it). When it replicates, it's two chromsomes, two dna molecules, but we call them chromatids, and when they're pulled apart, they're referred to as daughter chromosomes.

 

Hang on... that means that somewhere in there, between replication and division, there are 92 chromosomes. The heck?

 

Thank you for your time.

Edited by vosh
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They are not called differently, they are just different level of hierarchy. Think of it that way. The chromosome is always the whole structure, regardless whether it consists of one or two chromatids. Likewise during the S-phase (you really need to read up on cell cycles, it is basic knowledge) it is more correct to state that the chromatid replicates rather than that the chromosome duplicates. Because the result is still one single chromosome, only that it consists of two chromatids again. I know wikipedia describes it differently, but in my opinion that description is rather messy.

The confusion arises from the fact that chromatids, once separated are referred to as chromosomes, as each cell requires to have the full set of chromosomes. Referring to them as single-chromatid chromsome would be more precise.

Use this as starting point and revisit your assumptions once more.

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They are not called differently, they are just different level of hierarchy. Think of it that way. The chromosome is always the whole structure, regardless whether it consists of one or two chromatids. Likewise during the S-phase (you really need to read up on cell cycles, it is basic knowledge) it is more correct to state that the chromatid replicates rather than that the chromosome duplicates. Because the result is still one single chromosome, only that it consists of two chromatids again. I know wikipedia describes it differently, but in my opinion that description is rather messy.

The confusion arises from the fact that chromatids, once separated are referred to as chromosomes, as each cell requires to have the full set of chromosomes. Referring to them as single-chromatid chromsome would be more precise.

Use this as starting point and revisit your assumptions once more.

 

I see. "Chromosome" refers to the structure we can see under a light microscope of scrunched up dna, either as a single or pair of sister chromatids; is that a good way to say it? Perhaps this is because these are the structures that could be seen via dyes. I'll start from here... I can hear someone saying, "but that website defines a chromosome as a single molecule of DNA...". The websites and the books out there don't seem to have a good starting point. It's good to be able to talk to a phd; someone actually working in the field. Very much obliged.


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When shall we tell him about chromatin? >:D

 

Nooooooooooooooooooooooooooo!

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or does one DNA molecule = all the chromosomes, ie, all the genetic material in our cells?

 

Each chromosome is one DNA molecule. We have 23 pairs of chromosomes (1 of the pair from each parent) for 46 chromosomes. So we have 46 DNA molecules per cell.

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Actually it depends on the phase. After S (as I mentioned earlier) a single chromsome consists of two DNA (identical) molecules. So double the amount for that time.

 

Sigh. Sometimes you guys get a bit too nitpicky. :) The OP was clearly referring to G1 phase. paul clearly was confused about the relationship between chromosome and DNA molecules. If he knows that one chromosome = one DNA molecule, then paul (when he comes to learning S phase) will clearly realize that copying the chromosome will mean that there are 2 DNA molecules at that point. And, in the mitchondria, one chromosome = 1 DNA molecule there, too. So when paul gets to mitochondrial DNA, he will be able to work it out.

 

Please, try to understand what the question is asking and then answer it in the simplest terms that are still sufficient. That will be a lot more help than going into unnecessary details. Try to focus on the needs of the questioner and not either showing off how much you know or scoring "points" off me when I don't give every possible permutation.

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Sigh. Sometimes you guys get a bit too nitpicky.

This is the second time today you've called the comments of others "nitpicky" (the first time was in response to swansont). I can't help but to chuckle, as I wonder if you've ever read your own posts here which very frequently suffer from that very issue. :D

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what does nick picky mean

also DNA molecules are in chromosome when a fetus is first devloping in the womb

plus the egg and the sperm carry 23 chromosomes each or am i cumfused with something else

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what does nick picky mean

also DNA molecules are in chromosome when a fetus is first devloping in the womb

plus the egg and the sperm carry 23 chromosomes each or am i cumfused with something else

 

"Nits" are the eggs laid by lice. The eggs are tiny, and are attached to the base of a hair shaft. Thus, picking at nits (or "nit picking") refers to picking at something insignificant, something that does not matter.

 

DNA is normally in the form of euchromatin: it only condenses into chromosomes when it is time for the cell to divide. "Chromosome" refers to the condensed version, but euchromatin and chromosomes contain the same DNA, just different proteins are attached to wind it up.

 

The egg and sperm (of humans) each have 23 chromosomes.

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Each chromosome is one DNA molecule. We have 23 pairs of chromosomes (1 of the pair from each parent) for 46 chromosomes. So we have 46 DNA molecules per cell.

 

Technically, a double-stranded DNA is really two complementary DNA molecules. So then we have 92 DNA molecules composing our chromosomes, as 23 pairs of double-stranded DNA.

 

Sigh. Sometimes you guys get a bit too nitpicky.

 

As GDG said,

"Nits" are the eggs laid by lice. The eggs are tiny, and are attached to the base of a hair shaft. Thus, picking at nits (or "nit picking") refers to picking at something insignificant, something that does not matter.

So its only nit picking in the metaphorical sense :P

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