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Regarding the human genome, if the DNA molecule is two continuous strands, with each gene occupying a segment in it, and there are 46 chromosomes each with some genes, where is the physical boundary of each gene?

My understanding of a molecule is a collection of atoms bound together by covalent bonds thus giving the substance its properties.

My reading so far gives me the understanding that the nucleus of a cell contains DNA, whose molecule is that famous double helix. At the same time, I read that the genes are packed into 46 chromosomes. This got me confused. How is the DNA molecule split into 46 parts?

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closed as unclear what you're asking by Bryan Krause, David, canadianer, kmm, another 'Homo sapien' Jul 23 '17 at 10:47

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ I don't really understand what you're asking. $\endgroup$ – canadianer Jul 22 '17 at 3:52
  • $\begingroup$ Ehh! Can you make it clear what you are asking. $\endgroup$ – Mockingbird Jul 22 '17 at 3:56
  • $\begingroup$ Isn't DNA one molecule? $\endgroup$ – Old Geezer Jul 22 '17 at 8:30
  • $\begingroup$ What research have you done to try to answer this question from available sources on the internet or in books? If you had completed the introductory guided tour you would be aware that you need to demonstrate such prior effort. This question is standard introductory material that can be found in numerous places on the internet. $\endgroup$ – David Jul 22 '17 at 9:13
  • $\begingroup$ Yeah, DNA is one molecule, but its nearly impossible to pack one such long molecule into chromosome. So, its split into 23 parts, and each part has a duplicate with it. I suggest you should keep studying... $\endgroup$ – another 'Homo sapien' Jul 23 '17 at 10:46
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To cite @canadianer, I don't really understand what you're asking but the following may help you.

Genome composition

First of, in the genome of most eukaryote (incl. all plants, fungi and animals), in general, genes constitute only a small part of the genome. Typically, in humans genes occupy 26% of the genome (24.5 for introns and only 1.5 for exons).

What does a genome look like?

Introduction

You say

Isn't DNA one molecule?

I think this question shows an important misunderstand you have. Note that I will only talk about the nuclei of eukaryotes below.

DNA is a type of molecule, which is constituted of two anti-parallel strands. In the human genome, we have 46 chromosomes, that is 46 independent DNA molecules. Now, these exact numbers vary depending on the moment in the cell cycle you want to consider but this is a story for another time.

A little more advanced

Below is a karyotype of a human allowing you to visualize the chromosomes. The 46 chromosomes we have go by pairs (pair of homologuous chromosomes) and on the image, they are numbered by pairs, the last pair being the sex chromosomes. By the way, can you identify the sex?

enter image description here

Note now that, depending on the moment in the cell cycle you want to consider, each homologuous chromosome may be constituted one 1 or 2 DNA molecule, each of them being called a chromatid. Chromatids are physically attached by the centromere. Two physically attached chromatids are called sister chromatids.

More information

For the moment, I threw some vocabulary at you but it was probably a bit hard to follow. You can get more information at the posts

or simply an intro course to genetics such as the following three courses by Khan Academy

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  • $\begingroup$ Thanks, so it's 46 different DNA molecules and not just one DNA molecule in the cell nucleus. $\endgroup$ – Old Geezer Jul 22 '17 at 15:42
  • $\begingroup$ Indeed, there 46 double-stranded DNA molecule in a human cell nucleus (although this number may be doubled depending on the moment in the cell cycle). $\endgroup$ – Remi.b Jul 22 '17 at 16:02
  • $\begingroup$ Actually, each double-helix is two DNA molecules, if you define molecule as atoms held by covalent bonds (as is typical). Each strand of the double helix is a separate molecule, but the two strands fit together so well that the (non-covalent) hydrogen bonding holds the two very long strands tightly together. $\endgroup$ – mgkrebbs Jul 22 '17 at 16:09
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    $\begingroup$ @mkgrebbs. Sure, this is why I said double-stranded DNA molecule assuming hydrogen bounds are sufficient to consider the two strands together a single molecule. I avoided this discussion to avoid further complication given how introductory was the OP's question. $\endgroup$ – Remi.b Jul 22 '17 at 16:12
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    $\begingroup$ @John Indeed, this is why, I said in a human cell nucleus. The OP might be aware that there is DNA in the mitochondria as he says ` in the cell nucleus` himself. Again, I wanted to avoid further complications here (even more so that DNA is not organized into chromosomes in the mitochondria). $\endgroup$ – Remi.b Jul 22 '17 at 16:14

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