How is DNA arranged in a human cell?

[barryj]

I have read many sources on DNA and still have a few questions. Let me start with this one, and depending on the answers I will ask more.

Question 1. f I were to look into a human cell how would I see the DNA? Is there a single very long copy of the DNA string or are there two copies.

Question 2. Are the chromosomes located along the DNA string or are they individual pieces of DNA, i.e. are there 46 pieces of separate DNA each representing a single chromosome?

In summary, how many string would I initially see? 1, 2, 46 or how many?

 

[BillTre]

Each chromosome will be a single intact DNA molecule. In a diploid organism, there would be two for each chromosome pair (one for each individual chromosome).
In addition, there would be smaller DNA molecules in the mitochondria and if it has chloroplasts those would also have their own DNA molecules. These are endosymbionts derived from ancient bacteria that started living inside other cells and evolved into today's eukaryotic cells. These would not be fixed numbers since different cells have different numbers of mitochondria.
There might also be small pieces of DNA floating around in the cell like plasmids.

What would they look like?
Usually not like strings. This can be done in lab preps, but is not usually natural. To conveniently fit within the cell and protect from damage, the DNA would be packed into a smaller space by winding around histones, which in turn would get clumped together into higher level organizations.
conceptual picture (source: https://www.sciencelearn.org.nz/images/198-dna-chromosomes-and-cells):

Picture of fluorescently labeled DNA :

The mitochondria's amount of DNA is below the required resolution to shown up in this picture (source: https://en.wikipedia.org/wiki/Cell_nucleus).
The cell on the left has its chromosmes condensed and somewhat separated. It looks like it might be getting ready to divide

 

[Ygggdrasil]

Each chromosome is a separate piece of double-stranded DNA (in complex with the many proteins associated with the DNA that help package and organize the DNA). During most of the cell cycle (interphase, when the cell is not actively dividing), each chromosome is fairly spread out throughout the nucleus, so it is somewhat difficult to identify each distinct chromosome (though there is some organization to the individual chromosomes, see: https://www.nature.com/scitable/top...tories-the-arrangement-of-chromosomes-in-3025). Depending on the stage of the cell cycle, a normal human cell will contain either 46 (a parental and maternal copy of each of the 23 chromosomes) or 92 (after DNA duplication) individual double-stranded DNA molecules in the nucleus.

Understanding how DNA is organized within the interphase cell is a major area of active research.

 

[Andrew Mason]

Not all human cells contain nuclear DNA - red blood cells contain no nucleus at all.

The total human genome consists of 46 chromosomes in 23 pairs. Each chromosome contains separate strands of DNA. Each human nucleated cell, except for sperm and egg cells, contains all 46 chromosomes (23 pairs - diploid cells). Haploid (sperm or egg) have 23 single (unpaired) chromosomes.

Together the 46 chromosomes contain 3 billion nucleotide pairs and stretched out end to end the DNA would be about 3 m. or 10 feet long. So the DNA obviously has to be compacted in order to fit into a single cell (i.e. about 1/40th of a mm long or 1/120,000th of the stretched length). The DNA is strands are wrapped up around protein complexes (histones) and these are visible as chromosomes.

So, in summary, in a diploid cell you would see 46 separate pieces of DNA all wrapped up and seen as 23 chromosome pairs. In a haploid cell, you would see 23 separate pieces of DNA all wrapped up into 23 unpaired chromosomes.

AM

 

[barryj]

OK, so correct me where I am wrong. In one of my normal human cells, there are 46 individual pieces of DNA representing the 23 pairs of chromosomes.

So, if I wanted to knockout a specific gene, then I would have to identify the two chromosomes that contain the gene, then find the gene within the chromosome (somehow) and then take them out using CRISPR or other technique.

 

[Ygggdrasil]

OK, so correct me where I am wrong. In one of my normal human cells, there are 46 individual pieces of DNA representing the 23 pairs of chromosomes.

So, if I wanted to knockout a specific gene, then I would have to identify the two chromosomes that contain the gene, then find the gene within the chromosome (somehow) and then take them out using CRISPR or other technique.

Yes, that is correct. CRISPR is usually efficient enough to knockout both copies of a particular gene, though it is definitely possible to knock out only one copy. If the gene is essential to cell function or survival, one may find only heterozygous knockout cells (only one copy knocked out) and no homozygous knockout cells (both copies knocked out) after the CRISPR experiment (this is the case when I've tried to knock out one of the genes I study).