Chromosomes in DNA: Structure unclear

[one_raven]

DNA in Chromosomes: Structure unclear

I am a little unclear on the structure of chromosomes and the DNA within, and thought maybe you could point me in the right direction.
It's a little difficult to word this, so I hope it makes sense.

This site and most other sites basically state:

Chromosomes are paired threadlike "packages" of long segments of DNA contained within the nucleus of each cell. In humans there are 23 pairs of chromosomes. In 22 pairs, both members are essentially identical, one deriving from the individual's mother, the other from the father. The 23rd pair is different. In females this pair has two like chromosomes called "X". In males it comprises one "X" and one "Y," two very dissimilar chromosomes. It is these chromosome differences which determine sex.

OK, let's say there is a single complete strand of human DNA.
I read that it is about 6 feet long and has something like 3 billion base pairs.
There are 23 pairs of chromosomes in every cell (except reproductive cells).
What I am unclear on is whether each chromosome has a full six-foot strand of 3 billion base pairs, or if each chromosome has its own "section" of the full strand and if you add them all together you get a full strand (or, actually one maternal strand and one paternal strand).

I think each chromosome has it's own "section" of the full strand, but I can't find a definitive answer that says this.
If that is the case, are the chromosomes each their own individual piece as they are illustrated, or is that only done for illustration purposes and it is two long pieces in actuality?

If each chromosome has it's own separate section of the DNA strand, is there any place in the body (or any of our cells) that has a complete unbroken strand of DNA, or does it simply not come that way? Is the six-foot long 3 billion rung "ladder" just a visualization tool, rather DNA is actually 46 separate and distinct DNA molecules never actually being attached to each other?

Any help would be appreciated.

 

[Janitor]

I am pretty sure that a human somatic cell nucleus really does have a pair of chromosome number ones, and a pair of chromosome number twos, etc.

During one of the many waiting periods in the jury room of a trial I was called in for, I asked another juror, who was a community college biology teacher, if those pictures ("micrographs" or some such is probably better terminology) that show one chromosome as looking like a bulky letter X really are a single DNA molecule all woven together so that it doesn't stretch out too far in one dimension to fit in a cell. Her answer was, "Yes, it is one DNA plus some histones." I gather histones are a type of protein that bind to DNA.

So roughly 1/23 of the complete human DNA can be found by looking at just one chromosome. (I say "roughly" because chromosomes are not equally sized.)

Any errors in this explanation are the fault of the author alone.

 

[Moonbear]

Yes, Janitor, you have it right. Each chromosome contains a different segment of our total DNA. Each chromosome is a distinct "entity." A chromosome is DNA plus histones, which are proteins that help the DNA coil up, and they also are important for regulating gene transcription rates, but I suppose that's the next chapter ;-) However, that "X" shape you're thinking of is actually a replicated chromosome. That only happens as the cell prepares for division. Chromosome pairs each contain the same genes, but might have different alleles for those genes. However, the pair of chromosome ones is not attached to each other. And with regard to the first post, the pair of chromosome twos is not attached to any of the chromosome ones.

Does that help?

 

[one_raven]

Thank you both for your posts.

What about:

If each chromosome has it's own separate section of the DNA strand, is there any place in the body (or any of our cells) that has a complete unbroken strand of DNA, or does it simply not come that way?

 

[Monique]

The chromosomes are each separate strands of DNA, with their own identity. Everyone has a pair of chromosome 1, the largest of all, and a pair of all the other chromosomes. If you look through a microscope at a preparation of the chromosomes when they are condensed, you can tell the difference between chromosome 13 and 22 for instance. By directly looking at the chromosomes in this way you can actually determine if someone has Down syndrome or not: they will have three chromosome 21's.