[techmind (OP)]

I'm a physicist and electronics engineer. Please can you help me clarify the basics of all this molecular-biology/chromosomes/DNA stuff.   []

I know there's some good Wikipedia pages, but I haven't yet found one which gives a good overview - if you know of one, feel free to reply with a link to it.

(I'm principally interested in human side of things, although key differences in bacteria/viruses might be useful.)

Here goes:
As far as I can figure out, the chromosomes are at the top of the hierarchy. Apparently humans have 46 chromosomes, in 23 pairs.
There's some info here: http://en.wikipedia.org/wiki/Chromosome

Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 per cell.

This gives me several questions:

• Do all 46 chromosomes exist in (more or less) every cell?
• What's special about the (autosome) pairings? Are they physically paired in some way? Are the two chromosomes in each pair similar in some way?
• How does the X/Y sex chromosome fit in?
• Is there anything distinctive about the chromosomes - if someone gave you one of them, how would you know whether it was chromosome 5, 6, or 7? (I realise some of the lengths are more distinctive)

Each chromosome comprises one huge long chain of DNA - right?
Wikipedia has a very nice page on DNA: http://en.wikipedia.org/wiki/DNA
I'm reasonably confident I understand base-pairs G,A,T,C ; and that you have to pair A with T, and G with C. And that you can encode information by the sequence of base-pairs.

I kind-of get that you can make some sort of RNA copy that you then use to build proteins (reading the base-pairs in groups of three) - but I'm less clear about whether it matters which half (since they're complementary) of the DNA you use and what the implications are.

Now tell me, what is a 'gene'?

It seems to be a finite-length sequence of base-pairs within a DNA string...
So that begs the question What defines the start and end of a gene, within a DNA string?
http://en.wikipedia.org/wiki/Gene     doesn't seem too clear on it. Or is the answer just "it's complicated"?

In fact, what determines which end of the DNA string is the 'start' and 'end' (or head and foot in cinema-film parlance)? I understand that each half of the DNA has a 3'-end and a 5'-end and that that defines a directionality within half of it - but since the combined halves have one 3' and one 5' at each end, how do you define the overall 'start' and 'finish'?


Sorry if I'm asking some naive questions!

And finally, in principle (it might take another decade for the technology to be ready!) would it be possible for someone to extract the DNA from a flake of my skin or a drop of my blood and insert/swap it into another man's sperm, and make a baby for which I'd be the genetic father? Asked another way, is there anything 'special' about the DNA in a sperm, or is it exactly the same as in any other cell of my body?


I'm hoping you can give me a leg-up into this field. You might even have done a podcast on the topic. Links appreciated!  Thanks.

[CliffordK]

Most cells in your body will have the 46 chromosomes.
The major exception is that red blood cells have the nucleus removed, and thus no nuclear chromosomes.  Outer layers of skin cells, as well as hair cells are dead, and may not contain DNA.

Non-nuclear DNA?  The mitochondria in animals, and chloroplasts in plants have their own DNA, although it may not have the same chromosome organization that nuclear DNA has.  In humans, 100% of the mitochondria DNA comes from the mother, thus it can be used to follow maternal lineages.

There are 46 different strands of nuclear DNA in the cells.  They are really only visible as distinct chromosomes during cellular division. 

There are two different methods for recognizing chromosomes.  The classic method would be staining the chromosomes which creates a distinct banding pattern.  They can then be matched up with bands and length.

Perhaps the more modern method is DNA sequencing and PCR amplification.  Since the DNA sequence is relatively distinct, given a reasonable size chunk of DNA, one should be able to determine exactly where it came from.  The excepting, of course, is non-coding regions which can have high variability.

Classically genes are thought of what gives an organism distinct characteristics.  For example having blond hair, brown hair, or black hair.  Some features such as skin color may involve several genes.  Others such as cattle having horns or not would be controlled by a single gene.

As far as DNA, Genes would be DNA sequences that are transcribed to form various proteins, or have other cellular functions.  There is actually a precise translation between DNA and amino acid sequences.

Although, some of the genes may have other regulatory functions within the cells.

[techmind (OP)]

Thanks CliffordK.
I have more questions on chromosomes, but they'll (mostly) have to wait until this evening.

Of the chromosome-pairs, is it the case that one is from the father and one from the mother? Or is the separate identity of the parental DNA's completely lost when the child is made? Given that you can have genes whose effect 'skip' a generation, some remnants must live on?

And the effect of the parental DNA mixes on a gene-by-gene level in the next generation? Not chromosome-by-chromosome? How does that work?

[CliffordK]

For each chromosome pair, one chromosome is paternal, one is maternal, and could be matched up more or less with the chromosomes from the parents. 

However, due to gene crossover, the paternal chromosome will receive all the parts from the father, however, it would be a mix of the father's two chromosomes. 

Generally genes will be expressed on both chromosomes, so if one gene is defective, the other one will often take its place.

An interesting chromosome is the X chromosome.  Men get 1 copy, women get 2 copies.  So, to prevent over-expressing genes in women, one of the X chromosomes is mostly inactivated, so genes are generally only transcribed from one of the X chromosomes (either maternal or paternal).