[DoctorBeaver (OP)]

I'm reading up on them at the moment as I'm trying to learn more about Chromosome22 Ring. There's something basic that I don't understand. In the label 16p13.3, for instance, I know that the 16p refers to the shorter, upper arm of chromosome 16 & I think that the 13 is the specific locus. But what is the .3 on the end? What does that refer to? I can't find it anywhere.

[chris]

Hi Eth

you're quite right. Chromosomes usually have a long arm (termed q) and a shorter one, termed p. An easy way to remember it is that petit, in French, starts with p, and means small. A chromosome is an individual molecule of DNA containing up to several thousand genes, interspersed by non-coding (historically referred to as junk) DNA.

Normally cellular DNA is in the form of chromatin, which is thin and wispy and difficult to see. But when the cell divides it condenses (shrinks) the chromatin into a heavy chunky form that is easy for the cell to manipulate during the division process. These so-called metaphase chromosomes can be collected from a cell and stained with a special dye (known as Giemsa staining) which produces a characteristic banding pattern of light and dark areas along the chromosomes. This enables the chromosomes to be identified morphologically, and matched up with their parter (each adult cell, except red cells and germ cells) contains 2 copies of each chromosome, one from your mother and one from your father.

The chromosomes get their names according to the banding pattern. The first number (1-4) refers to the band region, and the second number (after the dot) refers to the individual bands within that region.

This website has some useful pictures which might help :

http://www-biology.ucsd.edu/classes/bimm110.SP05/lectures/L08.05_Cytogenetics.htm

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[DoctorBeaver (OP)]

quote:The chromosomes get their names according to the banding pattern. The first number (1-4) refers to the band region, and the second number (after the dot) refers to the individual bands within that region.

Thanks for that, Chris. 1 point, though - if the band number can only be 1-4, how do you get 16p13?

[chris]

Okay, I didn't make that clear, sorry.

The first number is the chromosome number. 16 is chr. 16

The second number is the band number, 1-4.

Each band is divided into smaller sub-bands, but these generally aren't referred to.

Sorry for the confusion

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[DoctorBeaver (OP)]

Sorry, Chris, but I'm still confused. If the band (2nd) number can only be 1-4, how come in the example I used the 2nd number is 13?
Can you use the 3 numbers in my example (16p13.3 - which I got direct from a chromo research website, so it's not made up) & explain what each of the 3 numbers means?
There was a diagram on the site I looked at which showed the different bandings & lines, but as you said there were only 3 bands in some cases & 4 in others.
I take your point about sub-bands as most of the literature I've read refers to, for instance, 16p13.

[chris]

Sorry - I was trying to remain consistent with the website I referred you to. it depends upon which chromosome you are talking about. Some are much larger (eg X) than others (eg Y).

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[DoctorBeaver (OP)]

OK, Chris, thanks. I get it now

[rtv3]

That is not entirely correct...
16p13.3

16    The chromosome number.
p     The position is on the chromosome's short arm (p for petit in French); q indicates the long arm.
13.3  The numbers that follow the letter represent the position on the arm: band 1, section 3, sub-band 3. The bands are visible under a microscope when the chromosome is suitably stained. Each of the bands is numbered, beginning with 1 for the band nearest
      the centromere. Sub-bands and sub-sub-bands are visible at higher resolution.

Don't comment on things you don't understand

[imatfaal]

Wow - six year wait and then a barbed comment. 

[chris]

Mmm - I'm quite surprised; the answer supplied was not actually incorrect, so I'm unclear what the provocative dig is all about.

Anyway, with an air of magnanimity, welcome, rtv3. I hope that you'll continue to contribute to our vibrant and friendly forum.

Chris