Expert: John Locke - 5/13/2007

Question
hi Mr.Locke,
I tried to ask barbara a question, but she thought it's my homework, so she didn't answer it. It's not my homework but its for my BIO AP which i will sit tomorrow morning. i would appreciate you answering my questions if you can, and maybe give me some advice of what to focus on for the AP (if you have any).

1) Is the barr body the structure that deactivates one of the female X-chromosomes, or is the barr body the actual inactive female X-chromosome ?!

2) What is gel electrophoresis and how can i solve proplems related to this topic ?!

3) what is theshold potential, and what does it have to do with membranes ?!

Answer
Hi Cristian. I understand that you are serious about having this answered, and also understand that this is not simply a homework question. Let me get that out of way right away: I will answer your questions, though this will wind up being a rather long answer (be forewarned!). Many of the experts here are rather zealous about not answering homework questions (me included); the unfortunate side effect is that non-homework questions sometimes get caught up in their refusal. You did exactly the right thing: if your question is refused once, ask someone else and indicate as you have why your question is not simply a homework question.

Now, to your specifics...

1. The Barr body is the deactivated X-chromosome itself. This question is actually on the leading edge of research in the genetic field, as the exact mechanism by which the deactivation occurs is not exactly certain. I'm providing a few links below with more information on the subject (full-text versions are available for each of these online). You may Google them to find the full-text versions.

Brown,C.J., Robinson,W.P., (1997), XIST Expression and X-Chromosome Inactivation in Human Preimplantation Embryos. Am. J. Hum. Genet. 61, 5-8

Alberts,B., Johnson,A., Lewis,J., Raff,M., Roberts,K., Walter,P., (2002), Molecular Biology of the Cell, Fourth Edition, (428-429) Garland Science, 0-8153-4072-9

2. Gel electrophoresis is an essential microbiology technique for separating the same cell components of different composition (i.e., proteins with different weights or strands of DNA or RNA with different weights; only DNA or RNA or proteins are separated in any single electrophoresis gel, but each separate piece of DNA/RNA/protein has a different weight). The components are separated by some other method like centrifugation or chromatography, then forced through a gel-like matrix by an electric current. They move different distances of the basis of their size--large, heavier segments get caught up in the matrix and don't move as far. Smaller, lighter segments move more quickly through the matrix. The distance that each segment moves can be compared to a standard--that is, a set of measurements that tell you how far certain proteins/DNA/RNA tend to migrate in a particular gel. On that basis, the protein/DNA/RNA can often be identified.

Usually, the pieces of DNA/RNA/protein are washed with a detergent before being loaded into the gel; this makes certain that they all have the same charge (the pieces will be forced through the gel by an electric current; if they had different charges, both charge and weight would determine how far they move, but we are usually only interested in their size, alone).  

In terms of questions related to electrophoresis, I suspect that you'll be comparing known diagrams to experimental results. This link contains a basic, animated overview of the process, which will also help with that type of question:
http://learn.genetics.utah.edu/units/biotech/gel/

3. The threshold potential is a concept related to neurons. Neurons, as I'm sure you know, fire an action potential by a flood of sodium and potassium ions exchanged across their cell membrane (if this process is unclear to you, you absolutely should review it, as the action of neurons is a basic concept of physiology and almost certain to be tested on the exam). This causes a change in the voltage between the inside and outside of the cell. At rest, when the neuron is not firing, there is also a difference of electric charge between the inside and outside (the resting potential). The proteins inside the cell carry a heavy negative charge, which affects the voltage such that the inside of the cell is negative relative to the outside. That's the role of the membrane in all this: we use the membrane to define the sign of the cell's voltage (inside vs. outside, negative vs. positive).

The neuron will fire its action potential only when its resting potential is altered to become slightly more positive. At a certain, predefined voltage, the neuron will fire an action potential. It always fires an AP once it reaches the threshold, and it never fires if it doesn't reach the threshold (the resting potential of a neuron is usually around -70 mV, and the threshold is usually around -55 mV, to illustrate the scale we're discussing here). That's the threshold voltage--the voltage at which the neuron fires. As before, there is a link below that discusses this in more detail:

http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb.figgrp.6213
http://www.cameron.edu/~gabrielr/PHYCH4/sld013.htm

Thank you for using AllExperts. I do wish you the best of luck tomorrow.