Whacked my back -- again :(

From: Michael Cooley <michael_at_newsummer.com>
Date: Thu, 23 Oct 2014 17:09:27 -0700

Eight years ago I had L4-L5 surgery--one of the worst things I've ever
endured. I don't think it's going to mean another go on the operating
table, but I hurt my back while making the bed. It was only sore for a few
days but I woke up a week ago Monday hardly able to walk. What is normally
a five minute walk from my car to the Human Genetics class took 20
minutes. I've since missed three classes and will probably need to drop
it. I waited three years to get into it. :(

Much of what I've learned is relevant to some of what we've been studying,
genetic-genealogy-wise. While it's somewhat fresh in my mind, I'll pass it
on here:

There's a misconception out there that genetic recombination happens after
conception. What happens is fertilization: ma's chromosome 1, for example,
meets pa's chromosome 1, but they do not mix. They are discrete entities
throughout the life of the child, cloning themselves from one cell
division to another through the process of mitosis.

Recombination happens through meiosis, a specialized form of cell division
that occurs in germ cells, those found in the testes and ovaries. It's not
the *parents'* DNA that mixes, it's the parents' parents--the
grandparents!

Through most of the cell's cycle, a chromosome is a single string of DNA.
(Again, one copy came from ma and one came from pa.) These single threads
are called chromotids. We can represent them like this: | | . Before cell
division, each of the two 'tids replicate and attach the copy at the
center, making up the vague X shape we associate with chromosomes. So, we
still have two chromosomes but four chromotids, each attached to its
"sister," like this: X X . (Again, remember, each is exactly as inherited
from each parent.) Now, this is the remarkable thing... Ma and Pa come
together and exchange DNA! It's almost as though they're embracing and
doing the deed! When they dilsodge from one another, all four chromotids,
still paired up with its sister 'tid, are unique from one another.

Now, all 23 chromosome pairs (we were looking only a chromosome 1), line
up at the axis of the cell. (There are 46 chromosomes--in pairs--and 92
chromotid "sisters." One member of each pair is pulled either to the
"north" or the "south" and the cell divides. (It's not predetermined which
direction they go. It's random. There's something like 8 million
north/south combinations.) We now have two cells, each with one chromosome
1, one ma and one ma, but having bits of the other in unique
combinations--and "X" (chromotid sisters) in each cell.

The last step is when the chromotid sisters split into four separate
cells. These are the gametes (sperm or egg).

To recap,

Cell #
Received on Thu Oct 23 2014 - 19:09:28 CDT