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 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 dislodge from one another, all four chromotids,
still paired up with its sister 'tid, are unique from one another.
Now, all 23 chromosome pairs (we are 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 pa, but having bits of the other in unique
combinations--an "X" (chromotid sisters) in each cell.
The last step occurs when the chromotid sisters split into four separate
cells. These are the gametes (sperm or egg).
To recap,
Again, say we're talking only about chromosome 1...
Cell #1 has | and | --ma and pa
They replicate: X and X
They crossover: X and X exchange DNA
New cells: X in one and X in another
Chromotids split from sisters and we now have four cells:
| and | and | and | each having bits from both parents
Remember, these derived from germ cells (testes or ovaries) and are now
four gametes (sperm or egg).
Then we have fertilization which forms a zygote and eventually becomes an
embryo. The zygote is composed of | from ma and | from pa, and one each
for each of the 23 chromosomes. This is *not* recombination.
But this is not all that is passed to the zygote. I'll try that in the
next email.
-Michael
--
Administrator, the Akins DNA Project
Administrator, the Ashenhurst DNA Project
Administrator, the Bishop DNA Project
Administrator, the Eldridge DNA Project
Administrator, the Fisk DNA Project
Administrator, the alt-McDowell DNA Project
Co-Administrator, the Cooley DNA Project
Co-Administrator, the McDougall DNA Project
Co-Administrator, the Pickens DNA Project
Co-Administrator, the Strother DNA Project
Instructor, the Osher Lifelong Learning Institute (OLLI)
B.A. Humboldt State University, History
Received on Thu Oct 23 2014 - 19:40:23 CDT