Scrambled eggs, mutant sperm

4 Oct

MEIOSIS: A cell with 46 chromosomes becomes four sperm, each with 23 chromosomes. Shown here, the "Chromosome 1" pair (top left) gets duplicated, shuffled, and divided into sperm cells (bottom).

In other posts, we’ve discussed meiosis and mitosis, the two little cellular dances that need to be considered on the way to a basic understanding of genetics.

Meiosis affords genes the opportunity to recombine.  Mitosis affords genes the opportunity to replicate, and replication gives genes the opportunity to mutate.

Both processes are involved in the production of eggs and sperm, but eggs recombine more than sperm, and sperm replicate more than eggs.  Hence the title of this post, in which we will describe how sperm and eggs are made inside of the testicles and ovaries of two randomly chosen celebrities.

Spermatogenesis

Bob Saget’s body will, over his lifetime, create billions of sperm.  Right now, inside of his testes, the sperm-making factory is running. (We invoke Bob Saget here as an example; we could have chosen any person with testicles.)

Inside of Bob Saget’s testicles are little tubes that are lined with spermatogonia, the germ cells that can divide and develop into sperm.  These cells proliferate through a process called mitosis, where one cell divides into two cells, each with the same set of chromosomes as the original.

Through the process of meiosis, each of these cells that contain 23 chromosomes from Bob Saget’s mom and 23 from Bob Saget’s dad will shuffle and divide, creating four cells with a total of 23 chromosomes in each that are a potpourri of Bob Saget’s parents.

These four cells, called spermatids, will develop into mature sperm, with the genetic information packaged in the heads and a tails propelling them forth into the gooey universe.

The development of sperm from spermatogonia happens within a couple of months.  The process is repeated over and over, between puberty and death, many, many times.   A single ejaculate might contain 200 to 300 million sperm.

Click here for an animated illustration of spermatogenesis (sperm-creation).

Oogenesis

The process of egg-formation looks a lot like sperm formation, but the timeline is very different.  The germ cells equipped to become mature eggs in, say, Betty White’s body, were formed, proliferated by mitosis and even began meiosis while she was a tiny embryo.  These cells remained in her ovaries and didn’t finish meiosis by dividing until ovulation took place, presumably some 12 to 50 years after they were formed.

Through meiosis, spermatocytes in men divide two times to produce four cells that become sperm.  The cellular goo known as the cytoplasm mostly falls away from each one, leaving four sperm equipped with necks and tails.

Through meiosis, oocytes (immature egg cells) in women similarly divide twice, but most of the cytoplasm goes to just one of the four cells, and only this cell will become a functional gamete.  The smaller cells, “polar bodies,” eventually degenerate.

The oocyte completes its first meiotic division during ovulation.  Usually one oocyte is ovulated per menstrual cycle, but fraternal twins result when two are released and fertilized.

If the egg is fertilized, it quickly completes meiosis as it passes down the fallopian tube.

Key differences

So in the process of egg and sperm formation we see two major differences: The timing, and, after meiosis, the viability of sex cells following meiosis (four in men, one in women).

Another important difference is that sperm arise from cells that have undergone 20 to 30 divisions, as opposed to eggs, that come from cells that divide only two or three times.  Bob Saget’s cells sat there dividing and dividing as his body grew; Betty White’s, not so much.

One of the wild things about this is that, although it seems perhaps merely mechanical, this difference between the gametes donated by men and women to the cause of creation actually gives rise to a kind of difference in diversity and evolutionary push.  So the sperm that could contribute to your child’s genes has more mutations in it– because of the opportunity for change that comes with mutating– than the egg.

Replication: The opportunity to mutate

There is always going to be mutation any time a large genome like the human genome is replicated for some reason.  Imagine that you had to transcribe with fidelity 6 billion base pairs of all As, Ts, Gs and Cs, and get them in the right order, with the right timing, in the right chromosomes– and do that in about an hour. You’re going to make some mistakes.  The cell has incredible proofreading capability; the enzymes that do this as their primary occupation are incredibly good at catching many of those errors.  But, the estimate is that every person on the planet has somewhere between 100-200 mutations that are uniquely theirs that represent the intimate relationship between parent and the gamete that they formed.

Because the pre-sperm cells divide 20-some times, there are 20-some opportunities for those reading and writing errors to accumulate in the sperm that men pass on to the next generation.  Those errors can mean nothing or they can be a life-and-death-changing event.  So for instance, if there is a mutation in one of the central developmental genes, say for example, the gene that directs the formation of internal organs is messed up, then the embryo won’t ever develop.  But most of the time, mutation is just kind of like a beer can in the forest.  Nobody ever knows that it’s there: You might walk upon it and say, “Somebody was drinking here,” and then you move on– there’s no huge impact.

So the cell division that happens during the formation of oocytes in a female embryo have fewer opportunities for mutation because these progenitor egg cells are just going through a few divisions.

But the eggs have more opportunity than sperm do for recombination– the “crossing over” that happens during meiosis.

For more information on sperm and egg creation, Google “spermatogenesis” and “oogenesis.”

This has been a longer post, so if you have read this far, you might be interested in a T-shirt depicting the maturation of the ovarian follicle.

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