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Pro-Life Academy every Tuesday and Thursday.

Fertilization of an egg cell by sperm, as shown above, is an example of sexual reproduction. This method, as we shall examine today, affords the greatest degree of genetic variation among the members of a species, including our own. Following our last lesson where we discussed the ultimate identity of an individual as residing in its genetic composition, we turn our attention today to exactly how it is that such unique individuality arises.

As we’ve discussed, all somatic cells, which are all body cells except the sperm and egg– called gametes, have 23 pairs of chromosomes. These chromosomes, recall, are long strands of DNA containing segments whose nucleotide sequences are instructions for building structural and functional proteins. We call these segments genes.

A somatic cell in humans has 23 distinctly different chromosomes. We get a set of 23 from our mother via the egg, and a set from our father via the sperm. This creates the 23 pairs found in each somatic cell. A word about what makes each of the 23 distinct chromosomes so distinct.

Each of those 23 unique chromosomes is unique because it contains a set of genes that cannot be found on other chromosomes. Further, the genes of a given chromosome reside at certain locations, or loci, such that when a chromosome from the mother finds its homologous partner chromosome from the father, they have the same genes at the same loci from top to bottom. Such a pair are called homologous pairs or homologous chromosomes (from homo meaning ‘same’ and logos, meaning ‘structure or form’).

In our last lesson, we considered how a somatic cell goes about dividing to make two genetically identical cells in a process called mitosis. Now we consider how a diploid cell (one with 23 pairs of chromosomes, 46 total) goes about making gametes, which are haploid (just 23 chromosomes) in a process called meiosis.

It’s really quite simple.

First a diploid stem cell for either egg or sperm will double its number of chromosomes. When it does this, each chromosome is stuck to its carbon copy at a point in the middle. Then, as the cell undergoes the first of two cell divisions, rather than the two new cels receiving a copy of each chromosome in a pair, the pair itself is separated during the division.

In this illustration to the right, we see an example involving a single homologous pair of chromosomes. At the top, the stem cell contains a single pair (we can imagine the yellow chromosome as coming from my mother and the blue one coming from my father).

Then the cells undergo DNA synthesis, making a carbon copy of each chromosome, joined at the center.

Next, we see that the pair is separated into separate cells, then, each of these cells divides to create four gametes.

Along the way, and not illustrated here, some genes were swapped between members of the pair-more on that next time in a lesson on genetic diversity.

Now, about a woman’s biological clock. Is that just a nasty manipulation by women to rope guys into marriage at a relatively early age, or is there merit to it?

Both!

Okay, just kidding. I wanted to see if you were still with me.

There is great scientific merit to the clock.

Go back to the illustration of meiosis above. Start at the top. We’ll use my wife as an example for show and tell today.

When Regina was a mere fetus in her fourth month of development, all of her organs were maturing in their development, including her ovaries and all of the eggs that she would ever carry. By her fifth month of fetal development, her eggs performed their DNA synthesis, as seen in the second illustration. The eggs remained that way, and still do today. The DNA carbon copies remain joined in every egg until a given egg is selected for a given menstrual cycle. Only then does the DNA separate. The longer the egg remains in the ovary, the greater the probability that the chromosomes will not separate.

So, when we were married, Regina was 24 years old, with an excellent chance that the chromosomes would separate, or disjoin, as we say.

Now at age XX (come on, you didn’t think I was that insane as to divulge her age, did you?) a great many of her chromosomes will not disjoin during the second round of cell division. That event is referred to as nondisjunction.

Okay, Regina’s turning 42 this year, I am that reckless and insane. And she’s more beautiful now than ever (which still won’t save me from the dog house 😉 ). But it’s an important milestone. By age 42, 90% of a woman’s eggs are chromosomally abnormal. Thus, at age eighteen, 1:2,000 live births results in Down Syndrome. By age 42 that rises to 1:25.

So, as a woman gets older, the greater the probability of nondisjunction occurring.

That’s a mouthful for one day.

Class dismissed. See you on Tuesday.
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Top photo: Quarandscience.com

Middle: bio.georgiasouthern.edu

Bottom photo:Growbrain.typepad.com