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Posts Tagged ‘Pro-Life Academy’

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Quite a few people have asked if we could use the book EMBRYO: A Defense of Human Life, by Robert P. George and Christopher Tollefsen, for our Pro-Life Academy. Beginning March 16, we’ll discuss a chapter per week from this extraordinary book. Written in plain language, it gives an excellent biological description of development, as well as the philosophical and ethical arguments in favor of the embryo’s personhood. Check it out at Amazon. Order soon!

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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

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Image via francis.edu

How long has it been since Biology class? The mere mention of it induces either sweaty palms or deep sleep. For many, it’s been too long and the material was neither clear nor interesting, yet the life issues demand a certain facility with biology. Beginning today, and on every Tuesday and Thursday this blog will have a column entitled Pro-Life Academy. It will begin with the fundamentals of biology necessary for discussing the life issues, and will advance over time into some more detailed aspects of science and medicine.

At all times, the lessons will be concise, friendly, accessible and non sleep-inducing (hopefully). Building a strong command of the fundamentals needn’t be painful or stressful. We’ll keep it light but informative. The posts will build on each previous post and will be stored under “Pro-Life Academy” in the Categories panel. Questions are encouraged. There’s no such thing as a dumb question!!!

Whether we are discussing sperm, egg, zygote, embryology, adult or embryonic stem cells, in vitro fertilization, the definition of death, it all comes back to a basic knowledge of the fundamentals-beginning with the most fundamental unit of life, the cell.

Cells are the fundamental, the smallest units of life. The human body begins as a single cell, which gives rise to some 200 specialized types of cells that make up its complexity. Below is an illustration of an animal cell, with a wedge cut-away to afford us a view inside.

Image via molecularexpressions.com

If it looks pretty complicated, it is. But we’ll just focus on a few features.

Nucleus– At the heart of the cell lies the nucleus. This organelle (little organ) is the vault where the blueprints of the cell (DNA) are stored.

DNA– These are long strands of a chemical called deoxyribonucleic acid. Certain segments on each strand contain instructions, called genes, for building proteins.

Proteins– are a large family of molecules in the cell whose members serve various functions, from cell skeleton, to receptors at the surface (like a satellite dish/transmitter tower) which enable the cell to communicate with the outside world, to enzymes which do the actual biochemical work of the cell.

ARE YOU STILL AWAKE?!?!?!

Genes– are the blueprints for building different proteins. The human cell contains over 30,000 different genes.

Ribosomes– When proteins need to be made, working copies of the genes, called RNA, are made and sent to the carpenters of the cell responsible for building proteins out of their amino acid building blocks. These carpenters are the Ribosomes.

Mitochondrion– The powerhouse of the cell where the energy contained in sugar is extracted and converted to useful form. (Sperm have long mitochondria to power the flagellum, which enables it to swim.)

That’s enough cell anatomy for now.

A few important observations here.

First, cells are extremely complex. Human cells carry on tens of thousands of biochemical reactions.

Next, not every gene is operational inside of every cell, even though every cell has a complete set of 23 pairs of chromosomes, shown in the karyotype below.

Image via bio.miami.edu

This is vital in understanding stem cell technology, as we shall see. Essentially, certain clusters of genes are responsible for determining what type of cell a certain cell is.

For example, a heart muscle cell’s genes for heart muscle function are turned on, while the genes responsible for making a nerve cell, or a skin cell are turned off. The very idea of skin cell reprogramming involves taking a skin cell and walking it backward developmentally to a point where one can direct the cell forward in a different developmental direction by aiding it in turning on certain genes, while turning off others. Perhaps we now want a cardiac muscle cell, instead of a skin cell. That’s the general idea that we’ll return to in the future.

Welcome to the world of the cell!

Thus endeth today’s lesson.

(Be prepared for a quiz next Tuesday.)

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