the genetic alphabet - A,T or U, C and G

Can we write instructions with only 4 (or 5) letters?

The answer to that is “yes”! Our cells’ genetic instructions are written using only the letters A, C, T or U and G. Can you believe that?

If that is the case, how come there are so many different cells? How come we are different from each other?

Well, these letters vary in number as well as in their sequence in every cell. Also, sometimes not all the sequences of these letters are active at the same time. If for example we designate numbers to the various sequence of these letters in a cell, it’s possible that only sequences 1, 2, and 3 are active in one cell while it can be sequences 1, 5, and 8 in another cell.

So if you think about it, just varying the number of A, T, C and G as well as their sequence in a cell are enough to produce almost limitless variety of combinations.

By the way, in the genetic alphabet, A always pairs with T or U and C always pairs with G. So that actually limits the combinations because there will always be the same number of A and T or U and equal number of C and G. Still even with this constraint, the possibilities are almost limitless.

Why do eukaryotic cells keep their genetic material inside a nuclear membrane?

If you recall, the major difference between eukaryotic and prokaryotic cells is the presence of an internal membrane system in the former (Nov 3, cell design 101.1).

This internal membrane system encloses the various metabolic centers and separates them into organelles. It also encloses the genetic material and we now recognize a nucleus in eukaryotic cells.

So, what is the advantage of this kind of design?

Well, as Sherlock Holmes would say, “elementary my dear Watson, elementary”. Just think about it, the genetic material contains the ‘blueprint’ of the cell’s life. So it’s but natural to ensure its safety, right?

The cell cannot leave its 'blueprint' lying around, exposed to all the enzymes and activities going on in the various metabolic centers. What if an enzyme will act on it and split it into pieces? What if it suddenly gets entangled in all the activities going on? The information in the blueprint might be destroyed or lost.

Now, do you wonder why prokaryotic cells mutate so fast? Their genetic material is not protected like that of eukaryotic cells.

Barr body

Do you know that the discovery of the Barr body was actually through serendipity? Yes, it was. Murray Llewellyn Barr was actually working on the effects of fatigue on the nerve cells of cats. However, he did not find any changes in the nerve cells of fatigued animals. Instead, he noticed a mass of chromatin material on the nuclear membrane of some nerve cells but not all cells. When he crossed checked the sources of those cells, he discovered that they actually all came from female cats. When he examined cells coming from other mammals including human, the same chromatin mass was also observed only in females. He later discovered that this mass of chromatin material is actually a sex chromatin.

This sex chromatin is now referred to as the “Barr body”. It represents an inactivated X chromosome. It is now known that in mammalian and human females, one of the X chromosomes becomes inactivated during development and it appears as a dark mass (Barr body) near the nuclear membrane of their cells. So a female with XX sex chromosome will always show one Barr body, while a male who has XY sex chromosome should have no Barr body on his cells.

The number of Barr bodies observed in cells is always a good indicator of the number of X chromosomes in an individual. Individuals with multiple X chromosomes will have all the X chromosome inactivated (will appear as Barr bodies) except one. Thus, as mentioned earlier, females with normal number of sex chromosomes will always show one Barr body. Some females however have XXX sex chromosomes. Their cells will thus show 2 Barr bodies. Males showing a Barr body in their cells therefore have XXY sex chromosome.

The discovery of the Barr body thus launched a new era of research on genetic disorders.

cell design 101

“If you were to design a cell, what would be your basic requirements or design?” This is the very first question I always ask my students about cells. Their answers always include: “nucleus, cytoplasm, cell membrane, organelles, etc.” When they answer this way, I then ask – “Is the bacterium a cell?” Of course they will answer “yes.” This answer is then perfect for my next question: “But the bacterium does not have a nucleus, so why did you list it as a basic requirement for your cell design?” I always follow this up with more questions and answers that eventually make students think really hard of what are the basic requirements of cells. This then sets the stage for my first lesson on “cell design 101” – wherein we will eventually agree that the basic requirements for cell design are simply: cell membrane, genetic material and biosynthetic machinery.

You can click on any of the links above and they will bring you to related post about these basic requirements for cell design.