Tuesday, August 31, 2010

Chromosomes-Get Yer Genes On!


by Geneva Coats

Why is Spot larger than Rover? Why does Trixie have a golden coat while Muffie’s is black? And why are Muffie’s ears floppy while Rover’s stand upright?

Specific characteristics of living organisms are determined by their “GENES”. Genes are “coding” segments made up of a substance called DNA. The DNA in your genes is arranged in specific patterns. Different genes are strung together in long rows to form a rope-like chain called a “chromosome”. Each chromosome contains thousands of genes.

Chromosomes are instruction panels; they provide the blueprint to make an organism what it is. They carry all of the information necessary to help living things grow, survive and reproduce. Chromosomes are located inside the cells of the body in a central control area called a “nucleus”. These chromosomes determine not only what you look like, but also how your body functions and, to a large extent, how you act, think and feel.

The DNA that makes up genes and chromosomes is like a computer code of instructions. Chromosomes in the nucleus build an instruction copy of themselves and send that instructions to other parts of the cell, the ribosomes, and the ribosomes in turn manufacture proteins according to instructions provided. These proteins might be enzymes for body metabolism, or proteins for building body tissues.

During normal cell division for growth or cell replacement and repair, chromosomes double and then split apart to form two cells form from one. Now both of these cells will end up with identical chromosomes within their nuclei. However, there is a special type of cell division that happens to produce the unique reproductive or “germinal” cells. Instead of doubling, the germinal cells are produced by by splitting up the original chromosomes. These reproductive germinal cells, the sperm and the eggs (ova), therefore will contain only HALF the number of chromosomes as do the other cells of the body. When a sperm cell combines with an ovum, VOILA! there is then a complete set of genes with a full set of instructions to create a new living being. This new creature will have half his genes originating from his father’s sperm, and the other half will have been contributed by his mother’s ovum.

This process involves something known as “random fertilization”. What does that mean?

The chromosome combination contributed by a sire to his offspring is random, and can vary considerably. Half his chromosomes will end up in that sperm cell…but how many different possible combinations of chromosomes can there be in any one sperm cell?

Let’s check it out. Humans have 46 chromosomes, arranged in 23 pairs, that divide and split up to form germinal cells, and they assort independently. To form a germinal cell, there are 2^23, or 8 million, possible different assortments of chromosomes that could be inherited for each individual cell!! The ovum also has 8 million possible different chromosome combinations. 8 million X 8 million = 64 trillion possible unique combinations of chromosomes for every human offspring created from any given mating! See how unique you are! Even your siblings may have quite a different genetic makeup than you do!

A human cell has 46 chromosomes, arranged in 23 pairs. A dog cell, however, has 78 chromosomes, arranged in 39 pairs. Each sire can produce roughly 550 BILLION different assortments of chromosomes in their sperm cells. Multiply that by the 550 billion possible combinations of chromosomes in the dam’s ova, and there is a possible 300,000,000,000,000,000,000,000 DIFFERENT combinations of chromosomes that can be produced for any individual dog created from any specific pairing.

WOW! that’s a lot of zeros. How do you read such a number? It is 30 billion trillion....roughly the same number as the estimate of the number of stars in the visible universe! Each dog from any certain mating is as unique in his genetic makeup as a star! That's a very nice comparison, I think.

But wait! There is another factor that can further increase genetic variety in offspring. This is the phenomenon known as “genetic crossover”. Crossover commonly happens during cell division to produce sperm and ova. What does “crossover” mean? Let’s see….remember we said that each chromosome has a partner chromosome with similar genes on it. During cell division, part of one chromosome may break off and swap material with its partner. This means that sometimes the chromosome that you inherit is totally different from the original one your parent has. The crossover process “shuffles the deck” so to speak, to produce even more variety in offspring. It would be impossible to estimate how much more variety this effect produces! But we would need millions more universes filled with billions more stars to get close to the number of unique combinations of chromosomes possible to achieve with any specific mating.

This vastly inconceivable number implies a rich potential to produce dogs who have a very unique and highly individualized genetic makeup. This inherent variety in the dog genome is how man has been able to create so many different breeds with characteristics as different as those noted between a Chihuahua and an Irish Wolfhound. Compare the variety in dogs to that of humans, who all look remarkably similar….even people of different races. We have fewer chromosomes to reassort and recombine, and less chance of isolating and promoting different specific traits.

Now do you still think that one or two litters is enough to judge what your dog can produce? Although, I am sure there are animal rights “overpopulation” handwringers out there who believe that every intact dog will produce billions of puppies in just seven years. Hmmm, I only wish I could get more than three or four in a litter to select from!

And just think, each and every chromosome contains thousands of individual genes!

Diagram of a chromosome, (looks like an "X"). It's looks chubby because it is composed of tightly coiled strands of DNA (see detail portion showing unravelled strand of DNA)

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