THE ROLE OF THE CHROMOSOME --- THE BICYCLE FACTORY: DESIGNERS AND WORKERS
{This analogy has been taken from an excellent
book written by Maitland A. Edey and Donald C. Johanson entitled BLUEPRINTS:
Solving the Mystery of Evolution. This is a very readable survey of genetics
that is highly recommended if you want to learn more about this subject
in the future.}
It is constructive to draw an analogy that will help you understand the nature of chromosomes and genes. We will use a bicycle factory. Inside a larger factory building there is a small, totally enclosed office. The factory makes bicycles. Inside that central office is a group of designers working on a complex set of blueprints. Out in the factory are a crowd of workers and piles of supplies. The designers' job is to issue to the workers instructions for keeping the building in repair and for making bicycles. The cell and the cell nucleus bear that relationship.
The workers in the factory never speak to the designers. Every once in a while a small grille in the office is opened and some instructions are handed out. But before those outside can say anything, the grille is slammed shut again. The workers can do anything - good or bad. Even disease can get into the factory and kill some of the workers. All those things can have an effect on how well they carry out the blueprint instructions given them. BUT THEY CAN HAVE NO SAY ABOUT THE BLUEPRINT. The might likes screens on the windows to keep out flies or blue seats on the bicycles they are making, but the workers have to endure if the designers don't order screens or blue seats. This is the "passive" nature to how the cell works. There is no active nature to change. The workers can't build a larger bicycle seat and then instruct the designers to change their blueprints. This would be similar to Jean Baptist Lamarck's acquired characteristics. Darwin and others recognized this wasn't the way things worked. Workers only "passively" accepted their instructions.
The concept of a factory is similar to that of a human body. A designer might put out a blueprint (genotype) for an excellent outer form (phenotype), the workers would get the supplies they needed and construction of the form would be completed. A similar factory down the road might not have such an excellent design. Ultimately, this factory might fail because of that fact. That would be an example of Darwin's survival of the fit in its strictest sense. Variation would have occurred and selection would have resulted.
By 1900, it was possible to recognize that there might be different outer forms (variation). It was also possible to produce a new design (a mutation) that would be faithfully followed by the factory workers whether they liked it or not.
Suppose we watched different bicycle
factories making their products. One made blue bikes with white seats and
another made red bikes with black seats. We would assume that the designers
in the first instance stipulated blue bikes and white seats on their blueprints
(that is, two genes (triats for blue bikes and white seats) were linked
on one chromosome (design instructions). 
After a long time watching
these factories make their products, one of them now began to make red bikes
with brown seats instead of black seats. This would be a change in the blueprint
-- another way of saying that a chromosome had undergone a mutation. It
could easily be the case that the factory making the blue bicycles had added
a bell to the bike. A different blueprint with new variation was in place
for workers to follow. The blue bike, white seat, and bell were on the same
blueprint and worked together. Each chromosome tends to work as a blueprint.
Workers are given different sets of instructions on each chromosome. Depending
where you are in the assembly line, your instructions may be different.
One of the ways instructions can get changed in through a process known as "crossover". In this case, instructions are shifted from one set of blueprints (one chromosome) to another. The other way is for one of the instructions to mutate -- change D to Q for example.
The concept of the assembly line can be used to envision how these changes
can take place. The assembly line is a double one; it has workers on both
sides standing along a counter. At a particular point along the line, a
worker reads the instructions and calls out in a loud voice "put on
wheels". He has a very loud or dominant voice and his opposite number
across the table agrees even though the instructions he is reading say "don't
put on wheels" (recessive). The bicycle gets wheels and is passed on
down the line.
Suppose that the designer in the central office is blinded by a flash of sunlight through the skylight and makes a mistake in the blueprint instructions. He passes to the first worker instructions that read "don't put on wheels". Now both sides of the assembly line have the same instructions and wheels are not put on the bicycle. In this analogy, the designer is the chromosome and the sunlight is background radiation.
It is possible that instructions on the blueprint are linked. For example, an instruction may read "if there is a primer coat, put on red paint". If the first instruction for a primer coat was missing, then the worker responsible for painting the bike red would not do so.
In review, genes are the workers responsible for carrying out a specific design. There are different kinds of workers, some dominant and some recessive. Each works on a separate part of the larger design. As we have seen, it is possible that instructions can be changed. It is the design that changes, not the workers (genes). The gene that works to govern eye color is always present along the assembly line. The instructions for red eyes, for example, may be standard for the design. If the design changes to white eyes, the worker (the gene) simply puts out white for the color of the eyes.
This is an idealized pair of chromosomes.
Two loci or genes have been shown. There are actually perhaps several million
such loci on a chromosome. If there were matching loci on the 
second gene, these would be "homologous pairs" (matching
genes). Note that there are loci on both of the strands of the one chromosome.
You always have two positions that govern a specific trait.
In order to understand the nature of the chromosome, it is necessary to introduce the concept of DNA. DNA in all organisms is made of the same materials: nucleotides composed of four different bases plus some sugar molecules and a phosphate molecule. Because everything is made of the same basic elements, the secret of DNA's marvels of creative diversity had to be sought not in its composition, but in its structure. Something in the way DNA was built had to account for the billions of different forms it could command.
In
order to understand how DNA is built, it first necessary to understand what
a base is. A base is a simple compound made up of nitrogen and carbons.
There are four different types of DNA bases: adenine, thymine, guanine,
and cytosine. These are generally referred to by the letters A, T, G, and
C.
One of the basic functions of DNA is to replicate or copy itself. It is more than a wallpaper pattern that programs life. Imagine, however, a typist trying to copy an entire encyclopedia. One would assume that mistakes would be made in each copy. Replication is designed to take the word "STEP" and have it retyped as "STEP". What happens if it is retyped "STOP" instead? This is a mutation. As DNA replicates, there is a chance for error since this replication process is such an immense task.
James Watson and Francis Crick discovered
that DNA is arranged as a double helix. They also discovered that bases
paired in only certain ways: A with T, T with A, C with G, or G with C on
either side of the "up" and "down" stairway ( the chromosome).
Remember that the cell nucleus where the chromosomes are located is very
small. This means that DNA has to be very compact. It is literally crammed
into the nucleus. If all forty-six chromosomes from the nearly trillion
cells in the human body 
were straightened out and connected, it would go
to the sun and back more than 100 times. Put another way, if you were to
extract all the DNA from just one cell in your body, you have an invisibly
thin hair of a molecule three feet long. It would be a string of C's, G's,
T's, and A's that would total 3 billion in just that three feet of DNA.
In essence, this means there are 3 billion bits of information coded in
the genetic library of that single cell and each cell of your body.
It is important to realize that by rearranging letters in this sequence, you spell out different words with entirely different meanings. For example, take the letters S T A E and L. Now, make a list of all the different words you can create. Mix up the sequencing of the bases along the helix and you configure a different protein. This is equivalent to a different design. DNA is the blueprint or design that instructs the gene what to build or create. DNA, in essence, builds us.
If the DNA has a function of duplication,
how is this accomplished? It involves a protein-maker known as RNA. RNA
is actually an amino acid. There is one particular type of RNA designated
as mRNA and known as messenger RNA. The problem that one faces with duplication
is that you don't want to lose the master because you may wish to make other
copies later. The mRNA literally attaches to a strand of the helix and creates
a complementary chain with one slight modification. The RNA is then free
to travel out into the cell and see that something is done with the message
it carries. The RNA strip is not just like the original DNA strip however.
It is a complement. Its actually a mirror image of the original. However,
it carries its message to another source that again turns the image backwards
to form the duplicate. 
There is another type of RNA that functions to "transfer" the message. This is known as tRNA or transfer RNA. This type of RNA attaches itself to the mRNA strands. It brings with it specific amino acids that again mirror image the partner mRNA thus giving you the pattern of the original DNA. There are two things that actually take place in this process. There is transcription or copying of the nuclear message onto a strip of RNA. There also is translation or transformations from protein into amino acids and then ultimately back to protein. This means that genetic information travels in one direction - from DNA to RNA and then to protein again or duplicated DNA. The central office issued a set of instructions through messenger RNA to duplicate the genetic information from the master design.
This process of duplication is important as cells divide. There are two types of duplication. The first is mitosis. This involves the duplication of cells in which the chromosome complement is identical to the master or original cell. This is a one for one duplication. The second type of duplication is meiosis. This involves cell division in which the original chromosome complement it reduced by half. The produce of meiosis are gametes (sperms for a male and eggs for a female).
MITOSIS occurs during a single cell division. It results in the production of two identical cells with a full complement of chromosomes. When the full complement of 46 chromosomes for humans occurs, this is referred to as the diploid number.
There are two primary phases to MEIOSIS. The first is similar to mitosis where the cell duplicates. There is a second cell division in meiosis. During this division, only half the complement (a haploid number) of the chromosomes is present in the daughter cells.
Daughter cells have half the number of chromosomes from the original. These cells are called gametes (sperm and egg.)
Members of different chromosome pairs sort independently when entering sex cells during meiosis. With two pais of chromosomes, four different combinations are equally possible.
MEIOSIS forms gametes or sex cells. The half complement then joins with a similar complement to form the normal diploid number during reproduction. The sperm and egg join to RECOMBINE genetic information. This leads to a new set of instructions for the workers of our factory. The design for the new bicycle are unique from either parent (the original factories). This is another source of continual variation. Keep in mind the "mission impossible" of retyping all those instructions. That too is a source of change (mutation or cross over).
Basic Terminology and Concepts Necessary to Understand Genetics | |
The Bicycle Factory Analogy |
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