DNA is a long fiber, like a hair, only thinner and longer(Fig. 1)(1). It is made from two strands that stick together with a slight twist. Proteins attach to the DNA and help the strands coil up into a chromosome when the cell gets ready to divide. The DNA is organized into stretches of genes, stretches where proteins attach to coil the DNA into chromosomes, stretches that "turn a gene on" and "turn a gene off".
Figure 1 DNA strand
The genes carry the instructions for making all the thousands of proteins that are found in a cell. The proteins in a cell determine what that cell will look like and what jobs that cell will do. The genes also determine how the many different cells of a body will be arranged.
In these ways, DNA controls how many fingers you have, where your legs are placed on your body, and the color of your eyes.
What's the difference between DNA and a chromosome?
A chromosome is made up of DNA and the proteins attached to it. There are 23 pairs of chromosomes in a human cell(Fig 2). One of each pair was inherited from your mother and the other from your father. DNA is a particular bio-molecule. All of the DNA in a cell is found in individual pieces, called chromosomes.
Figure 2 human chromosome
Why do you want to learn about DNA?
If you have gotten this far, you already have some curiosity about DNA. That curiosity may have come from hearing about it in the news or in the movies. A revolution has occurred in the last few decades that explains how DNA makes us look like our parents and how a faulty gene can cause disease. This revolution opens the door to curing illness, both hereditary and contracted. The door has also been opened to an ethical debate over the full use of our new knowledge. In the end, curiosity is the reason to learn about DNA. Fittingly, curiosity is the driving force behind science itself.
What’s Genes?
I’ll tell you a bit about genes. Genes are stretches of DNA and all have assigned places on the cell's chromosomes. There's all sorts of other fragments that have assigned places, but they're not genes. Think about there's the on switches that tell a gene when its time to go to work. For example, right after you eat, the insulin-on-switch tells me to get busy!
And then there's the off switches that give a gene time off. For instance, when enough insulin has been made, the insulin-off-switch works.
Not to mention the docking sites for special proteins to bind when its time to wind up into a chromosome before cell division. And what about all those stretches of DNA that do who knows what?
How are genes all fit on the chromosome? Let's take a peek at the DNA Ladder. Can you find gene? For example, I suppose A is a pretty small gene as far as that goes. It is only about 230 nucleotides long. When you consider that all the DNA in one nucleus of one human cell adds up to over two and a half billion (2,500,000,000) nucleotides long, well, I'm kind of trivial. However the size has nothing to do with importance. And the existence of gene is important.
Think about how to make hemoglobin and myosin transcribing gene.
The B gene , who knows how to make hemoglobin, is about twice A size. And then, there is the C gene, who knows how to make myosin, the protein that makes muscles work, which is 20 times bigger than A gene product! It’s amazing.
How do genes do it? Keep the blueprints for making proteins? After all, we are made from only 4 different nucleotides! Fact is, genes are so simple, scientists used to call them the 'stupid substance'. Genes had a good laugh over that announcement! However eventually those scientists figured out our secret.
You know proteins are made from building blocks, too. Only their building blocks are different from gene’s and are called 'amino acids'. There are 20 different amino acids that can be used to make enzyme like polymerase. That's what threw those scientists off a clue for a while. They were thinking, since protein is made from 20 kinds of building blocks and genes are only made from 4 kinds. Is it possible?
DNA STRUCTURE
In chromosome, there are a lot of genes here, so it can get pretty confusing. Genes are the brains behind the whole operation - not just the nucleus, but the entire cell and even the entire body. Each of genes have only one job to do. That's to remember exactly how to construct a single protein. Gene A, for instance, keep the blueprint for making insulin. Insulin tells your body that the glucose (sugar) levels are too high and that your cells should begin using it to make fat. Gene B lives on another chromosome, keeps the blueprint for making hemoglobin. Hemoglobin is the protein that carries oxygen around in your bloodstream. Your body needs to make hemoglobin all the time. However gene A doesn’t have to work every minute. Your body doesn't need insulin except right after you eat. So, I get some time off between meals.
That's why I can show you around chromosome.
Let me introduce you to polymerase. Polymerase is not a gene. It's not even DNA. Polymerase is a protein: a special protein called an enzyme. Gene may be the brains around here. Like Gene said, polymerase’s full name is DNA Polymerase. It’s job is to construct an exact copy of all the chromosomes just before the cell divides. Actually, it takes a whole team to do that job, and it’s part is to super-glue the nucleotides together.
Gene and polymerase are made out of small pieces hooked together to make a long strand; like train cars are the small pieces that are hooked together to make a long train. Polymerase is made out of small pieces called 'amino acids'. Gene and the rest of the chromosomes are made from small pieces called 'nucleotides'.
Here, let me show you what a nucleotide is.
Here you can see nucleotides being made from a base, a sugar, and a P (phosphate)(2). There are four factories like this, each making one type of nucleotide for gene to use in making a new chromosome. One factory makes a nucleotide with the name 'adenine' that we just call 'A'. There are also factories for making 'thymine' ('T'), 'cytosine' ('C'), and 'guanine' ('G').
That's what all the Genes and all the rest of the chromosomes are made of. See the left hand end of the nucleotides? Those are sticky spots that cause the two DNA strands of a chromosome to stick together. Scientists call them 'hydrogen bonds'. And see how A and T have two sticky spots and C and G have three? That makes A and T pair up and C and G pair up(3).
1. Alberts, Bruce; Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walters (2002). Molecular biology of the cell:New York and London: Garland Science.
2. Ghosh A, Bansal M (2003). "A glossary of DNA structures from A to Z". Acta Crystallogr D Biol Crystallogr: 620 – 6.
3. James D. Watson(2005). Molecular biology of the gene: fifth edition. Cold spring harbor laboratory press: 100 – 2.
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