Roger's Equations Blog

Roger's Equations

This blog is all about science and technology (with occasional math thrown in for fun). The goal of this blog is to try and pass on the sense of excitement and wonder I feel when I read about these topics. I hope you enjoy the posts.

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The Chemistry of DNA, Part 2

Posted February 11, 2009 5:14 PM by Bayes

Last time I discussed the chemical structure of DNA including the Phosphate-Sugar Backbone and the Nucleotides (Adenine, Thymine, Guanine, Cytosine). But you have all heard about DNA before, and even if you didn't know the particular names of the parts, you already knew that DNA is a double helix of complimentary base pairs (complementary nucleotides). So we all know what DNA is.

But how does DNA work? How does something so small determine what we look like? How fast we can run? Our blood type? I mean aside from the nebulous arm waving explanation that it "passes on genetic information".

To understand that, we need to further investigate the structure of DNA. Last time we saw that DNA is a double helix polymer with hydrogen bonded base pairs. But how many base pairs? How long is a human DNA?

DNA Facts

Human DNA consists of about 3.17 billion base pairs. That's not one continuous polymer of DNA, rather it is broken up into 23 paired structures called chromosomes. So what is a chromosome?

Chromosome is an organized structure of DNA and proteins. Please take a look at the picture below. As you go from bottom to top, the scale becomes larger and larger with DNA farthest to the left being the key building block (along with proteins) of the chromosomes.

As you can see from above, it takes a lot of DNA to make a chromosome. The chromosomes are found in the cell nucleus. As mentioned earlier, there are 23 pairs of chromosomes in the nucleus of a cell. Here they are. The last pair are the sex chromosomes.

Every cell in the human body has these 46 chromosomes, except for sex cells which have only 23 chromosomes (unpaired). The average human body has approximately 50 trillion cells. Go ahead, let that sink in. I'll even repeat it:

3.17 billion unique base pairs of DNA per cell
50 trillion cells in the human body
So for each human walking around, that's over 2 billion miles of DNA walking around with them. Of course, there are many other animals, bacteria, plants, etc. out there. All have DNA. Lets see how long their DNA is, please note that the unit Mb means Millions of Base Pairs:

Oryza sativa (rice) genome = 441 Mb
Musa sp. (banana) genome = 873 Mb
Spinacia oleracea (spinach) genome = 989 Mb
Gallus gallus (chicken) genome = 1,200 Mb
Zea mays (corn) genome = 2,500 Mb
Homo sapiens (human) genome = 3,000 Mb
Nicotiana tabacum (tobacco) genome = 4,434 Mb
Vanilla planifolia (vanilla) genome = 7,672 Mb
Avena sativa (oat) genome = 11,315 Mb
Triticum aestivum (wheat) genome = 15,966 Mb
Triturus cristatus (crested newt) genome = 18,600 Mb
Necturus maculosus (mudpuppy) genome = 50,000 Mb
Lilium longiflorum (Easter lily) genome = 90,000 Mb
Fritillaria assyriaca (butterfly) genome = 124,900 Mb
Protopterus aethiopicus (lungfish) genome = 139,000 Mb

That's right, the lungfish genome is more than 40 times larger than the human genome. In fact, there are lots of animals and plants and even protozoa with larger DNA than us (C-Value is just a measure of DNA size):

Kind of makes you rethink your understanding of evolution, doesn't it? Well, don't panic, evolution is safe, it's just a bit more complicated than you were led to believe. Let's take a deep breath and go back to the beginning to see if we can't make sense of what we are seeing.
Let's zoom back in on the chromosome again, way down to the DNA scale. See the picture below:

Notice a segment of DNA in the picture above is being called a "gene". We've all heard the term "gene". It's what DNA is supposed to pass on. But what is a gene? and more importantly, how does it pass on traits to us? All that and more in my next blog entry.

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