Deoxyribonucleic Acid

For starters, you should know where the DNA is in a cell. In a prokaryotic cell, the DNA is in a part of the cell called the nucleoid. In a eukaryotic cell, the DNA is in the nucleus of the cell.

Chromosomes

Introduction to Chromosomes

Cells sometimes use a “fork” to wind up DNA into a structure called a chromosome. Chromosomes are tightly coiled DNA. (Cells are pretty sensible—a cell contains about two meters of DNA!) In the picture, note how the “DNA” (the white line-like objects) “coils” into a mass. The “mass” is a picture of a chromosome taken with a very powerful microscope. Although not all chromosomes look like X’s, they are usually represented as them.

Examples:
Note: These are not real chromosomes, but rather demonstrative drawings of ones.
Note: Chromosomes aren’t always X’s, and they aren’t really colored.

Diploid Numbers

Chromosomes often come in pairs. This is because animals get one chromosome from their mom, and one from their dad. The number of chromosomes an animal has is called a diploid number.
Below is a table showing the diploid number of some species:

Species (Common Name) Diploid Number

Homo sapiens (human) 46

Myrmecia pilosula (an ant) 2

Parascaris equorum var. univalens (parasitic roundworm) 2

Muntiacus muntjac (the Chinese muntjac’s Indian cousin) 6

Drosophila melanogaster (fruit fly) 8

Arabidopsis thaliana (plant in the mustard family) 10

Caenorhabditis elegans (microscopic roundworm) 12

Zea mays (corn or maize) 20

Muntiacus reevesi (the Chinese muntjac, a deer) 23

(Tomatoes) 24

Saccharomyces cerevisiae (budding yeast) 32

Xenopus laevis (South African clawed frog) 36

Mus musculus (house mouse) 40

Canis familiaris (domestic dog) 78

Cambarus clarkii (a crayfish) 200

Equisetum arvense (field horsetail, a plant) 216

Chromosomes are numbered from largest to smallest.

Normal and Abnormal Chromosomes

X and Y chromosomes are sex chromosomes. If you get X chromosomes from both of your parents, you are a girl. If you get a Y chromosome from your dad, you are a boy. (Note: You always get an X chromosome from your mom.)
Below is a picture of a normal man's chromosomes.

Normal Chromosomes

Below is a picture of a woman with Down syndrome’s chromosomes.

Down Syndrome Chromosomes

Down syndrome, as you can see, is when a person has one extra chromosome 21. The inheritance of an extra chromosome is called trisomy. In this case, this is trisomy 21.
Another example of trisomy is the XYY syndrome, nicknamed the “Super Male Syndrome”. (I guess that that is Y trisomy!) (Visit http://www.alumni.ca/%7Ebernd3a/ to see a slide show on “Super Male Syndrome”.

Structure of DNA

Nucleotides

DNA is in the shape of a double helix (an image of the double helix is on the right). It looks like a twisted rope ladder. DNA is made up of nucleotides (I always get this confused with nucleoids), which are made up of phosphate, a sugar called deoxyribose, and a base.


Phosphate	Deoxyribose

The phosphate and the deoxyribose are the same, but the base may vary. This is because the bases carry the genetic information.

Bases

There are four bases: adenine, thymine, guanine, and cytosine. Abbreviated, adenine is A, thymine is T, guanine is G, and cytosine is C.


Adenine	Thymine	Guanine	Cytosine

In other words, the “rungs” of DNA are made of adenine, thymine, guanine, and cytosine, and the sides are made of alternating sections of phosphate and deoxyribose.

How Bases Connect

The bases connect to each other to form a base pair. This is done by hydrogen bonds. However, they connect only in a specific way—for example, thymine cannot—I repeat, cannot—connect to cytosine. Bases have to connect in a specific order. Adenine has to connect to thymine, and guanine to cytosine. In other words:

A=T; G=C

This principle is called Chargaff’s rule and is really important for DNA replication.

Chargaff’s Rule

DNA Replication

   DNA replication is when a replica—an identical copy—is made. It occurs whenever a cell divides so that that the DNA’s information can be preserved and handed down to an animal’s (including humans’) babies. DNA replication has to happen when a cell divides. DNA replication is similar to (analogy time!) copying a file onto a disk (the one shown is a floppy) so you can transfer the file to a different computer.
   DNA replication begins with a partial unwinding of the double helix at a part known as the replication fork. An enzyme known as DNA helicase does this. This unwound section is seen under electron microscopes as a “bubble” and therefore is known as a replication bubble.
   As you know, DNA replication starts when the helicase breaks the hydrogen bond. The DNA is broken in half and the new half-strands become templates for the new two DNA strands.
   (Note: DNA doesn't always wait for itself to complete separation before moving on to the next step of DNA replication.)
   There are many extra nucleotides in the cell’s nucleus. Now is when these come into use. The bases of the nucleotide connect to the template DNA according to Chargaff’s rule. The same thing happens to both templates until there are two copies of the DNA. Now cell division can begin.

Note: If you wish to see a movie on DNA replication, go to http://www.ncc.gmu.edu/dna/repanim.htm. (The movie has more advanced concepts.) (The movie requires Shockwave!)

Interesting Facts

If you unravelled all your chromosomes from all your cells and laid the DNA end to end, the DNA would stretch from the Earth to the Moon about 6000 times!!!! [That's far, because the Moon is at average 384,403 kilometres (238,857 miles) away from the Earth!!]
The Y chromosome is about one-third the size of the X chromosome and contains only a fraction of the number of genes. At one time when humans were evolving, the X and Y chromosomes were the same in size and gene number, but the two chromosomes gradually changed over the time of 300 million years!!!!
Viruses (in biology, not those ones you get on a computer) is either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid) surrounded by a protective layer of protein called a capsid!!!!
A cell, which is less than a millimeter long, contains 2 metres of DNA!!!!
DNA is very important, and tells your body and your cells everything it needs to know: to put the skin of your body on the outside, whether you like math (I do) or not, what color your hair is, and even to build your brain!!!!

Bibliography For "Deoxyribonucleic Acid

Deoxyribonucleic Acid (DNA)

Contents

Introduction