This article first appeared in the July/August 2005 issue of the Boxer Ring.
The Genetic Code
DNA is like a code. DNA is a two-strand molecule, built like a twisted ladder with the "rungs" being the code. The code is made up of four chemical bases abbreviated: A, T, C and G (Adenine, Thymine, Cytosine and Guanine).
These letters are arranged in three-letter words, known as codons.
Say a DNA Molecule is made up like this, which is depicted only by the chemical bases
1. Strand #1-AUGTGTTGA
2. Strand #2-TGCACAACT
Each three-letter sequence codes for, an amino acid (parts of a protein), START or STOP. Think of DNA as an old fashion Morse code. Taps are arranged in a fashion as to convey a message. In this case the message is a three-letter code for a certain amino acid. Amino acids link together to make a protein. Proteins make up much of the body's tissue (muscle, heart, liver, etc) in addition to being the main part of enzymes, hormones, and antibodies. To read the code a small protein opens the DNA strand like a zipper, making it single stranded, then another small protein "reads" the bases of Strand #1 and builds a mirror chain. This chain is the same as Strand #2 but it uses U instead of T and is called mRNA (messenger RNA). This mRNA strand then travels off with a "message" which is from the DNA section it mirrors.
Strand #1 TACTGTTTTTGA
The mRNA travels to a place within the cell that can "read" the message it is carrying. In our Morse code example, this is now the station that has received the message and must translate it into useable information. The apparatus that reads the message is a "ribosome". The ribosome recognizes the code for START, which universally is AUG. The code is in threes so the next would be ACA. ACA codes for an amino acid, threonine that is brought into line and held, then the next threesome is read, UUU. This codes for another amino acid phenylalanine, which is added to the threonine in a chain form. When this process gets done, the chain is usually a large part of a protein. The process continues until a STOP code is reached, ACU in our example code. After the STOP is reach, the partial protein chain is cleaved off. Many times, it combines with another part of a protein making a fully functional protein. Still other times this chain IS the complete molecule and it is ready to go off and work (insulin is an example).
So what is a DNA MARKER?
In the DNA code is a large amount of non-coding DNA or "junk". In the junk are long repeats of bases. These long repeated sequences are Microsatellites. They consist of two to four bases, and can be repeated 10 to 100 times. A Microsatellite might be "TAGTAGTAGTAGTAGTAGTAG." These areas are useful because Markers basically are areas within the DNA that we know the location and can consistently find. Markers are not actual genes but usually Microsatellites. These areas are associated with the presence of a certain trait. For example in a coat color analysis, a TAGTAGTAG marker may be frequently found on a specific piece of DNA (out of all the DNA in the dog) in DNA samples from black dogs but not white dogs. If a DNA marker is often found in Black dogs, it must be close to or include the section of DNA that actually has the gene for black coat color. The more often the marker is found in dogs that have black coats, the closer it is to the actual DNA that controls coat color. A marker that is associated with a characteristic 90 percent of the time is closer than a marker found 60 percent of the time. The goal is to locate markers at regular intervals along the DNA, like the mile-markers along the highway. Once the "mile markers" are established, we can screen DNA samples for common markers associated with a specific trait.
Some Simple & Quick Terms
Chromosome = a structure of compact, intertwined DNA that carry genetic information (Genes)
Locus = is the position on a chromosome where a gene, or some other sequence, is located. A specific location on a chromosome.
Gene = the ordered sequence of chemical bases located in a certain position on a chromosome that encodes a specific functional product.
DNA Marker = a piece of the DNA that is associated with a certain trait and used to help scientists determine the location of genes that control important traits.
By Christa Cook-Chiaurro