Genetics 101 Part 2
Posted by Kristal Louie on Mon, Feb 01, 2010 @ 02:00 PM
Blog 2: What's a gene?
If DNA is the genetic material, then what are genes? To understand this, we need to go back to the 1850's to the father of genetics, Gregor Mendel and his experiments studying traits in pea plants. In analyzing his data, he suggested factors exist that pass traits from parents to offspring, and called those factors genes. When DNA was discovered years later, many scientists quickly made the association that Mendel's factors, or genes, were composed chemically of DNA molecules.
Still not clear? Let's start with the textbook definition of a gene: A hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic in an organism. Put more simply, a gene is just the stretch of DNA that when read by the cell, is the code for a specific trait or cellular function. More specifically, genes specify the sequence of amino acids within proteins, which then work with numerous other proteins to generate the trait or function. Eye color is a good way to explain this. There really isn't a blue-eye gene. Rather, there is a collection of genes that encode a collection of unique proteins, that when present together in the eye, produce a blue color.
A gene is made up of combinations of the four nucleotides (A, G, C and T) in a long sequence. These unique combinations determine the gene's function, much like letters join together to form words. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. DNA is copied into the related nucleic acid mRNA, in a process called transcription, as one step to making a protein. Humans have approximately 23,000 genes person encoded by billions of base pairs of DNA.
Genes are aligned precisely all along the length of 23 pairs of much larger structures: the chromosomes. One chromosome in each pair comes from the mother and the other one from the father. The chromosomes in any particular pair look like each other, except in a boy. There is one pair of chromosomes (the sex chromosomes), which usually settles the sex of the individual. In females, the sex chromosomes are both X-chromosomes. In males, the sex chromosomes are a mismatched pair consisting of one X-chromosome and one Y-chromosome. The Y-chromosome is transmitted only by the father.
Did you know? In 2001, the DNA sequence for humans, called the human genome, was completed. This scientific accomplishment represented the largest biomedical research project of the 20th Century. Interestingly, there were actually two competing human genome projects - one financed by the US government through the NIH and one financed by a private company, Celera Genomics. There was a bitter rivalry between NIH and Celera which was finally settled by President Clinton when he, at the White House, ceremoniously declared both groups "the winner". Nonetheless, most scientists believe that the competition between the two groups greatly enhanced and accelerated the project and led to its completion years sooner than anticipated. Celera had hoped to build a commercial enterprise around their version of the genome, but was unable to build any significant business due to the availability of the public sequence. In 2004, Celera made their sequence available, without charge, to all scientists world-wide, bringing an end to what had become known as the Genome War. For a great history of the project and to access the human genome reference sequence, visit the Department of Energy's Human Genome Project site.
Dennis Gilbert
www.sbgenetics.org
support@sbgenetics.org