Saturday, February 26, 2011

Genes, DNA, Chromosomes, Genome:

Genes, DNA, Chromosomes, Genome:


So, what’s a gene? Inside each of our cells (except red blood cells, which is another story), we have deoxyribonucleic acid (DNA) molecules. Genes are pieces of DNA that serve as the units of heredity. In cells the DNA is organized into chromosomes, and the entire chromosome complement of a cell is called its genome. Genes contain the code for proteins.

In fact, every living cell, be it a human cell, animal cell, plant cell, yeast cell, fungus cell, protozoan cell, or a bacterial cell has DNA in it. It’s the blueprint of life. It’s what guides every aspect of life development. Our DNA is the guidebook that told our cells to develop as human. My dog’s DNA has many similarities to mine, but enough differences so that she’s not human, she’s a dog (although sometimes it’s uncanny how “human” she can act—also, another story).

DNA guides the chemistry of life. Each gene codes for something—mostly proteins. Another molecule, RNA (ribonucleic acid) is the immediate result of the biochemical reading of DNA. RNA is the carrier of the message to the workhorse of the cell, the ribosome. The ribosome reads the RNA, and a protein is made from the messenger RNA (mRNA).

It’s those proteins that do the work that keeps everything going. They keep the cells burning energy, determine your eye color, determine your development and growth, determine if your earlobes are attached or not, and build the machinery that is your body. All things mundane and fabulous.

DNA is an organic molecule (there’s that word again), meaning that the molecule is constructed on a backbone of Carbon molecules. The molecule is a very long chain that consists of 2 strands organized into a double helix structure. The two strands are bound together along their length. The long DNA chains are made up of nucleotides along a backbone made up of sugars (dexoyribose) and phosphate molecules. Attached to these sugar molecules are organic bases. There are 4 bases in DNA: Adenine (A), Guanine (G), Cytosine (T), and Thymine (T). See the pictures at the top for cartoons of DNA structure and the chemical structure of a C-T base pair.

RNA is a single helix made up of nucleotides along a backbone made up of the sugar ribose with phosphate molecules. Three of the bases in RNA are the same as those in DNA: Adenine (A), Guanine (G), and Cytosine (C); however, there is no thymine in RNA. It is replaced with another base named Uracil (U).

One analogy for DNA is that it’s an alphabet. However, this alphabet only has 4 letters: AGCT. All the variability in life on Earth is derived from different combinations of AGCT.

The two helices of the DNA molecule are connected to each other through bonds between the base molecules on each strand. However, A’s only pair up with T’s, and C’s only pair up with G’s. In that way, the 2 strands complement each other.

Length in DNA is measured in bases. A gene can be hundreds to thousands (kilobases) of bases long, so those 4 bases can combine into countless unique combinations. Hence the genetic variability of life on Earth. Each cell that contains a genome is like a hard drive on a computer. All the information necessary to run all of the functions is contained there. That’s a lot of information! So, there’s a lot of DNA in each cell. How much? If the contents of one human cell’s genome was arranged in a straight line, it would be over 6 feet long! And there are anywhere between 10 and 100 trillion cells in our bodies! (60 – 600 trillion feet of DNA/person! -- 113 billion miles or 182 billion kilometers!)

DNA is the molecule (and an elegant one at that). Genes are pieces of DNA that code for proteins (mostly). The genes and DNA are organized into chromosomes (humans have 23 pairs of chromosomes). All of the genetic information in the nucleus is the genome. So, when they say that they’ve sequenced the human genome, that means they’ve determined the DNA sequence (the combination of AGCT) for each chromosome. In a human male, that’s 3 billion DNA nucleotide pairs!

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