There are many compelling reasons to determine the complete genetic information of the human organism. “The sequence of the human genome would be perhaps the most useful tool ever developed to explore the mysteries of human development and disease
,” said Dr. Lee Hood
A genome is all of a living thing's genetic material. It is the entire set of hereditary instructions for building, running, and maintaining an organism, and passing life on to the next generation. Here is a description of human genome in top-down order:
- A chromosome is a package containing a chunk of a genome.
- In a few very simple forms of life, such as bacteria, the entire genome is packaged into a single chromosome.
- Other organisms with larger genomes divide their hereditary material among a number of different chromosomes.
- For example, a human genome has 23 pairs of chromosomes.
- We each possess two sets of each chromosome, with one set inherited from each parent. It's why we look a bit like our parents, but also different.
- Chromosomes are composed of strands of DNA.
- DNA is made up of tens of thousands of genes
- DNA double helix was first discovered by Francis Crick and James Watson in 1953
- What makes DNA so amazing is that it contains just four letters but all sorts of combinations of those four letters contains all the information for making all the creatures that are on the planet.
- Approximately 99.9% of DNA sequences are similar across the human population.
- There are approximately 3-billion-plus chemical bases that make up the human DNA code
- Four chemical bases, or nucleotides, are known most commonly by the letters:
- A (adenine), G (guanine), C (cytosine), and T (thymine)
- A,G, C, and T are key structural elements for the genes that individually or in combination determine everything from a person's hair color to their predisposition for Parkinson's disease.
- Protein-coding genes
- Many genes get translated into proteins, and these proteins make the stuff of our bodies. One protein makes hair; another makes cartilage; others make muscle.
- The vast bulk of the double helix, some 98 percent of it, doesn't code for proteins. The genes which do comprise just two percent. However, a decade-long project, the Encyclopedia of DNA Elements (ENCODE), has found that 80% of the human genome serves some purpose, biochemically speaking.
- Totally 20,000 to 25,000 protein-coding genes were found in a human genome, the same number as a chicken, less than an ear of corn.
- It wasn't just that we had so few genes, but many of our key genes were identical to those of other animals.
- Q: "How do you get all these differences, if you have really similar sets of genes?"
- A: It's not the number of genes that counts. Scientists now realize that not all genes are created equal. Some make the stuff of our bodies, and switches are needed to turn many of these stuff genes on and off. The body-plan genes are what throw these switches, which tell the stuff genes what to do and when.
- Switches are not genes. They don't make stuff like hair, cartilage or muscle, but they turn on and off the genes that do.
- Switches are very powerful parts of DNA, because they allow animals to use genes in one place and not another; at one time, and not another.
- We now know these genetic switches exist. But they're still very hard to find.
- Body-plan genes (i.e., Hox genes)
- Hox genes have been found in all complex animals, from the velvet worm that dates back some 600 million years, to the modern human. And in all that time, the letters of their DNA have remained virtually unchanged.
- They give orders that cascade through a developing embryo, activating entire networks of switches and genes that make the parts of the body. They are absolutely critical to the shape and form of a developing creature.
- All it needed was a few mutations, a few changes to the timing and order of what was turned off and on, and a fin could become a limb.
Many genes get translated into proteins, and these proteins make the stuff of our bodies. One protein makes hair; another makes cartilage; others make muscle. Besides those protein-coding genes, some genes are switches which turn other genes on and off. And, still other genes (i..e, body-plan genes) that give those switches orders. Together, in a complex cascade of timing and intensity, they combine to produce the amazing diversity of life on this planet.