The Recipe for Life

The machinery of every human mind and body is built and run with fewer than 100,000 kinds of protein molecules. 

And for each of these proteins, we can imagine a single corresponding gene whose job it is to ensure an adequate and timely supply.

In a material sense, all of the subtlety of our species, all of our art and science, is ultimately accounted for by a surprisingly small set of discrete genetic instructions. 

The differences between two unrelated individuals, between the man next door and Mozart, may reflect a mere handful of differences in their genomic recipes -- perhaps one altered word in five hundred. 

We are far more alike than we are different. At the same time, there is room for near-infinite variety. 

 

 

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It is no overstatement to say that to decode our 100,000 genes in some fundamental way would be an epochal step toward unraveling the manifold mysteries of life.

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The human genome is the full complement of genetic material in a human cell. (Despite five and a half billion variations on a theme, the differences from one genome to the next are minute; hence, we hear about the human genome -- as if there were only one.) 

The genome is distributed among 23 sets of chromosomes have been replicated and re-replicated since the fusion of sperm and egg that marked our conception. The source of our personal uniqueness, our full genome, is therefore preserved in each of our body's several trillion cells. 

At a more basic level, the genome is DNA, deoxyribonucleic acid, a natural polymer built up of repeating nucleotides, each consisting of a simple sugar, a phosphate group, and one of four nitrogenous bases. 

In the chromosomes, two DNA strands are twisted together into an entwined spiral -- the famous double helix -- held together by weak bonds between complementary bases, adenine (A) in one strand to thymine (T) in the other, and cytosine to guanine (C-G). 

In the language of molecular genetics, each of these linkages constitutes a base pair. If we count only one of each pair of chromosomes, the human genome comprises about three billion base pairs. 

 

 

DNA  ---->  RNA  -----> Protein 

Every protein is made up of one or more polypeptide chains, each a series of (typically) several hundred molecules known as amino acids, linked by so-called peptide bonds. 

Only 20 amino acids suffice as the building blocks for all human proteins. The synthesis of a protein chain, then, is simply a matter of specifying a particular sequence of amino acids. 

Each linear sequence of three bases (both in RNA and in DNA) corresponds uniquely to a single amino acid. The RNA sequence AAU thus dictates that the amino acid asparagine should be added to a polypeptide chain, GCA specifies alanine -- and so on. 

A segment of the chromosomal DNA that directs the synthesis of a single type of protein constitutes a single gene.