Your hidden code.

The human body is made of 10,000 trillion cells, most of which are too small to see with the naked eye.

At the heart of each cell in the body lies the nucleus, or the cell's control center.

Inside the nucleus are 23 pairs of tightly coiled, threadlike structures called chromosomes. One chromosome in each pair comes from the person's father, and the other from the mother.

Coiled up in each chromosome is a long, double-stranded DNA molecule. If you unravel all of the chromosomes in a single nucleus, you get a stretch of DNA about a meter (3 ft) long.

The human DNA contains about 80,000 to 100,000 genes. Each gene tells a cell how to make a specific protein to build bones, fight infections, etc.

DNA consists of two long strands that twist around each other like a spiral staircase, the strands are held together by four chemical bases: adenine (A), thymine (T), guanine (G), and cytosine (C). Each "step" in the DNA staircase is made up of two bases paired together--A with T, and G with C.

breaking the code

Scientists cut DNA into small, random pieces that serve as templates, or patterns. They then clone, or make exact replicas of the templates.

Scientists use the template clones to generate a set of copies. Each copy will be one base letter longer than the next. This helps ensure that scientists don't miss a single letter in the DNA's sequence.

To make the copies, scientists put the clones in four different solutions where they start to replicate. Each solution contains a specific chemical that stops the process when it reaches a particular base letter.

Now, each solution contains fragments of different sizes. The fragments are put in a sequencing machine. Inside, a gel acts as a mesh that sorts the fragments by size.

Applying a negative charge at the top of gel-filled tubes and a positive charge at the bottom pulls DNA fragments down by size. A laser and detector read the sequence as the fragments pass by and send it to a computer. The machine sequences 450 to 800 bases at a time. From beginning to end, it takes about a week to sequence a 1,500-base-long strand of DNA.

your body your genes

The goal of the Human Genome Project is to produce a detailed map of all the genes in the human body by 2003. To do this, scientists first need to figure out the exact order or sequence of the 3 billion base letters that make up human DNA.

Just as letters in the alphabet can form words, the sequence of base letters in DNA spells out a list of instructions known as the genetic code. Within this genetic code are smaller, more specific sets of instructions called genes. Scientists estimate that the entire human DNA contains about 80,000 to 100,000 genes.

Even though all the cells in the body contain the exact same genes, the genes that are "turned on" in each cell vary depending on the cell's function. The diagram below shows the number of working genes in different parts of the body.

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