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How are new DNA molecules made?

New DNA molecules are made by copying, using old DNA molecules as a template.

Our understanding of this process also goes back to Watson and Crick's observation that only certain base pairings can exist in DNA. Each DNA strand contains the information necessary to reconstruct the other, or complementary, strand.

When a cell needs to copy a DNA molecule, it "unzips" part of the double helix, breaking the rungs of the ladder in half so that the molecule separates down the middle. New nucleotides, floating free in the cell, can then hook up with complementary nucleotides along each strand. Gradually the unzipping proceeds, and the new strands continue to grow until one DNA molecule becomes two identical DNA molecules.

A cell copies all of its DNA in this fashion each time it divides. In the cells of complex organisms such as humans, this process takes an average of 8 hours. In other words, each human cell can read and reproduce the entire genome sequence in one working day.

Scientists use a similar method to make copies of DNA in the laboratory. They put a piece of DNA in a test tube along with a bunch of free nucleotides, short DNA sequences called primers, and some enzymes that help the process along. Given the right conditions of chemistry and temperature, up to a billion DNA molecules, all identical to the original template molecule, may be produced in a matter of hours.

How does DNA tell a cell about making proteins?

DNA tells a cell how to make proteins through the genetic code.

Both DNA and proteins are long molecules made from strings of shorter building blocks. While DNA is made of nucleotides, proteins are made of amino acids, a group of 20 different chemicals with names like alanine, arginine, and serine. The genetic code enables a cell to translate the nucleotide language of DNA into the amino acid language of proteins.

In the genetic code, each group of three nucleotides—known as a "triplet" or "codon"—stands for a specific amino acid. For example, GCA stands for alanine, AGA stands for arginine, and AGC stands for serine. There are 64 possible codons, but only 20 amino acids, so more than one codon may code for a single amino acid. For example, GCA, GCC, and GCG all mean alanine.

For the most part, the genetic code is the same across every form of life, from bacteria to sea stars to German shepherds to humans. A few species might translate a codon or two differently—GCA means alanine for most species, but could mean valine in a few organisms. But everyone uses three-letter codons and most of the same codon-amino acid relationships.

How much DNA is in a gene? How much is in a genome?

Both genes and genomes come in a variety of sizes.

About 1,000 base pairs would be enough DNA to encode most proteins. But introns—"extra" or "nonsense" sequences inside genes—make many genes longer than that. Human genes are commonly around 27,000 base pairs long, and some are up to 2 million base pairs.

Very simple organisms tend to have relatively small genomes. The smallest genomes, belonging to primitive, single-celled organisms, contain just over half a million base pairs of DNA.

But among multicellular species, the size of the genome does not correlate well with the complexity of the organism. The human genome contains 3 billion base pairs of DNA, about the same amount as frogs and sharks. But other genomes are much larger. A newt genome has about 15 billion base pairs of DNA, and a lily genome has almost 100 billion.

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Updated on January 15, 2003