DNA: The Code of Life

The double helix

DNA stores its information as a code made of four chemical bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The order of the bases carries the information, a bit like the order of letters in words and sentences.

Each base is attached to a sugar and a phosphate. Together, a base, a sugar, and a phosphate make a nucleotide. Nucleotides line up in two long strands that twist around each other into a spiral called a double helix.

The double helix is a bit like a twisted ladder. The sugars and phosphates form the two sides. The bases form the rungs, and they always pair the same way: A with T, and C with G. Each rung is called a base pair. Because the pairing rule never changes, each strand can serve as a pattern for copying DNA when a cell divides.

DNA in nearly every cell

Nearly every cell in a person’s body has the same DNA. Most of it is in the cell’s nucleus. A small amount is also found in the mitochondria, the parts of the cell that turn the energy from food into a form the cell can use.

Human DNA has about 3 billion bases, and more than 99 percent of those bases are the same in all people.

Chromosomes, genes, and the genome

A DNA molecule is very long, so cells pack it up. In the nucleus, DNA is wound tightly around proteins into thread-like structures called chromosomes. Most of the time, chromosomes cannot be seen, even under a microscope. They pack more tightly and become visible when a cell divides.

Human cells normally have 23 pairs of chromosomes, 46 in all.

A gene is a section of DNA. It is the basic unit of heredity. Some genes are instructions for making proteins, which the body needs to work. Many other genes do not make proteins; instead, they help control other genes. Human genes range from a few hundred base pairs to more than 2 million.

How many genes make proteins in humans? The Human Genome Project estimated 20,000 to 25,000. Later studies counted about 19,900, so “roughly 20,000” is a fair summary, and the number may still change.

All of the DNA instructions in a cell make up its genome. The human genome includes the 23 pairs of chromosomes in the nucleus, plus a small chromosome in the mitochondria.

How do genes pass from parents to children? That is the topic of our Heredity and Genetics unit.

From gene to protein

How does a gene become a protein? It happens in two main steps.

1. Transcription. In the nucleus, the gene’s information is copied into a similar molecule called RNA. This copy is called messenger RNA (mRNA) because it carries the message out of the nucleus. RNA uses the base uracil (U) where DNA uses thymine.
2. Translation. Out in the cytoplasm, a ribosome reads the messenger RNA. Each group of three bases, called a codon, usually stands for one amino acid. Another kind of RNA brings the amino acids, and they are joined one at a time into a chain. Building stops when the ribosome reaches a “stop” codon.

The chain of amino acids becomes a protein. Proteins are built from 20 common amino acids, a little like words built from an alphabet. The flow of information from DNA to RNA to protein is one of the main ideas in biology.

Mutations

A mutation is a change in the DNA sequence. For a change in a gene, many scientists now say variant, because changes in DNA do not always cause disease. Mutations can come from mistakes when DNA is copied before a cell divides, from things in the environment that damage DNA, or from some virus infections.

Mutations happen in our cells all the time, but almost none of them affect our health. Cells have machinery that repairs many changes quickly. Some changes do not even change the protein, because more than one codon can stand for the same amino acid.

• Most mutations have no effect on health.
• Some can help. For example, a change could make a protein that protects against a new kind of bacteria. Over many generations, natural selection can make helpful changes more common.
• Some are harmful and can cause genetic disorders.

Changes in body cells, such as skin or muscle cells, are not passed on to children. Changes in egg or sperm cells can be.

Who found the double helix?

Rosalind Franklin was a researcher in Maurice Wilkins’s lab. She used a method called X-ray diffraction, studying the patterns that X-rays make when they pass through DNA, to learn about its structure.

James Watson and Francis Crick built models of DNA. They were able to piece together its structure on the basis of Franklin’s data. The double helix was worked out in 1953, thanks to the work of Watson, Crick, Wilkins, and Franklin.

In 1962, Watson, Crick, and Wilkins were awarded the Nobel Prize. By then Franklin had died, and Nobel Prizes are not awarded after a person’s death. Today, she is recognized as one of the four scientists whose work revealed the double helix.

Key words

• DNAthe molecule that carries hereditary information in nearly every cell
• Baseone of the four chemical “letters” of DNA: A, T, C, or G
• Base pairtwo bases joined across the DNA ladder: A with T, or C with G
• Double helixthe twisted-ladder shape of DNA
• ChromosomeDNA wound tightly around proteins into a thread-like structure
• Genea section of DNA; the basic unit of heredity
• Genomethe entire set of DNA instructions in a cell
• Transcriptioncopying a gene into messenger RNA
• Translationreading messenger RNA to build a protein
• Codonthree bases in a row that usually stand for one amino acid
• Mutationa change in the DNA sequence

Game: Structure, Protein, or Mutation?

You will see twelve clues, one at a time. Decide whether each one is about the structure of DNA, making a protein, or mutations. A clue you sort on your first try earns 1 point, for a total of 12. Your best score is saved on this device.

Quick quiz

Six questions. Pick one answer for each, then press Check my answers.