DNA is made up of two strands wound around each other and takes on a shape like that of a spiral staircase or a twisted ladder with rungs. The two strands are connected by combinations of four compounds called bases. Each base of one strand is paired with a base on the other strand. These base pairs form the rungs of the twisting DNA ladder. The exact order of the bases in the DNA molecule is what determines the genetic information it carries. Simply put, this sequence determines virtually everything about you, from the color of your hair to the shape of your nose.

Each protein carries out a specific function that is determined by its DNA gene. But how is the genetic information in a DNA gene decoded so that a particular protein is made? As shown in the accompanying diagram “How Proteins Are Made,” the genetic information stored in the DNA must first be transferred from the nucleus of the cell into the cytoplasm, where the ribosomes, or protein-producing factories, are located. This transfer is accomplished by means of an intermediary called ribonucleic acid (RNA). The ribosomes in the cytoplasm “read” the RNA instructions and assemble the proper sequence of amino acids to form a particular protein. Thus, there exists an interdependent relationship between DNA, RNA, and the formation of proteins.

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How DNA Replicates

For the sake of visual simplicity, the twisted DNA helix has been flattened

1 Before cells divide to produce the next generation of cells, they must replicate (make a copy of) the DNA. First, proteins help to unzip sections of the double-stranded DNA

Protein

2 Then, following strict base-pairing rules, free (available) bases in the cell are linked together with their matching bases on the two original strands

Free bases

3 Finally, two duplicate codebooks are made. So when the cell divides, each new cell gets an identical DNA codebook

Protein

Protein

The DNA base-pair rule:

A always with T

A T Thymine

T A Adenine

C always with G

C G Guanine

G C Cytosine

[Diagram on page 8, 9]

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How Proteins Are Made

For simplicity, we illustrate a protein made of 10 amino acids. Most proteins have more than 100

1 A special protein zips open a section of the DNA strands

Protein

2 Free RNA bases link up with the exposed DNA bases on one strand only, thus forming a strand of messenger RNA

Free RNA bases

3 The newly made messenger RNA peels off and moves away to the ribosomes

4 A transfer RNA picks up an amino acid and brings it to the ribosome

Transfer RNA

Ribosome

5 As the ribosome sweeps across the messenger RNA, a chain of amino acids is linked together

Amino acids

6 As it is being formed, the protein chain begins to fold into the shape needed to function properly. Then the chain is released by the ribosome

Transfer RNA has two important ends:

One recognizes the messenger RNA code

The other carries the correct amino acid

Transfer RNA

RNA bases use U rather than T, so U pairs with A

A U Uracil

U A Adenine

Blind Chance?

Recent findings of two British scientists confirm that the genetic code is not simply the product of random chance. “Their analysis has shown [the genetic code] to be among the best of more than a billion billion possible codes,” notes New Scientist magazine. Of the roughly 10²⁰ (1 followed by 20 zeros) possible genetic codes, only one was selected early in the history of life. Why this specific one? Because it minimizes errors made during the protein-making process or errors caused by genetic mutations. In other words, the specific code guarantees that laws of heredity are strictly followed. Although some ascribe the selection of this genetic code to “strong selective pressures,” the two researchers have concluded that “it is extremely unlikely that such an efficient code arose by chance.”