We've all learned that genes reside on the chromosomes, and chromosomes are made of DNA (deoxyribose nucleic acid), the gigantic macromolecule commonly called the "code of life." Fewer of us probably understand how that code becomes the proteins and enzymes essential to our physiology. Knowing generally how the code is revealed is a key step in solving the mystery surrounding gene transfer between organisms.
DNA is actually a long chain (or strand) of molecules called nucleotides that consist of one of four nitrogren-containing organic bases (named adenine, thymine, cytosine and guanine) with attached sugar (deoxyribose) and phosphate groups (Figure 1). The sequential arrangement of the nucleotides holds the directions for making the proteins. DNA actually consists of two helically woven strands, and these essentially comprise the chromosome. The bases in one DNA strand line up and pair with the bases in the opposite chain; however, the pairing is restricted to specific combinations of bases. Adenine only lines up (i.e., pairs) with thymine in the complementary DNA chain, and cytosine only pairs with guanine.
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| DNA is a polymer of nucleotides consisting of nitrogen containing bases (letter coded). Each chromosome has a double strand of DNA. The strands are held together by pairing of complementary bases through hydrogen bonding. The paired polymers assume a helical shape. |
Three major steps--replication, transcription, translation--lead from DNA to functional proteins. First the DNA must replicate itself before a cell divides. The complementary chains unwind, and each chain serves as a template upon which a new DNA chain is assembled nucleotide by nucleotide with the aid of an enzyme called DNA polymerase (Figure 2). For example, when the template strand of DNA contains the base adenine, the DNA polymerase links the base thymine onto the growing new strand. The newly synthesized strand remains associated with the old strand, essentially forming a "new" replicated chromosome.
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| DNA strand unwinds for replication. Each strand serves as a template for synthesis of a new strand by DNA polymerase. |
Proteins are not synthesized from the DNA code
directly. First, a macromolecule called messenger RNA is transcribed
from the DNA (Figure 3). RNA is made up of linked nucleotides
identical to those found in DNA with two exceptions. The sugar
molecule is ribose and the base uracil subsitutes for thymine,
but uracil still pairs with adenine. Transcription of the DNA
code starts with unwinding of the double-stranded DNA. Only one
of the strands serves as a template to make the RNA. The enzyme
responsible for linking the bases together, RNA polymerase, starts
synthesizing the mRNA at a site on the DNA template called the
promoter. The promoter is actually a sequence of DNA bases whose
only function is to serve as a recognition site for the polymerase.
In bacteria, the mRNA strand is synthesized nucleotide by nucleotide
until the polymerase comes across another region of DNA bases
called the terminator sequence. Plant and animal cells do not
use a specific base termination sequence, but they contain enzymes
that clip the mRNA to the correct size and add a sequence of adenine
bases at the tail end.
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| DNA strands unwind for messenger RNA synthesis. Only one strand is copied. The complementary RNA basis are linked together by the enzyme RNA polymerase. The polymerase recognizes a sequence of DNA bases called the promoter and starts assembling the mRNA after this sequence. Synthesis ends when a termination sequence is reached. |
Two other types of RNA are also produced from DNA templates--ribosomal RNA and transfer RNA. After its synthesized the mRNA moves to cell organelles called the ribosome that are made of wound up strands of rRNA (Figure 4). Like an old stock market ticker tape, the mRNA strand moves along the ribosome while a set of three bases on the tRNA, known as the anticodon, pair up with complementary three bases on the mRNA. On its end opposite the anticodon, each tRNA holds an amino acid. As each amino acid comes into the vicinity of the mRNA held by the ribosome, the amino acids are linked together to form the protein.
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| Messenger RNA is bound to a ribosome unit for translation of the DNA code into proteins. Transfer RNA carry bound amino acids to the ribosome and line up on the mRNA by codon and anticodon base pairing. An enzyme links the chain of amino acids to form the protein. |