f nucleotides in the DNA is transcribed into a molecule of RNA in the cell’s nucleus (purple area). The RNA travels to the cytoplasm (blue-green area), where it is translated into the specific am
its specific sequence of bases. The red strand underneath represents the results of transcription: an RNA molecule. Its base sequence is complementary to that of the DNA. The purple chain re
st amino acids are specified by two or more codons. For example, both UUU and UUC stand for the amino acid phenylalanine (Phe). Notice that the codon AUG not only stands for the amino a
cleotides, making up the cap and tail, are attached at the ends of the RNA transcript. The exons are spliced together. The product, a molecule of messenger RNA (mRNA), then travels to the cy
t some of the bases. The site where an amino acid will attach is a three-nucleotide segment at one end (purple). Note the three-base anticodo
eotides that are not part of the message; that is, they are not translated. These nucleotides, along with the cap and tail (yellow), help the mRNA attach to the ribosome.
he mRNA. The initiator tRNA carries the amino acid methionine (Met); its anticodon, UAC, binds to the start codon, AUG.
A large ribo
counterpart, a gene for hemoglobin, by only one nucleotide. This difference changes the mRNA codon from one that codes for the amino acid glutamic acid (Glu) to one that codes for valine (V
RNA. The result in the polypeptide is a serine (Ser) instead of a glycine (Gly). This amino acid substitution may or may not affect the protein’s function.
When a nucleotide is deleted (or i
otein shell shaped like a 20-sided polyhedron, shown here in a computer-generated model. At each vertex of the polyhedron is a protein spik
osed within an elaborate structure made of proteins. The “legs” of the phage (called tail fibers) bend when they touch the cell surface. The tail is a hollow rod enclosed in a springlike sheath. A
mosome (lysogenic cycle) or immediately start the production of progeny phages (lytic cycle), destroying the cell. In most cases, the phage follows the lytic pathway, but once it enters a lysoge
ows the mottling of leaves in tobacco mosaic disease. The rod-shaped virus causing the disease has RNA as its genetic material.
t separate molecules of RNA, each wrapped in a protein coat. Around the outside of the virus is an envelope made of membrane, studded with protein spikes.
s the one that causes mumps. Like the flu virus, it has a membranous envelope with protein spikes, but its genome is a single molecule of RNA.
AIDS, acquired immunodeficiency syndrome, is one of the most significant health challenges facing the world today. The cause of AIDS is
infection by HIV, the human immunodeficiency virus. Since it was first recognized in 1981, HIV has infected an estimated 40 million people
worldwide and caused 3 million deaths. While there is no cure for AIDS, its spread can be slowed by anti-HIV drugs. One such drug, AZT, has