Genes, Description of Table 657 The genetic code that shows how 20 amino acids are specified by three-base-pair codonsa First position (5 end) U C A G Third position (3 end) Second position U C A G UUU UUC PHE UUA UUG LEU CUU CUC CUA LEU CUG AUU AUC ILE AUA AUG MET GUU GUC GUA VAL GUG UCU UCC UCA SER UCG CCU CCC CCA PRO CCG ACU ACC ACA THR ACG GCU GCC GCA ALA GCG UAU UAC TYR UAA UAG STOP CAU CAC HIS CAA CAG GLN AAU AAC ASN AAA AAG LYS GAU GAC ASP GAA GAG GLU UGU UGC CYS UGA STOP UGG TRP CGU CGC CGA ARG CGG AGU AGC SER AGA AGG ARG GGU GGC GGA GLY GGG U C A G U C A G U C A G U C A G a The three stop codons (UAA, UAG, and UGA) and the start codon for methionine (AUG) are shown in bold Most amino acids are specified by more than a single codon, with several (arginine, leucine, and serine) specified by six codons There is a weakly positive general relationship between the frequency of incorporation of an amino acid into proteins and the number of codons specifying the amino acid In most cases, a change in the third position in a codon does not result in a change in the amino acid because most tRNA anticodons will still bind to mRNA codons if the first two positions on the mRNA match the tRNA This is the so-called wobble effect and redundancy in the genetic code acid molecule, and the genetic code must follow this direction for the language of transcription and translation to be faithfully transmitted For instance, RNA polymerases transcribe by adding new nucleotides to the 30 end of a growing RNA molecule, and so create the RNA in the 50 –30 direction However, transcription is from a complementary DNA chain that is read in the 30 –50 direction In general, reading frames of DNA are written by placing the 50 end on the left, and protein sequences are written in the same left-to-right direction specified by the nucleotide sequence (e.g., Table 2) Prokaryotic Genomes The genomes of most prokaryotes (bacteria and blue-green algae) are composed of one double-stranded circular DNA molecule attached to a central core of proteins in a series of supercoiled loops emanating from the center like the cotton fibers of a dust mop Although the nucleoid ‘‘chromosome’’ is not enclosed within a nuclear membrane, it tends to be found in a particular part of the cytoplasm An additional feature of prokaryotes is their ability to acquire smaller circular DNA molecules, called plasmids Plasmids contain numerous genes, including genes found in the chromosome Transfer of plasmids into and out of bacterial cells makes them useful in DNA-based biotechnology and is responsible for the rapid transfer of antibiotic resistance genes between bacterial strains Genes in prokaryotes are collinear and uninterrupted, in that one to several genes are expressed in the order in which they follow a promoter Transcription of mRNA leads directly to translation with little modification (Figure 3) This arrangement of several genes being expressed as a group is sometimes referred to as polycistronic, and is restricted to prokaryotic and organelle genomes The control of gene expression in prokaryotes is through either positive or negative regulation Under negative regulation, like that found in the tryptophan operon of E coli, a repressor protein combines with a tryptophan molecule and then binds to the repressor region of the gene, turning off transcription when tryptophan becomes common within the cell Under positive control, a transcription factor must bind with the regulatory site to initiate transcription This is part of the lac operon of E coli, one of the earliest metabolic pathways described at the molecular level by Jacques Monod and colleagues at the University of Paris The lac operon is mainly under the negative control of a repressor that is removed when lactose is present in the cell However, transcription also depends upon a second protein that must bind for transcription of RNA This double control allows the bacterial cell to turn off the lac operon when glucose, a preferred food source to lactose, is present in the cell Eukaryotic Genomes FChromosomes The genomes of eukaryotic organisms are characterized by DNA arranged in long linear molecules and associated proteins in a complex structure known as chromatin, which is contained within a membrane-bound nucleus Chromatin is in a diffused state during interphase of the cell cycle when transcription and translation occur When the chromatin coils and condenses during cell division, it forms itself into microscopically visible threads in the nucleus These threads are called chromosomes The number of chromosomes found in different eukaryotic groups varies considerably, but each group displays a characteristic range of chromosome numbers For example, humans and great apes have haploid chromosome numbers of N ¼ 23 and N ¼ 25, respectively, while