15.1 The Central Dogma traces the flow of gene-encoded information.
Cells Use RNA to Make Protein
• Ribosomes are composed of a large and a small subunit. (p. 302)
• Ribosomal RNA (rRNA) functions in polypeptide synthesis; transfer RNA (tRNA) transports amino acids to the ribosome for use in building polypeptides; and messenger RNA (mRNA) is transcribed from DNA and travels to ribosomes to direct polypeptide assembly. (p. 302)
• The Central Dogma is that information passes from DNA to mRNA (transcription), and then mRNA directs the sequential assembly of amino acids into proteins (translation). (p. 303)
15.2 Genes encode information in three-nucleotide code words.
The Genetic Code
• The genetic code is nearly universal. It is read in codons or triplets, increments of three nucleotides on mRNA, with each triplet coding for an amino acid. (pp. 304-305)
• There are 64 possible codons that code for 20 amino acids. (p. 304)
15.3 Genes are first transcribed, then translated.
Transcription in Prokaryotes
• Only the template strand of DNA is transcribed, while the coding strand is not. (p. 306)
• Transcription begins at the promoter site, and the transcription bubble moves along the DNA segment until the termination sequence is reached; the RNA strand then dissociates, and the DNA rewinds. (pp. 306-307)
Transcription in Eukaryotes
• Eukaryotic transcription differs from prokaryotic transcription because there are three different RNA polymerase enzymes, an initiation complex forms at the promoter, and posttranscriptional modification of the RNA occurs. (pp. 308-309)
Translation
• Translation begins when the initial portion of an mRNA molecule binds to an rRNA molecule in a ribosome. (p. 310)
• Each step of the ribosome's progress exposes a codon to which a tRNA molecule with the complementary anticodon binds. The amino acid carried by each tRNA molecule is added to the end of the growing polypeptide chain. (pp. 311-312)
15.4 Eukaryotic gene transcripts are spliced.
The Discovery of Introns
• Eukaryotic genes are interrupted by sequences not represented in the mature mRNA and the protein. Introns are noncoding sequences that interrupt coding sequences, or exons. (p. 313)
• Spliceosomes are responsible for splicing and removing the introns prior to mRNA translation. (p. 314)
Differences Between Prokaryotic and Eukaryotic Gene Expression
• Gene expression is similar in both prokaryotes and eukaryotes, although some differences do exist, such as ribosomal size and the possession of introns. (p. 316)