Chapter 15

Summary		Questions		Media Resources
15.1 The Central Dogma traces the flow of gene-encoded information.
• The Central Dogma, DNA Æ RNA Æ protein requires three principal kinds of RNA: messenger RNA (mRNA), transcripts of genes used to direct the assembly of amino acids into proteins; ribosomal RNA (rRNA), which combines with proteins to make up the ribosomes that carry out the assembly process; and transfer RNA (tRNA), molecules that transport the amino acids to the ribosome for assembly into proteins.		1.  What are the three major classes of RNA? What is the function of each type? 2.  What is the function of RNA polymerase in transcription? What determines where RNA polymerase begins and ends its function?		• Gene Activity •Discovery of Messenger RNA (mRNA) (Jacob/Meselson/Brenner) •Proving the tRNA Hypothesis (Chapeville)
15.2 Genes encode information in three-nucleotide code words.
• The sequence of nucleotides in DNA encodes the sequence of amino acids in proteins. The mRNA transcribed from the DNA is read by ribosomes in increments of three nucleotides called codons.		3.  How did Crick and his colleagues determine how many nucleotides are used to specify each amino acid? What is an anticodon?		•The Genetic Code is Read in Three Bases at a Time (Crick) •Breaking the Genetic Code (Nierenberg/Khorana)
15.3 Genes are first transcribed, then translated.
• During transcription, the enzyme RNA polymerase manufactures mRNA molecules with nucleotide sequences complementary to particular segments of the DNA. • During translation, the mRNA sequences direct the assembly of amino acids into proteins on cytoplasmic ribosomes. • The information in a gene and in an mRNA molecule is read in three-nucleotide blocks called codons. • On the ribosome, the mRNA molecule is positioned so that only one of its codons is exposed at any time. • This exposure permits a tRNA molecule with the complementary base sequence (anticodon) to bind to it. • Attached to the other end of the tRNA is an amino acid, which is added to the end of the growing polypeptide chain.		4.  During protein synthesis, what mechanism ensures that only one amino acid is added to the growing polypeptide at a time? What mechanism ensures the correct amino acid is added at each position in the polypeptide? 5.  How does an mRNA molecule specify where the polypeptide it encodes should begin? How does it specify where the polypeptide should end? 6.  What roles do elongation factors play in translation?		•Transcription •Translation •Polyribosomes •Transcription •Translation *Activity: Transcription *Activity: Translation •Proving the tRNA Hypothesis (Chapeville) • Translation
15.4 Eukaryotic gene transcripts are spliced.
• Most eukaryotic genes contain noncoding sequences (introns) interspersed randomly between coding sequences (exons). • The portions of an mRNA molecule corresponding to the introns are removed from the primary RNA transcript before the remainder is translated.		7.  What is an intron? What is an exon? How is each involved in the mRNA molecule that is ultimately translated?		•Discovery of Introns (Chambon) •Introns and Exons •Gene Information Processing-Prokaryotes •Gene Information Processing-Eukaryotes

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