Gene Expression: From Transcription to Translation PDF

CHAPTER 11 Gene Expression: From Transcription to Translation 11.1 The Relationship between Genes and Proteins (1) • & Genes store information for producing all cellular proteins. Early observation suggested a direct relationship between genes and proteins. - • – – Garrod studied the relation...

CHAPTER 11 Gene Expression: From Transcription to Translation 11.1 The Relationship between Genes and Proteins (1) • & Genes store information for producing all cellular proteins. Early observation suggested a direct relationship between genes and proteins. - • – – Garrod studied the relationship between a specific gene, a specific enzyme, and a metabolic condition (alcaptonuria). Beadle and Tatum formulated the “one gene–one enzyme” hypothesis. e - n The Beadle-Tatum experiment Loading… The Relationship between Genes and Proteins (2) • • Beadle and Tatum’s hypothesis was later modified to “one gene–one polypeptide chain” Mutation in a single gene causes a single substitution in an amino acid sequence of a single protein. & e - e The Relationship between Genes and Proteins (3) • – – – An Overview of the Flow of Information through the Cell Messenger RNA (mRNA) is an intermediate between a gene and a polypeptide. Transcription is the process by which RNA is formed from a DNA template. Translation is the process by which proteins are synthesized in the cytoplasm from an mRNA template. Loading… mRNA - RNA-DNA-protein Overview of the flow of information in eukaryotes The Relationship between Genes and Proteins (4) • • • There are three classes of RNA in a cell: mRNA, ribosomal RNA (rRNA), and transfer RNA (tRNA). rRNA recognizes other molecules, provide structural support, and catalyzes the chemical reaction in which amino acids are linked to one another. tRNAs are required to translate information in the mRNA code into amino acids. Structure of a bacterial ribosomal RNA • • – – 11.2 An Overview of Transcription and Translation in Both Prokaryotic and Eukaryotic Cells (1) DNA-dependent RNA polymerases (or RNA polymerases) are responsible for transcription in both prokaryotes and eukaryotes. These enzymes incorporate nucleotides into a strand of RNA from a DNA template. The promoter is where the enzyme binds prior to initiating transcription. The enzyme require the help of transcription factors to recognize the promoter. Chain elongation during transcription Chain elongation during transcription Loading… An Overview of Transcription and Translation in Both Prokaryotic and Eukaryotic Cells (2) • – – • • The newly synthesized RNA chain grows in a 5’ to 3’ direction antiparallel to the DNA. RNA polymerase must be processive – remain attached to DNA over long stretches. RNA polymerase must be able to move from nucleotide to nucleotide. Nucleotides enter the polymerization reaction as trinucleotide precursors. The reaction is driven forward by the hydrolysis of a pyrophosphate: PPi 2Pi An Overview of Transcription and Translation in Both Prokaryotic and Eukaryotic Cells (3) • • Once polymerase has finished adding nucleotides, the DNA-RNA hybrid dissociates and the DNA double helix reforms. There are two enzymatic activities of RNA polymerase: digestion of incorrect nucleotides and polymerization. An Overview of Transcription and Translation in Both Prokaryotic and Eukaryotic Cells (4) • – – Transcription in Bacteria There is only one type of RNA polymerase in prokaryotes: five subunits associated to form a core enzyme. Transcription-competent cells also have a sigma factor attached to the RNA polymerase before attaching to DNA. Initiation of trancription in bacteria An Overview of Transcription and Translation in Both Prokaryotic and Eukaryotic Cells (5) • Bacterial promoters are located upstream from the site of initiation. – Two conserved regions: –35 element (consensus sequence) and Pribnow box. • • Differences in the DNA sequences at both –35 element and the Pribnow box may regulate gene expression. Termination in bacteria can either require a rho factor protein or may reach a terminator sequence without rho. The basic element of a promoter region in bacteria An Overview of Transcription and Translation in Both Prokaryotic and Eukaryotic Cells (6) • – • • • Transcription and Processing in Eukaryotic Cells There are three types of RNA polymerases in eukaryotes. Most rRNAs are transcribed by RNA polymerase I. mRNAs are transcribed by RNA polymerase II. tRNAs are transcribed by RNA polymerase III. Eukaryotic Nuclear RNA Polymerases A comparison of prokaryotic and eukaryotic RNA polymerase structure It An Overview of Transcription and Translation in Both Prokaryotic and Eukaryotic Cells (7) • • – – – Transcription factors regulate the activity of RNA polymerases. Newly transcribed RNAs are processed. A primary transcript (or pre-RNA) is the initial RNA molecule synthesized. A transcription unit is the DNA segment corresponding to a primary transcript. A variety of small RNAs are required for RNA processing. 11.3 Synthesis and Processing of Ribosomal and Transfer RNAs (1) • • • A eukaryotic cell may contain millions of ribosomes. The DNA sequence encoding rRNA are called rDNA, and is typically clustered in the genome. In nondividing cells, rDNA are clustered in the nucleoli, where ribosomes are produced. The composition of a mammalian ribosome The nucleolus