Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Document
Document related concepts
Transposable element wikipedia , lookup
Community fingerprinting wikipedia , lookup
Metalloprotein wikipedia , lookup
Transcription factor wikipedia , lookup
DNA supercoil wikipedia , lookup
Alternative splicing wikipedia , lookup
Two-hybrid screening wikipedia , lookup
Point mutation wikipedia , lookup
Endogenous retrovirus wikipedia , lookup
Gene regulatory network wikipedia , lookup
Vectors in gene therapy wikipedia , lookup
Real-time polymerase chain reaction wikipedia , lookup
RNA interference wikipedia , lookup
Genetic code wikipedia , lookup
Artificial gene synthesis wikipedia , lookup
Non-coding DNA wikipedia , lookup
Promoter (genetics) wikipedia , lookup
Biosynthesis wikipedia , lookup
Polyadenylation wikipedia , lookup
Messenger RNA wikipedia , lookup
Nucleic acid analogue wikipedia , lookup
Eukaryotic transcription wikipedia , lookup
Silencer (genetics) wikipedia , lookup
RNA polymerase II holoenzyme wikipedia , lookup
RNA silencing wikipedia , lookup
Deoxyribozyme wikipedia , lookup
Transcriptional regulation wikipedia , lookup
Transcript
Overview: The Flow of Genetic Information • The information content of DNA is in the form of specific sequences of nucleotides • The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins • Proteins are the links between genotype and phenotype • RNA is the manager • Gene expression, the process by which DNA directs RNA and protein synthesis, includes two stages: transcription(RNA) and translation(protein) Evidence from the Study of Metabolic Defects • In 1902, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions • He thought symptoms of an inherited disease reflect an inability to synthesize a certain enzyme • Linking genes to enzymes required understanding that cells synthesize and degrade molecules in a series of steps, a metabolic pathway Nutritional Mutants in Neurospora: • George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal media • Using crosses, they and their coworkers identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine • They developed a “one gene–one enzyme” hypothesis, which states that each gene dictates production of a specific enzyme Lactose Operon of E. coli: • In the 1950s, Jacob and Monod worked with bacterial mutants to dissect gene control circuits • They and others developed evidence of a shortlived intermediate between a gene and the protein that it coded for • This intermediate was required for protein synthesis • Shown to be an RNA molecule-was named messenger RNA Basic Principles of Transcription and Translation • RNA is the bridge and the gatekeeper between genes and the proteins for which they code • Transcription is the synthesis of RNA using coded information in DNA • Transcription produces many classes of RNA • Translation is the synthesis of a polypeptide, using information in one class: messenger RNA • Ribosomes are the sites of translation • The “Central Dogma” is the old-fashioned concept that cells are governed by a cellular chain of command: DNA RNA protein • Idea developed in 1960s • It is far more complicated than this in real life Figure 17.3 Overview-steps in gene Expression in Prokaryotes and Eukaryotes mRNA = messenger RNA Nuclear envelope TRANSCRIPTION RNA PROCESSING DNA Pre-mRNA mRNA TRANSCRIPTION DNA mRNA Ribosome TRANSLATION Ribosome TRANSLATION Polypeptide (a) Bacterial cell Polypeptide (b) Eukaryotic cell Figure 17.4 DNA template strand 5 3 A C C A A A C T T T G G T C G A G G G C T T C A 3 5 DNA molecule Gene 1 TRANSCRIPTION Gene 2 U G G +mRNA U U U G G C U C A 5 3 Codon TRANSLATION Protein Trp Phe Gly Ser Gene 3 Amino acid Codons in an mRNA molecule are read by translation machinery in the 5 to 3 direction The Genetic Code is a triplet code • How are the instructions for assembling amino acids into proteins encoded into DNA? • There are 20 amino acids, but there are only four nucleotide bases in DNA • Three nucleotides correspond to an amino acid? • codon The Genetic Code is Universal (a) Tobacco plant expressing a firefly gene (b) Pig expressing a jellyfish gene Second mRNA base UUU U UUC First mRNA base (5 end of codon) UUA C Phe Leu UAU UCC UAC UCA Ser Tyr UGU UGC Cys U C UAA Stop UGA Stop A UCG UAG Stop UGG Trp G CUU CCU CAU CUC CCC CAC Leu CCA Pro CAA CUG CCG CAG AUU ACU AAU ACC AAC AUC Ile AUA AUG G UCU G UUG CUA A A C ACA Met or start Thr AAA His Gln Asn Lys CGU U CGC C CGA Arg CGG AGU G Ser AGC AGA A Arg U C A ACG AAG AGG G GUU GCU GAU GGU U GUC GCC GAC GGC C GAA GGA GUA GUG Val GCA GCG Ala GAG Asp Glu GGG Gly A G The code is redundant!(>1 codon/aa) Third mRNA base (3 end of codon) U Second mRNA base UUU U UUC First mRNA base (5 end of codon) UUA C Phe Leu UAU UCC UAC UCA Ser Tyr UGU UGC Cys U C UAA Stop UGA Stop A UCG UAG Stop UGG Trp G CUU CCU CAU CUC CCC CAC Leu CCA Pro CAA CUG CCG CAG AUU ACU AAU ACC AAC AUC Ile AUA AUG G UCU G UUG CUA A A C ACA Met or start Thr AAA His Gln Asn Lys CGU U CGC C CGA Arg CGG AGU G Ser AGC AGA A Arg U C A ACG AAG AGG G GUU GCU GAU GGU U GUC GCC GAC GGC C GAA GGA GUA GUG Val GCA GCG Ala GAG Asp Glu GGG Gly Third mRNA base (3 end of codon) U A G The code is punctuated (start and stop) 4 Important Characteristics of the Genetic Code • • • • Triplet: 5’ to 3’ in mRNA Universal Punctuated Redundant Molecular Components of Transcription • RNA synthesis is catalyzed by RNA polymerase, which separates the DNA strands apart and links together the RNA nucleotides (condensation reaction) • The RNA is complementary to the DNA template strand • RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine Nontemplate strand of DNA RNA nucleotides RNA polymerase A 3 T C C A A 5 3 end C A U C C A T A G G T 5 5 C 3 T Direction of transcription Template strand of DNA Newly made RNA Figure 17.7-4 Promoter Transcription unit 5 3 Start point RNA polymerase 3 5 DNA 1 Initiation Nontemplate strand of DNA 3 5 5 3 Unwound DNA RNA transcript Template strand of DNA 2 Elongation Rewound DNA 5 3 3 5 3 5 RNA transcript 3 Termination 3 5 5 3 5 Completed RNA transcript 3 Direction of transcription (“downstream”) Eukaryotic complexities • Nucleus has 3 types of RNA polymerase: (Rpol I, Rpol II, R pol III) • All 3 need a lot of help to initiate RNA synthesis • Eukaryotic control signals are very complicated Promoter Nontemplate strand DNA 5′ 3′ T A T AAAA AT AT T T T TATA box Start point Transcription factors 3′ 5′ 1 A eukaryotic promoter 3′ 5′ 2 Several transcription factors bind to DNA. Template strand 5′ 3′ RNA polymerase II Transcription factors 5′ 3′ 5′ 3′ RNA transcript Transcription initiation complex 3′ 5′ 3 Transcription initiation complex forms. Transcription Terminology • • • • • • • • • • RNA polymerase Template/non-template +/- sense Initiation, Elongation, Termination Upstream/downstream Promoter/terminator Transcription unit Rpol II (for messenger RNA) Transcription factor TATA box Eukaryotes modify RNA after transcription • A newly made RNA is called a primary transcript • When a new RNA molecule is first made it is not RTU • It has to be changed or modified prior to use • Enzymes in the eukaryotic nucleus modify primary transcripts before they are sent to the cytoplasm (RNA processing aka RNA modification) • Pre-RNA, mature RNA RNA molecules are usually modified after transcription • Enzymes catalyze changes to the RNA molecule before it is ready to be used. • Changes or modifications can be at the ends or in the middle. • Changes or modifications can involve a single nucleotide at a time or a group. • Modifications help to control gene expression Split Genes and RNA Splicing • Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions • These noncoding regions are called intervening sequences, or introns • The other regions are called exons because they are eventually expressed, usually translated into amino acid sequences • RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence Splicing is the most dramatic modification: Many genes are organized into “expressed” sections or exons separated by “unexpressed” sections or introns 5 Exon Intron Exon Pre-mRNA 5 Cap Codon 130 31104 numbers Intron Exon 3 Poly-A tail 105 146 Introns cut out and exons spliced together mRNA 5 Cap Poly-A tail 1146 5 UTR Coding segment 3 UTR The exons and introns are transcribed into RNA and then the exons are joined together at the RNA level: Splicing Diverse splicing mechanisms exist • Spliceosomes consist of a variety of proteins and several small nuclear ribonucleoproteins (snRNPs) that recognize the splice sites • The RNAs of the spliceosome also catalyze the splicing reaction Small RNAs Spliceosome 5′ Pre-mRNA Exon 2 Exon 1 Intron Spliceosome components mRNA 5′ Exon 1 Exon 2 Cut-out intron Ribozymes • Ribozymes are catalytic RNA molecules that function as enzymes and some can splice RNA • The discovery of ribozymes rendered obsolete the belief that all biological catalysts were proteins • Three properties of RNA enable it to function as a catalyst • It can form a three-dimensional structure because of its ability to base-pair with itself • Some bases in RNA contain functional groups that may participate in catalysis • RNA may hydrogen-bond with other nucleic acid molecules RNA secondary structure-illustrations U1 snRNA and snRNP tRNA The Functional and Evolutionary Importance of Introns • Some introns contain sequences that may regulate gene expression • Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during splicing • This is called alternative RNA splicing • Consequently, the number of different proteins an organism can produce is much greater than its number of genes • More then one product from each gene • Adds flexibility Gene DNA Exon 1 Intron Exon 2 Intron Exon 3 Transcription RNA processing Alternate splicing works Because genes and proteins are made of modules Translation Exons = gene modules Domains = protein modules Domain 3 Domain 2 Domain 1 Polypeptide RNA has more roles and functions than any other component • • • • • • Ribsosomal RNA (rRNA) Messenger RNA (mRNA) Transfer RNA (tRNA) Catalytic RNA (ribozymes) Structural RNA Regulatory RNA • The RNA World Hypothesis – Did the first life forms evolve as RNA-based systems? – Did DNA and protein evolve later? Note Card Question (Review) Promoter RNA splicing/alternative splicing Intron/exon RNA secondary structure Ribozyme Protein domain RNA World Hypothesis
