Presentation on theme: "Chapter 17: From Gene to Protein"— Presentation transcript:
1 Chapter 17: From Gene to Protein Gene: A segment of DNA that specifies the amino acid sequence of a polypeptideDNA does not directly control protein synthesis, instead its information is transcribed into RNA
2 Overview: The Flow of Genetic Information The information content of DNAIs in the form of specific sequences of nucleotides along the DNA strands
3 Genes specify proteins via transcription and translation The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteinsThe process by which DNA directs protein synthesis is called gene expressionIncludes two stages, called transcription and translationThe Central Dogma of Molecular Genetics:There are 3 major classes of genetic biopolymers: DNA and RNA (both nucleic acids), and protein.
4 One Gene, One Enzyme hypothesis Synthesis of all substances in living things is dictated by enzymesRemember that enzymes are proteins whose 1' structure (sequence of linked amino acids) are coded for by DNA base triplets.Beadle and Tatum experiments (1941)Purpose: "to determine if and how genes control known biochemical reaction"Work with red bread mold Neurospora crassa to find “nutritional mutants”Used radiation to create “auxotrophs”, organisms, such as a strain of bacteria, that have lost the ability to synthesize certain substances required for its growth and metabolism as the result of mutational changes.
5 Beadle and Tatum’s experiment X-ray mutations loss of enzyme lack of an AA (ex. Arg.) mold could only grow on arginine-supplemented mediaBeadle and Tatum proposed that a single gene (thru a single mutation) codes for a single specific enzyme = Nobel Prize (1958)
6 The Products of Gene Expression: A Developing Story Beadle and Tatum developed the :"One Gene - One Enzyme" correlation:Which states that the function of a gene is to dictate the production of a specific enzymeLater found out not necessarily true: only some proteins are enzymes. It is also true of structural proteins, chains of polypeptides, or hormones.
7 Ribonucleic AcidWhy would the cell want to have an intermediate between DNA and the proteins it encodes?The DNA can then stay pristine and protected, away from the caustic chemistry of the cytoplasm.Gene information can be amplified by having many copies of an RNA made from one copy of DNA.Regulation of gene expression can be effected by having specific controls at each element of the pathway between DNA and proteins.The more elements there are in the pathway, the more opportunities there are to control it in different circumstances.
9 24.2 Gene Expression Three Classes of RNA Messenger RNA (mRNA) Takes a message from DNA to the ribosomesstrandRibosomal RNA (rRNA)Makes up ribosomes (along with proteins)globularTransfer RNA (tRNA)Transfers amino acids to ribosomesHairpin shape
10 24.2 Gene Expression Gene Expression Requires Two Steps: Transcription Is the synthesis of RNA under the direction of DNAProduces messenger RNA (mRNA)TranslationIs the actual synthesis of a polypeptide, which occurs under the direction of mRNAOccurs on ribosomes
11 24.2 Gene Expression Transcription Messenger RNA (mRNA) During transcription, a segment of the DNA serves as a template for the production of an RNA moleculeMessenger RNA (mRNA)RNA polymerase (enzyme) binds to a promoter (“start” sequence)DNA helix is opened so complementary base pairing can occurRNA polymerase joins new RNA nucleotides in a sequence complementary to that on the DNA, in a 5’ to 3’ direction
13 Messenger RNA mRNA - of the 64 possible 3-base combinations: 61 code for the twenty different amino acids3 code for "stop"; i.e. chain terminationSpecific nucleotide sequences call for “start” of transcription (usually AUG = methionine) = PROMOTOR sequence“stop” of mRNA synthesis = TERMINATION sequence (UAA, UGA, UAG)Finished mRNA strands are ~500-10,000 nucleotides long
14 Cracking the Code A codon in messenger RNA Is either translated into an amino acid or serves as a translational stop signalFigure 17.5Second mRNA baseUCAGUUUUUCUUAUUGCUUCUCCUACUGAUUAUCAUAAUGGUUGUCGUAGUGMet orstartPheLeulleValUCUUCCUCAUCGCCUCCCCCACCGACUACCACAACGGCUGCCGCAGCGSerProThrAlaUAUUACUGUUGCTyrCysCAUCACCAACAGCGUCGCCGACGGAAUAACAAAAAGAGUAGCAGAAGGGAUGACGAAGAGGGUGGCGGAGGGUGGUAAUAGStopUGATrpHisGlnAsnLysAspArgGlyFirst mRNA base (5 end)Third mRNA base (3 end)Glu
15 During transcriptionThe gene determines the sequence of bases along the length of an mRNA moleculeFigure 17.4DNAmoleculeGene 1Gene 2Gene 3DNA strand(template)TRANSCRIPTIONmRNAProteinTRANSLATIONAmino acidACGTUTrpPheGlySerCodon35The Process of Transcription
16 transfer RNASmall, ~80 nucleotides long. tRNA exists as a single-stranded molecule. However, regions of double helix can form where there is some base pair complementation (U and A , G and C), resulting in hairpin loops. The RNA molecule with its hairpin loops is said to have a secondary structure.It can bind an amino acid at one end, and mRNA (anticodon) at the other end. It acts as an adaptor to carry the amino acid elements of a protein to the appropriate place as coded for by the mRNA codon (complementary).The "Wobble Phenomenon": There are only 40 different types of t-RNA and 64 codons. This means that some of the t-RNA can pair up with several different codons. This can occur because there is some third base “flexibility”.
18 rRNA Ribosomal RNA is the most abundant type of RNA in cells Ribosomes: comprised of subunits 2/3 RNA, 1/3 proteinTwo populations of ribosomes are evident in cells, Free and boundFree ribosomes in the cytosol initiate the synthesis of all proteins
19 The ribosome has three binding sites for tRNA The P siteThe A siteThe E siteP site (Peptidyl-tRNAbinding site)E site(Exit site)mRNAbinding siteA site (Aminoacyl-tRNA binding site)LargesubunitSmallSchematic model showing binding sites. A ribosome has an mRNA binding site and three tRNA binding sites, known as the A, P, and E sites. This schematic ribosome will appear in later diagrams.(b)EPAFigure 17.16b
20 Translation (Building a polypeptide) requires Three Steps: Initiation (requires energy)Elongation (requires energy)TerminationAnimation: How Translation Works.Amino endGrowing polypeptideNext amino acidto be added topolypeptide chaintRNAmRNACodons35Schematic model with mRNA and tRNA. A tRNA fits into a binding site when its anticodon base-pairs with an mRNA codon. The P site holds the tRNA attached to the growing polypeptide. The A site holds the tRNA carrying the next amino acid to be added to the polypeptide chain. Discharged tRNA leaves via the E site.(c)
22 Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide: a closer look Narrated animation: Protein Synthesis (with quiz)Interactive practice: Transcribe & Translate a Gene
23 Polyribosomes Several ribosomes may move along the same mRNA Multiple copies of a polypeptide may be madeThe entire complex is called a polyribosomeFigure 17.20a, bGrowingpolypeptidesCompletedpolypeptideIncomingribosomalsubunitsStart ofmRNA(5 end)End of(3 end)PolyribosomeAn mRNA molecule is generally translated simultaneouslyby several ribosomes in clusters called polyribosomes.(a)RibosomesThis micrograph shows a large polyribosome in a prokaryoticcell (TEM).0.1 µm(b)
24 Overview of Gene Expression Simple Gene Expression animationDetailed Protein Synthesis animation
26 Regulation of gene expression Genes are activated in some cells, but not othersGenes can be active some of the time, but not othersThe mechanics of the “on/off” switch for genes was first identified in bacteria.1965 Nobel Prize in Medicine François Jacob, Jacques Monod and André Lwoff.This operon enables the metabolism of lactose in Escherichia coliAnimation of the lac operon
27 Regulation of Gene Expression in Eukaryotes Animation:4 levels:1. Transcriptional control (nucleus):e.g. chromatin density and transcription factors2. Posttranscriptional control (nucleus)e.g. mRNA processing3. Translational control (cytoplasm)e.g. differential ability of mRNA to bind ribosomes4. Posttranslational control (cytoplasm)e.g. changes to the protein to make it functional
28 Regulation of gene expression Transcriptional control (nucleus):e.g. chromatin density and transcription factorsEuchromatin: Loosely packed form of DNA; genes are transcibedHeterochromatin: tightly packed form of DNA; genes are “silenced”A transcription factor (sometimes called a sequence-specific DNA-binding factor) is a protein that binds to specific DNA sequences, thereby controlling the flow (or transcription) of genetic information from DNA to mRNA. Transcription factors perform this function alone or with other proteins in a complex, by promoting (as an activator), or blocking (as a repressor) the recruitment of RNA polymerase (the enzyme that performs the transcription of genetic information from DNA to RNA) to specific genes.
29 Transcription factors in Prokaryotes InductionAnimation: The lac operon inductionRepressionAnimation: The trp operonTranscription factors in Eukaryotes, animation/quiz
30 Posttranscriptional control (nucleus) Processing of mRNA After TranscriptionPrimary “Pre-”mRNA must be modified into mature mRNAIntrons are intragene segments (often, junk)Exons are the portion of a gene that is expressedIntron sequences are removed, and a poly-A tail is addedRibozyme splices exon segments togetherEukaryotic mRNA modification: RNA splicing animation
31 mRNA Processing pre-RNA must be modified before translation
32 The Functional and Evolutionary Importance of Introns The presence of intronsAllows for alternative RNA splicingAdditional animations of RNA processing:Processing of Gene Information: Prokaryotes –vs- Eukaryotes:How Spliceosomes Process RNA:
33 Transposons “jumping genes” Sections of DNA that can move to new locations and disrupt gene sequencesAnimationSee Barbara McClintockChromosome 11 flyover