Genetic Expression Genotype => Phenotype

DNA Functions Information Storage –sequence of bases Information Transmission –replication Information Expression –DNA =>Proteins & RNAs

Genetic Expression Beadle & Tatum - early 1940’s –examined the relationship between genes and enzymes in biochemical pathways –used Neurospora crassa haploid fungus –each allele is expressed in phenotype prototrophs grow on minimal medium auxotrophs require supplements

Genetic Expression Beadle & Tatum - early 1940’s –Neurospora crassa auxotrophs different auxotrophs may require the same supplement –some map to the same genetic locus –some map to different genetic loci »arginine auxotrophs map to 3 loci >arg-A, arg-B, arg -C

3 different arginine auxotrophs lack 3 different enzymes in the arginine synthesis pathway Figure 12.1 arg-C arg-B arg-A

Genetic Expression Beadle & Tatum - early 1940’s –“one gene - one protein” hypothesis later –“one gene - one polypeptide” or –“one gene - one functional product” [polypeptide or RNA]

Genetic Expression Central Dogma of Molecular Biology DNA => RNA => protein information flow is one-way from nucleic acids to proteins information handling is done by nucleic acids

The Central Dogma & Its Elaborations Figure 12.2 RNA-dependent RNA Polymerase RNA-dependent DNA Polymerase

Genetic Expression information flows from DNA to protein by transcription and translation –transcription copies information from a large document to a small document by complementary base pairing

Genetic Expression information flows from DNA to protein by transcription and translation –translation converts information from a storage format to a functional format by complementary base pairing using an adaptor molecule (tRNA) at a dedicated work station (ribosome)

cellular information flow Figure 12.3

Genetic Expression transcription –occurs in three stages initiation elongation termination

Genetic Expression transcription –initiation ingredients –DNA template –ATP, GTP, CTP, UTP (NTPs, nucleoside triphosphates) –RNA polymerase –transcription factors

Genetic Expression transcription –initiation events –RNA polymerase binds DNA at a promoter –DNA near the start site is denatured –the first nucleotide forms a base pair –the next few nucleotides are added –promoter clearance

Transcription Initiation Figure 12.4

Genetic Expression transcription –elongation the “transcription bubble” moves along the template in the 3’ to 5’ direction RNA grows in the 5’ to 3’ direction RNA polymerase links nucleotides covalently

Transcription Elongation Figure 12.4

Genetic Expression transcription –termination at the end of a gene a termination signal causes RNA polymerase to release the RNA

Transcription Termination Figure 12.4

Genetic Expression products of transcription –tRNA –rRNA –mRNA (prokaryotes) –primary transcript (eukaryotes) modified to make mRNA ready for translation

Genetic Expression RNA ready for translation is messenger RNA –mRNA contains information needed to synthesize a polypeptide information is encoded in the sequence of bases translation converts information from base sequence to amino acid sequence –4 bases specify 20 amino acids –each amino acid is specified by a 3 base codon

The “Universal” Genetic Code Figure 12.5

Genetic Expression The genetic code –64 codons 61 codons specify amino acids 1 codon specifies “start” (& Met) 3 codons specify “stop” 2 amino acids have 1 codon other amino acids have 2, 3, 4 or 6 codons –the code is degenerate –the code is not ambiguous

Genetic Expression translation –conversion of codons to amino acids requires a “decoder” tRNAs bind specific amino acids –each tRNA binds only one amino acid tRNAs can decode specific codons –some tRNAs decode more than one codon

Three views of tRNA Figure 12.7

Genetic Expression tRNAs –75-80 nucleotides long –complex tertiary structure –3’ end binds amino acid –3 bases at the other end form the anticodon –“charged” by aminoacyl-tRNA synthetases

aminoacyl-tRNA synthetase charges tRNA Figure 12.8

Genetic Expression ribosome - the site of polypeptide synthesis –large & small subunits each have RNA and protein components –any mRNA can be translated on any ribosome –two sites are central to polypeptide synthesis A site -binds the arriving Aminoacyl-tRNA P site -binds the growing Polypeptide

Ribosome Structure Figure 12.9

Genetic Expression translation –process occurs in three stages –initiation –elongation –termination

Genetic Expression initiation –initiation complex forms mRNA at the AUG codon ribosome small subunit charged tRNA MET initiation factors ribosome large subunit

Translation Initiation Figure 12.10

Genetic Expression elongation –appropriate aminoacyl-tRNA binds A site –LSU transfers the growing polypeptide to the amino acid in the A site –tRNA in the P site leaves to be recharged –ribosome moves to next codon –energy is supplied by GTP

Translation Elongation Figure 12.11

Genetic Expression termination –stop codon enters A site –release factors bind stop codon –ribosome releases mRNA, disassembles

Translation Termination Figure 12.12

Genetic Expression translation –reads codons from 5’ to 3’ on mRNA –assembles polypeptide from N to C terminus –multiple ribosomes may translate the same mRNA at one time

Polysomal translation Figure 12.13

Genetic Expression Translation –many antibiotics target bacterial translation processes

Translation is a Common Target of Antibiotics Figure 12.2

translation –a polypeptide destined for a particular organelle carries a signal sequence –the signal sequence allows specific binding to a receptor protein on the surface of the target organelle –the receptor opens a channel for import of the properly ‘labeled’ polypeptide Genetic Expression

signal sequences target proteins to organelles Figure 12.14

translation –a polypeptide destined for the ER has a signal peptide at its N terminus –translation begins in cytoplasm –Signal Recognition Particle (SRP) transports ribosome and mRNA to ER –polypeptide is inserted into ER as it is made –signal peptide is removed in ER Genetic Expression

recruitment of ribosome to the rough ER by the Signal Recognition Particle Figure 12.15

Genetic Expression following translation many polypeptides are modified –proteolytic activation –attachment of nonprotein molecules

post-translational modifications of polypeptides Figure 12.16

Genetic Expression mutations alter information encoded in DNA base sequence –mutations are faithfully replicated mutations are passed to the next generation –in single celled organisms –if they occur in germ cells of multicellular organisms mutations may or may not change phenotype

Genetic Expression mutations may be conditional –phenotype is normal under permissive conditions –phenotype is altered under restrictive conditions

Frame Shift Mutation Silent Mutation Missense Mutation Nonsense Mutation

Genetic Expression mutations of many kinds occur –point mutation - change in a single base insertion/deletion –add or lose one or more bases –can cause a frame-shift substitution of a wrong base –silent mutations do not alter a.a. –missense mutations change a.a. –nonsense mutations introduce stops

Genetic Expression mutations of many kinds occur –chromosomal mutations are extensive changes deletion duplication inversion translocation

mutations alter information encoded in DNA base sequence Figure 12.18

Genetic Expression mutations have many causes –natural error in replication tautomerization –induced caused by an external agent

natural or induced mutation mechanisms Figure 12.19