Chapter 17 - Gene Expression PDF

suggested genes dictate phenotypes through enzymes...

suggested genes dictate phenotypes through enzymes Garrod inherited diseases may reflect enzyme inability linking genes to enzymes necessitates understanding of metabolic pathway exposed bread mold to X-rays, creating mutants unable to survive on minimal media identified three classes of arginine-deficient mutants, each lacking a specific enzyme states that each gene dictates specific enzyme production scientists doubted hypothesis because some proteins aren’t enzymes developed hypothesis one gene-one enzyme many proteins are polypeptides with their own genes reformulate hypothesis to one gene-one polypeptide hypothesis it’s common to refer to gene products as proteins rather than polypeptides Beadle, Tatum idea of gene has evolved through history of genetics discrete unit of inheritance process by information from gene (DNA) to synthesize functional gene region of specific nucleotide sequence in chromosome considered gene as Gene proteins links between genotype and phenotype product DNA sequence that codes for specific polypeptide chain RNA bridge between genes and proteins for which they code gene can be defined as region of DNA that can be expressed to produce final functional product, either polypeptide or RNA molecule 1. transcription 2. RNA processing are changes in genetic material of cell or virus 3. translation replication 4. post-translational modifications recombination spontaneous mutations can occur during DNA repair are physical or chemical agents mutagens it cause by stage are chemical changes in just one base pair of gene change of single nucleotide in a DNA template strand can lead to production of abnormal protein replaces one nucleotide and its partner with another pair of nucleotides mutations have no effect on amino acid produced by codon because of redundancy in genetic code gene expression silent nucleotide-pair substitutions point mutations still code for amino acid, but not correct amino acid missense types allow mRNA translation before transcription prokaryote can simultaneously transcribe and translate same gene are prokaryotes, but share many features of gene expression with eukaryotes archaea transcription and translation are likely coupled comparing in separate transcription and translation via nuclear envelope change amino acid codon into stop codon, nearly always leading to nonfunctional protein eukarya RNA transcripts undergo RNA processing to produce finished mRNA nonsense prokaryote and eukarya differ in RNA polymerases, termination of transcription, and ribosomes types DNA contains 20 amino acids but only four nucleotide bases a series of nonoverlapping, three-nucleotide words information flow from gene to protein is based on triplet code gene words are transcribed into complementary mRNA words mRNA words are translated into a chain of amino acids forming polypeptide are additions or losses of nucleotide pairs in gene addition of one or more nucleotides insertions removal of one or more nucleotides deletions have disastrous effect on resulting protein more often than substitutions do when nucleotides are read in codons during protein synthesis occurs shift in reading frame and altering interpretation of sequence causing frameshift mutation effects detrimental protein change result one or more nucleotide-pair insertions or deletions during transcription one of two DNA strands template strand provides template for ordering sequence of complementary nucleotides in RNA transcript always same strand for given gene genetic code during translation mRNA base triplets codon read in 5' to 3' direction each codon specifies amino acid to be placed along polypeptide at corresponding position situation it must be read in correct reading frame (correct grouping) for specified polypeptide to be produced more than one codon may specify particular amino acid redundant not ambiguous no codon specifies more than one amino acid genetic code is nearly universal, shared by simplest bacteria to complex animals genes transcribed and translated after being transplanted from one species to another polypeptide chains spontaneously coil and fold into three-dimensional shape during and after synthesis proteins may require post-translational modifications before function some polypeptides are activated by enzymes that cleave them, other form subunits of protein in cytosol From Gene to Protein free ribsomes mostly synthesize proteins that function in cytosol populations of ribosomes in cells attached to endoplasmic reticulum (ER) bound ribosomes make proteins of endomembrane system and proteins that are secreted from cell ribosomes can switch from free to bound is framework describes flow of genetic information within biological system synthesis starts in cytosol, ends in ER unless polypeptide signals ribosome attachment central dogma concept that cells are governed by cellular chain of command polypeptides marked by signal peptide are destined for ER or secretion post-translational modifications flow of information DNA → RNA → protein signal-recognition particle stands for binds to signal peptide SRP is synthesis of RNA under direction of DNA brings signal peptide and its ribosome to ER produces messenger RNA (mRNA) pries DNA strands apart RNA synthesis catalyzed by RNA polymerase hooks together RNA nucleotides RNA is complementary to DNA template strand molecular components follows same base-pairing rules as DNA, substituting uracil for thymine promoter DNA sequence is attached by RNA polymerase terminator sequence signals end of transcription in bacteria transcription unit is transcribed DNA stretch promoters signal transcriptional start point, extending nucleotide pairs transcription factors mediate RNA polymerase binding and transcription initiation transcription initiation complex completed assembly of transcription factors and RNA polymerase Il bound to promoter TATA box crucial promoter, forms complex in eukaryotes is synthesis of polypeptide, using information in mRNA initiation genetic information flows from mRNA to protein through process of translation often insufficient for creating functional proteins, and polypeptide chains are modified or targeted to specific cell sites after translation it’s an enzyme aminoacyl-tRNA synthetase correct match between a tRNA and amino acid correct match between tRNA anticodon and mRNA codon accurate requires two steps transcription RNA polymerase untwists double helix 10-20 bases per DNA move transcription progresses at 40 nucleotides per second in eukaryotes process multiple RNA polymerases can simultaneously transcribe gene nucleotides added to 3' end of growing RNA molecule elongation flexible pairing at third base of codon wobble allows some tRNAs to bind to more than one codon transfer RNA help to cell translates mRNA message into protein transfer amino acids to growing polypeptide in ribosome each carries specific amino acid on one end molecules aren’t identical each has anticodon base pairs with complementary mRNA codon on other end tRNA formation of primary transcript is initial RNA transcript from any gene prior to processing polymerase stops transcription at end of terminator in prokaryote mRNA can be translated without further modification termination RNA polymerase II transcribes polyadenylation signal sequence in eukarya RNA transcript is released 10-35 nucleotides past this polyadenylation sequence facilitate coupling of tRNA anticodons with mRNA codons in protein synthesis proteins ribosomal subunits (large and small) are made of ribosomal RNA (rRNA) some antibiotic drugs (tetracycline and streptomycin) specifically target bacterial ribosomes without harming eukaryotic ribosomes aminoacyl site A site holds tRNA that carries next amino acid to be added to chain integral components peptidyl site P site binding sites ribosome holds tRNA that carries growing polypeptide chain translation exit site E site where discharged tRNAs leave ribosome crucial process in eukaryotic gene expression that involves modification of pre-mRNA (precursor mRNA) eukaryotic genes and RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions during RNA processing, both ends of primary transcript are usually altered some interior parts of molecule are cut out, and other parts spliced together seem to facilitate export of mRNA share function protect mRNA from hydrolytic enzymes help ribosomes attach to 5' end pre-mRNA molecule modifications 5' end receives modified nucleotide 5' cap end way know as polysome 3' end gets poly-A tail multiple ribosomes translate single mRNA simultaneously enable cell to make many copies of polypeptide very quickly polyribosome removing non-coding sequences from pre-mRNA introns contain sequences that may regulate gene expression some genes can encode multiple polypeptides through alternative RNA splicing functional importance organisms produce more proteins than their number of genes joining together coding sequences to produce mature mRNA eventually expressed process exons mRNA usually translated into amino acid sequences tRNA with first amino acid combines shuffling may result in evolution of new proteins 2 ribosomal subunits small ribosomal subunit binds with mRNA and initiator tRNA subunit moves along mRNA until start codon (AUG) is a protein initiation factors initiation RNA processing bring large subunit that completes translation initiation complex removes introns and joins exons creating mRNA molecule with continuous coding sequence proteins consist of snRNPs small nuclear ribonucleoproteins recognize splice sites RNA splicing spliceosome add amino acids to preceding amino acid at C-terminus of growing chain it’s a proteins that involve codon recognition elongation factors peptide bond formation step translocation molecules translation along mRNA in 5' to 3' direction elongation process catalytic RNA molecules that function as enzymes and can splice RNA ribozyme form three-dimensional structure because of its ability to base-pair with itself RNA function as enzyme some bases in RNA contain functional groups that may participate in catalysis RNA may hydrogen-bond with other nucleic acid molecules proteins often have modular architecture consisting of discrete regions different exons code for different domains in protein domains stop codon in mRNA reaches ribosome A site A site accepts release factor protein release factor causes add water molecule instead of amino acid releases polypeptide and translation assembly then comes apart termination