Chapter 17 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 Beadle, Tatum states that each gene dictates specific enzyme production doubted hypothesis because some proteins aren’t enzymes developed hypothesis one gene-one enzyme many proteins are polypeptides with their own genes but scientists reformulate hypothesis to one gene-one polypeptide hypothesis it’s common to refer to gene products as proteins rather than polypeptides process by information from gene (DNA) to synthesize functional gene proteins links between genotype and phenotype product RNA bridge between genes and proteins for which they code idea of gene has evolved through history of genetics 1. transcription discrete unit of inheritance 2. RNA processing stage region of specific nucleotide sequence in chromosome considered gene as Gene gene expression 3. translation DNA sequence that codes for specific polypeptide chain 4. post-translational modifications gene can be defined as region of DNA that can be expressed to produce final functional product, either polypeptide or RNA molecule allow mRNA translation before transcription prokaryote can simultaneously transcribe and translate same gene are changes in genetic material of cell or virus are prokaryotes, but share many features of gene expression with eukaryotes archaea replication transcription and translation are likely coupled comparing in recombination spontaneous mutations can occur during DNA separate transcription and translation via nuclear envelope eukarya repair RNA transcripts undergo RNA processing to produce finished mRNA are physical or chemical agents mutagens it cause by mutations prokaryote and eukarya differ in RNA polymerases, termination of transcription, and ribosomes 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 DNA contains 20 amino acids but only four nucleotide bases replaces one nucleotide and its partner with another pair of nucleotides a series of nonoverlapping, three-nucleotide words have no effect on amino acid produced by codon because of redundancy in genetic code silent nucleotide-pair substitutions point mutations information flow from gene to protein is based on triplet code gene words are transcribed into complementary mRNA words still code for amino acid, but not correct amino acid missense types mRNA words are translated into a chain of amino acids forming polypeptide change amino acid codon into stop codon, nearly always leading to nonfunctional protein nonsense during transcription types are additions or losses of nucleotide pairs in gene one of two DNA strands template strand addition of one or more nucleotides insertions provides template for ordering sequence of complementary nucleotides in RNA transcript removal of one or more nucleotides deletions always same strand for given gene one or more nucleotide-pair insertions or deletions have disastrous effect on resulting protein more often than substitutions do during translation when nucleotides are read in codons during protein synthesis occurs mRNA base triplets shift in reading frame and altering interpretation of sequence causing frameshift mutation effects genetic code read in 5' to 3' direction detrimental protein change result each codon specifies amino acid to be placed along polypeptide at corresponding position codon it must be read in correct reading frame (correct grouping) for specified polypeptide to be produced polypeptide chains spontaneously coil and fold into three-dimensional shape during and after synthesis start AUG proteins may require post-translational modifications before function UAA some polypeptides are activated by enzymes that cleave them, other form subunits of protein stop UAG in cytosol free ribsomes UGA mostly synthesize proteins that function in cytosol populations of ribosomes in cells more than one codon may specify particular amino acid attached to endoplasmic reticulum (ER) redundant make proteins of endomembrane system and proteins that are secreted from cell bound ribosomes post-translational modifications From Gene to Protein genetic code is not ambiguous no codon specifies more than one amino acid nearly universal, shared by simplest bacteria to complex animals ribosomes can switch from free to bound genes transcribed and translated after being transplanted from one species to another synthesis starts in cytosol, ends in ER unless polypeptide signals ribosome attachment polypeptides marked by signal peptide are destined for ER or secretion is framework describes flow of genetic information within biological system signal-recognition particle stands for central dogma concept that cells are governed by cellular chain of command binds to signal peptide SRP flow of information DNA → RNA → protein brings signal peptide and its ribosome to ER is synthesis of RNA under direction of DNA is synthesis of polypeptide, using information in mRNA produces messenger RNA (mRNA) genetic information flows from mRNA to protein through process of translation pries DNA strands apart RNA synthesis catalyzed by RNA polymerase often insufficient for creating functional proteins, and polypeptide chains are modified or targeted to specific cell sites after translation hooks together RNA nucleotides it’s an enzyme RNA is complementary to DNA template strand aminoacyl-tRNA synthetase correct match between a tRNA and amino acid accurate requires two steps molecular components follows same base-pairing rules as DNA, substituting uracil for thymine correct match between tRNA anticodon and mRNA codon promoter DNA sequence is attached by RNA polymerase flexible pairing at third base of codon terminator sequence signals end of transcription in bacteria wobble transcription allows some tRNAs to bind to more than one codon transcription unit is transcribed DNA stretch transfer RNA promoters signal transcriptional start point, extending nucleotide pairs help to cell translates mRNA message into protein transcription factors mediate RNA polymerase binding and transcription initiation initiation transfer amino acids to growing polypeptide in ribosome tRNA transcription initiation complex completed assembly of transcription factors and RNA polymerase Il bound to promoter each carries specific amino acid on one end TATA box crucial promoter, forms complex in eukaryotes molecules aren’t identical each has anticodon base pairs with complementary mRNA codon on other end RNA polymerase untwists double helix 10-20 bases per DNA move facilitate coupling of tRNA anticodons with mRNA codons in protein synthesis transcription progresses at 40 nucleotides per second in eukaryotes process elongation proteins multiple RNA polymerases can simultaneously transcribe gene ribosomal subunits (large and small) are made of ribosomal RNA (rRNA) nucleotides added to 3' end of growing RNA molecule some antibiotic drugs (tetracycline and streptomycin) specifically target bacterial ribosomes without harming eukaryotic ribosomes ribosome formation of primary transcript is initial RNA transcript from any gene prior to processing aminoacyl site integral components polymerase stops transcription at end of terminator A site translation in prokaryote holds tRNA that carries next amino acid to be added to chain mRNA can be translated without further modification termination peptidyl site RNA polymerase II transcribes polyadenylation signal sequence P site binding sites in eukarya holds tRNA that carries growing polypeptide chain RNA transcript is released 10-35 nucleotides past this polyadenylation sequence exit site E site where discharged tRNAs leave ribosome crucial process in eukaryotic gene expression that involves modification of pre-mRNA (precursor mRNA) know as polysome eukaryotic genes and RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions multiple ribosomes translate single mRNA simultaneously polyribosome during RNA processing, both ends of primary transcript are usually altered enable cell to make many copies of polypeptide very quickly some interior parts of molecule are cut out, and other parts spliced together mRNA seem to facilitate export of mRNA tRNA with first amino acid combines share function protect mRNA from hydrolytic enzymes 2 ribosomal subunits help ribosomes attach to 5' end pre-mRNA molecule modifications small ribosomal subunit binds with mRNA and initiator tRNA initiation 5' end receives modified nucleotide 5' cap end way subunit moves along mRNA until start codon (AUG) 3' end gets poly-A tail is a protein removing non-coding sequences from pre-mRNA initiation factors bring large subunit that completes translation initiation complex introns contain sequences that may regulate gene expression add amino acids to preceding amino acid at C-terminus of growing chain some genes can encode multiple polypeptides through alternative RNA splicing functional importance it’s a proteins that involve organisms produce more proteins than their number of genes process codon recognition elongation factors elongation process joining together coding sequences to produce mature mRNA RNA processing peptide bond formation step eventually expressed exons translocation usually translated into amino acid sequences translation along mRNA in 5' to 3' direction shuffling may result in evolution of new proteins removes introns and joins exons stop codon in mRNA reaches ribosome A site creating mRNA molecule with continuous coding sequence A site accepts release factor protein termination proteins release factor causes add water molecule instead of amino acid RNA splicing consist of snRNPs small nuclear ribonucleoproteins releases polypeptide and translation assembly then comes apart spliceosome recognize splice sites molecules 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 domains different exons code for different domains in protein