Transcript CHAPTER 17
From Gene To Protein Chapter 17 The Connection Between Genes and Proteins • Proteins - link between genotype (what DNA says) and phenotype (physical expression) • Beadle and Tatum – 1st to make connection between genes and enzymes that carry out genes through bread mold experiments. http://fig.cox.miami.edu/~cmallery/150/gene/17x3.jpg • Bridge between DNA, proteins RNA. • RNA similar to DNA - sugar ribose; contains uracil instead of thymine. • RNA single-stranded. http://gibk26.bse.kyutech.ac.jp/jouhou/image/nucleic/rna/rna_bb_st.gif • Nucleotides found in DNA and RNA - code - determines order of amino acids. • 2 steps - transcription and translation. http://www.ktf-split.hr/glossary/image/nucleotide.gif • Transcription - DNA serves as template for complementary RNA strand. • Transcription produces mRNA strand (messenger RNA). • Translation uses mRNA sequence to determine order of amino acids creates polypeptide. http://www.brooklyn.cuny.edu/bc/ahp/BioInfo/graphics/Transcription.02.GIF • Bacteria - transcription and translation occur at once. • Eukaryotes, most transcription occurs in nucleus, translation occurs at ribosome. • Before primary transcript can leave nucleus - modified during RNA processing before enters cytoplasm. • Genetic code consists of triplet code - series 3 nitrogen bases that code for specific amino acid. • 64 possible combinations of nitrogen bases. • Only 20 amino acids = each amino acid has more than 1 code. http://www.dls.ym.edu.tw/lesson/gen.files/codon.jpg • 61 of 64 codes specific to an amino acid. • Other 3 - stop codons - determine when process stops. • Specific code that signals start of translation - also codes for amino acid. • Start begins correct reading frame of polypeptide. • Transcription, 1 DNA strand template strand, provides template for ordering sequence of nucleotides in RNA transcript. • Translation, blocks of 3 nucleotides, codons, decoded into sequence of amino acids. • Possible to take genetic code of 1 organism, place it into another nearly universal. • Firefly gene for luminescence transplanted into tobacco plant. • Bacteria can be inserted with specific genes to synthesize genes in large amounts. Synthesis and Processing of RNA • mRNA transcribed from template of original gene. • RNA polymerase separates DNA strands and bonds RNA bases along complementary strand. • Bases can only be added to 3’ end. http://www.csu.edu.au/faculty/health/biomed/subjects/molbol/images/7_9.jpg • Specific sequences determine where transcription starts and where it ends. • Promoter – initiates; terminator ends. • 3 stages in transcription. • Presence of promotor sequence determines which strand of DNA helix is template. • Proteins (transcription factors) recognize promotor region, especially a TATA box, bind to promotor. http://www.nslij-genetics.org/pic/promoter.gif • After they have bound to promotor, RNA polymerase binds to transcription factors to create transcription initiation complex. • RNA polymerase starts transcription. • RNA polymerase moves along nucleotides added to 3’ end. • Single gene can be transcribed simultaneously by several RNA polymerases at a time. • Growing strand of RNA trails off from each polymerase. • RNA splicing - removal of large portion of RNA molecule. • Most eukaryotic genes and RNA transcripts have long noncoding stretches of nucleotides. • Noncoding segments, introns, lie between coding regions. • Coding regions - exons. http://ghs.gresham.k12.or.us/science/ps/sci/ibbio/chem/nucleic/chpt15/introndeletion.gif • RNA splicing removes introns, joins exons to create mRNA molecule with continuous coding sequence. • Splicing done by spliceosome. The Synthesis of Proteins • Translation - cell interprets series of codons along mRNA molecule. • Transfer RNA (tRNA) transfers amino acids from cytoplasm’s pool to ribosome. • Ribosome adds each amino acid carried by tRNA to growing end of polypeptide chain. • Translation - tRNA links mRNA codon with appropriate amino acid. • tRNA arriving at ribosome carries specific amino acid at 1 end, has specific nucleotide triplet, anticodon, at other. • Anticodon base-pairs with complementary codon on mRNA. http://bioweb.uwlax.edu/GenWeb/Molecular/Theory/Translation/ribosome.jpg • tRNA synthesized like other forms of RNA. • Once in cytoplasm, each tRNA used repeatedly to pick up and drop off that amino acid. • Anticodons of tRNA recognize more than one codon. • Rules for base pairing between 3rd base of codon and anticodon relaxed (wobble). http://www.geneticengineering.org/chemis/Chemis-NucleicAcid/Graphics/tRNA.jpg • Each ribosome has binding site for mRNA 3 binding sites for tRNA molecules. • P site holds tRNA carrying growing polypeptide chain. • A site carries tRNA with next amino acid. • Discharged tRNAs leave ribosome at E site. • Translation divided into 3 stages: initiation elongation termination http://nobelprize.org/educational_games/medicine/dna/a/translation/pics/translation2.gif • Initiation brings together mRNA, tRNA with 1st amino acid, 2 ribosomal subunits. • Elongation - series of 3-step cycles as each amino acid added to previous one. • 3 steps of elongation continue codon by codon to add amino acids until polypeptide chain completed. Fig. 17.18 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Termination - 1 of 3 stop codons reaches A site. • Release factor binds to stop codon, hydrolyzes bond between polypeptide and tRNA in P site. • Frees polypeptide, translation complex disassembles. • 2 types of ribosomes active in process. • 1Free ribosomes suspended in cytosoll synthesize proteins in cytosol. • 2Bound ribosomes attached to cytosolic side of endoplasmic reticulum. Fig. 17.21 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Bacteria and eukaryotes have differences in cellular machinery and in details of processes. • Eukaryotic RNA polymerases differ from prokaryotic; require transcription factors. http://www.personal.psu.edu/faculty/r/c/rch8/workmg/TxnPromotersCh11_files/image052.png • Differ in how transcription terminated. • Ribosomes also different. • Prokaryotes can transcribe and translate same gene simultaneously. • Mutations - changes in genetic material of cell (or virus). • Chemical change in 1 base pair of gene causes point mutation. • Occur in gametes or cells producing gametes - may be transmitted to future generations. http://staff.jccc.net/PDECELL/evolution/mutations/mutypes.gif • Point mutation that results in replacement of pair of complementary nucleotides with another nucleotide pair - base-pair substitution. • Some base-pair substitutions have little or no impact on protein function (silent mutations). http://fajerpc.magnet.fsu.edu/Education/2010/Lectures/26_DNA_Transcription_files/image008.jpg • Other base-pair substitutions cause readily detectable change in protein. • Missense mutations - still code for amino acid, change indicated amino acid. • Nonsense mutations change amino acid codon into stop codon, nearly always leading to nonfunctional protein. Fig. 17.24 Copyright © Pearson Education, Inc., publishing as Benjamin Cummings • Insertions and deletions - additions or losses of nucleotide pairs in gene. • Unless these mutations occur in multiples of 3 - cause frameshift mutation. Fig. 17.24 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Mutations can occur in number of ways. Errors can occur during DNA replication, DNA repair, or DNA recombination. • Mutagens - chemical or physical agents that interact with DNA to cause mutations. • Physical agents - high-energy radiation like X-rays and ultraviolet light.