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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.
• 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.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 17.18
• 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.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 17.24
• 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.