From Gene to Protein - PowerPoint PPT Presentation

from gene to protein n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
From Gene to Protein PowerPoint Presentation
Download Presentation
From Gene to Protein

play fullscreen
1 / 119
From Gene to Protein
243 Views
Download Presentation
mareo
Download Presentation

From Gene to Protein

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. From Gene to Protein Campbell and Reece Chapter 17

  2. Gene Expression • process by which DNA directs the synthesis of proteins or RNA • synthesis of proteins • transcription • translation

  3. How Gene to Protein Figured Out • Evidence from study of metabolic disorders: • 1902: British physician 1st to suggest genes responsible for phenotype thru enzymes that catalyze specific chemrx in the cell

  4. Inborn Errors of Metabolism • Garrod hypothesized that symptoms of an inherited disease are due to a gene that leads to inability to make a certain enzyme • 1 of 1st to realize Mendel’s principle’s of heredity applied to more than pea plants

  5. Alkaptonuria • Signs & Symptoms: • urine turns black when alkapton (chemical in urine) reacts with air • missing enzyme in pathway that degrades phenylalanine (a.a.)

  6. Beadle & Tatum Experiment • worked with a bread mold Neurosporacrassa • bombarded it with radiation (already known to cause genetic changes) • then checked for survivors who had different nutritional needs from wild-type mold

  7. Beadle & Tatum Experiment • individually put yeast in different mediums (agar with different nutrients) • identified mutants that could not survive on minimal nutrients placed them in complete growth medium (minimal med. + all 20 a.a. + few vitamins & minerals)

  8. 1-Gene-1-Polypeptide • Beadle & Tatum’s results supported their hypothesis • 1958: Nobel prize

  9. 1-Gene-1-Polypeptide • revised over time: • not all proteins are enzymes • some proteins have >1 polypeptide • now: 1- gene-1-protein hypothesis • not 100%: some eukaryotic genes can each code for a set of closely related polypeptides via alternative splicing

  10. Transcription: short version • the synthesis of RNA using information in DNA • mRNA made using complimentary base pairing

  11. Translation: short version • synthesis of a polypeptide using the information in mRNA • “translates” message in mRNA  a.a.

  12. The Genetic Code • 4 nucleotide bases to code for 20 a.a. • triplet code: 3 consecutive bases code for 1 of the a.a./ stop

  13. Template Strand • during transcription: • DNA helix unwound • 1 strand only transcribed (could be either side depending on the gene)

  14. mRNA • uracil added as compliment to adenine • ribose as its 5-carbon sugar • single stranded

  15. Codons • nucleotide triplets of DNA or mRNA that specifies a particular amino acid or termination signal • basic unit of the genetic code • written in 5’  3’ direction (in DNA 3 bases read in 3’  5’ direction)

  16. Genetic Code

  17. Cracking the Code • early 1960’s • Nirenberg: synthesized mRNA using only uracil (UUUUUUU…) • added it to test tube with all 20 a.a., ribosomes • translated into polypeptide made up of phenyalanine • now knew UUU = Phe • did same for AAA= Lys, CCC = Pro, GGG = Gly

  18. Cracking the Code • all 64 a.a. deciphered by mid-1960’s • 3 codons code for “stop” marking end of translation • AUG functions as “start” & Met • Met may or may not be clipped off later

  19. Genetic Code is Redundant • >1 triplet codes for each of the a.a. but any 1 triplet codes for only 1 a.a • redundant triplets usually only differ in the 3rd base

  20. Reading Frame • translating the code in correct groupings • example: Did the red dog eat the bug? Idt her eddogeatthebug?

  21. Reading Frame

  22. Evolution of Genetic Code • code is nearly universal: • bacteria  complex multicellular organisms CAU = His • insert genes into other species & get same result (human insulin gene in bacteria) • exceptions: certain unicellular eukaryotes & in organelle genes of some species

  23. RNA Polymerase • unwinds 2 strands of DNA • binds nucleotides together as build mRNA • only in 5’  3’ direction (like DNA polymerase)

  24. 3 Stages of Transcription • Initiation • Elongation • Termination

  25. Initiation • After RNA polymerase binds to promoter, ¤ DNA strands unwind • polymerase begins RNA synthesis @ start pt. on template strand

  26. Initiation • promoter: usually includes w/in it the transcription start point (a nucleotide where transcription begins) & extends several dozen or more nucleotide pairs upstream from start pt. • RNAP can assemble nucleotides only in 5’  3’ direction (just like DNA polymerase) • unlike DNAP, RNAP does not require a primer

  27. Start Point • nucleotide where RNA synthesis actually begins • RNAP binds in precise location & orientation on the promoter  where determines where transcription starts & which of the 2 strands will be transcribed

  28. RNA Polymerase • Bacteria: • 1 single RNAP used to make all types RNA • Eukaryotic Cells: • @ least 3 types RNA polymerase • II used for RNA synthesis • I and III used to transcribe RNA not used for protein synthesis

  29. RNA Polymerase • Prokaryotes : • RNAP recognizes & binds to the promoter by itself • Eukayotes: • collection of proteins , transcription factors, mediate the binding of RNAP & initiation of transcription

  30. Transcription Factors • must 1st attach to promoter b/4 RNAP II can bind to it • RNAP II + transcription factors = Transcription Initiation Complex • TATA box: DNA sequence in eukaryotic promoters crucial in forming the transcription initiation complex

  31. Elongation • RNAP moves downstrean, unwinding the DNA & elongating the RNA transcript 5’  3’ • ~ 10 – 20 nucleotides exposed • in wake of transcription the 2 DNA strands spontaneously rewind • length of DNA transcribed = transcription unit

  32. Elongation

  33. Termination • mechanism differs between prokaryotes & eukaryotes • Bacteria: transcription proceeds thru terminator sequence in the DNA  the transcribed RNA functions as the terminator sequence  causing RNAP to detach • prokaryotes have no further modification

  34. Termination in Eukaryotes • RNAP II transcribes a portion of DNA called the polyadenylation signal (AAUAAA) in the pre-mRNA • ~10 – 35 nucleotides downstream from that sequence proteins ass’c with transcription cut the pre-mRNA free from the polymerase • pre-mRNA then  modified

  35. RNA Processing • in eukaryotes only • both ends of primary transcript altered • certain interior sections cut out & remaining parts spliced back together

  36. mRNA Ends • 5’ end receives a 5’cap: modified G is added after ~ 20 – 40 nucleotides in mRNA • 3’ end modified: enzyme adds 50 -250 A’s to the AAUAAA forming a poly-A tail

  37. Functions of Modified Ends of mRNA • facilitate exit of mRNA from nucleus • protect mRNA from degradation of hydrolytic enzymes • help ribosomes attach to the 5’ end

  38. RNA Splicing • cut-and-paste job removing segments of RNA that were transcribed • average size transcript: 27,000 nucleotides • average size protein: 1,200 nucleotides (400 a.a.)

  39. Introns • noncoding, intervening sequence w/in primary transcript that is removed from the transcript during RNA processing; also refers to the region of DNA from which this sequence was transcribed