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Chapter 7 Gene Expression and Control

Chapter 7 Gene Expression and Control. 7.1 Ricin and Your Ribosomes. The ability to make proteins is critical to all life processes Seeds of the castor-oil plant contain the protein ricin, a deadly poison that inactivates ribosomes that assemble proteins

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Chapter 7 Gene Expression and Control

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  1. Chapter 7 Gene Expression and Control

  2. 7.1 Ricin and Your Ribosomes • The ability to make proteins is critical to all life processes • Seeds of the castor-oil plant contain the protein ricin, a deadly poison that inactivates ribosomes that assemble proteins • Ricin has been used by assassins, and is banned as a weapon under the Geneva Protocol

  3. Seeds of the castor-oil plant

  4. 7.2 DNA, RNA, and Gene Expression • A gene is a DNA sequence that encodes an RNA or protein product in the sequence of its nucleotide bases (A, T, G, C) • In transcription, enzymes use the gene’s DNA sequence as a template to assemble a strand of messenger RNA (mRNA) • In translation, the protein-building information in mRNA is decoded into a sequence of amino acids • The result is a polypeptide chain that folds into a protein

  5. DNA and RNA sugar– phosphate backbone base pair nucleotide base DNA RNA deoxyribonucleic acid ribonucleic acid

  6. Gene Expression • Gene expression involves transcription (DNA to mRNA), and translation (mRNA to protein) • Gene expression • Process by which the information in a gene becomes converted to an RNA or protein product • Proteins (enzymes) assemble other molecules and perform many functions that keep the cell alive

  7. 7.3 Transcription: DNA to RNA • During transcription, a strand of DNA acts as a template upon which a strand of RNA is assembled from nucleotides • Base-pairing rules in DNA replication apply to RNA synthesis in transcription, but RNA uses uracil in place of thymine • The enzyme RNA polymerase, not DNA polymerase, adds nucleotides to the end of a growing RNA strand

  8. Base Pairing in Transcription

  9. The Process of Transcription • In transcription, RNA polymerase binds to a promoter in the DNA near a gene • Polymerase moves along the DNA, unwinding the DNA so it can read the base sequence • RNA polymerase links RNA nucleotides in the order determined by the base sequence of the gene • The new mRNA is a copy of the gene from which it was transcribed

  10. RNA polymerase binds to a promoter RNA polymerase gene region binding site in DNA The enzyme RNA polymerase binds to a promoter in the DNA. The binding positions the polymerase near a gene. Only one of the two strands of DNA will be transcribed into RNA. 1

  11. RNA nucleotides are linked RNA DNA unwinding DNA winding up The polymerase begins to move along the gene and unwind the DNA. As it does, it links RNA nucleotides in the order specified by the nucleotide sequence of the template DNA strand. The DNA winds up again after the polymerase passes. The structure of the “opened” DNA at the transcription site is called a transcription bubble, after its appearance. 2

  12. RNA nucleotides are linked direction of transcription Zooming in on the transcription bubble, we can see that RNA polymerase covalently bonds successive nucleotides into an RNA strand. The new strand is an RNA copy of the gene. 3

  13. Three Genes Being Transcribed • Many polymerases transcribe a gene region at the same time RNA molecules DNA molecule

  14. RNA Modifications • Eukaryotic cells modify their RNA before it leaves the nucleus • Sequences that stay in the RNA are exons • Introns are sequences removed during RNA processing • Exons can be spliced together in different combinations, so one gene may encode different proteins • After splicing, a tail of 50 to 300 adenines (poly-A tail) is added to the end of a new mRNA

  15. Post-transcriptional modification of RNA gene intron exon exon promoter exon intron DNA transcription exon exon intron exon intron newly transcribed RNA exon exon exon finished mRNA poly-A tail

  16. ANIMATED FIGURE: Pre-mRNA transcript processing To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

  17. ANIMATED FIGURE: Gene transcription details To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

  18. ANIMATED FIGURE: Negative control of the lactose operon

  19. 7.4 RNA Players in Translation • Three types of RNA are involved in translation: mRNA, rRNA, and tRNA • mRNA produced by transcription carries protein-building information from DNA to the other two types of RNA for translation

  20. mRNA and the Genetic Code • Information in mRNA consists of sets of three nucleotides (codons) that form “words” spelled with bases A, C, G, U • Sixty-four codons, most of which specify amino acids, constitute the genetic code • The sequence of three nucleotides in a base triplet determines which amino acid the codon specifies • The order of codons in mRNA determines the order of amino acids in the polypeptide that will be translated from it

  21. Genetic Code • Twenty amino acids are encoded by the sixty-four codons in the genetic code • Some amino acids are specified by more than one codon • Other codons signal the beginning and end of a protein-coding sequence • Most organisms use the same code

  22. The Genetic Code

  23. Correspondence between DNA, RNA, and proteins a gene region in DNA transcription codon codon codon mRNA translation amino acid sequence methionine (met) tyrosine (tyr) serine (ser)

  24. rRNA and tRNA – the Translators • Ribosomes consist of two subunits of rRNA and structural proteins • Ribosomes and transfer RNAs (tRNA) interact to translate an mRNA into a polypeptide • tRNA has two attachment sites • An anticodon base-pairs with an mRNA codon • An attachment site binds to an amino acid specified by the codon

  25. Ribosome Structure + = large subunit small subunit intact ribosome

  26. tRNA for Tryptophan anticodon A) Icon and model of the tRNA that carries the amino acid tryptophan. Each tRNA’s anticodon is complementary to an mRNA codon. Each also carries the amino acid specified by that codon.

  27. tRNAs dock at a ribosome B) During translation, tRNAs dock at an intact ribosome (for clarity, only the small subunit is shown, in tan). Here, the anticodons of two tRNAs have base-paired with complementary codons on an mRNA (red).

  28. ANIMATED FIGURE: Structure of a ribosome To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

  29. 7.5 Translating the Code: RNA to Protein • Translation (second part of protein synthesis) occurs in the cytoplasm of all cells: • mRNA is transcribed in the nucleus • In the cytoplasm a small ribosomal subunit binds to mRNA • Initiator tRNA base-pairs with the first mRNA codon • Large ribosomal subunit joins the small subunit • Ribosome assembles a polypeptide chain • Translation ends when the ribosome encounters a stop codon

  30. Translation in Eukaryotes 1 Transcription ribosome subunits 2 RNA transport tRNA Convergence of RNAs 3 Translation 4 mRNA polypeptide

  31. Ribosome assembles a polypeptide chain

  32. Ribosome assembles a polypeptide chain

  33. Ribosome assembles a polypeptide chain

  34. ANIMATED FIGURE: Translation To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

  35. 7.6 Mutated Genes and Their Products • Mutations are permanent changes in the nucleotide sequence of DNA, which may alter a gene product • A mutation that changes a gene’s product may have harmful effects • Example: Mutations that affect the proteins in hemoglobin reduce blood’s ability to carry oxygen

  36. Types of Mutations • Base-pair substitution • Type of mutation in which a single base-pair changes • Example: Sickle cell anemia • Mutations that shift the reading frame of the mRNA codons: • Deletion of one or more base pairs • Insertion of one or more base pairs • Example: Beta thalassemia

  37. Mutations in Hemoglobin A) Hemoglobin, an oxygen-binding protein in red blood cells. This protein consists of four polypeptides: two alpha globins (blue) and two beta globins (green). Each globin forms a pocket that cradles a type of cofactor called a heme (red ). Oxygen gas binds to the iron atom at the center of each heme.

  38. Mutations in Hemoglobin B) Part of the DNA (blue), mRNA (brown), and amino acid sequence (green) of human beta globin. Numbers indicate the position of the nucleotide in the coding sequence of the mRNA.

  39. Mutations in Hemoglobin C) A base-pair substitution replaces a thymine with an adenine. When the altered mRNA is translated, valine replaces glutamic acid as the sixth amino acid of the polypeptide. Hemoglobin with this form of beta globin is called HbS, or sickle hemoglobin.

  40. Mutations in Hemoglobin D) A deletion of one nucleotide causes the reading frame for the rest of the mRNA to shift. The protein translated from this mRNA is too short and does not assemble correctly into hemoglobin molecules. The result is beta thalassemia, in which a person has an abnormally low amount of hemoglobin.

  41. Mutations in Hemoglobin E) An insertion of one nucleotide causes the reading frame for the rest of the mRNA to shift. The protein translated from this mRNA is too short and does not assemble correctly into hemoglobin molecules. As in D, the outcome is beta thalassemia.

  42. Sickle-Cell Anemia: A Base-Pair Substitution glutamic acid valine A) A base-pair substitution results in the abnormal beta globin chain of sickle hemoglobin (HbS). The sixth amino acid in such chains is valine, not glutamic acid. The difference causes HbS molecules to form rod-shaped clumps that distort normally round blood cells into sickle shapes.

  43. Sickle-Cell Anemia sickled cell normal cell B) Left, the sickled cells clog small blood vessels, causing circulatory problems that result in damage to many organs. Destruction of the cells by the body’s immune system results in anemia. Right, Tionne “T-Boz” Watkins of the music group TLC is a celebrity spokesperson for the Sickle Cell Disease Association of America. She was diagnosed with sickle-cell anemia as a child.

  44. What Causes Mutations? • Most mutations result from unrepaired DNA polymerase errors during DNA replication • Some natural and synthetic chemicals cause mutations in DNA (example: cigarette smoke) • Insertion mutations may be caused by transposable elements, which move within or between chromosomes

  45. ANIMATED FIGURE: Base-pair substitution To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

  46. ANIMATION: Deletion To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

  47. ANIMATION: Frameshift mutation To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

  48. ANIMATED FIGURE: Sickle-cell anemia To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

  49. ANIMATED FIGURE: Controls of eukaryotic gene expression To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

  50. ANIMATION: X-chromosome inactivation To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

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