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Chapter 11

Chapter 11. Gene Expression and Regulation. 11.1 How Is The Information In DNA Used In A Cell?. Most genes contain information for the synthesis of a single protein. A gene is a stretch of DNA encoding the instructions for the synthesis of a single protein.

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Chapter 11

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  1. Chapter 11 Gene Expression and Regulation

  2. 11.1 How Is The Information In DNA Used In A Cell? • Most genes contain information for the synthesis of a single protein. • A gene is a stretch of DNA encoding the instructions for the synthesis of a single protein. • Proteins form cellular structures and the enzymes that catalyze cellular chemical reactions.

  3. 11.1 How Is The Information In DNA Used In A Cell? • Proteins are synthesized through the processes of transcription and translation. • DNA does not directly guide protein synthesis, but rather, an intermediary—ribonucleic acid (RNA)—carries information from the nucleus to the cytoplasm.

  4. 11.1 How Is The Information In DNA Used In A Cell? • RNA is different than DNA in three respects: • RNA is single stranded; DNA is double stranded. • RNA has the sugar ribose; DNA has deoxyribose. • RNA contains the base uracil; DNA has thymine.

  5. 11.1 How Is The Information In DNA Used In A Cell?

  6. 11.1 How Is The Information In DNA Used In A Cell? • Protein synthesis occurs in two steps, called transcription and translation. gene DNA (nucleus) (cytoplasm) Transcription of thegene produces an mRNA with anucleotide sequencecomplementary to oneof the DNA strands (a) Transcription messenger RNA Translation of the mRNAproduces a protein moleculewith an amino acid sequencedetermined by the nucleotidesequence in the mRNA (b) Translation ribosome protein Fig. 11-1

  7. 11.1 How Is The Information In DNA Used In A Cell? • Transcription: the information contained in the DNA of a specific gene is copied into one of three types of RNA • Messenger RNA (mRNA) • Transfer RNA (tRNA) • Ribosomal RNA (rRNA) • In eukaryotic cells, transcription occurs in the nucleus.

  8. 11.1 How Is The Information In DNA Used In A Cell? • Translation: ribosomes convert the base sequence in mRNA to the amino acid sequence of a protein • In eukaryotic cells, translation occurs in the cytoplasm.

  9. 11.2 What Are The Functions Of RNA? • Messenger RNA carries the code for a protein from the nucleus to the cytoplasm. • All RNA is produced by transcription from DNA, but only mRNA carries the code for amino acid sequence of a protein. • mRNA is synthesized in the nucleus and enters the cytoplasm through nuclear envelope pores. • In the cytoplasm, mRNA binds to ribosomes, which synthesize a protein specified by the mRNA base sequence; DNA remains in the nucleus.

  10. 11.2 What Are The Functions Of RNA? • Messenger RNA (mRNA) The base sequenceof mRNA carries theinformation for theamino acid sequenceof a protein G C U G A G U A U A U G (a) Messenger RNA (mRNA) Fig. 11-2a

  11. 11.2 What Are The Functions Of RNA? • Ribosomal RNA and proteins form ribosomes. • Each ribosome consists of two subunits—one small and one large. • The small subunit has binding sites for mRNA, a “start” tRNA, and other proteins that cooperate to read mRNA to start protein synthesis. • The large subunit has two binding sites for tRNA molecules, and one catalytic site where peptide bonds join amino acids together into a protein. • During protein synthesis, the two subunits come together, clasping an mRNA molecule between them.

  12. 11.2 What Are The Functions Of RNA? • Ribosomal RNA (rRNA) catalytic site rRNA combines withproteins to form ribosomes;the small subunit bindsmRNA; the large subunitbinds tRNA and catalyzespeptide bond formationbetween amino acidsduring protein synthesis 1 2 largesubunit tRNA/amino acidbinding sites smallsubunit (b) Ribosome: contains ribosomal RNA (rRNA) Fig. 11-2b

  13. 11.2 What Are The Functions Of RNA? • Transfer RNA molecules carry amino acids to the ribosomes. • Each cell synthesizes many different kinds of transfer RNA, one or more for each amino acid. • Twenty different kinds of enzymes in the cytoplasm, one for each amino acid, recognize the rRNA and attach the correct amino acid. • These “loaded” tRNA molecules deliver their amino acids to the ribosome, where they are incorporated into the growing protein chain.

  14. 11.2 What Are The Functions Of RNA? • Transfer RNA (tRNA) Each tRNA carries a specificamino acid (in this example,tyrosine [tyr]) to a ribosomeduring protein synthesis;the anticodon of tRNA pairswith a codon of mRNA,ensuring that the correctamino acid is incorporatedinto the protein tyr tRNA attachedamino acid anticodon (c) Transfer RNA (tRNA) Fig. 11-2c

  15. 11.3 What Is The Genetic Code? • The genetic code translates the sequence of bases in nucleic acids into the sequence of amino acids in proteins. • A sequence of three bases codes for an amino acid; the triplet is called a codon. • There are 64 possible combinations of codons, which is more than enough to code for the 20 amino acids in proteins.

  16. 11.3 What Is The Genetic Code?

  17. 11.3 What Is The Genetic Code? • How does a cell recognize where codons start and stop, and where the code for an entire proteins starts and stops? • Most codons specify a specific amino acid in a protein sequence, but others are punctuation marks that indicate the end of one protein sequence and the start of another. • All proteins begin with the start codon AUG (methionine), and all end with UAG, UAA, or UGA, called stop codons. • Almost all amino acids are coded for by more than one codon (e.g., six codons code for leucine).

  18. 11.4 How Is The Information In A Gene Transcribed Into RNA? • Transcription copies the genetic information of DNA into RNA in the nucleus of eukaryotic cells. • Transcription is made up of three different processes: • Initiation: the promotor region at the beginning of a gene starts transcription • Elongation: the main body of a gene is where the RNA strand is elongated • Termination: the termination signal at end of a gene is where RNA synthesis stops

  19. 11.4 How Is The Information In A Gene Transcribed Into RNA? • Transcription begins when RNA polymerase binds to the promotor of a gene. • RNA polymerase catalyzes the transcription of DNA to RNA. • RNA polymerase first finds the promoter region (a non-transcribed sequence of DNA bases) that marks the start of a gene, and then binds to it, opening up the DNA as it does. • Transcription of the gene begins after the promoter is bound to RNA polymerase.

  20. 11.4 How Is The Information In A Gene Transcribed Into RNA? • Initiation DNA gene 3 gene 1 gene 2 RNApolymerase DNA promoter Initiation: RNA polymerase binds to the promoter region of DNA nearthe beginning of a gene, separating the double helix near the promoter. Fig. 11-3(1)

  21. 11.4 How Is The Information In A Gene Transcribed Into RNA? • Elongation generates a growing strand of RNA. • RNA polymerase adds complementary bases to those in the DNA template strand, to make a growing RNA strand that has uracil rather than thymine complementary to adenine. • The two strands of DNA re-form the original double helix. • One end of the growing RNA strand drifts away from the DNA molecule, while the other remains attached to the DNA template strand by the RNA polymerase.

  22. 11.4 How Is The Information In A Gene Transcribed Into RNA? • Elongation RNA DNA template strand Elongation: RNA polymerase travels along the DNA template strand (blue), unwindingthe DNA double helix and synthesizing RNA by catalyzing the addition of ribose nucleotidesinto an RNA molecule (red). The nucleotides in the RNA are complementary to the templatestrand of the DNA. Fig. 11-3(2)

  23. 11.4 How Is The Information In A Gene Transcribed Into RNA? • RNA transcription in action gene growingRNAmolecules end ofgene DNA direction of transcription beginningof gene Fig. 11-4

  24. 11.4 How Is The Information In A Gene Transcribed Into RNA? • Transcription stops when RNA polymerase reaches the termination signal. • RNA polymerase continues along the DNA template strand until it comes to the termination signal (a specific sequence of DNA bases). • At the termination signal, RNA polymerase drops off the DNA and releases the completed RNA molecule. • The enzyme is ready to bind to another promoter, to start the process over.

  25. 11.4 How Is The Information In A Gene Transcribed Into RNA? • Termination termination signal Termination: At the end of the gene, RNA polymerase encounters a DNAsequence called a termination signal. RNA polymerase detaches from theDNA and releases the RNA molecule. Fig. 11-3(3)

  26. 11.4 How Is The Information In A Gene Transcribed Into RNA? • Conclusion of transcription RNA Conclusion of transcription: After termination, the DNA completely rewinds into a doublehelix. The RNA molecule is free to move from the nucleus to the cytoplasm for translation,and RNA polymerase may move to another gene and begin transcription once again. Fig. 11-3(4)

  27. 11.4 How Is The Information In A Gene Transcribed Into RNA? PLAY Animation—Transcription

  28. 11.4 How Is The Information In A Gene Transcribed Into RNA? • Transcription is selective. • Some genes are transcribed in all cells because they encode essential proteins, like the electron transport chain of mitochondria. • Other genes are transcribed only in specific types of cells.

  29. 11.4 How Is The Information In A Gene Transcribed Into RNA? • Transcription is selective (continued). • How do cells regulate which genes are transcribed? • Proteins bind to “control regions” near gene promotors and block or enhance the binding of RNA polymerase. • By this means, the amount of a specific protein encoded by a specific gene in a cell can be controlled.

  30. 11.5 How Is The Information In Messenger RNA Translated Into Protein? • mRNA, with a specific base sequence, is used during translation to direct the synthesis of a protein with the amino acid sequence encoded by the mRNA. • Decoding the base sequence of mRNA is the job of tRNA and ribosomes in the cytoplasm. • The ability of tRNA to deliver the correct amino acid to the ribosomes depends on base pairing between each codon of mRNA and a set of three complementary bases in tRNA, called the anticodon.

  31. 11.5 How Is The Information In Messenger RNA Translated Into Protein? • Like transcription, translation has three steps: • Initiation of protein synthesis • Elongation of the protein chain • Termination of translation

  32. 11.5 How Is The Information In Messenger RNA Translated Into Protein? • Initiation: translation begins when tRNA and mRNA bind to a ribosome • The first amino acid in all proteins is a methionine (AUG codon). • An initiation complex—a small ribosomal subunit, a methonine tRNA, and a methionine amino acid—binds to an AUG codon in an mRNA molecule. • The large subunit of the ribosome joins the complex to complete the assembly of the ribosome. • The methionine tRNA binds to the first binding site on the large ribosome subunit.

  33. 11.5 How Is The Information In Messenger RNA Translated Into Protein? • Initiation Initiation: second tRNA binding site amino acid catalytic site met met met anticodon tRNA first tRNAbindingsite methioninetRNA largeribosomalsubunit initiationcomplex A U C A U C mRNA A U C smallribosomalsubunit G C A U G G U U C A G C A U G G U U C A start codon A tRNA with an attachedmethionine amino acid bindsto a small ribosomal subunit,forming an initiation complex. The initiation complex binds to anmRNA molecule. The methionine (met)tRNA anticodon (UAC) base-pairs withthe start codon (AUG) of the mRNA. The large ribosomal subunitbinds to the small subunit. Themethionine tRNA binds to the firsttRNA site on the large subunit. Fig. 11-5(1,2,3)

  34. 11.5 How Is The Information In Messenger RNA Translated Into Protein? • Elongation: amino acids are added one at a time to the growing protein chain • Assembled ribosomes have two binding sites and a catalytic site. • The first binding site has methionine and its rRNA attached. • The second binding site accepts another tRNA with an anticodon complementary to the codon on the mRNA associated with the second binding site.

  35. 11.5 How Is The Information In Messenger RNA Translated Into Protein? • Elongation Elongation: catalytic site met initiatortRNA detaches met met val peptidebond val val U A C C A A C C A C A A U A A G C A U G G U U C A G C A U G G U U C A G C A U G G U U C A U A G ribosome moves one codon to the right The second codon of mRNA(GUU) base-pairs with theanticodon (CAA) of a secondtRNA carrying the amino acidvaline (val). This tRNA binds tothe second tRNA site on thelarge subunit. The catalytic site on thelarge subunit catalyzes theformation of a peptide bondlinking the amino acidsmethionine and valine. Thetwo amino acids are nowattached to the tRNA in thesecond binding site. The “empty” tRNA is releasedand the ribosome moves down themRNA, one codon to the right. ThetRNA that is attached to the twoamino acids is now in the first tRNAbinding site and the second tRNAbinding site is empty. Fig. 11-5(4,5,6)

  36. 11.5 How Is The Information In Messenger RNA Translated Into Protein? • Elongation (continued) • The catalytic site forms a peptide bond between the two amino acids. • The ribosome moves to the next codon on mRNA and shifts the growing amino acid chain from the second to the first binding site. • The third amino acid is then added to the chain. • The ribosome moves along mRNA, adding one amino acid to the next. • The process repeats over and over as the ribosome moves along the mRNA, one codon at a time.

  37. 11.5 How Is The Information In Messenger RNA Translated Into Protein? • Elongation (continued) met met val his his val C A A G U A C A A G U A G C A U G G U U C A G G C A U G G U U C A G U A U A The third codon of mRNA (CAU)base-pairs with the anticodon (GUA) ofa tRNA carrying the amino acid histidine(his). This tRNA enters the second tRNAbinding site on the large subunit. The catalytic site forms a peptide bondbetween valine and histidine, leaving the peptideattached to the tRNA in the second binding site.The tRNA in the first site leaves, and theribosome moves one codon over on the mRNA. Fig. 11-5(7,8)

  38. 11.5 How Is The Information In Messenger RNA Translated Into Protein? • Termination: a stop codon signals the end of translation • Ribosome encounters a stop codon in the mRNA sequence that signals that protein synthesis is complete. • Stop codons do not bind tRNA, but rather, they bind proteins that cause the ribosome to release the complete amino acid chain. • The large and small subunits of the ribosome also come apart once the stop codon is reached.

  39. 11.5 How Is The Information In Messenger RNA Translated Into Protein? • Termination Termination: met val his arg completedpeptide arg ile stop codon C G A A U C U A G U A A This process repeats untila stop codon is reached; themRNA and the completedpeptide are released from theribosome, and the subunitsseparate. Fig. 11-5(9)

  40. 11.5 How Is The Information In Messenger RNA Translated Into Protein? PLAY Animation—Translation

  41. 11.5 How Is The Information In Messenger RNA Translated Into Protein? • Summing up: transcription and translation • With a few exceptions, each gene codes for a single protein. • Transcription of a protein-coding gene produces an mRNA that is complementary to the template strand of the DNA for the gene. • Enzymes in the cytoplasm attach the appropriate amino acid to each tRNA. • The mRNA moves from the nucleus to the cytoplasm. • tRNAs carry their attached amino acids to the ribosome.

  42. 11.5 How Is The Information In Messenger RNA Translated Into Protein? • Summing up: transcription and translation (continued) • At the ribosome, the bases in tRNA anticodons bind to the complementary bases in mRNA codons. • The amino acids attached to the tRNAs line up in the sequence specified by the codons. • The ribosome joins the amino acids together with peptide bonds to form a protein. • When a stop codon is reached, the finished protein is released from the ribosome.

  43. 11.5 How Is The Information In Messenger RNA Translated Into Protein? • Complementary base pairing is critical to decoding genetic information. gene (a) DNA A T G G G A G T T etc. complementaryDNA strand template DNAstrand etc. T A C C C T C A A codons (b) mRNA etc. A U G G G A G U U anticodons (c) tRNA etc. U A C C C U C A A amino acids (d) protein etc. methionine glycine valine Fig. 11-6

  44. Protein Synthesis Suggested Media Enhancement: Protein Synthesis To access this animation go to folder C_Animations_and_Video_Filesand open the BioFlix folder.

  45. 11.6 How Do Mutations Affect Gene Function? • Changes in the sequence of DNA nucleotide bases as a result of replication errors, ultraviolet light, chemicals, and many other environmental factors are called mutations. • Sometimes during DNA replication, an incorrect pair of nucleotides is incorporated into the growing DNA double helix. • This is called nucleotide substitution, or point mutation, because the nucleotides in the DNA sequence are changed.

  46. 11.6 How Do Mutations Affect Gene Function? • Mutations (continued) • A insertion mutation occurs when one or more new nucleotide pairs are inserted into a gene. • A deletion mutation occurs when one or more nucleotide pairs are removed from a gene.

  47. 11.6 How Do Mutations Affect Gene Function? • Mutations may have a variety of effects on protein structure and function. • The protein may be unchanged. • The new protein may be functionally equivalent to the original one. • Protein function may be changed by an altered amino acid sequence. • Protein function may be destroyed by a premature stop codon.

  48. 11.6 How Do Mutations Affect Gene Function?

  49. 11.6 How Do Mutations Affect Gene Function? • Mutations are the raw material for evolution. • Mutations are the ultimate source of all genetic differences among individuals. • Without mutations, individuals would share the same DNA sequence. • Most mutations are harmful; some improve the individual’s ability to survive and reproduce. • The mutation may be passed from generation to generation and become more common over time. • This process is known as natural selection, and is the major cause of evolutionary change.

  50. 11.7 Are All Genes Expressed? • All of the genes in the human genome are present in each body cell, but individual cells express only a small fraction of them. • The particular set of genes that is expressed depends on the type of cell and the needs of the organism. • This regulation of gene expression is crucial for proper functioning of individual cells and entire organisms.

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