Chapter 10 Gene Expression and Regulation Fact 1: DNA contains information but is unable to carry out actions Fact 2: Proteins are the workhorses but contain no information Information in DNA must be linked with proteins Enzyme A THUS HOW? Substrate 1 Substrate 2 Substrate 3 Gene A Enzyme B Gene B Beadle & Tatum: Bread mold experiments (1940s) 1
Generally, one gene codes for one protein (polypeptide) Figure 10.1 Audesirk 2 & Byers Figure 3.6 Audesirk 2 & Byers How does information travel from DNA to ribosomes? Answer: RNA (ribonucleic acid) Structural Differences between RNA and DNA (See Table 10.1): 1) RNA is single-stranded 2) RNA has ribose sugar in backbone (DNA = deoxyribose) 3) RNA has base Uracil instead of Thymine (A U) Types of RNA: Messenger RNA (mrna) Carries code from DNA to ribosomes Ribosomal RNA (rrna) Combines with proteins to form ribosomes Transfer RNA (trna) Carries appropriate amino acids to ribosomes Figure 10.2 Audesirk 2 & Byers 2
Transcription DNA RNA Protein Nucleus Central Dogma of Biology: Translation Cytoplasm Figure 10.3 Audesirk 2 & Byers The Genetic Code: The Language of Life S O S Most organisms synthesize 20 unique amino acids A T G C = 4 AA TA CA GA AT TT CT GT AC TC CC GC AG TG CG GG = 16 How many possible three-base sequences (e.g., AAA) are there? Answer: 64 3
The Genetic Code: The Language of Life The genetic code is a triplet code: Three bases (called a codon) code for 1 amino acid More than 1 codon exists for most amino acids (see Table 10.3) START A U G Alanine Lysine Arginine Alanine G C G A A G A G G G C A STOP U A G Punctuation codons (start / stop) exist in genetic code Start = AUG Stop = UAG, UAA, UGA Transcription DNA RNA Protein Nucleus Central Dogma of Biology: Translation Cytoplasm Figure 10.3 Audesirk 2 & Byers 4
Transcription (DNA RNA): Promoter T A C Body GENE Termination Signal Step 1: Initiation Transcription produces a single strand of RNA that is complementary to one strand of DNA Non-coding region; protein binding sites RNA Polymerase binds to promoter region of gene Different version of RNA polymerase synthesizes each RNA type Step 2: Elongation RNA Polymerase unwinds DNA; synthesizes complementary copy Base pair rules apply except uracil replaces thymine A T C G A A A T C G C G A G G T DNA U A G C U U U A G C G C U C C A RNA Transcription: Initiation Elongation Strand which is transcribed Figure 10.4 Audesirk 2 & Byers 5
Transcription (DNA RNA): Promoter T A C Body GENE Termination Signal Transcription produces a single strand of RNA that is complementary to one strand of DNA Step 3: Termination RNA Polymerase reaches termination signal RNA detaches from RNA Polymerase RNA Polymerase detaches from DNA DNA zips back up Figure 10.4 Audesirk 2 & Byers Figure 10.5 Audesirk 2 & Byers Transcription (DNA RNA): Multiple RNA polymerase can transcribe a single gene at the same time The transcription of genes into RNA is selective: 1) Only certain cells transcribe certain genes Insulin (hormone) Pancreas 2) Only one strand of DNA (template strand) is copied 6
Transcription DNA RNA Protein Nucleus Central Dogma of Biology: Translation Cytoplasm Figure 10.3 Audesirk 2 & Byers Figure 10.2 / 10.7 Audesirk 2 & Byers Translation (RNA Protein): Step 1: mrna processed in nucleus Exons = Coding segments Introns = Non-coding segments Importance: A) Multiple proteins from single gene B) Quick / efficient protein evolution Step 2: mrna enters cytoplasm Exits nucleus via nuclear pores Composed of rrna and proteins Step 3: mrna binds to ribosome (protein factories) Composed of two (2) sub-units: Binds mrna and part of trna Large Sub-unit Binds part of trna; contains catalytic site 7
Translation (RNA Protein): Step 4: trna delivers first amino acid to ribosome trna contains three bases (anticodon) that form base pairs with mrna codon trna has correct amino acid attached for the mrna codon (61 unique trnas) Large Sub-unit Met Methionine U A C Amino Acid Anticodon U A C A U G A A G G C A U C U U A G Translation (RNA Protein): Step 5: Elongation of protein The next trna with proper anticodon binds to mrna Catalytic site joins amino acids together (peptide bond) Large Sub-unit Met Lys U A C U U C A U G A A G G C A U C U U A G 8
Translation (RNA Protein): Step 5: Elongation of protein The next trna with proper anticodon binds to mrna Catalytic site joins amino acids together (peptide bond) 1st trna leaves and ribosome moves down one spot Large Sub-unit Met Lys U A C U U C A U G A A G G C A U C U U A G Translation (RNA Protein): Step 5: Elongation of protein The next trna with proper anticodon binds to mrna Catalytic site joins amino acids together (peptide bond) 1st trna leaves and ribosome moves down one spot Large Sub-unit Met Lys Ala Cycle repeated U A C U U C C G U A U G A A G G C A U C U U A G 9
Translation (RNA Protein): Step 6: Termination of protein Process continues until stop codon is reached Finished protein is released from ribosome Met Lys Ala Ser A G A A U G A A G G C A U C U U A G Translation (RNA Protein): Step 6: Termination of protein Process continues until stop codon is reached Finished protein is released from ribosome Sub-units of ribosome separate from mrna Met Lys Ala Ser A G A A U G A A G G C A U C U U A G 10
Translation (RNA Protein): Step 6: Termination of protein Process continues until stop codon is reached Finished protein is released from ribosome Sub-units of ribosome separate from mrna Met Lys Ala Ser A U G A A G G C A U C U U A G Figure 10.9 Audesirk 2 & Byers Transcription / Translation Review: 11
Once Again - Mistakes Happen... Mutation: Changes in the sequence of bases in DNA Types of Mutations: 1) Point Mutation: A pair of bases is incorrectly matched T A T C T A A T A T A T Possible outcomes of a point mutation (active gene): 1) Protein is unchanged (codes for same amino acid) CTC and CTT still code for leucine 2) New protein equivalent to original protein Replace hydrophobic AA with hydrophobic AA (neutral mutation) 3) Protein structure is changed (problem causing e.g., sickle cell anemia) 4) Protein function destroyed due to stop codon insertion AAG codes of amino acid; ATG is a stop codon Once Again - Mistakes Happen... Mutation: Changes in the sequence of bases in DNA Types of Mutations: 2) Insertion Mutation: One or more nucleotide pairs are inserted into a gene T A T C A T A A T A G T A T 3) Deletion Mutation: One or more nucleotide pairs are deleted from a gene T A T T A? A T A A T? 12
Once Again - Mistakes Happen... Mutation: Changes in the sequence of bases in DNA Mutations provide the raw material for evolution... 13
Proper regulation of gene expression crucial for survival For Example: 30,000 genes in human genome Individual cells express small fraction of genes Gene expression changes over time Some genes never exptressed Regulation of Genes Occur Across Central Dogma: 1) Rate of transcription controlled: A) Regulatory proteins Assist / block binding of RNA polymerase B) Chromosome condensation (tightly packed areas) RNA polymerase can t access regions C) Chromosome inactivity (XX vs. XY chromosomes) Barr Body: Inactive X chromosome in females Random during development for which X chromosome inactivates 14
Proper regulation of gene expression crucial for survival For Example: 30,000 genes in human genome Individual cells express small fraction of genes Gene expression changes over time Some genes never exptressed Regulation of Genes Occur Across Central Dogma: 1) Rate of transcription controlled 2) Genes may produce different protein products 3) Rate of translation controlled (linked to mrna stability) 4) Protein requires activation modifications 5) Life span of protein controlled Figure 10.11 Audesirk 2 & Byers Gene Regulation: 15