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RESULTS

This experiment investigates the growth of different classes of Neurospora crassa mutants in a minimal medium. Results show the effect of mutations in genes A, B, and C on cell growth. The study concludes that class II mutants (gene B mutation) have impaired growth compared to wild-type and class I mutants (gene A mutation) and class III mutants (gene C mutation).

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RESULTS

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  1. EXPERIMENT No growth: Mutant cells cannot grow and divide Growth: Wild-type cells growing and dividing Minimal medium RESULTS Classes of Neurospora crassa Wild type Class I mutants Class III mutants Class II mutants Minimal medium (MM) (control) MM + ornithine Condition MM + citrulline MM + arginine (control) CONCLUSION Class II mutants (mutation in gene B) Class I mutants (mutation in gene A) Class III mutants (mutation in gene C) Wild type Precursor Precursor Precursor Precursor Gene A Enzyme A Enzyme A Enzyme A Enzyme A Ornithine Ornithine Ornithine Ornithine Gene B Enzyme B Enzyme B Enzyme B Enzyme B Citrulline Citrulline Citrulline Citrulline Gene C Enzyme C Enzyme C Enzyme C Enzyme C Arginine Arginine Arginine Arginine

  2. DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide (a) Bacterial cell Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA TRANSLATION Ribosome Polypeptide (b) Eukaryotic cell

  3. Gene 2 DNA molecule Gene 1 Gene 3 DNA template strand TRANSCRIPTION mRNA Codon TRANSLATION Protein Amino acid

  4. Second mRNA base First mRNA base (5 end of codon) Third mRNA base (3 end of codon)

  5. (a) Tobacco plant expressing a firefly gene (b) Pig expressing a jellyfish gene

  6. Promoter Transcription unit 5 3 3 5 DNA Start point RNA polymerase Initiation Nontemplate strand of DNA 1 Elongation RNA nucleotides 5 3 RNA polymerase 5 3 Template strand of DNA RNA transcript Unwound DNA 3 Elongation 2 3 end Rewound DNA 5 3 5 3 5 3 5 5 Direction of transcription (“downstream”) RNA transcript Template strand of DNA Termination 3 Newly made RNA 3 5 5 3 5 3 Completed RNA transcript

  7. A eukaryotic promoter includes a TATA box 1 Promoter Template 5 3 3 5 TATA box Template DNA strand Start point Several transcription factors must bind to the DNA before RNA polymerase II can do so. 2 Transcription factors 5 3 3 5 Additional transcription factors bind to the DNA along with RNA polymerase II, forming the transcription initiation complex. 3 RNA polymerase II Transcription factors 3 5 5 5 3 RNA transcript Transcription initiation complex

  8. Polyadenylation signal Protein-coding segment 5 3 … G AAA AAA P P P AAUAAA 5 Cap Poly-A tail Start codon 5 UTR 3 UTR Stop codon

  9. Exon Exon Intron 5 Exon Intron 3 Pre-mRNA Poly-A tail 5 Cap 146 31 104 1 105 30 Introns cut out and exons spliced together Coding segment mRNA 5 Cap Poly-A tail 1 146 5 UTR 3 UTR

  10. RNA transcript (pre-mRNA) 5 Exon 1 Intron Exon 2 Protein Other proteins snRNA snRNPs Spliceosome 5 Spliceosome components Cut-out intron mRNA 5 Exon 2 Exon 1

  11. Gene DNA Exon 1 Intron Exon 2 Intron Exon 3 Transcription RNA processing Translation Domain 3 Domain 2 Domain 1 Polypeptide

  12. Amino acids Polypeptide tRNA with amino acid attached Ribosome Trp Phe Gly tRNA Anticodon Codons 5 3 mRNA

  13. 3 Amino acid attachment site 5 Hydrogen bonds Anticodon (a) Two-dimensional structure Amino acid attachment site 5 3 Hydrogen bonds 3 5 Anticodon Anticodon (c) Symbol used in this book (b) Three-dimensional structure

  14. Aminoacyl-tRNA synthetase (enzyme) Amino acid P P P Adenosine ATP P Adenosine tRNA P P i Aminoacyl-tRNA synthetase P i P i tRNA P Adenosine AMP Computer model Aminoacyl-tRNA (“charged tRNA”)

  15. Growing polypeptide Exit tunnel tRNA molecules Large subunit E P A Small subunit 5 3 mRNA (a) Computer model of functioning ribosome P site (Peptidyl-tRNA binding site) A site (Aminoacyl- tRNA binding site) E site (Exit site) E Large subunit A P mRNA binding site Small subunit (b) Schematic model showing binding sites Growing polypeptide Amino end Next amino acid to be added to polypeptide chain E tRNA mRNA 3 Codons 5 (c) Schematic model with mRNA and tRNA

  16. Large ribosomal subunit 3 U 5 C A P site Met Met 5 3 A G U Initiator tRNA GDP GTP E A mRNA 5 5 3 3 Start codon Small ribosomal subunit Translation initiation complex mRNA binding site

  17. Amino end of polypeptide E 3 mRNA P site A site Ribosome ready for next aminoacyl tRNA 5 GTP GDP E E P A A P GDP GTP E P A

  18. Release factor Free polypeptide 5 3 3 3 2 GTP 5 5 Stop codon (UAG, UAA, or UGA) 2 GDP

  19. Completed polypeptide Growing polypeptides Incoming ribosomal subunits Polyribosome Start of mRNA (5 end) End of mRNA (3 end) (a) Ribosomes mRNA (b) 0.1 µm

  20. Ribosome mRNA Signal peptide ER membrane Signal peptide removed Signal- recognition particle (SRP) Protein CYTOSOL Translocation complex ER LUMEN SRP receptor protein

  21. Wild-type hemoglobin DNA Mutant hemoglobin DNA C T T C 3 3 A T 5 5 T 5 A G G A A 3 5 3 mRNA mRNA G A A 5 G U A 3 5 3 Normal hemoglobin Sickle-cell hemoglobin Val Glu

  22. Wild-type DNA template strand 3 5 3 5 mRNA 5 3 Protein Stop Amino end Carboxyl end A instead of G Extra A 5 3 5 3 5 5 3 3 U instead of C Extra U 5 5 3 3 Stop Stop Silent (no effect on amino acid sequence) Frameshift causing immediate nonsense (1 base-pair insertion) T instead of C missing 3 3 5 5 5 5 3 3 A instead of G missing 5 3 5 3 Stop Missense Frameshift causing extensive missense (1 base-pair deletion) missing A instead of T 5 3 5 3 5 3 5 3 U instead of A missing 5 3 5 3 Stop Stop Nonsense No frameshift, but one amino acid missing (3 base-pair deletion) (a) Base-pair substitution (b) Base-pair insertion or deletion

  23. RNA polymerase DNA mRNA Polyribosome 0.25 µm Direction of transcription RNA polymerase DNA Polyribosome Polypeptide (amino end) Ribosome mRNA (5 end)

  24. DNA TRANSCRIPTION 3 Poly-A RNA polymerase RNA transcript 5 RNA PROCESSING Exon RNA transcript (pre-mRNA) Intron Aminoacyl-tRNA synthetase Poly-A NUCLEUS Amino acid AMINO ACID ACTIVATION CYTOPLASM tRNA mRNA Growing polypeptide 3 Cap A Poly-A Activated amino acid P Ribosomal subunits E Cap 5 TRANSLATION E A Anticodon Codon Ribosome

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