Ch. 17 From Gene to Protein Thought Questions Ms. Whipple – Brethren Christian High School

1. What is Gene Expression? Gene expression is the process by which DNA directs protein synthesis, includes two stages called transcription and translation. Proteins are the link between genotype and phenotype. Section 17.1

2. Briefly explain how Beadle and Tatum’s experiment supported the one-gene to one-enzyme hypothesis? George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal media Using crosses, they and their coworkers identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine They developed a one gene–one enzyme hypothesis, which states that each gene dictates production of a specific enzyme Section 17.1

Figure 17.2 EXPERIMENT RESULTS Classes of Neurospora crassa Growth: Wild-type cells growing and dividing No growth: Mutant cells cannot grow and divide Wild type Class I mutants Class II mutants Class III mutants Minimal medium (MM) (control) Minimal medium MM  ornithine Condition MM  citrulline MM  arginine (control) Can grow with or without any supplements Can grow on ornithine, citrulline, or arginine Can grow only on citrulline or arginine Require arginine to grow Summary of results Figure 17.2 Inquiry: Do individual genes specify the enzymes that function in a biochemical pathway? CONCLUSION Gene (codes for enzyme) Class I mutants (mutation in gene A) Class II mutants (mutation in gene B) 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 Section 17.1 Arginine Arginine Arginine Arginine

3. How did this one gene to one-enzyme hypothesis develop over the years of research? How is it understood now? Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one protein Many proteins are composed of several polypeptides, each of which has its own gene Therefore, Beadle and Tatum’s hypothesis is now restated as the one gene–one polypeptide hypothesis Note that it is common to refer to gene products as proteins rather than polypeptides Section 17.1

4. When making a protein from a gene, the cell will go through Transcription and then Translation. Please describe the process of Transcription. What is the role of Messenger RNA (mRNA) in this process? Getting from DNA to protein requires two major stages: transcription and translation. During transcription, a DNA strand provides a template for the synthesis of a complementary RNA strand. Just as a DNA strand provides a template for the synthesis of each new complementary strand during DNA replication, it provides a template for assembling a sequence of RNA nucleotides. Transcription of a protein-coding gene produces a messenger RNA (mRNA) molecule. Section 17.1

5. Describe the process of Translation. Translation is the synthesis of a polypeptide, using the information in mRNA. During translation, there is a change of language. The nucleotides from mRNA will now be read in groups of three called codon The sites of translation are the ribosomes, complex particles that facilitate the orderly assembly of amino acids into polypeptide chains. Transcription and translation occur in all organisms, from all three domains of life. Section 17.1

Nuclear envelope DNA Pre-mRNA TRANSCRIPTION (b) Eukaryotic cell Figure 17.3b-1 Nuclear envelope DNA TRANSCRIPTION Pre-mRNA Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information. (b) Eukaryotic cell Section 17.1

Nuclear envelope DNA Pre-mRNA mRNA TRANSCRIPTION RNA PROCESSING Figure 17.3b-2 Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information. (b) Eukaryotic cell Section 17.1

Nuclear envelope DNA Pre-mRNA mRNA Ribosome Polypeptide TRANSCRIPTION Figure 17.3b-3 Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information. TRANSLATION Ribosome Polypeptide (b) Eukaryotic cell Section 17.1

6. What is a Codon? Why is your genetic code “read” this way? There are 20 amino acids, but there are only four nucleotide bases in DNA so each amino acid cannot be linked to one nucleotide. The flow of information from gene to protein is based on a triplet code: a series of non-overlapping, three-nucleotide words The words of a gene are transcribed into complementary non- overlapping three-nucleotide words of mRNA These words are then translated into a chain of amino acids, forming a polypeptide Section 17.1

DNA template strand 5 DNA 3 molecule Gene 1 5 3 TRANSCRIPTION Figure 17.4 DNA template strand 5 DNA 3 A C C A A A C C G A G T molecule T G G T T T G G C T C A Gene 1 5 3 TRANSCRIPTION Gene 2 U G G U U U G G C U C A mRNA 5 3 Codon TRANSLATION Figure 17.4 The triplet code. Protein Trp Phe Gly Ser Gene 3 Amino acid Section 17.1

7. What is a Template Strand? Is it the same side for every gene? During transcription, one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript The template strand is always the same strand for a given gene but not the same side for all genes. Section 17.1

8. Critical Thinking: Why do you think many of the amino acids have multiple codon options for translation? (Hint: what would this protect against?) Having multiple codon options for amino acids helps protect against detrimental mutations of the DNA code. For example, Leucine is coded for by CUU; CUA; CUC; and CUG. If there is a mutation of the last nucleotide, the protein will remain the same. Section 17.1

First mRNA base (5 end of codon) Third mRNA base (3 end of codon) Figure 17.5 Second mRNA base U C A G UUU UCU UAU UGU U Phe Tyr Cys UUC UCC UAC UGC C U Ser UUA UCA UAA Stop UGA Stop A Leu UUG UCG UAG Stop UGG Trp G CUU CCU CAU CGU U His CUC CCC CAC CGC C C Leu Pro Arg CUA CCA CAA CGA A Gln CUG CCG CAG CGG G First mRNA base (5 end of codon) Third mRNA base (3 end of codon) AUU ACU AAU AGU U Asn Ser AUC Ile ACC AAC AGC C A Thr Figure 17.5 The codon table for mRNA. AUA ACA AAA AGA A Lys Arg AUG Met or start ACG AAG AGG G GUU GCU GAU GGU U Asp GUC GCC GAC GGC C G Val Ala Gly Gly GUA GCA GAA GGA A Glu GUG GCG GAG GGG G Section 17.1

9. What is a Reading Frame? Why is this so important when it comes to sending messages? To extract the message from the genetic code requires specifying the correct starting point. The starting point establishes the reading frame; subsequent codons are read in groups of three nucleotides. The cell’s protein-synthesizing machinery reads the message as a series of non-overlapping three-letter words. Section 17.1

10. What evidence from the Genetic code & Protein synthesis supports the theory of the Evolution of Species? The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals Genes can be transcribed and translated after being transplanted from one species to another

(a) Tobacco plant expressing (b) Pig expressing a jellyfish Figure 17.6 Figure 17.6 Expression of genes from different species. (a) Tobacco plant expressing (b) Pig expressing a jellyfish a firefly gene gene Section 17.1