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CHAPTER 13 GENE EXPRESSION DISCOVERY OF THE GENE-PROTEIN RELATIONSHIP Archibald Garrod discussed alkaptonuria in his book o Rare genetic disease that causes urine to turn black when it is exposed to air o Hypothesized that people with alkaptonuria lack the enzyme that normally oxidizes homogentisic acid James Summer purified a different enzyme, urease, and showed it to be a protein o First clear identification of an enzyme as a protein Beadle and Tatum proposed the one-gene, one-enzyme hypothesis It was not clear whether the genes themselves were acting as enzymes or whether they controlled the workings of enzymes in more intricate ways o Decided to look for mutations interfering with known metabolic reactions that produce essential molecules, such as amino acids and vitamins o Exposed thousands of haploid wild-type Neurospora asexual spores to X- rays or ultraviolet radiation to induce mutant strains Each mutant strain had a mutation in only one gene and that each gene affected only one enzyme One-gene, one enzyme hypothesis INFORMATION FLOW FROM DNA TO PROTEIN: AN INTERVIEW Ribonucleic acid (RNA) is the link between DNA and protein o RNA copy is made of the information in the DNA RNA copy provides the information that directs polypeptide synthesis o RNA is a polymer of nucleotides o Usually single-stranded o Sugar in RNA is ribose Similar to deoxyribose except it has a hydroxyl group on 2’ o Uracil replaces thymine and is pyrimidine that can form two hydrogen bonds with adenine DNA is transcribed to form RNA Sequence of bases in the RNA strand is determined by complementary base pairing with one of the DNA strands o Template strand o Process is called transcription o Three types of RNA are transcribed Messenger RNA (mRNA) Single strand of RNA that carries the information for making a protein Transfer RNA (tRNA)
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Single strand of RNA that folds back on itself to form a specific shape Ribosomal RNA (rRNA) Globular form of RNA that is an important part of the structure of ribosomes and has catalytic functions needed during protein synthesis RNA is translated to form a polypeptide In translation , transcribed information in the mRNA is used to specify the amino acid sequence of polypeptides o Sequence of three consecutive bases in mRNA ( codon ) specifies one amino acid Code is referred to as triplet code o Assignment of codons for amino acids and for start and stop signals are called the genetic code Transfer RNAs act as adapters that connect amino acids and nucleic acids o Each tRNA links with a specific amino acid Recognizes the appropriate mRNA codon for that particular amino acid o tRNA has a sequence of three bases ( anticodon ) that hydrogen bonds with the mRNA codon by base pairing Translation requires: o Each tRNA anticodon to be hydrogen-bonded to the complementary mRNA codon o The amino acids carried by the tRNAs to be linked in the order specified by the sequence of codons in the mRNA
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This note was uploaded on 04/04/2008 for the course BIO 101 taught by Professor Martin during the Fall '08 term at Rutgers.

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