Study guide: Chapter 15 - How Genes Work? • Describe the central dogma. DNA, RNA, Protein and exceptions to it. • Describe the nature of the genetic code. • Predict the sequence of amino acids in a protein when given a sequence of bases in DNA or RNA. • Explain the causes and impact of point mutations. I. What Do Genes Do? II. The Central Dogma of Molecular Biology A. The genetic code hypothesis1. F. Crick proposed that the nucleotide sequence of DNA is a code; DNA is only an information storage molecule. 2. Different combinations of bases specify each of the 20 amino acids. 3. The amino acid sequence of an enzyme is coded for by a specific stretch of DNA sequence. 4. The information in DNA can be copied for the next generation by complementary base pairing of nucleotides. B. RNA as the intermediary between genes and proteins1. Information does not flow from DNA directly to proteins because DNA is enclosed in the nucleus (in eukaryotes) and proteins are made on ribosomes in the cytoplasm. 2. Jacob and Monod hypothesis: RNA molecules act as intermediaries between genes and ribosomes. (Fig. 15.3) a. Prediction: Short-lived messenger RNA carries the information of DNA to the ribosomes. b. Follow-up research confirmed the mRNA hypothesis. (1) RNA polymerase, the enzyme that synthesizes RNA, was discovered. (2) RNA polymerase uses the information in DNA to determine the RNA sequence. (3) RNA polymerase does not need a primer. (4) RNA polymerase pairs uracil (not thymine) to adenine. C. Dissecting the central dogma1. The central dogma summarizes the flow of information in a cell. 2. The central dogma states that DNA codes for RNA and RNA codes for protein.a. DNA is hereditary material. b. Genes are stretches of DNA that code for proteins. c. The gene sequence in DNA specifies an RNA sequence. d. The RNA sequence specifies a sequence of amino acids in the protein. e. Ultimately, genes code for proteins. 3. The roles of transcription and translation a. The process of making RNA from the DNA code is called transcription; DNA is transcribed to RNA by RNA polymerase. b. The process of using the RNA code to make a protein iscalled translation; mRNA is translated to proteins in ribosomes.4. The central dogma states that an organism’s genotype is determinedby its DNA sequences and its phenotype is a product of the proteins itproduces. Example: coat color in Oldfield mice. (Fig. 15.4) 5. Exceptions to the central dogma a. Many genes code for RNA molecules that are not translated into proteins. (1) Some RNAs are important during translation. (2) Some RNAs regulate transcription. (3) Some RNAs regulate which proteins are active in a cell. 1b. Reverse transcriptase: Some RNA viral polymerases synthesize DNA from the RNA template. Example: human immunodeficiency virus (HIV). III. The Genetic Code How long is a word in the genetic code?1. The genetic code is a set of rules that specify the relationship between a sequence of nucleotides in DNA or RNA and the amino acid sequence in the primary structure of a protein.  3-base code. a. A 1-base code (4 different nucleotides) could specify only 4 amino acids. b. A 2-base code (4 × 4) could specify 16 amino acids. c. A 3-base code (4 × 4 × 4) could specify 64 different amino acids, more than the 20 found in living organisms. d. Prediction: The code is redundant; some amino acids are specified by more than one triplet of nucleotides.2. Francis Crick and Sydney Brenner confirmed that codons are three bases long. (1) The code is read in triplets of bases. Each triplet is a codon. (2) Additions or deletions of one or two nucleotides disturb the reading frame (the groups of three nucleotides that are read as codons) and scramble the message. (3) Additions or deletions of three nucleotides maintain the reading frame (although the sequence either gains a triplet or loses one). -The code is redundant: All amino acids except methionine are specified by more than one codon. -The code is nearly universal: All organisms, with few exceptions, use the same codons to specify thesame amino acids. START codon and STOP codons?-The code is conservative: When several codons specify the same amino acid, the first two bases in the codons are almost always identical. (1) If a transcription or translation error occurs in the third position of the codon, it is less likely to affect the amino acid in the final protein. (2) This minimizes the phenotypic effects of small changes in DNA and increases the efficiency of the genetic code. 3. Using the code a. One can predict the amino acid sequence of a protein from the gene sequence. (Fig. 15.7) b. You should be able to identify the codons in Figure 15.7a and check to make sure that they are translated correctly. You should also be able to complete the exercises in Figure 15.7b. IV. What Is the Molecular Basis of Mutation? A. What is a mutation? 1. A mutation is a permanent change in an organism’s DNA, a change in its genotype. Point mutation (Fig. 15.8) DNA polymerase mistakenly inserts the wrong base. Proofreading and mismatch repair fail to correct the error. A change is now present in one of the bases in the DNA. This change can cause a change in an amino acid in the protein and thus a change in the phenotype. (Fig. 15.4) Explain 4 known types of POINT mutations?- Impact of point mutations (Table 15.1) a. A beneficial mutation may increase fitness-thatis, the ability to survive in certain environments. b. A neutral mutation has no effect on fitness.Example: silent mutations. c. A deleterious mutation lowers fitness becausemost organisms are well adapted to their currenthabitat.