Chapter 16 How Genes Work
Chapter 16 How Genes Work BIOL 2311
Popular in Biology 2311
Popular in Biology
verified elite notetaker
This 8 page Class Notes was uploaded by Ming-Han Lu on Sunday July 24, 2016. The Class Notes belongs to BIOL 2311 at University of Texas at Dallas taught by Dr. Mehmet Candas in Summer 2016. Since its upload, it has received 39 views. For similar materials see Biology 2311 in Biology at University of Texas at Dallas.
Reviews for Chapter 16 How Genes Work
Report this Material
What is Karma?
Karma is the currency of StudySoup.
Date Created: 07/24/16
Chapter 16 – How Genes Work MingHan Lu Gene expression: The process of converting archived information into molecules that actually do thing in the cell. 16.2 What Do Genes Do? Beadle and Tatum discovered what genes did by making them defective. The idea was to knock out a gene by damaging it and then infer what the gene does by observing the phenotypes of the mutant individual. Today, alleles that do not function at all are called knockout, null, or lossoffunction alleles. The OneGene, One Enzyme Hypothesis Beadle and Tatum exposed a large number of N. crassa cells to radiation. o Radiation can damage the doublehelical structure of DNA—often in a way that makes the affected gene nonfunctional. Results showed that mutant cells could not make specific compounds. Showed that the inability to synthesize pyridoxine was due to a defect in a single gene, and that the inability to synthesized other molecules was due to defects in other genes. o Results inspired their onegene, oneenzyme hypothesis. The mutant N. Crassa could not make pyridoxine because it lacked an enzyme required to synthesize the compound. They further proposed that the lack of the enzyme was due to a genetic defect. Based on analysis of knockout mutants, the onegene, oneenzyme hypothesis claimed that each gene contains the information needed to make an enzyme. An Experimental Test of the Hypothesis N. Crassa can also synthesize the amino acid arginine. o Organisms synthesize arginine in a series of steps called a metabolic pathway. Compounds called ornithine and citrulline are intermediate products in the metabolic pathway leading to arginine. Specific enzymes are required to synthesize ornithine, convert orthinine to citrulline, and change citrulline to arginine. N. crassa genes are responsible for producing each of the three enzymes involved. To test the idea, Srb and Horowitz used radiation o They used genetic screen (any tchnique for picking certain types of mutants out of many randomly generated mutants) to find the mutants. Biologists finally understood what most genes do: They contain the instructions for making proteins. Chapter 16 – How Genes Work MingHan Lu 16.2 The Central Dogma of Molecular Biology How does a gene specify the production of a protein? A part of the answer lays in the molecular structure of the gene. The primary components of DNA were 4 nitrogencontaining bases: the pyrimidines thymine (T) and cytosine (C), and the purines adenine (A) and guanine (G). The Genetic Code Hypothesis Crick proposed that different combinations of bases could specify the 20 amino acids, just as different combinations of dots and dashes specify the 26 letters of the alphabet. o A particular stretch of DNA, then, could contain the information needed to produce the amino acid sequence of a particular polypeptide. In code form, the tremendous quantity of information required to build and operate a cell could be stored compactly. This information could also be copied through complementary base pairing and transmitted efficiently from one generation to the next. The information encoded in the base sequence of DNA is not translated into amino acid sequence of proteins directly. Instead, the link between DNA as information repository and proteins as cellular machines is indirect. RNA as the Intermediary between Genes and Proteins RNA molecules act as a link between genes and the proteinmanufacturing centers. o Jacob and Monod predicted that shortlived molecules of RNA, which they called messenger RNA, or mRNA for short, carry information out of the nucleus from DNA to the site of protein synthesis. Messenger RNA is one of the several distinct types of RNA in cells. The protein called RNA polymerase polymerizes ribonucleotides into strands of RNA. o RNA polymerase synthesizes RNA molecules according to the information provided by the sequence of bases in a particular stretch of DNA. RNA polymerase does not require a primer to begin connecting ribonucleotides together to produce a strand of RNA. Chapter 16 – How Genes Work MingHan Lu Dissecting the Central Dogma The central dogma summarizes the flow of information in cells. It simply states that DNA codes for RNA, which codes for proteins. DNA RNA proteins The sequence of bases in DNA specifies the sequence of bases in an RNA molecule, which specifies the sequence of amino acids in a protein. In this way, genes ultimately code for proteins. Proteins are the workers of cells, functioning not only as enzymes but also as motors, structural elements, transporters, and molecular signals. The Roles of Transcription and Translation 1. DNA is transcribed to RNA by RNA polymerase. Transcription is the process of copying hereditary information in DNA to RNA. 2. Messenger RNA is translated to proteins in ribosomes. Translation is the process of using the information in nucleic acids to synthesize proteins. DNA acts as a permanent record—an information archive or blueprint. This permanent record is copied, during transcription, to produce the shortlived form called mRNA. Translation refers to converting information from one language to another (in English); in Biology, translation is the transfer of information from one type of molecule to another— from the “language” of nucleic acids to the “language” of protein. o Translation is also referred to simply as protein synthesis. Linking Genotypes and Phenotypes An organism’s genotype is determined by the sequence of bases in its DNA, while its phenotype is a product of the proteins its produces. Chapter 16 – How Genes Work MingHan Lu The proteins encoded by genes are what make the “stuff” of the cell and dictate which chemical reactions occur inside. The alleles of a gene differ in their DNA sequence. As a result, the proteins produced by different alleles of the gene may differ in their amino acid sequence. o If the primary structure of proteins varies, their functions are likely to vary as well. Exceptions to the Central Dogma 1. Many genes code for RNA molecules that do not function as mRNAs—they are not translated into proteins. 2. In some cases, information flows from RNA back to DNA. When some RNA viruses infect a cell, a specialized viral polymerase called reverse transcriptase synthesizes a DNA version of the RNA genes. 16.3 The Genetic Code Genetic code – the rules that specify the relationship between a sequence of nucleotides in DNA or RNA and the sequence of amino acids in a protein. How Long Is a Word in the Genetic Code? There are only four different bases in ribonucleotides (A, U, G, and C), so a onebase code could specify only four different amino acids. A twobase code could represent just 4x4, or 16, different amino acids A threebase code could specify 4x4x4, or 64, different amino acids. A threebase code provides more than enough words to code for all 20 amino acids. A threebase code is known as a triplet code. The group of three bases that specifies a particular amino acid is called a codon. Many of the 64 codons that are possible might specify the same amino acids. o Onebase addition or deletion in the base sequence led to a loss of function in the gene. This is because a single addition or deletion mutation throws the sequence of codons, or the reading frame, out of register. The only time functional proteins were produced was when three bases were added or removed. How Did Researchers Crack the Code? There is one start codon (AUG), which signals that protein synthesis should begin at that point on the mRNA molecule. The start codon specifies the amino acid methionine. There are three stop codons, also called termination codons (UAA, UAG, and UGA). The stop codons signal that the protein is complete, they do not code for any amino acid, and they end translation. Chapter 16 – How Genes Work MingHan Lu Analyzing the Code The code is redundant. All amino acids except methionine and tryptophan are coded by more than one codon. The code is unambiguous. A single codon never codes for more than one amino acid. The code is nonoverlapping. Once the ribosome locks onto the first codon, it then reads each separate codon one after another. The code is nearly universal. With a few minor exceptions, all codons specify the same amino acids in all organisms. The code is conservative. When several codons specify the same amino acid, the first two bases in those codons are almost always identical. If a mutation in DNA or an error in transcription or translation affects the third position in a codon, it is less likely to change the amino acid in the final protein. The genetic code does not represent a random assemblage of bases, like letters drawn from a hat. It has been honed by natural selection and is remarkably efficient. Using the Code Using the genetic code and the central dogma, biologists can: 1. Predict the codons and amino acid sequence encoded by a particular DNA sequence. 2. Determine the set of mRNA and DNA sequences that would code for a particular sequence of amino acids. 16.4 How Can Mutation Modify Genes and Chromosomes? A mutation is any permanent change in an organism’s DNA. It is a modification in a cell’s information archive—a change in its genotype. Mutations create new alleles. Point Mutation A singlebase change is called a point mutation. A change in a single base in DNA is associated with a difference in coat color in populations of oldfield mice. o Point mutations that cause these types of changes in the amino acid sequence of proteins are called missense mutations. Chapter 16 – How Genes Work MingHan Lu A point mutation that does not change the amino acid sequence of the gene product is called a silent mutation. Frameshift mutations – a single addition/deletion mutation throws the sequence of codons out of register and alters the meaning of all subsequent codons. o Another type of point mutation with a large effect is a nonsense mutation. Nonsense mutations occur when a codon that specifies an amino acid is changed by mutation to one that specifies a stop codon. This causes early termination of the polypeptide chain and often results in a nonfunctional protein. In terms of the impact on organisms, biologists divide mutations into three categories: 1. Beneficial Some mutations increase the fitness of the organism—meaning, its ability to survive and reproduce—in certain environments. The GtoA mutation is beneficial in beach habitats because it camouflages mice. 2. Neutral If a mutation has no effect on fitness, it is termed neutral. Silent mutations are usually neutral. 3. Deleterious Because organisms tend to be well adapted to their current habitat, and because mutations are random changes in the genotype, many mutations are random changes in the genotype, many mutations lower fitness. These mutations are termed harmful or deleterious. The GtoA mutation would be deleterious in the forest habitat. Chapter 16 – How Genes Work MingHan Lu If point mutations alter DNA sequences that are important for gene expression, they can have important effects on phenotype even though they do not change the amino acid sequence of a protein. Chromosome Mutations Largescale mutations that change chromosomes. Polyploidy is an increase in the number of each type of chromosome, while aneuploidy is the addition or deletion of individual chromosomes. Polyploidy and aneuploidy are forms of mutations that don’t change DNA sequences, but alter the number of chromosome copies. In addition to changes in overall chromosome number, the composition of individual chromosomes can change in important ways. o Chromosome segments can become detached when accidental breaks in chromosomes occur. o The segments may be flipped and rejoined—a phenomenon known as a chromosome inversion, or become attached to a different chromosome, an event called chromosome translocation. When a segment of chromosome is lost, this is a deletion, and when additional copies of a segment are present, this is a duplication. Could be beneficial, neutral, or deleterious. o However…. Chromosomes of cancer cells exhibit deleterious chromosome mutations that include aneuploidy, inversions, translocations, deletions, and duplications! Karyotype—the complete set of chromosomes in a cell. Chapter 16 – How Genes Work MingHan Lu SUMMARY – point mutations and chromosome mutations are random changes in DNA that can produce new genes, new alleles, and new traits. At the level of individuals, mutations can cause disease or death or lead to increases in fitness.