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112.1 Identifying the Substance of Genes Lesson Overview12.1 Identifying the Substance of Genes
2Griffith’s Experiments Griffith isolated two different strains of the same bacterial species.Only one of the strains caused pneumonia.The discovery of the chemical nature of the gene began in 1928 with British scientist Frederick Griffith, who was trying to figure out how certain types of bacteria produce pneumonia.Griffith isolated two different strains of the same bacterial species.When Griffith injected mice with disease-causing bacteria, the mice developed pneumonia and died.When he injected mice with harmless bacteria, the mice stayed healthy.Perhaps the S-strain bacteria produced a toxin that made the mice sick? To find out, Griffith ran a series of experiments.Both strains grew very well in culture plates in Griffith’s lab, but only one of the strains caused pneumonia.The disease-causing bacteria (S strain) grew into smooth colonies on culture plates, whereas the harmless bacteria (R strain) produced colonies with rough edges.
3Griffith’s Experiments When injecting mice with disease-causing bacteria, the mice developed pneumonia and died.When injecting mice with harmless bacteria, the mice stayed healthy.Perhaps the S-strain bacteria produced a toxin that made the mice sick? To find out, Griffith ran a series of experiments.
4Griffith’s Experiments First, Griffith took the S strain, heated the cells to kill them, and then injected the heat-killed bacteria into mice.Mice survived, suggesting that the cause of pneumonia was not a toxin from disease-causing bacteria.
5Griffith’s Experiments In Griffith’s next experiment, mixed the heat-killed S-strain with live, harmless R strain and injected the mixture into mice. The injected mice developed pneumonia, and died.The lungs of these mice were filled with the disease-causing bacteria. How could that happen if the S strain cells were dead?
6TransformationProcess called transformation - one type of bacteria is changed permanently into another.Because the ability to cause disease was inherited by the transformed bacteria, Griffith concluded that the transforming factor had to be a gene.Griffith reasoned that some chemical factor that could change harmless bacteria into disease-causing bacteria was transferred from the heat-killed cells of the S strain into the live cells of the R strain.
7The Molecular Cause of Transformation Avery extracted molecules from heat-killed bacteria and destroyed proteins, lipids, carbohydrates, and RNA.Transformation still occurred.A group of scientists at the Rockefeller Institute in New York, led by the Canadian biologist Oswald Avery, wanted to determine which molecule in the heat-killed bacteria was most important for transformation.treated this mixture with enzymes
8The Molecular Cause of Transformation Then destroyed DNA and transformation did not occur.Therefore, DNA was the transforming factor.This led to the discovery that DNA stores and transmits genetic information.
9Bacteriophages Bacteriophage - virus that infects bacteria means “bacteria eater.”Several different scientists repeated Avery’s experiments. Alfred Hershey and Martha Chase performed the most important of the experiments relating to Avery’s discovery.Hershey and Chase studied viruses—nonliving particles that can infect living cells.When a bacteriophage enters a bacterium, it attaches to the surface of the bacterial cell and injects its genetic information into it.The viral genes act to produce many new bacteriophages, which gradually destroy the bacterium.When the cell splits open, hundreds of new viruses burst out.
10The Hershey-Chase Experiment Hershey and Chase studied a bacteriophage with a DNA core and a protein coat.Wanted to determine if the protein coat or the DNA core entered the bacterial cellHershey and Chase grew viruses containing radioactive isotopes of phosphorus-32 (P-32) and sulfur-35 (S-35)Their results would either support or disprove Avery’s finding that genes were made of DNA.Since proteins contain almost no phosphorus and DNA contains no sulfur, these radioactive substances could be used as markers, enabling the scientists to tell which molecules actually entered the bacteria and carried the genetic information of the virus.
11The Hershey-Chase Experiment Bacteria contained phosphorus P-32 , the marker found in DNA.Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein.Experiment confirmed Avery’s results - that DNA was the genetic material found in genes.The two scientists mixed the marked viruses with bacterial cells, waited a few minutes for the viruses to inject their genetic material, and then tested the bacteria for radioactivity.
12The Role of DNA DNA can store, copy, and transmit genetic information When a cell divides, each daughter cell must receive a complete copy of the genetic information.Careful sorting is especially important during the formation of reproductive cells in meiosis.The loss of any DNA during meiosis might mean a loss of valuable genetic information from one generation to the next.
14Nucleic Acids and Nucleotides Located in the nucleus.Made up of nucleotides, linked to form long chains.Three components: a 5-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base.NA are slightly acidic.
15Nucleic Acids and Nucleotides Nucleotides joined by covalent bondsDNA has four nitrogenous bases: adenine, guanine, cytosine, and thymine, or AGCTThe nucleotides can be joined together in any order, meaning that any sequence of bases is possible.between sugar and phosphate groups.
16Chargaff’s RulesChargaff discovered the percentages of [A] and [T] bases are almost equal in any sample of DNA. The same thing is true for the other two nucleotides, guanine [G] and cytosine [C].The observation that [A] = [T] and [G] = [C] became known as one of “Chargaff’s rules.”
17Franklin’s X-RaysRosalind Franklin used X-ray diffraction that showed:DNA has 2 strands that are twisted around each other.The nitrogen bases are near the center.In the 1950s, British scientist Rosalind Franklin used a technique called X-ray diffraction to get information about the structure of the DNA molecule.
18The Work of Watson and Crick Franklin’s X-ray pattern enabled Watson and Crick to build a model of the specific structure and properties of DNA.Built three-dimensional model of DNA in a double helixAt the same time, James Watson, an American biologist, and Francis Crick, a British physicist, were also trying to understand the structure of DNA.Early in 1953, Watson was shown a copy of Franklin’s X-ray pattern.
19Antiparallel StrandsDNA strands are “antiparallel”— they run in opposite directions.Enables the nitrogenous bases to come into contact at the center.It also allows each strand to carry nucleotides.
20Hydrogen BondingHydrogen bonds form between certain nitrogenous bases, holding the two DNA strands together.Hydrogen bonds are weak forces that allow the two strands to separate.Ability to separate is critical to DNA’s functions.Watson and Crick discovered
21Base PairingWatson and Crick realized that base pairing explained Chargaff’s rule. It gave a reason why [A] = [T] and [G] = [C].Fit between A–T and G–C nucleotides called base pairing.Watson and Crick’s model showed that hydrogen bonds could create a nearly perfect fit between nitrogenous bases along the center of the molecule.These bonds would form only between certain base pairs—adenine with thymine, and guanine with cytosine.For every adenine in a double-stranded DNA molecule, there had to be exactly one thymine. For each cytosine, there was one guanine.This nearly perfect fit between A–T and G–C nucleotides is known as base pairing, and is illustrated in the figure.
38The Role of RNAFirst step in decoding genetic instructions is to copy DNA into RNA.RNA, like DNA, is a nucleic acid that consists of a long chain of nucleotides.RNA uses the base sequence copied from DNA to produce proteins.
39Comparing RNA and DNAEach nucleotide in both DNA and RNA is made up of a 5-carbon sugar, a phosphate group, and a nitrogenous base.Three important differences between RNA and DNA:(1) Sugar in RNA is ribose(2) RNA is single-stranded.(3) RNA contains uracil (U) in place of thymine (T).
40Comparing RNA and DNAThe cell uses DNA “master plan” to prepare RNA “blueprints.”DNA stays in the cell’s nucleus, while RNA goes to the ribosomes.A master plan has all the information needed to construct a building. Builders never bring a valuable master plan to the building site, where it might be damaged or lost. Instead, they prepare inexpensive, disposable copies of the master plan called blueprints.
41Functions of RNARNA is like a disposable copy of a segment of DNA, a working copy of a single gene.RNA controls the assembly of amino acids into proteins.
42Functions of RNA Three main types of RNA: messenger RNA, ribosomal RNA, and transfer RNA.
43Messenger RNAThe RNA molecules that carry copies of instructions to other parts of the cell are known as messenger RNA (mRNA)
44Ribosomal RNARibosomal RNA (rRNA) make up ribosomes and assemble proteins.
45Transfer RNATransfer RNA (tRNA) transfers each amino acid to the ribosome as specified by the mRNA to make proteins.
46Making RNA - Transcription Transcription – DNA serves as templates to produce complementary RNA molecules.
47TranscriptionIn prokaryotes, RNA synthesis and protein synthesis take place in the cytoplasm.In eukaryotes, RNA is produced in the nucleus and moves to the cytoplasm to produce proteins.
48TranscriptionRequires RNA polymerase, which separates DNA strands, then uses one strand of DNA as a template to assemble complementary strand of RNA.
49PromotersRNA polymerase binds to promoters - regions of DNA with specific base sequences.Promoters show RNA polymerase where to begin making RNA.Similar signals cause transcription to stop when a new RNA molecule is completed.
50RNA EditingPortions of RNA are cut out and stay in the nucleus are called introns.The remaining pieces, known as exons, are spliced together to form the final mRNA, which exits the nucleus.Introns stay in the nucleus. Exons exit nucleusSome pre-mRNA molecules may be cut and spliced in different ways in different tissues, making it possible for a single gene to produce several different forms of RNA.Introns and exons may also play a role in evolution, making it possible for very small changes in DNA sequences to have dramatic effects on how genes affect cellular function.
5113.2 Ribosomes and Protein Synthesis Lesson Overview13.2 Ribosomes and Protein Synthesis
52The Genetic CodeFirst step in decoding genetic messages is to transcribe DNA to RNA.Transcribed information contains a code for making proteins.The genetic code is read three “letters” at a time, so that each “word” is three bases long and corresponds to a single amino acid.
53The Genetic CodeProteins are made by joining amino acids together into long chains, called polypeptides.There are about 20 amino acids.
54The Genetic CodeThe amino acids and their order determine the properties of proteins.Sequence of amino acids affects the shape of the protein, which determines its function.
55The Genetic Code Each three-letter “word” in mRNA is known as a codon. A codon consists of three consecutive bases that specify a single amino acid.
56Start and Stop CodonsThe methionine codon AUG serves as the “start” codon for protein synthesis.Following the start codon, mRNA is read, three bases at a time, until it reaches one of three different “stop” codons, which end translation.
57TranslationThe decoding of mRNA into amino acids and eventually a protein is known as translation.The forming of a protein requires the folding of one or more polypeptide chains.The sequence of nucleotide bases in an mRNA molecule is a set of instructions that gives the order in which amino acids should be joined to produce a polypeptide.Ribosomes use the sequence of codons in mRNA to assemble amino acids into polypeptide chains.
58Steps in TranslationmRNA is transcribed in the nucleus and then translated in the cytoplasm.
59Steps in TranslationTranslation begins when a ribosome attaches to mRNA.As the ribosome reads each codon of mRNA, it directs tRNA to bring the amino acid to the ribosome.One at a time, the ribosome then attaches each amino acid to the growing chain.
60Steps in Translation Each tRNA molecule carries one amino acid. In addition, each tRNA has three unpaired bases, called the anticodon — which is complement to one mRNA codon.The tRNA molecule for methionine has the anticodon UAC, which pairs with the methionine codon, AUG.
61Steps in TranslationThe ribosome forms a peptide bond between the amino acidsAt the same time, the bond holding tRNA to its amino acid is broken.If that next codon is UUC, a tRNA molecule with an AAG anticodon brings the amino acid phenylalanine into the ribosome.
62Steps in TranslationThe polypeptide chain grows until the ribosome reaches a “stop” codon.When it reaches a stop codon, it releases both the newly formed polypeptide and the mRNA molecule, completing translation.
63The Roles of tRNA and rRNA in Translation rRNA holds ribosomal proteins in place and locates the beginning of mRNA.They may even join amino acids together.Ribosomes are made of many proteins and four different rRNA molecules.
64The Molecular Basis of Heredity Genes contain instructions for assembling proteins.Many proteins are enzymes, which catalyze and regulate chemical reactions.Proteins are microscopic tools, each specifically designed to build or operate a component of a living cell.Molecular biology seeks to explain living organisms by studying them at the molecular level, using molecules like DNA and RNA.The central dogma of molecular biology is that information is transferred from DNA to RNA to protein.
65The Molecular Basis of Heredity Gene expression - the way DNA, RNA, and proteins put genetic information into action in living cells.DNA carries information for specifying the traits of an organism.The cell uses the sequence of bases in DNA as a template for making mRNA.The codons of mRNA specify the sequence of amino acids in a protein.Proteins, in turn, play a key role in producing an organism’s traits.
66The Molecular Basis of Heredity There is a near-universal nature in the genetic code.Although some organisms show slight variations in the amino acids assigned to particular codons, the code is always read three bases at a time and in the same direction.Despite their enormous diversity in form and function, living organisms display remarkable unity at life’s most basic level, the molecular biology of the gene.
68Types of MutationsNow and then cells make mistakes in copying their own DNA, inserting the wrong base or even skipping a base as a strand is put together.These variations are called mutationsMutations are heritable changes in genetic information.from the Latin word mutare, meaning “to change.”
69Types of Mutations All mutations fall into two basic categories: Gene mutations - produce changes in a single geneChromosomal mutations - produce changes in whole chromosomes.
70Gene MutationsPoint mutations - involve changes in one or a few nucleotides.If a gene in one cell is altered, the alteration can be passed on to every cell that develops from the original one.because they occur at a single point in the DNA sequence. They generally occur during replication.
71Gene MutationsPoint mutations include substitutions, insertions, and deletions.
72SubstitutionsIn a substitution, one base is changed to a different base.Usually affect a single amino acid, and sometimes they have no effect at all.In this example, the base cytosine is replaced by the base thymine, resulting in a change in the mRNA codon from CGU (arginine) to CAU (histidine).However, a change in the last base of the codon, from CGU to CGA for example, would still specify the amino acid arginine.
73Insertions and Deletions Insertions and deletions are point mutations in which one base is inserted or removed.Called frameshift mutations because they shift the “reading frame” of the genetic message and can change the protein so much that it won’t be functional.If a nucleotide is added or deleted, the bases are still read in groups of three, but now those groupings shift in every codon that follows the mutation.
74Chromosomal Mutations Chromosomal mutations involve changes in the number or structure of chromosomes.Can change the location of genes and the number of copies of some genes.Four types: deletion, duplication, inversion, and translocation.
75Chromosomal Mutations Deletion involves the loss of all or part of a chromosome.
76Chromosomal Mutations Duplication produces an extra copy of all or part of a chromosome.
77Chromosomal Mutations Inversion reverses the direction of parts of a chromosome.
78Chromosomal Mutations Translocation occurs when part of one chromosome breaks off and attaches to another.
79Effects of MutationsGenetic material can be altered by natural or artificial means.Resulting mutations may or may not affect an organism, most do not.Some mutations that affect individual organisms can also affect a species or even an entire ecosystem.
80Effects of MutationsMany mutations are produced by errors in genetic processes.During DNA replication, an incorrect base is inserted roughly once in every 10 million bases.Small changes in genes can accumulate over time.For example, some point mutations are caused by errors during DNA replication.Stressful environmental conditions may cause some bacteria to increase mutation rates.This can actually be helpful to the organism, since mutations may sometimes give such bacteria new traits, such as the ability to consume a new food source or to resist a poison in the environment.
81Some mutations arise from mutagens - chemical or physical agents in the environment. Chemical mutagens include certain pesticides, plant alkaloids, tobacco smoke, and environmental pollutants.Physical mutagens include forms of electromagnetic radiation, such as X-rays and UV light. Stress can also be a factor.MutagensIf these mutagens interact with DNA, they can produce mutations at high rates.Some compounds interfere with base-pairing, increasing the error rate of DNA replication.Others weaken the DNA strand, causing breaks and inversions that produce chromosomal mutations.Cells can sometimes repair the damage; but when they cannot, the DNA base sequence changes permanently.
82Harmful EffectsThe most harmful mutations dramatically change protein structure or gene activity.Example: Sickle Cell DiseaseThe effects of mutations on genes vary widely. Some have little or no effect; and some produce beneficial variations. Some negatively disrupt gene function.Whether a mutation is negative or beneficial depends on how its DNA changes relative to the organism’s situation.The defective proteins produced by these mutations can disrupt normal biological activities, and result in genetic disorders.Some cancers, for example, are the product of mutations that cause the uncontrolled growth of cells.Mutations are often thought of as negative because they disrupt the normal function of genes.However, without mutations, organisms cannot evolve, because mutations are the source of genetic variability in a species.Sickle cell disease is a disorder associated with changes in the shape of red blood cells. Normal red blood cells are round. Sickle cells appear long and pointed.Sickle cell disease is caused by a point mutation in one of the polypeptides found in hemoglobin, the blood’s principal oxygen-carrying protein.Among the symptoms of the disease are anemia, severe pain, frequent infections, and stunted growth.
83Beneficial EffectsSome mutations can be highly advantageous to an organism or species.Example: Pesticide Resistance and PolyploidyMutations often produce proteins with new or altered functions that can be useful to organisms in different or changing environments.For example, mutations have helped many insects resist chemical pesticides.Some mutations have enabled microorganisms to adapt to new chemicals in the environment.Plant and animal breeders often make use of “good” mutations.For example, when a complete set of chromosomes fails to separate during meiosis, the gametes that result may produce triploid (3N) or tetraploid (4N) organisms.The condition in which an organism has extra sets of chromosomes is called polyploidy.Polyploid plants are often larger and stronger than diploid plants.Important crop plants—including bananas and limes—have been produced this way.Polyploidy also occurs naturally in citrus plants, often through spontaneous mutations.