Chapter 10 THE STRUCTURE OF THE GENETIC MATERIAL Experiments showed that DNA is the genetic material

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1 Chapter 10 Molecular Biology of the Gene THE STRUCTURE OF THE GENETIC MATERIAL oweroint Lectures for Biology: Concepts & Connections, Sixth Edition Campbell, Reece, Taylor, Simon, and Dickey Lecture by Mary C. Colavito 10.1 Experiments showed that is the genetic material 10.1 Experiments showed that is the genetic material! Frederick Griffith discovered that a transforming factor could be transferred into a bacterial cell! Alfred Hershey and Martha Chase used bacteriophages to show that is the genetic material Disease-causing bacteria were killed by heat Harmless bacteria were incubated with heat-killed bacteria Some harmless cells were converted to diseasecausing bacteria, a process called transformation The disease-causing characteristic was inherited by descendants of the transformed cells Bacteriophages are viruses that infect bacterial cells hages were labeled with radioactive sulfur to detect proteins or radioactive phosphorus to detect Bacteria were infected with either type of labeled phage to determine which substance was injected into cells and which remained outside

2 10.1 Experiments showed that is the genetic material The sulfur-labeled protein stayed with the phages outside the bacterial cell, while the phosphorus-labeled was detected inside cells Batch 1 Radioactive protein hage Bacterium Radioactive protein Empty protein shell hage Centrifuge Radioactivity in liquid Cells with phosphorus-labeled produced new bacteriophages with radioactivity in but not in protein 1 Mix radioactively labeled phages with bacteria. The phages infect the bacterial cells. 2 Agitate in a blender to separate phages outside the bacteria from the cells and their contents. 3 Centrifuge the mixture so bacteria form a pellet at the bottom of the test tube. ellet 4 Measure the radioactivity in the pellet and the liquid. Animation: Hershey-Chase Experiment Batch 2 Radioactive Radioactive Centrifuge ellet Radioactivity in pellet Animation: hage T2 Reproductive Cycle 10.2 and are polymers of nucleotides 10.2 and are polymers of nucleotides! The monomer unit of and is the nucleotide, containing Nitrogenous base 5-carbon sugar hosphate group! and are polymers called polynucleotides A sugar-phosphate backbone is formed by covalent bonding between the phosphate of one nucleotide and the sugar of the next nucleotide Nitrogenous bases extend from the sugar-phosphate backbone Animation: and Structure

3 Sugar-phosphate backbone 10.3 is a double-stranded helix hosphate group Nitrogenous base Sugar nucleotide hosphate group Nitrogenous base (A, G, C, or T)! James D. Watson and Francis Crick deduced the secondary structure of, with X-ray crystallography data from Rosalind Franklin and Maurice Wilkins Thymine (T) Sugar (deoxyribose) nucleotide polynucleotide 10.3 is a double-stranded helix! is composed of two polynucleotide chains joined together by hydrogen bonding between bases, twisted into a helical shape Base pair Hydrogen bond The sugar-phosphate backbone is on the outside The nitrogenous bases are perpendicular to the backbone in the interior Specific pairs of bases give the helix a uniform shape A pairs with T G pairs with C Ribbon model artial chemical structure Computer model Animation: Double Helix

4 RELICATION 10.4 replication depends on specific base pairing! replication follows a semiconservative model The two strands separate Each strand is used as a pattern to produce a complementary strand, using specific base pairing Each new helix has one old strand with one new strand Animation: Replication Overview 10.5 replication proceeds in two directions at many sites simultaneously! replication begins at the origins of replication unwinds at the origin to produce a bubble Replication proceeds in both directions from the origin arental molecule of Nucleotides Both parental strands serve as templates Two identical daughter molecules of Replication ends when products from the bubbles merge with each other! replication occurs in the 5 3 direction Replication is continuous on the 3 5 template Replication is discontinuous on the 5 3 template, forming short segments

5 10.5 replication proceeds in two directions at many sites simultaneously! roteins involved in replication polymerase adds nucleotides to a growing chain ligase joins small fragments into a continuous chain Origin of replication Bubble arental strand Daughter strand Animation: Origins of Replication Animation: Leading Strand Animation: Lagging Strand Animation: Replication Review Two daughter molecules 5! end 5! 4! 3! 1! 2! 3! end 2! 3! 1! 4! 5! 5! 3! arental polymerase molecule 3! 5! 3! 5! Daughter strand synthesized continuously Daughter strand synthesized in pieces 5! 3! 3! end 5! end ligase Overall direction of replication

6 THE FLOW OF GENETIC INFORMATION FROM TO TO ROTEIN 10.6 The genotype is expressed as proteins, which provide the molecular basis for phenotypic traits! A gene is a sequence of that directs the synthesis of a specific protein is transcribed into is translated into protein! The presence and action of proteins determine the phenotype of an organism 10.6 The genotype is expressed as proteins, which provide the molecular basis for phenotypic traits! Demonstrating the connections between genes and proteins The one gene one enzyme hypothesis was based on studies of inherited metabolic diseases The one gene one protein hypothesis expands the relationship to proteins other than enzymes The one gene one polypeptide hypothesis recognizes that some proteins are composed of multiple polypeptides Transcription Nucleus Cytoplasm Translation rotein

7 10.7 Genetic information written in codons is translated into amino acid sequences! The sequence of nucleotides in provides a code for constructing a protein rotein construction requires a conversion of a nucleotide sequence to an amino acid sequence Transcription rewrites the code into, using the same nucleotide language Each word is a codon, consisting of three nucleotides Translation involves switching from the nucleotide language to amino acid language Each amino acid is specified by a codon 64 codons are possible Some amino acids have more than one possible codon strand Transcription Codon Translation olypeptide Amino acid molecule Gene 1 Gene 2 Gene The genetic code is the Rosetta stone of life 10.8 The genetic code is the Rosetta stone of life! Characteristics of the genetic code Triplet: Three nucleotides specify one amino acid 61 codons correspond to amino acids AUG codes for methionine and signals the start of transcription 3 stop codons signal the end of translation Redundant: More than one codon for some amino acids Unambiguous: Any codon for one amino acid does not code for any other amino acid Does not contain spacers or punctuation: Codons are adjacent to each other with no gaps in between Nearly universal

8 Second base Strand to be transcribed Transcription First base Third base Start codon Stop codon Translation olypeptide Met Lys he 10.9 Transcription produces genetic messages in the form of! Overview of transcription The two strands separate One strand is used as a pattern to produce an chain, using specific base pairing For A in, U is placed in polymerase catalyzes the reaction 10.9 Transcription produces genetic messages in the form of! Stages of transcription Initiation: polymerase binds to a promoter, where the helix unwinds and transcription starts Elongation: nucleotides are added to the chain Termination: polymerase reaches a terminator sequence and detaches from the template Animation: Transcription

9 romoter polymerase 1 Initiation 2 Elongation of gene Terminator Area shown in Figure 10.9A Growing 3 Termination Completed polymerase Eukaryotic is processed before leaving the nucleus! Messenger () contains codons for protein sequences! Eukaryotic has interrupting sequences called introns, separating the coding regions called exons! Eukaryotic undergoes processing before leaving the nucleus Cap added to 5 end: single guanine nucleotide Tail added to 3 end: oly-a tail of adenines splicing: removal of introns and joining of exons to produce a continuous coding sequence transcript with cap and tail Cap Exon Intron Exon Intron Exon Coding sequence Transcription Addition of cap and tail Introns removed Exons spliced together Nucleus Tail Transfer molecules serve as interpreters during translation! Transfer (t) molecules match an amino acid to its corresponding codon t structure allows it to convert one language to the other An amino acid attachment site allows each t to carry a specific amino acid An anticodon allows the t to bind to a specific codon, complementary in sequence A pairs with U, G pairs with C Cytoplasm

10 Amino acid attachment site Ribosomes build polypeptides Hydrogen bond! Translation occurs on the surface of the ribosome Ribosomes have two subunits: small and large Each subunit is composed of ribosomal s and proteins Ribosomal subunits come together during translation polynucleotide chain Ribosomes have binding sites for and ts Anticodon Next amino acid to be added to polypeptide An initiation codon marks the start of an message! Initiation brings together the components needed to begin synthesis Growing polypeptide! Initiation occurs in two steps 1. binds to a small ribosomal subunit, and the first t binds to at the start codon Codons t The start codon reads AUG and codes for methionine The first t has the anticodon UAC 2. A large ribosomal subunit joins the small subunit, allowing the ribosome to function The first t occupies the site, which will hold the growing peptide chain The A site is available to receive the next t

11 10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation Initiator t 1 Met Start codon Small ribosomal subunit site 2 Met Large ribosomal subunit A site! Each cycle of elongation has three steps 1. Codon recognition: next t binds to the at the A site 2. eptide bond formation: joining of the new amino acid to the chain Amino acids on the t at the site are attached by a covalent bond to the amino acid on the t at the A site 3. Translocation: t is released from the site and the ribosome moves t from the A site into the site Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation! Elongation continues until the ribosome reaches a stop codon! Termination The completed polypeptide is released The ribosomal subunits separate is released and can be translated again movement olypeptide site Stop codon Codons A site New peptide bond Amino acid Anticodon 1 Codon recognition 2 eptide bond formation Animation: Translation 3 Translocation

12 10.16 Mutations can change the meaning of genes Mutations can change the meaning of genes! A mutation is a change in the nucleotide sequence of Base substitutions: replacement of one nucleotide with another Effect depends on whether there is an amino acid change that alters the function of the protein Deletions or insertions Alter the reading frame of the, so that nucleotides are grouped into different codons Lead to significant changes in amino acid sequence downstream of mutation Cause a nonfunctional polypeptide to be produced! Mutations can be Spontaneous: due to errors in replication or recombination Induced by mutagens High-energy radiation Chemicals Normal gene Normal hemoglobin Mutant hemoglobin rotein Met Lys he Gly Ala Base substitution Met Lys he Ser Ala Normal hemoglobin Glu Sickle-cell hemoglobin Val Base deletion Missing Met Lys Leu Ala His

13 MICROBIAL GENETICS Viral may become part of the host chromosome! Viruses have two types of reproductive cycles Lytic cycle Viral particles are produced using host cell components The host cell lyses, and viruses are released Viral may become part of the host chromosome! Viruses have two types of reproductive cycles Lysogenic cycle Viral is inserted into the host chromosome by recombination Viral is duplicated along with the host chromosome during each cell division The inserted phage is called a prophage Most prophage genes are inactive Environmental signals can cause a switch to the lytic cycle Animation: hage Lambda Lysogenic and Lytic Cycles Cell lyses, releasing phages 4 hages assemble Lytic cycle hage Attaches to cell hage New phage and proteins are synthesized 3 hage injects 2 1 hage circularizes OR Bacterial chromosome 5 7 Lysogenic cycle rophage hage inserts into the bacterial chromosome by recombination Many cell divisions Lysogenic bacterium reproduces normally, replicating the prophage at each cell division 6 Animation: hage T4 Lytic Cycle

14 10.18 CONNECTION: Many viruses cause disease in animals and plants! Both viruses and viruses cause disease in animals! Reproductive cycle of an virus Entry Glycoprotein spikes contact host cell receptors Viral envelope fuses with host plasma membrane Uncoating of viral particle to release the genome synthesis using a viral enzyme rotein synthesis synthesis of new viral genome Assembly of viral particles Viral (genome) VIRUS lasma membrane of host cell Viral (genome) Entry Uncoating Glycoprotein spike rotein coat Membranous envelope synthesis by viral enzyme 4 rotein 5 synthesis synthesis (other strand) Template New viral proteins 6 Assembly 7 Exit New viral genome CONNECTION: Many viruses cause disease in animals and plants! Most plant viruses are viruses They breach the outer protective layer of the plant and spread from cell to cell Infection can spread to other plants by animals, humans, or farming practices! Some animal viruses are viruses and reproduce in the cell nucleus EVOLUTION CONNECTION: Emerging viruses threaten human health! How do emerging viruses cause human diseases? Mutation viruses mutate rapidly Contact between species Viruses from other animals spread to humans Spread from isolated populations! Examples of emerging viruses HIV, Ebola virus, West Nile virus, SARS virus, Avian flu virus Animation: Simplified Viral Reproductive Cycle

15 10.20 The AIDS virus makes on an template! AIDS is caused by HIV, human immunodeficiency virus! HIV is a retrovirus, containing an genome Reverse transcriptase, an enzyme that produces from an template The AIDS virus makes on an template! HIV duplication Reverse transcriptase uses to produce the complementary strand Viral enters the nucleus and integrates into the chromosome, becoming a provirus rovirus is used to produce is translated to produce viral proteins Viral particles are assembled and leave the host cell Animation: HIV Reproductive Cycle Viral strand Doublestranded Viral and proteins CYTOLASM NUCLEUS Chromosomal 4 rovirus Viroids and prions are formidable pathogens in plants and animals! Some infectious agents are made only of or protein Viroids: circular molecules that infect plants Replicate within host cells without producing proteins Interfere with plant growth rions: infectious proteins that cause brain diseases in animals Misfolded forms of normal brain proteins Convert normal protein to misfolded form