Chapter 10 Molecular Biology of the Gene Power
Chapter 10 Molecular Biology of the Gene Power. Point Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko
Introduction § Viruses infect organisms by – binding to receptors on a host’s target cell, – injecting viral genetic material into the cell, and – hijacking the cell’s own molecules and organelles to produce new copies of the virus. § The host cell is destroyed, and newly replicated viruses are released to continue the infection. © 2012 Pearson Education, Inc.
Introduction § Viruses are not generally considered alive because they – are not cellular and – cannot reproduce on their own. § Because viruses have much less complex structures than cells, they are relatively easy to study at the molecular level. § For this reason, viruses are used to study the functions of DNA. © 2012 Pearson Education, Inc.
Figure 10. 0_1 Chapter 10: Big Ideas The Structure of the Genetic Material DNA Replication The Flow of Genetic Information from DNA to RNA to Protein The Genetics of Viruses and Bacteria
THE STRUCTURE OF THE GENETIC MATERIAL © 2012 Pearson Education, Inc.
10. 1 SCIENTIFIC DISCOVERY: Experiments showed that DNA is the genetic material § Until the 1940 s, the case for proteins serving as the genetic material was stronger than the case for DNA. – Proteins are made from 20 different amino acids. – DNA was known to be made from just four kinds of nucleotides. § Studies of bacteria and viruses – ushered in the field of molecular biology, the study of heredity at the molecular level, and – revealed the role of DNA in heredity. © 2012 Pearson Education, Inc.
10. 1 SCIENTIFIC DISCOVERY: Experiments showed that DNA is the genetic material § In 1928, Frederick Griffith discovered that a “transforming factor” could be transferred into a bacterial cell. He found that – when he exposed heat-killed pathogenic bacteria to harmless bacteria, some harmless bacteria were converted to disease-causing bacteria and – the disease-causing characteristic was inherited by descendants of the transformed cells. © 2012 Pearson Education, Inc.
10. 1 SCIENTIFIC DISCOVERY: Experiments showed that DNA is the genetic material § In 1952, Alfred Hershey and Martha Chase used bacteriophages to show that DNA is the genetic material of T 2, a virus that infects the bacterium Escherichia coli (E. coli). – Bacteriophages (or phages for short) are viruses that infect bacterial cells. – Phages were labeled with radioactive sulfur to detect proteins or radioactive phosphorus to detect DNA. – Bacteria were infected with either type of labeled phage to determine which substance was injected into cells and which remained outside the infected cell. © 2012 Pearson Education, Inc.
10. 1 SCIENTIFIC DISCOVERY: Experiments showed that DNA is the genetic material – The sulfur-labeled protein stayed with the phages outside the bacterial cell, while the phosphorus-labeled DNA was detected inside cells. – Cells with phosphorus-labeled DNA produced new bacteriophages with radioactivity in DNA but not in protein. © 2012 Pearson Education, Inc.
Figure 10. 1 A Head Tail fiber DNA
Figure 10. 1 B Phage Empty protein shell Radioactive protein Bacterium Centrifuge Pellet 1 Batch 2: Radioactive DNA labeled in green Phage DNA Batch 1: Radioactive protein labeled in yellow The radioactivity is in the liquid. 2 3 4 Radioactive DNA Centrifuge Pellet The radioactivity is in the pellet.
Figure 10. 1 C 1 A phage attaches itself to a bacterial cell. 2 The phage injects 3 The phage DNA directs its DNA into the bacterium. the host cell to make more phage DNA and proteins; new phages assemble. 4 The cell lyses and releases the new phages.
10. 2 DNA and RNA are polymers of nucleotides § DNA and RNA are nucleic acids. § One of the two strands of DNA is a DNA polynucleotide, a nucleotide polymer (chain). § A nucleotide is composed of a – nitrogenous base, – five-carbon sugar, and – phosphate group. § The nucleotides are joined to one another by a sugar-phosphate backbone. © 2012 Pearson Education, Inc.
10. 2 DNA and RNA are polymers of nucleotides § Each type of DNA nucleotide has a different nitrogen-containing base: – adenine (A), – cytosine (C), – thymine (T), and – guanine (G). © 2012 Pearson Education, Inc.
Figure 10. 2 A T A C T G Sugar-phosphate backbone A C G T A C G A G T T Covalent bond joining nucleotides T C A C A A G Phosphate group Nitrogenous base Sugar Nitrogenous base (can be A, G, C, or T) C G T A A DNA double helix DNA nucleotide T Thymine (T) T Phosphate group G G Two representations of a DNA polynucleotide Sugar (deoxyribose) DNA nucleotide
Figure 10. 2 B Thymine (T) Cytosine (C) Pyrimidines Guanine (G) Adenine (A) Purines
10. 2 DNA and RNA are Polymers of Nucleotides § RNA (ribonucleic acid) is unlike DNA in that it – uses the sugar ribose (instead of deoxyribose in DNA) and – RNA has the nitrogenous base uracil (U) instead of thymine. © 2012 Pearson Education, Inc.
Figure 10. 2 C Nitrogenous base (can be A, G, C, or U) Phosphate group Uracil (U) Sugar (ribose)
Figure 10. 2 D Cytosine Uracil Adenine Guanine Ribose Phosphate
10. 3 SCIENTIFIC DISCOVERY: DNA is a double-stranded helix § In 1952, after the Hershey-Chase experiment demonstrated that the genetic material was most likely DNA, a race was on to – describe the structure of DNA and – explain how the structure and properties of DNA can account for its role in heredity. © 2012 Pearson Education, Inc.
Figure 10. 3 A
10. 3 SCIENTIFIC DISCOVERY: DNA is a double-stranded helix § In 1953, James D. Watson and Francis Crick deduced the secondary structure of DNA, using – X-ray crystallography data of DNA from the work of Rosalind Franklin and Maurice Wilkins and – Chargaff’s observation that in DNA, – the amount of adenine was equal to the amount of thymine and – the amount of guanine was equal to that of cytosine. © 2012 Pearson Education, Inc.
10. 3 SCIENTIFIC DISCOVERY: DNA is a double-stranded helix § Watson and Crick reported that DNA consisted of two polynucleotide strands wrapped into a double helix. – 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, forming two hydrogen bonds, and – G pairs with C, forming three hydrogen bonds. © 2012 Pearson Education, Inc.
Figure 10. 3 C Twist
Figure 10. 3 D Hydrogen bond Base pair Ribbon model Partial chemical structure Computer model
10. 3 SCIENTIFIC DISCOVERY: DNA is a double-stranded helix § In 1962, the Nobel Prize was awarded to – James D. Watson, Francis Crick, and Maurice Wilkins. – Rosalind Franklin probably would have received the prize as well but for her death from cancer in 1958. Nobel Prizes are never awarded posthumously. § The Watson-Crick model gave new meaning to the words genes and chromosomes. The genetic information in a chromosome is encoded in the nucleotide sequence of DNA. © 2012 Pearson Education, Inc.
DNA REPLICATION © 2012 Pearson Education, Inc.
10. 4 DNA replication depends on specific base pairing § In their description of the structure of DNA, Watson and Crick noted that the structure of DNA suggests a possible copying mechanism. § DNA replication follows a semiconservative model. – The two DNA strands separate. – Each strand is used as a pattern to produce a complementary strand, using specific base pairing. – Each new DNA helix has one old strand with one new strand. © 2012 Pearson Education, Inc.
Figure 10. 4 A_s 3 A T A C G C G A T A T A parental molecule of DNA T A G C C A Free nucleotides The parental strands separate and serve as templates T A T G C G C G C T A T A T A Two identical daughter molecules of DNA are formed
Figure 10. 4 B A T G A A T Parental DNA molecule T A G C Daughter strand T C G T C A G C C Parental strand G C G G T C C T A G T A C C T A T G A T A A C A T G T Daughter DNA molecules
10. 5 DNA replication proceeds in two directions at many sites simultaneously § DNA replication begins at the origins of replication where – DNA unwinds at the origin to produce a “bubble, ” – replication proceeds in both directions from the origin, and – replication ends when products from the bubbles merge with each other. © 2012 Pearson Education, Inc.
10. 5 DNA replication proceeds in two directions at many sites simultaneously § DNA replication occurs in the 5 to 3 direction. – Replication is continuous on the 3 to 5 template. – Replication is discontinuous on the 5 to 3 template, forming short segments. © 2012 Pearson Education, Inc.
10. 5 DNA replication proceeds in two directions at many sites simultaneously § Two key proteins are involved in DNA replication. 1. DNA ligase joins small fragments into a continuous chain. 2. DNA polymerase – adds nucleotides to a growing chain and – proofreads and corrects improper base pairings. © 2012 Pearson Education, Inc.
10. 5 DNA replication proceeds in two directions at many sites simultaneously § DNA polymerases and DNA ligase also repair DNA damaged by harmful radiation and toxic chemicals. § DNA replication ensures that all the somatic cells in a multicellular organism carry the same genetic information. © 2012 Pearson Education, Inc.
Figure 10. 5 A Parental DNA molecule Origin of replication “Bubble” Two daughter DNA molecules Parental strand Daughter strand
Figure 10. 5 B 3 end 5 end P 4 3 P 5 2 1 2 A T 5 C P P G C P P T 3 end 3 4 G P OH 1 HO A P 5 end
Figure 10. 5 C DNA polymerase molecule 5 3 Parental DNA Replication fork 5 3 DNA ligase Overall direction of replication 3 5 This daughter strand is synthesized continuously This daughter strand is 3 synthesized 5 in pieces
THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN © 2012 Pearson Education, Inc.
10. 6 The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits § DNA specifies traits by dictating protein synthesis. § The molecular chain of command is from – DNA in the nucleus to RNA and – RNA in the cytoplasm to protein. § Transcription is the synthesis of RNA under the direction of DNA. § Translation is the synthesis of proteins under the direction of RNA. © 2012 Pearson Education, Inc.
Figure 10. 6 A_s 3 DNA Transcription RNA NUCLEUS Translation Protein CYTOPLASM
10. 6 The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits § The connections between genes and proteins – The initial one gene–one enzyme hypothesis was based on studies of inherited metabolic diseases. – The one gene–one enzyme hypothesis was expanded to include all proteins. – Most recently, the one gene–one polypeptide hypothesis recognizes that some proteins are composed of multiple polypeptides. © 2012 Pearson Education, Inc.
10. 7 Genetic information written in codons is translated into amino acid sequences § The sequence of nucleotides in DNA provides a code for constructing a protein. – Protein construction requires a conversion of a nucleotide sequence to an amino acid sequence. – Transcription rewrites the DNA code into RNA, using the same nucleotide “language. ” © 2012 Pearson Education, Inc.
10. 7 Genetic information written in codons is translated into amino acid sequences – The flow of information from gene to protein is based on a triplet code: the genetic instructions for the amino acid sequence of a polypeptide chain are written in DNA and RNA as a series of nonoverlapping threebase “words” called codons. – Translation involves switching from the nucleotide “language” to the amino acid “language. ” – Each amino acid is specified by a codon. – 64 codons are possible. – Some amino acids have more than one possible codon. © 2012 Pearson Education, Inc.
Figure 10. 7 DNA molecule Gene 1 Gene 2 Gene 3 DNA A C C G G C A A Transcription RNA Translation U U U G G C Codon Polypeptide Amino acid C G U U
10. 8 The genetic code dictates how codons are translated into amino acids § Characteristics of the genetic code – 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. © 2012 Pearson Education, Inc.
10. 8 The genetic code dictates how codons are translated into amino acids § The genetic code is – redundant, with more than one codon for some amino acids, – unambiguous in that any codon for one amino acid does not code for any other amino acid, – nearly universal—the genetic code is shared by organisms from the simplest bacteria to the most complex plants and animals, and – without punctuation in that codons are adjacent to each other with no gaps in between. © 2012 Pearson Education, Inc.
Figure 10. 8 A Third base First base Second base
Figure 10. 8 B_s 3 Strand to be transcribed DNA T A C T T C A A T A T G A A G T T T C T A G Transcription RNA A U G A A G U U A G Translation Start codon Polypeptide Met Stop codon Lys Phe
10. 9 Transcription produces genetic messages in the form of RNA § Overview of transcription – An RNA molecule is transcribed from a DNA template by a process that resembles the synthesis of a DNA strand during DNA replication. – RNA nucleotides are linked by the transcription enzyme RNA polymerase. – Specific sequences of nucleotides along the DNA mark where transcription begins and ends. – The “start transcribing” signal is a nucleotide sequence called a promoter. © 2012 Pearson Education, Inc.
10. 9 Transcription produces genetic messages in the form of RNA – Transcription begins with initiation, as the RNA polymerase attaches to the promoter. – During the second phase, elongation, the RNA grows longer. – As the RNA peels away, the DNA strands rejoin. – Finally, in the third phase, termination, the RNA polymerase reaches a sequence of bases in the DNA template called a terminator, which signals the end of the gene. – The polymerase molecule now detaches from the RNA molecule and the gene. © 2012 Pearson Education, Inc.
Figure 10. 9 A Free RNA nucleotides RNA polymerase A T C C A A T Direction of transcription Newly made RNA A T A G U G T C C A U C C A G T A G G T U T A C C Template strand of DNA
Figure 10. 9 B RNA polymerase DNA of gene Terminator DNA Promoter DNA 1 Initiation 2 Elongation Area shown in Figure 10. 9 A 3 Termination Growing RNA Completed RNA polymerase
10. 10 Eukaryotic RNA is processed before leaving the nucleus as m. RNA § Messenger RNA (m. RNA) – encodes amino acid sequences and – conveys genetic messages from DNA to the translation machinery of the cell, which in – prokaryotes, occurs in the same place that m. RNA is made, but in – eukaryotes, m. RNA must exit the nucleus via nuclear pores to enter the cytoplasm. – Eukaryotic m. RNA has – introns, interrupting sequences that separate – exons, the coding regions. © 2012 Pearson Education, Inc.
10. 10 Eukaryotic RNA is processed before leaving the nucleus as m. RNA § Eukaryotic m. RNA undergoes processing before leaving the nucleus. – RNA splicing removes introns and joins exons to produce a continuous coding sequence. – A cap and tail of extra nucleotides are added to the ends of the m. RNA to – facilitate the export of the m. RNA from the nucleus, – protect the m. RNA from attack by cellular enzymes, and – help ribosomes bind to the m. RNA. © 2012 Pearson Education, Inc.
Figure 10. 10 Exon Intron Exon DNA Cap RNA transcript with cap and tail Transcription Addition of cap and tail Introns removed Tail Exons spliced together m. RNA Coding sequence NUCLEUS CYTOPLASM
10. 11 Transfer RNA molecules serve as interpreters during translation § Transfer RNA (t. RNA) molecules function as a language interpreter, – converting the genetic message of m. RNA – into the language of proteins. § Transfer RNA molecules perform this interpreter task by – picking up the appropriate amino acid and – using a special triplet of bases, called an anticodon, to recognize the appropriate codons in the m. RNA. © 2012 Pearson Education, Inc.
Figure 10. 11 A Amino acid attachment site Hydrogen bond RNA polynucleotide chain Anticodon A t. RNA molecule, showing its polynucleotide strand hydrogen bonding A simplified schematic of a t. RNA
Figure 10. 11 B Enzyme t. RNA ATP
10. 12 Ribosomes build polypeptides § Translation occurs on the surface of the ribosome. – Ribosomes coordinate the functioning of m. RNA and t. RNA and, ultimately, the synthesis of polypeptides. – Ribosomes have two subunits: small and large. – Each subunit is composed of ribosomal RNAs and proteins. – Ribosomal subunits come together during translation. – Ribosomes have binding sites for m. RNA and t. RNAs. © 2012 Pearson Education, Inc.
Figure 10. 12 A Growing polypeptide t. RNA molecules Large subunit Small subunit m. RNA
Figure 10. 12 B t. RNA binding sites Large subunit P A site Small subunit m. RNA binding site
Figure 10. 12 C The next amino acid to be added to the polypeptide Growing polypeptide m. RNA t. RNA Codons
10. 13 An initiation codon marks the start of an m. RNA message § Translation can be divided into the same three phases as transcription: 1. initiation, 2. elongation, and 3. termination. § Initiation brings together – m. RNA, – a t. RNA bearing the first amino acid, and – the two subunits of a ribosome. © 2012 Pearson Education, Inc.
10. 13 An initiation codon marks the start of an m. RNA message § Initiation establishes where translation will begin. § Initiation occurs in two steps. 1. An m. RNA molecule binds to a small ribosomal subunit and the first t. RNA binds to m. RNA at the start codon. – The start codon reads AUG and codes for methionine. – The first t. RNA has the anticodon UAC. 2. A large ribosomal subunit joins the small subunit, allowing the ribosome to function. – The first t. RNA occupies the P site, which will hold the growing peptide chain. – The A site is available to receive the next t. RNA. © 2012 Pearson Education, Inc.
Figure 10. 13 A Start of genetic message Cap End Tail
Figure 10. 13 B Met Large ribosomal subunit Initiator t. RNA P site m. RNA U A C A U G Start codon 1 Small ribosomal subunit 2 U A C A U G A site
10. 14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation § Once initiation is complete, amino acids are added one by one to the first amino acid. § Elongation is the addition of amino acids to the polypeptide chain. © 2012 Pearson Education, Inc.
10. 14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation § Each cycle of elongation has three steps. 1. Codon recognition: The anticodon of an incoming t. RNA molecule, carrying its amino acid, pairs with the m. RNA codon in the A site of the ribosome. 2. Peptide bond formation: The new amino acid is joined to the chain. 3. Translocation: t. RNA is released from the P site and the ribosome moves t. RNA from the A site into the P site. © 2012 Pearson Education, Inc.
10. 14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation § Elongation continues until the termination stage of translation, when – the ribosome reaches a stop codon, – the completed polypeptide is freed from the last t. RNA, and – the ribosome splits back into its separate subunits. © 2012 Pearson Education, Inc.
Figure 10. 14_s 4 Polypeptide P site m. RNA Amino acid A site Anticodon Codons 1 Codon recognition m. RNA movement Stop codon 2 New peptide bond 3 Translocation Peptide bond formation
10. 15 Review: The flow of genetic information in the cell is DNA RNA protein § Transcription is the synthesis of RNA from a DNA template. In eukaryotic cells, – transcription occurs in the nucleus and – the m. RNA must travel from the nucleus to the cytoplasm. © 2012 Pearson Education, Inc.
10. 15 Review: The flow of genetic information in the cell is DNA RNA protein § Translation can be divided into four steps, all of which occur in the cytoplasm: 1. amino acid attachment, 2. initiation of polypeptide synthesis, 3. elongation, and 4. termination. © 2012 Pearson Education, Inc.
Figure 10. 15 Transcription DNA 1 m. RNA Transcription RNA polymerase CYTOPLASM Translation Amino acid attachment 2 Enzyme t. RNA ATP Anticodon Initiator t. RNA Large ribosomal subunit Start Codon m. RNA Initiation of polypeptide synthesis 3 Small ribosomal subunit New peptide bond forming Growing polypeptide 4 Elongation Codons m. RNA Polypeptide 5 Stop codon Termination
10. 16 Mutations can change the meaning of genes § A mutation is any change in the nucleotide sequence of DNA. § Mutations can involve – large chromosomal regions or – just a single nucleotide pair. © 2012 Pearson Education, Inc.
10. 16 Mutations can change the meaning of genes § Mutations within a gene can be divided into two general categories. 1. Base substitutions involve the replacement of one nucleotide with another. Base substitutions may – have no effect at all, producing a silent mutation, – change the amino acid coding, producing a missense mutation, which produces a different amino acid, – lead to a base substitution that produces an improved protein that enhances the success of the mutant organism and its descendant, or – change an amino acid into a stop codon, producing a nonsense mutation. © 2012 Pearson Education, Inc.
10. 16 Mutations can change the meaning of genes 2. Mutations can result in deletions or insertions that may – alter the reading frame (triplet grouping) of the m. RNA, so that nucleotides are grouped into different codons, – lead to significant changes in amino acid sequence downstream of the mutation, and – produce a nonfunctional polypeptide. © 2012 Pearson Education, Inc.
10. 16 Mutations can change the meaning of genes § Mutagenesis is the production of mutations. § Mutations can be caused by – spontaneous errors that occur during DNA replication or recombination or – mutagens, which include – high-energy radiation such as X-rays and ultraviolet light and – chemicals. © 2012 Pearson Education, Inc.
Figure 10. 16 A Normal hemoglobin DNA C T Mutant hemoglobin DNA C A T T m. RNA G A A G U A Normal hemoglobin Sickle-cell hemoglobin Val Glu
Figure 10. 16 B Normal gene m. RNA Protein Nucleotide substitution A U G A A G U Met A U G A Met Lys U U G G C Phe Gly Ala U A G C A G U U Lys Phe Ser G C A A Ala U Deleted Nucleotide deletion A U G A A G Met U U G G C G Ala Leu Lys C A U His Inserted Nucleotide insertion A U G A A G Met Lys U U G Leu U G G C Ala His
THE GENETICS OF VIRUSES AND BACTERIA © 2012 Pearson Education, Inc.
10. 17 Viral DNA may become part of the host chromosome § A virus is essentially “genes in a box, ” an infectious particle consisting of – a bit of nucleic acid, – wrapped in a protein coat called a capsid, and – in some cases, a membrane envelope. § Viruses have two types of reproductive cycles. 1. In the lytic cycle, – viral particles are produced using host cell components, – the host cell lyses, and – viruses are released. © 2012 Pearson Education, Inc.
10. 17 Viral DNA may become part of the host chromosome 2. In the Lysogenic cycle – Viral DNA is inserted into the host chromosome by recombination. – Viral DNA is duplicated along with the host chromosome during each cell division. – The inserted phage DNA is called a prophage. – Most prophage genes are inactive. – Environmental signals can cause a switch to the lytic cycle, causing the viral DNA to be excised from the bacterial chromosome and leading to the death of the host cell. © 2012 Pearson Education, Inc.
Figure 10. 17_s 2 Phage Attaches to cell Phage DNA 4 The cell lyses, releasing phages 1 Bacterial chromosome The phage injects its DNA 7 Lytic cycle Phages assemble Environmental stress Lysogenic cycle 2 The phage DNA circularizes Prophage 6 The lysogenic bacterium replicates normally OR 3 Many cell divisions New phage DNA and proteins are synthesized 5 Phage DNA inserts into the bacterial chromosome by recombination
10. 18 CONNECTION: Many viruses cause disease in animals and plants § Viruses can cause disease in animals and plants. § DNA viruses and RNA viruses cause disease in animals. § A typical animal virus has a membranous outer envelope and projecting spikes of glycoprotein. § The envelope helps the virus enter and leave the host cell. § Many animal viruses have RNA rather than DNA as their genetic material. These include viruses that cause the common cold, measles, mumps, polio, and AIDS. © 2012 Pearson Education, Inc.
10. 18 CONNECTION: Many viruses cause disease in animals and plants § The reproductive cycle of the mumps virus, a typical enveloped RNA virus, has seven major steps: 1. 2. 3. 4. 5. entry of the protein-coated RNA into the cell, uncoating—the removal of the protein coat, RNA synthesis—m. RNA synthesis using a viral enzyme, protein synthesis—m. RNA is used to make viral proteins, new viral genome production—m. RNA is used as a template to synthesize new viral genomes, 6. assembly—the new coat proteins assemble around the new viral RNA, and 7. exit—the viruses leave the cell by cloaking themselves in the host cell’s plasma membrane. © 2012 Pearson Education, Inc.
10. 18 CONNECTION: Many viruses cause disease in animals and plants § Some animal viruses, such as herpesviruses, reproduce in the cell nucleus. § Most plant viruses are RNA viruses. – To infect a plant, they must get past the outer protective layer of the plant. – Viruses spread from cell to cell through plasmodesmata. – Infection can spread to other plants by insects, herbivores, humans, or farming tools. § There are no cures for most viral diseases of plants or animals. © 2012 Pearson Education, Inc.
Figure 10. 18 Glycoprotein spike Protein coat Membranous envelope Viral RNA (genome) Plasma membrane of host cell 1 Entry 2 Uncoating 3 RNA synthesis by viral enzyme CYTOPLASM Viral RNA (genome) 4 Protein synthesis 5 m. RNA New viral proteins 6 RNA synthesis (other strand) Template Assembly Exit 7 New viral genome
10. 19 EVOLUTION CONNECTION: Emerging viruses threaten human health § Viruses that appear suddenly or are new to medical scientists are called emerging viruses. These include the – AIDS virus, – Ebola virus, – West Nile virus, and – SARS virus. © 2012 Pearson Education, Inc.
10. 19 EVOLUTION CONNECTION: Emerging viruses threaten human health § Three processes contribute to the emergence of viral diseases: 1. mutation—RNA viruses mutate rapidly. 2. contact between species—viruses from other animals spread to humans. 3. spread from isolated human populations to larger human populations, often over great distances. © 2012 Pearson Education, Inc.
10. 20 The AIDS virus makes DNA on an RNA template § AIDS (acquired immunodeficiency syndrome) is caused by HIV (human immunodeficiency virus). § HIV – is an RNA virus, – has two copies of its RNA genome, – carries molecules of reverse transcriptase, which causes reverse transcription, producing DNA from an RNA template. © 2012 Pearson Education, Inc.
Figure 10. 20 A Envelope Glycoprotein Protein coat RNA (two identical strands) Reverse transcriptase (two copies)
10. 20 The AIDS virus makes DNA on an RNA template § After HIV RNA is uncoated in the cytoplasm of the host cell, 1. reverse transcriptase makes one DNA strand from RNA, 2. reverse transcriptase adds a complementary DNA strand, 3. double-stranded viral DNA enters the nucleus and integrates into the chromosome, becoming a provirus, 4. the provirus DNA is used to produce m. RNA, 5. the viral m. RNA is translated to produce viral proteins, and 6. new viral particles are assembled, leave the host cell, and can then infect other cells. © 2012 Pearson Education, Inc.
Figure 10. 20 B Reverse transcriptase Viral RNA 1 DNA strand CYTOPLASM NUCLEUS Chromosomal DNA 2 Doublestranded DNA 3 Provirus DNA 4 5 Viral RNA and proteins RNA 6
10. 21 Viroids and prions are formidable pathogens in plants and animals § Some infectious agents are made only of RNA or protein. – Viroids are small, circular RNA molecules that infect plants. Viroids – replicate within host cells without producing proteins and – interfere with plant growth. – Prions are infectious proteins that cause degenerative brain diseases in animals. Prions – appear to be misfolded forms of normal brain proteins, – which convert normal protein to misfolded form. © 2012 Pearson Education, Inc.
10. 22 Bacteria can transfer DNA in three ways § Viral reproduction allows researchers to learn more about the mechanisms that regulate DNA replication and gene expression in living cells. § Bacteria are also valuable but for different reasons. – Bacterial DNA is found in a single, closed loop, chromosome. – Bacterial cells divide by replication of the bacterial chromosome and then by binary fission. – Because binary fission is an asexual process, bacteria in a colony are genetically identical to the parent cell. © 2012 Pearson Education, Inc.
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