The Structure and Function of DNA

The Structure and Function of DNA
Chapter 10 The Structure and Function of DNA

Overview DNA Structure and Replication
Flow of Genetic Information from DNA to RNA to Protein Viruses and other Noncellular Infectious Agents When a cell reproduces, the complete set of genetic instructions must pass from one generation to the next. Each DNA strand serves as a mold or template to guide reproduction of the other strand. The nucleotides are lined up one at a time along the template strand in accordance with the base pairing rules Enzymes link the nucleotides to form the new DNA strand

Biology and Society: Mix-and-Match Viruses
In 2009, a cluster of unusual flu cases broke out around Mexico City. In June 2009, the World Health Organization (WHO) declared H1N1 a pandemic (global epidemic) and unveiled a massive effort to contain it. Scientists determined that H1N1 was a hybrid flu strain, made when a known flu virus mixed with an Asian swine flu virus. H1N1 had a combination of genes that infected young, healthy people instead of the elderly or people already sick. Many countries produced a coordinated response, and the WHO declared the pandemic over in August 2010. 3

Figure 10.0 Figure 10.0 These women are taking precautions against the 2009 H1N1 flu virus in Mexico City.

DNA and RNA Structure DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are Nucleic acids and consist of chemical units called nucleotides. A nucleotide polymer is a polynucleotide The nucleotides are joined by covalent bounds between the sugar of one nucleotide and the phosphate of the next sugar forming a sugar-phosphate backbone. Polynucleotide Phosphate group Nucleotide Sugar DNA double helix Nitrogenous base Nitrogenous base Number of strands Sugar DNA RNA Ribose Deoxy- ribose C G A T U 1 2 DNA: Was known to be a chemical in cells by the end of the nineteenth century; Has the capacity to store genetic information and can be copied and passed from generation to generation

Chargaff 1947 A = 30.9% T = 29.4% G = 19.9% C = 19.8%
DNA composition: “Chargaff’s rules” varies from species to species all 4 bases not in equal quantity bases present in equal amounts In humans: A = 30.9% T = 29.4% G = 19.9% C = 19.8%

Structure of DNA 1953 | 1962 James Watson & Francis Crick developed double helix model. other scientists working on question: Rosalind Franklin ( ) Maurice Wilkins Linus Pauling Franklin Wilkins Pauling Watson & Crick’s model was inspired by 3 recent discoveries: Chargaff’s rules Pauling’s alpha helical structure of a protein X-ray crystallography data from Franklin & Wilkins

Watson and Crick’s discovery of the double helix
Watson and Crick used X-ray crystallography data () to reveal the basic shape of DNA. Rosalind Franklin collected the X-ray crystallography data. The model of DNA is like a rope ladder twisted into a spiral. The ropes at the sides represent the sugar-phosphate backbones. Each wooden rung represents a pair of bases connected by hydrogen bonds. Twist The X-ray image of the DNA revealed that A rope-ladder model of a double helix. The X-ray image of the DNA revealed that the basic shape of DNA to be a helix and the diameter of the helix is uniform The thickness of the helix suggested that it was made of two polynucleotide – double helix

Practice Evidence for the spiral nature of DNA came from ______. A) X-ray crystallography studies B) studies of pathogenic bacteria C) base rule studies D) transforming studies E) bacteriophage studies

Watson and Crick’s discovery of the double helix
The four nucleotides found in DNA differ in their nitrogenous bases. These bases are: Thymine (T) , Cytosine (C) , Adenine (A) , Guanine (G) RNA has uracil (U) in place of thymine. Base pairing in DNA DNA bases pair in a complementary fashion Adenine (A) pairs with thymine (T) Cytosine (C) pairs with guanine (G) It is apparent that a double-ringed base on one strand must always be paired with a single-ringed base on the opposite strand Individual structures of the bases dictated/ordered the pairings more specifically: each base has chemical side groups that can best form hydrogen bonds with one appropriate partner- A with T and G with C. Though the base pairing rules dictates the complementary bases, there is no restrictions on the sequences of bases along the length of DNA strand

Practice Which one of the following is a list of the four bases contained in DNA? ( DNA: Structure and Replication) A. cytosine, guanine, thymine, uracil B. adenine, guanine, uracil, thymine C. guanine, pyroline, thymine, uracil D. adenine, guanine, purine, thymine E. adenine, guanine, cytosine, thymine

Practice In analyzing the number of different bases in a DNA sample, which result would be consistent with the base-pairing rules? A = G A + G = C + T A + T = G + T A = C G = T

Practice If one strand of a DNA double helix has the sequence GTCCAT, what is the sequence of the other strand? If adenine makes up 20% of the bases in a DNA double helix, what percent of the bases are guanine?

Practice Fourteen percent of the DNA from a certain organism contains T. What percent of the other bases would you expect to be present in this particular DNA? Which one of the following lists the chemical components of a nucleotide? A. a sugar and a phosphate group only B. a nitrogenous base and a sugar only C. a nitrogenous base, a sugar, and a phosphate D. group a sugar and a pyrimidine E. a sugar and a purine 14%A 36%G 36%C

Three representatives of DNA
(c) Computer model (b) Atomic model (a) Ribbon model Hydrogen bond the structure of DNA suggested a mechanism for how DNA is copied by the cell Figure 10.5 Three representatives of DNA. Ribbon model: the sugar phosphates are backbones, blue ribbons enclosing the bases Atomic model: this is more chemically detailed structure with individual hydrogen bonds linking the complementary bases in the 2 strands Computer model: each atom is represented by spheres

Directionality of DNA & The DNA backbone
You need to number the carbons! Putting the DNA backbone together refer to the 3 and 5 ends of the DNA 5 PO4 base CH2 O nucleotide PO4 C O –O P O N base O base 5 CH2 CH2 O O ribose 4 1 OH 3 2 3 OH

Practice The backbone of DNA consists of ______. A) nitrogenous bases B) a repeating sugar-nucleotide-sugar-nucleotide pattern C) a repeating sugar-phosphate-sugar-phosphate pattern D) a repeating ACTGACTG pattern E) paired nucleotides DNA and RNA are polymers composed of ______ monomers. A) nucleotide B) carbohydrate C) fatty acid D) nucleic acid E) amino acid

Practice What type of chemical bond joins the bases of complementary DNA strands? A) ionic B) covalent C) hydrophobic D) hydrophilic E) hydrogen The DNA double helix is ______. A) made up of two polynucleotide strands B) composed of adenine and thymine C) a puzzle to geneticists D) composed of adenine and guanine E) composed of two chromosomes

DNA Replication When a cell reproduces, a complete copy of the DNA must pass from one generation to the next. Watson and Crick’s model for DNA suggested that DNA replicates by a template mechanism. base pairing allows each strand to serve as a pattern for a new strand When a cell reproduces, the complete set of genetic instructions must pass from one generation to the next. Each DNA strand serves as a mold or template to guide reproduction of the other strand. The nucleotides are lined up one at a time along the template strand in accordance with the base pairing rules Enzymes link the nucleotides to form the new DNA strand

DNA Replication DNA can be damaged by X-rays and ultraviolet light.
DNA replication ensures that all the body cells in multicellular organisms carry the same genetic information DNA can be damaged by X-rays and ultraviolet light. DNA polymerases are enzymes make the covalent bonds between the nucleotides of a new DNA strand and are involved in repairing damaged DNA DNA replication in eukaryotes: begins at specific sites on a double helix (Origins of replication) proceeds in both directions (bidirectional) Figure 10.7 Multiple "bubbles" in replicating DNA.

DNA Replication Large team of enzymes coordinates DNA replication
more than a dozen enzymes & other proteins participate in DNA replication

Multiple “bubbles” in replicating DNA
Origin of replication Origin of replication Parental strands Origin of replication Parental strand Daughter strand Bubble Two daughter DNA molecules Figure 10.7 Figure 10.7 Multiple “bubbles” in replicating DNA

Practice DNA replication occurs Replication forks tend to
A. prior to prophase B. when the chromosomes align at the cell's equator C. after telophase D. in both meiosis I and meiosis II. Replication forks tend to A. speed up replication. B. slow down replication. C. be more depleted in prokaryotic DNA. D. increase errors during replication. E. be more plentiful in prokaryotic DNA

How an Organism’s Genotype Determines Its Phenotype
An organism’s genotype is its genetic makeup, the sequence of nucleotide bases in DNA. The phenotype is the organism’s physical traits, which arise from the actions of a wide variety of proteins. DNA specifies the synthesis of proteins in two stages: Transcription, the transfer of genetic information from DNA into an RNA molecule Translation, the transfer of information from RNA into a protein The function of a gene is to dictate the production of a polypeptide. A protein may consist of two or more different polypeptides (chapter 3). Keeping in mind the structure of DNA, we can define genotype and phenotype more accurately. The function of a gene is to dictate the production of a polypeptide

DNA RNA protein The “Central dogma”
flow of genetic information within a cell DNA RNA Protein transcription translation DNA RNA protein replication Figure 17.UN01 In-text figure, p. 328

The flow of genetic information in a eukaryotic cell
TRANSLATION Protein RNA TRANSCRIPTION DNA Cytoplasm Nucleus What is the connection between genotype and protein that more directly determine the phenotype Figure 10.8 The flow of genetic information in a eukaryotic cell. (Step 3) A sequence of nucleotides in the DNA is transcribed into a molecules of RNA in the cell’s nucleus.

Gene expression in Eukaryotes:
Overview the roles of transcription and translation in the flow of genetic information Nuclear envelope Transcription is the synthesis of complementary strand of RNA using DNA as a template RNA processing is the modification of the primary RNA transcript (addition of 5’ cap, addition of 3’ poly A tail and splicing) to make mature mRNA Translation is the synthesis of a polypeptide, using mRNA as a template DNA TRANSCRIPTION RNA PROCESSING Pre-mRNA mRNA Ribosome TRANSLATION Polypeptide (b) Eukaryotic cell 29

Practice Which of these is a difference between a DNA and an RNA molecule? (Activity: DNA and RNA Structure) DNA contains uracil, whereas RNA contains thymine. DNA is a polymer composed of nucleotides, whereas RNA is a polymer composed of nucleic acids. DNA is double-stranded, whereas RNA is single-stranded. DNA contains five-carbon sugars, whereas RNA contains six-carbon sugars. DNA contains nitrogenous bases, whereas RNA contains phosphate groups.

Practice What is this monomer? How do you know? What are the parts of this monomer? In a nucleotide, the nitrogenous base is attached to the sugar's _____ carbon and the phosphate group is attached to the sugar's _____ carbon.

Practice Which of these correctly illustrates the pairing of DNA and RNA nucleotides? a. GTTACG CAATCG b. GTTACG CAAUGC c. GTTACG GTTACG d. GTTACG ACCGTA e. GTTACG UAACAU

Transcription 33

Transcription: From DNA to RNA
From nucleus to cytoplasm Where are the genes? genes are on chromosomes in nucleus Where are proteins synthesized? proteins made by ribosomes in cytoplasm How does the information get from nucleus to cytoplasm? Messenger RNA

Transcription: From DNA to RNA
Makes RNA from a DNA template Uses a process that resembles DNA replication Substitutes uracil (U) for thymine (T) RNA nucleotides are linked by the enzyme RNA polymerase. This enzyme find the starting and stopping points of the gene It attaches RNA nucleotides in the correct order to make the RNA molecule transcription DNA RNA

Transcription of DNA and translation of codons
From nucleotide to Amino acids The “language” of nucleic acids is translated into the language of protein The language of DNA and RNA is the sequence of nucleotide bases. The language of proteins is the sequence of amino acids Each triplet of nucleotide bases in the RNA is called a codon Polypeptide TRANSLATION TRANSCRIPTION mRNA DNA Gene TRANSLATION Amino acid RNA TRANSCRIPTION DNA strand Polypeptide Codon The DNA of a gene is transcribed into RNA using the usual base-pairing rules except that an A in DNA pairs with U in RNA. In the translation of a genetic message, each triplet of nucleotide bases in the RNA, called a codon, specifies one aa in the polypeptide

The transcription of an entire gene occurs in 3 phases.
From DNA to RNA 1. The first phase is called Initiation RNA synthesis begins when RNA polymerase attaches to the promoter = the “start transcribing” signal which is a nucleotide sequence Role of promoter Where to start reading = initiation site 2. Which strand to read = template strand 3. Direction on DNA = reads DNA 3’ 5’ 2. During the 2nd phase RNA elongation: The RNA grows longer as RNA polymerase decodes DNA ~20 base pairs at a time reads DNA 3’ to 5’ builds RNA 5’3 The RNA strand peels away from the DNA template The 2 strands of DNA come back together The transcription of an entire gene occurs in 3 phases.

The transcription of an entire gene occurs in 3 phases.
3. During the 3rd phase, called Termination: RNA polymerase reaches a sequence of DNA bases called a terminator ( termination sequence) signaling the end of the gene RNA Polymerase detaches from the RNA and the gene, and The DNA strands rejoins mRNA leaves nucleus through pores The transcription of an entire gene occurs in 3 phases.

Transcription of a gene
RNA polymerase Completed RNA Growing RNA Termination Elongation Initiation Terminator DNA Area shown in next slide RNA Promoter DNA DNA of gene The transcription of an entire gene occurs in 3 phases. The section of DNA where the RNA polymerase starts is called the promoter, the place where it stops is called the terminator 1. Initiation of Transcription The “start transcribing” signal is a nucleotide sequence called a promoter. RNA polymerase attaches to the promoter RNA synthesis begins 2. RNA Elongation The RNA grows longer The RNA strand peels away from the DNA template 3. Termination of Transcription RNA polymerase reaches a sequence of DNA bases called a terminator Polymerase detaches from the RNA The DNA strands rejoin

A close-up view of transcription
RNA nucleotides RNA polymerase Newly made RNA Direction of transcription Template strand of DNA As RNA nucleotides base pair one by one with DNA bases on one DNA strand (called the template strand), the enzyme RNA polymerase links the RNA nucleotides into an RNA chain

Practice If a strand of DNA has the sequence AAGCTC, transcription will result in a(n) ______. A) single RNA strand with the sequence TTCGAG B) DNA double helix with the sequence AAGCTC for one strand and TTCGAG for the complimentary strand C) single DNA strand with the sequence TTCGAG D) single RNA strand with the sequence UUCGAG E) RNA double helix with the sequence UUCGAG for one strand and AAGCUC for the complimentary strand Transcription begins when RNA polymerase binds to the A) terminator on DNA B) initiator on DNA C) promoter on DNA D) promoter on RNA TRUE or FALSE : RNA polymerase copies the template strand of DNA making single-stranded mRNA complementary to the DNA template.

The Processing of Eukaryotic RNA
After transcription Eukaryotic cells process RNA whereas Prokaryotic cells do not The eukaryotic cell localizes transcription in the nucleus and modifies, or processes, the RNA transcripts in the nucleus before they move to the cytoplasm for translation by ribosomes. RNA processing refers to protecting RNA from RNase (Ribonuclease or hydrolytic enzymes) in the cytoplasm and it includes: Adding a 5’ cap and 3’ poly-A tail consisting of extra nucleotides at the ends of the RNA transcript Removing introns Splicing exons (the parts of the gene that are expressed) together to form messenger RNA (mRNA)

The Processing of Eukaryotic RNA

The production of messenger RNA (mRNA) in a eukaryotic cell
Transcription Addition of cap and tail Coding sequence mRNA DNA Cytoplasm Nucleus Exons spliced together Introns removed Tail Cap RNA transcript with cap and tail Exon Intron Figure The production of messenger RNA (mRNA) in a eukaryotic cell.

ALTERNATIVE SPLICING Alternative mRNAs produced from same gene
when is an intron not an intron… different segments treated as exons 45

Practice What protects mRNA from attack by cytoplasmic enzymes?
A) the excision of introns B) RNA splicing C) the excision of exons D) the lack of RNA-digesting enzymes in the cytoplasm E) Cap and tail The expressed (coding) regions of eukaryotic genes are called ______. A) caps B) promoters C) exons D) tails E) introns

Practice Which of the following statements about RNA splicing is FALSE? it removes the introns it is performed by the spliceosome it shortens the RNA molecule it always occurs in the nucleus all of the above statements are true Splicing joins together two introns. two exons. an intron and an exon. any two RNA molecules. a DNA and an RNA molecule.

Translation 48

How does mRNA code for proteins?
What was the coding puzzle? How can you code for 20 amino acids with only 4 nucleotide bases (A,U,G,C)? How can an alphabet of 4 letters translate into an alphabet of 20 letters? Think about building sentences?

From Nucleotides to Amino Acids: An Overview
Genetic information in DNA is: Transcribed into RNA molecule, then RNA is Translated into polypeptides (proteins) What is the language of nucleic acids? DNA is the linear sequence of nucleotide bases A typical gene consists of thousands of nucleotides A single chromosome may contain thousands of genes What is the language of nucleic acids? Both DNA and RNA are polymers made of nucleotide monomers tied together in specific sequence that convey the conformation much as specific sequences of letters

The Genetic Code What are the rules for translating the RNA message into a polypeptide? A codon is a triplet of bases, which codes for one amino acid The genetic code is: the set of rules relating nucleotide sequence to amino acid sequence shared by all organisms Of the 64 triplets: 61 code for amino acids One Start codon (AUG, methionine) 3 are stop codons (UAA, UAG, UGA) indicating the end of a polypeptide Translation is the conversion of NA language to the polypeptide language. In translation, the sequence of the nucleotides of RNA dictates the sequence of AA of the polypeptide. Talk about genetic code………..

Second base of RNA codon
Phenylalanine (Phe) Tyrosine (Tyr) Cysteine (Cys) Serine (Ser) Stop Stop Leucine (Leu) Stop Tryptophan (Trp) Histidine (His) Leucine (Leu) Proline (Pro) Arginine (Arg) Glutamine (Gln) First base of RNA codon Third base of RNA codon Asparagine (Asn) Serine (Ser) Isoleucine (Ile) Threonine (Thr) Lysine (Lys) Arginine (Arg) Met or start Aspartic acid (Asp) Valine (Val) Alanine (Ala) Glycine (Gly) Glutamic acid (Glu) Figure The dictionary of the genetic code, listed by RNA codons. Almost all of the genetic code is shared by all organism, from the simplest bacteria to the most complex plants and animals

Practice Original DNA Sequence: T A C A C C T T G G C G A C G A C T What would be the mRNA sequence: ??? What would be the amino acid sequence: ????

Translation: The Players
Translation is the conversion from the nucleic acid language to the protein language. Translation requires: Messenger RNA (mRNA), Transfer RNA (tRNA), Ribosomes, Energy (ATP), Enzymes Ribosome reads mRNA; start codon is always = AUG tRNA (interpreter) brings in correct amino acid and matches codon of mRNA = anticodon Amino acids assembled into polypeptide chain Start of genetic message Tail End Cap Once mRNA is present, the machinery used to translate mRNA requires 2 major components, the ribosomes and tRNA in addition to enzyme and energy

Transfer RNA (tRNA) Transfer RNA (tRNA):
Acts as a molecular interpreter Carries amino acids and Matches amino acids with codons in mRNA using anticodons a special triplet of bases that is complementary to a codon triplet on mRNA tRNA polynucleotide (ribbon model) RNA polynucleotide chain Anticodon Hydrogen bond Amino acid attachment site tRNA (simplified representation) Translation of genetic message carried in mRNA into the aa language of proteins requires an interpreter. To convert the 3 letter words (codon) of nucleic acids to the aa words of protein, cell uses a molecular protein, a type of RNA called tRNA. tRNA has 2 distinct function Pick up the appropriate aminoacids Recognize the appropriate codons in the mRNA

tRNA structure “clover leaf” structure
anticodon located on “clover leaf” end amino acid on 3’ end

Practice The DNA codon AGT codes for an amino acid carried by a tRNA with the anticodon ______. A) UCA B) TCU C) AGU D) TCA E) AGT

Ribosomes Ribosomes are organelles that:
coordinate the functions of mRNA and tRNA are made of two protein subunits contain ribosomal RNA (rRNA) A fully assembled ribosome holds tRNA and mRNA for use in translation. Next amino acid to be added to polypeptide Growing polypeptide tRNA mRNA tRNA binding sites Codons Ribosome (b) The “players” of translation (a) A simplified diagram of a ribosome Large subunit Small P site binding site A site A fully assembled ribosome has a binding site for mRNA on its small subunit and a binding site for tRNA on its large subunit

Translation: The Process
Translation is divided into three phases: Initiation Elongation and Termination

1. Initiation Initiation brings together mRNA, the first amino acid, methionine, with its attached tRNA, two ribosome subunits The mRNA molecule has a cap and tail that help it bind to the ribosome

1. Initiation (cont) Initiation occurs in two steps:
First, an mRNA molecule binds to a small ribosomal subunit, then an initiator tRNA binds to the start codon. Second, a large ribosomal subunit binds, creating a functional ribosome. The initiation of translation Initiator tRNA mRNA Start codon Met P site Small ribosomal subunit A site Large ribosomal Figure The initiation of translation

2. Elongation: growing a polypeptide
Elongation occurs in three steps. Step 1, codon recognition: the anticodon of an incoming tRNA pairs with the mRNA codon at the A site of the ribosome. Step 2, peptide bond formation: The polypeptide leaves the tRNA in the P site and attaches to the amino acid on the tRNA in the A site The ribosome catalyzes the bond formation between the two amino acids Step 3, translocation: The P site tRNA leaves the ribosome The tRNA carrying the polypeptide moves from the A to the P site

Peptide bond formation
Amino acid Polypeptide P site mRNA Anticodon A site Codons Codon recognition ELONGATION Stop codon Peptide bond formation New peptide bond mRNA movement Translocation Step 1, codon recognition: the anticodon of an incoming tRNA pairs with the mRNA codon at the A site of the ribosome. Step 2, peptide bond formation: The polypeptide leaves the tRNA in the P site and attaches to the amino acid on the tRNA in the A site The ribosome catalyzes the bond formation between the two amino acids Step 3, translocation: The P site tRNA leaves the ribosome The tRNA carrying the polypeptide moves from the A to the P site

3. Termination: release of polypeptide
Elongation continues until the ribosome reaches a stop codon / release factor (UAA, UAG, UGA) “release protein” bonds to A site the completed polypeptide is freed the ribosome splits into its subunits Now what happens to the protein?

Review: DNA RNA Protein
In a cell, genetic information flows from DNA to RNA in the nucleus and RNA to protein in the cytoplasm. Transcription RNA polymerase mRNA DNA Intron Nucleus Tail Cap RNA processing tRNA Amino acid attachment Enzyme ATP Initiation of translation Ribosomal subunits Elongation Anticodon Codon Termination Polypeptide Stop codon Figure A summary of transcription and translation (step 6) Here the gene (DNA) dictates the transcription of a complementary sequence of nucleotides in mRNA. In turn, mRNA specifies the linear sequence of aa in a polypeptide Finally, the proteins that form from the polypeptides determine the appearance and capabilities of the cell and organism

Review: DNA RNA Protein
As it is made, a polypeptide: coils and folds assumes a three-dimensional shape, its tertiary structure Several polypeptides may come together, forming a protein with quaternary structure. Transcription and translation are how genes control: the structures the activities of cells

tRNA, anticodon, small subunit of ribosome, large subunit of ribosome, mRNA, aminoacid

Practice What is the correct order of the stages of translation? A) initiation, codon recognition, peptide bond formation, translocation, termination B) initiation, translocation, codon recognition, peptide bond formation, termination C) initiation, codon recognition, translocation, peptide bond formation, termination D) initiation, peptide bond formation, codon recognition, translocation, termination E) initiation, peptide bond formation, translocation, codon recognition, termination

Practice Translation converts the information stored in ______ to ______. A) DNA RNA B) RNA a polypeptide C) protein DNA D) DNA a polypeptide E) RNA DNA Peptide bonds form between ______. A) amino acids B) an mRNA codon and a tRNA anticodon C) the small ribosomal subunit and the large ribosomal subunit D) a tRNA and the amino acid it is carrying E) an mRNA transcript and the small ribosomal subunit

Mutations A mutation is any change in the nucleotide sequence of DNA.
Mutations especially in the 1st or 2nd base can change the amino acids in a protein. Mutations can involve: Large regions of a chromosome Just a single nucleotide pair, as occurs in sickle cell anemia Sickle cell anemia Normal hemoglobin DNA mRNA Normal hemoglobin Mutant hemoglobin DNA Sickle-cell hemoglobin

Types of Mutations Mutations within a gene can occur as a result of:
Base substitution, the replacement of one base by another Nucleotide deletion, the loss of a nucleotide Nucleotide insertion, the addition of a nucleotide Insertions and deletions can: Change the reading frame of the genetic message Lead to disastrous effects Deleted Inserted (b) Nucleotide deletion (c) Nucleotide insertion mRNA and protein from a normal gene (a) Base substitution

Mutations Point mutations 1 base pair change base-pair substitution
silent mutation no amino acid change redundancy in code missense change amino acid nonsense change to stop codon How can mutations affect the next generation?

Mutations Insertions adding base(s) Deletions losing base(s)
both cause frameshift How can mutations affect the next generation?

Practice A frameshift mutation replaces one amino acid with another.
removes part of the protein. introduces a section of amino acids not normally found. joins two different proteins. The codon for leucine is CUC. How many different amino acids could possibly result from single-base substitutions? 1 3 5 7

Practice If a frameshift mutation causes a stop codon to be inserted into the DNA sequence A) the resulting protein will not be affected B) the phenotype will change but not the genotype C) the resulting protein will be too short and non-functional D) the resulting protein will be too long and non-functional

Challenge Mutated DNA Sequence #2
T A C A T C T T G G C G A C G A C T Mutated DNA Sequence #2 T A C G A C C T T G G C G A C G A C T Mutated DNA Sequence #3 T A C A C C T T A G C G A C G A C T Questions What’s the mRNA sequence? (Circle the change on your paper) What will be the amino acid sequence? What will be the effect on the protein? What type of mutation is it?

Challenge Which mutation would you want from the last slide. WHY?

Summary: Mutations Figure 10.UN7 Summary: Mutations

Mutagens Mutations may result from: errors in dna replication
physical or chemical agents called mutagens Although mutations are often harmful, they are the source of genetic diversity, which is necessary for evolution by natural selection. Mutations and diversity What causes mutagens? Can occur in number of ways 1. Mutations resulting from errors during DNA replication or recombination are known as spontaneous mutation

Viruses And Other Noncellular Infectious Agents
Viruses exhibit some, but not all, characteristics of living organisms. Viruses: possess genetic material in the form of nucleic acids wrapped in a protein coat are not cellular and cannot reproduce on their own. Protein coat DNA Bacteriophages, or phages, are viruses that attack bacteria. Phages consist of a molecule of DNA, enclosed within an elaborate structure made of proteins Adenovirus Figure Adenovirus (part 1)

Bacteriophages (viruses) infecting a bacterial cell
Head Tail Tail fiber DNA of virus Bacteriophage (200 nm tall) Colorized TEM Figure Bacteriophages (viruses) infecting a bacterial cell.

Phages have two reproductive cycles. (1) In the lytic cycle:
Bacteriophages Phages have two reproductive cycles. (1) In the lytic cycle: Many copies of the phage are made within the bacterial cell, and then The bacterium lyses (breaks open) (2) In the lysogenic cycle: The phage DNA inserts into the bacterial chromosome and The bacterium reproduces normally, copying the phage at each cell division

a) Alternative phage reproductive cycles
Phage lambda E. coli Phage Phage DNA Phage injects DNA Phage attaches to cell. Bacterial chromosome (DNA) Phage DNA circularizes. Phages assemble New phage DNA and proteins are synthesized. LYTIC CYCLE Cell lyses, releasing phages. Figure 10.26a Alternative phage reproductive cycles.

b) Alternative phage reproductive cycles
Phage DNA Phage injects DNA Phage attaches to cell. Bacterial chromosome (DNA) circularizes. LYSOGENIC CYCLE Many cell divisions Prophage Occasionally a prophage may leave the bacterial chromosome. Lysogenic bacterium reproduces normally, replicating the Prophage at each cell division. Phage DNA is inserted into the bacterial chromosome. Phage Figure 10.26b Alternative phage reproductive cycles.

Plant Viruses Viruses that infect plants can: Viral plant diseases:
Stunt growth Diminish plant yields Spread throughout the entire plant Viral plant diseases: Have no cure and are best prevented by producing plants that resist viral infection Tobacco mosaic virus RNA Protein Most plant viruses have RNA rather than DNA as their genetic material

Animal Viruses Viruses that infect animals are:
Common causes of disease May have RNA or DNA genomes Some animal viruses steal a bit of host cell membrane as a protective envelope. Protein coat RNA Protein spike Membranous envelope Virus that have RNA as their genetic material are flu virus, common cold, measles, mumps, AIDS and polio Virus that have DNA as their genetic material are hepatitis, chicken pox and herpes infection

The influenza virus Figure 10.0a The influenza virus.
The influenza virus is one of the deadliest pathogens in the world. Each year in the United States, over 20,000 people die from influenza infection. In the flu of 1918–1919, about 40 million people died worldwide. Vaccines against the flu are the best way to protect public health. Because flu viruses mutate quickly, new vaccines must be created every year.

Animation: Simplified Viral Reproductive Cycle
The reproductive cycle of an enveloped virus- Mumps Protein coat mRNA Protein spike Plasma membrane of host cell Envelope Template New viral genome New viral proteins Exit Virus Viral RNA (genome) Viral RNA (genome) Entry Uncoating Assembly RNA synthesis by viral enzyme RNA synthesis (other strand) Protein synthesis Mumps virus Animation: Simplified Viral Reproductive Cycle The reproductive cycle of an enveloped RNA virus can be broken into seven steps. The viral envelope fuses with the cell’s membrane, allowing the protein-coated RNA to enter the cytoplasm Enzymes then remove the protein coat An enzyme that entered the cell as part of the virus uses the virus’s RNA as template for making complementary strands of RNA They serve 2 functions – serve as mRNA for the synthesis of viral protein Serve as template for synthesizing new viral genome RNA The new coat proteins assemble around the new viral DNA Finally, the virus leave the cells by covering themselves in PM

HIV, the AIDS Virus HIV is a retrovirus, an RNA virus that reproduces by means of a DNA molecule. Retroviruses use the enzyme reverse transcriptase to synthesize DNA on an RNA template. HIV steals a bit of host cell membrane as a protective envelope. Envelope Reverse transcriptase Surface protein Protein coat RNA (two identical strands) HIV, the AIDS virus

HIV (red dots) infecting
The behavior of HIV nucleic acid in an infected cell The behavior of HIV nucleic acid in an infected cell can be broken into six steps. Reverse transcriptase HIV (red dots) infecting a white blood cell DNA strand Chromosomal Cytoplasm Nucleus Provirus RNA Viral RNA Double stranded Viral and proteins SEM The behavior of HIV nucleic acid in an infected cell can be broken into six steps. Uses the RNA as template to make a DNA strand Adds a second complementary strand The resulting ds-viral DNA enters the host nucleus and inserts itself into the chromosomal DNA, becoming provirus Transcribed into RNA Translated into viral proteins

HIV, the AIDS Virus AIDS (acquired immune deficiency syndrome) is:
Caused by HIV infection and Treated with drugs that interfere with the reproduction of the virus Part of a T nucleotide Thymine (T) AZT While there is no cure for AIDS, its progression can by slowed down by two types of drugs The first type inhibits the action of enzyme called proteases which help produce the final versions of HIV protein The second type inhibits the action of the HIV enzyme, reverse transcriptase: The anti-HIV drug AZT has a chemical shape very similar to part of the T nucleotide of DNA but they can not be incorporated into a growing polypeptide chain

Viroids and Prions Two classes of pathogens are smaller than viruses:
Viroids are small circular RNA molecules that do not encode proteins-infect plants Prions are misfolded proteins that somehow convert normal proteins to the misfolded prion version Prions are responsible for neurodegenerative diseases including: Mad cow disease Scrapie in sheep and goats Chronic wasting disease in deer and elk Creutzfeldt-Jakob disease in humans

Emerging Viruses Emerging viruses are viruses that have: Avian flu:
Appeared suddenly or Have only recently come to the attention of science Avian flu: Infects birds Infected 18 people in 1997 Since has spread to Europe and Africa infecting 300 people and killing 200 of them If avian flu mutates to a form that can easily spread between people, the potential for a major human outbreak is significant New viruses can arise by: Mutation of existing viruses Spread to new host species © 2010 Pearson Education, Inc.

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