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

2. 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

3. 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

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

5. 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

6. Watson and Crick’s discovery of the double helix
James Watson and Francis Crick determined that DNA is a 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.
A rope-ladder model of a double helix.
Twist
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

7. 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.
DNA bases pair in a complementary fashion:
Adenine (A) pairs with thymine (T) and
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

8. Three representatives of DNA
(c) Computer
model
(b) Atomic model
(a) Ribbon model
Hydrogen bond
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

11. DNA Replication
Parental (old)
DNA molecule
Daughter
(new) strand
DNA molecules
(double helices)
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.
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

12. DNA Replication 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 ensures that all the body cells in multicellular organisms carry the same genetic information.
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.

13. 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

14. Central dogma
DNA
RNA
Protein
Figure 17.UN01 In-text figure, p. 328

15. 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.
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

16. The flow of genetic information in a eukaryotic cell
TRANSLATION
Protein
RNA
TRANSCRIPTION
DNA
Cytoplasm
Nucleus
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.

17. Gene expression in Eukaryotes: Overview
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 17

18. Nuclear envelope
Overview: the roles of transcription and translation in the flow of genetic information
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
TRANSLATION
Ribosome
Polypeptide
(b) Eukaryotic cell
Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information.

19. Transcription 19

20. 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

21. 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

22. 1. Initiation of Transcription
The “start transcribing” signal is a nucleotide sequence called a promoter.
The first phase of transcription is called as initiation, in which:
RNA polymerase attaches to the promoter
RNA synthesis begins
2. RNA Elongation
During the second phase of transcription, called elongation:
The RNA grows longer
The RNA strand peels away from the DNA template
The transcription of an entire gene occurs in 3 phases.

23. During the third phase of transcription, called termination:
3. Termination of Transcription
During the third phase of transcription, called termination:
RNA polymerase reaches a sequence of DNA bases called a terminator signaling the end of the gene
Polymerase detaches from the RNA and the gene, and
The DNA strands rejoin
The transcription of an entire gene occurs in 3 phases.

24. 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

25. 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

26. 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 includes:
Adding a cap and 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)

27. 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.

28. Translation 28

29. From Nucleotides to Amino Acids: An Overview
Genetic information in DNA is:
Transcribed into RNA, then
Translated into polypeptides (proteins)
What is the language of nucleic acids?
In DNA, it is the linear sequence of nucleotide bases.
A typical gene consists of thousands of nucleotides.
A single chromosome may contain thousands of genes.
When DNA is transcribed, the result is an RNA molecule.
RNA is then translated into a sequence of amino acids in a polypeptide.
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

30. The Genetic Code The genetic code is:
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
3 are stop codons, 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………..

31. 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

32. Translation: The Players
Translation is the conversion from the nucleic acid language to the protein language.
Translation requires:
mRNA
Transfer RNA (tRNA)
Ribosomes
ATP
Enzymes
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

33. 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

34. 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

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

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

37. 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

38. 2. Elongation 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

39. 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

40. Elongation continues until:
3. Termination
Elongation continues until:
The ribosome reaches a stop codon
The completed polypeptide is freed
The ribosome splits into its subunits

41. 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

42. 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

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

45. 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
Normal hemoglobin DNA
mRNA
Normal hemoglobin
Mutant hemoglobin DNA
Sickle-cell hemoglobin

46. 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

47. Summary: Mutations
Figure 10.UN7 Summary: Mutations

48. 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

49. 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.
Adenovirus
Bacteriophages, or phages, are viruses that attack bacteria.
Phages consist of a molecule of DNA, enclosed within an elaborate structure made of proteins
Protein coat
DNA
Figure Adenovirus (part 1)

50. 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.

51. 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

52. 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.

53. 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.

54. 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

55. 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

56. 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.

57. 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

58. 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

59. 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

60. 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

61. Two classes of pathogens are smaller than viruses:
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

62. 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
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