Chapter 10 Structure and Function of DNA

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CHAPTER 10
THE STRUCTURE AND FUNCTION
OF DNA
CM Lamberty
General Biology
BIOLOGY AND SOCIETY
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Flu Virus
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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.
DNA STRUCTURE AND REPLICATION
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Molecular Biology
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Was known to be a chemical in cells by the end of the
nineteenth century
Has the capacity to store genetic information
Can be copied and passed from generation to generation
DNA AND RNA STRUCTURE
Nucleotides
 Polynucleotides
 Sugar-phosphate backbone
 DNA:
 4 nucleotides
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Thymine, T
 Cytosine, C
 Adenine, A
 Guanine, G
Uracil instead of thymine in RNA
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WATSON AND CRICK (AND FRANKLIN)
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Determination of DNA double helix structure
From photo of X-ray image taken by Rosalind
Franklin: diameter of helix is uniform, thickness
suggested 2 polynucleotide strands
 Combined info from x-ray photos and known data
 Backbones allowed for the swivel in the interior
 Base-pairs for support
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originally AA or CC but that would cause bulges
 Became apparent for double ring with single ring
 Hydrogen bonding to link pairs
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dasd
ROPE LADDER ANALOGY
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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.
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DNA REPLICATION
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DNA
Known to be a chemical in cells at end of 19th century
 Has capacity to store genetic information
 Can be copied and passed from generation to
generation
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DNA and RNA are nucleic acids
Consist of chemical unit called nucleotides
 Nucleotides joined by sugar-phosphate backbone
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DNA REPLICATION
When a cell
reproduces, a
complete copy of the
DNA must be passed
on
 Watson & Crick’s
model suggested DNA
replicates by a
template mechanism
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DNA REPLICATION
DNA can be damaged by UV light
 DNA polymerase:
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Are enzymes
 Make the covalent bond b/t nucleotides of a new DNA
strand
 Are invovled in repairing damaged DNA
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DNA repliation in eukaryotes:
Begins at specific sties on a double helix
 Proceeds in both directions
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FLOW OF GENETIC INFORMATION
DNA functions as the inherited directions for a
cell or organism
 How are the directions carried out?
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HOW GENOTYPE DETERMINES 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 synthesis of proteins in 2 stages:
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Transcription, the transfer of genetic information
from DNA into an RNA molecule
 Translation, the transfer of information from RNA
into a protein
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Nucleus
DNA
Cytoplasm
Figure 10.8-1
Nucleus
DNA
TRANSCRIPTION
RNA
Cytoplasm
Figure 10.8-2
Nucleus
DNA
TRANSCRIPTION
RNA
TRANSLATION
Protein
Cytoplasm
Figure 10.8-3
HOW GENOTYPE DETERMINES PHENOTYPE
The function of a gene is to dictate the production
of polypeptide
 A protein may consist of two or more different
polypeptides
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FROM NUCLEOTIDES TO AMINO ACIDS
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Genetic information in DNA is
Transcribed into RNA then
 Translated into polypeptides
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What is the language of nucleic acids?
In DNA, it is the linear sequence of nucleotide bases
 A typical gene consists of 1000s of nucleotides
 A single DNA molecule may contains 1000s of genes
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When DNA is transcribed, the result is an RNA
 RNA is then translated into sequence of amino
acids into a polypeptide
 What are the rules for translating RNA message?
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A codon is a triplet of bases which codes for one
amino acid
THE GENETIC CODE
Genetic code is the set of rules relating to
nucleotide sequence to amino acid sequence
 Is shared by all organisms
 Of the 64 triplets
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61 code for amino acids
 3 are stop codons, indicating the end of the
polypeptide
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TRANSCRIPTION:
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FROM
DNA TO RNA
Transcription
Makes RNA from a DNA template
 Uses a process that resembles DNA replication
 Substitutes uracil (U) for thymine (T)
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RNA nucleotides are linked by RNA polymerase
 Initiation
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The “start transcribing” signal is a nucleotide
sequence called a promoter
 The 1st phase of transcription is initiation in which
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RNA polymerase attaches to the promoter
 RNA synthesis begins
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TRANSCRIPTION:
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FROM
DNA TO RNA
RNA Elongation
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2nd phase
RNA grows longer
 RNA strand peels away form the DNA template
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Termination
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3rd phase
RNA polymerases reaches a sequence of DNA bases called
terminator
 Polymerases detaches from RNA
 The DNA strands rejoin
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THE PROCESSING OF EUKARYOTIC RNA
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After transcription
Eukaryotic cells process RNA
 Prokaryotic cells do not
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RNA processing include
Adding a cap and tail
 Removing introns
 Splicing exons together to form messenger RNA (mRNA)
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TRANSLATION: THE PLAYERS
Translation is the conversion from the nucleic
acid language to the protein language
 Translation requires
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mRNA
ATP
Enzymes
Ribosomes
Transfer RNA (rRNA)
TRANSFER RNA (TRNA)
Acts as a molecular interpreter
 Carries amino acids
 Matches amino acids with codons in mRNA using
anticodons
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RIBOSOMES
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Organelles that
Coordinate the functions of mRNA and tRNA
 Are made of two protein strands
 Contain ribosomal RNA (rRNA)
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A fully assembled ribosome holds tRNA and
mRNA for using in translation
TRANSLATION: THE PROCESS
Divided into three phases
 1. Initiation
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Brings together
mRNA
 The first aa, Met, with its attached tRNA
 2 subunits of ribosome
 mRNA has cap and tail to help it bind to ribosome
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Occurs in two steps
mRNA moleucle binds to a small ribosomal subunti
then an initiator tRNA binds to the start codon
 Large ribosomal subunit binds, creating a funcitonal
ribosome
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TRANSLATION: THE PROCESS
Divided into three phases
 2. Elongation
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Codon recognation:
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Occurs in 3 steps
the anticodon of an incomoing tRNA pairs w/ mRNA codon
at the A site of the ribosome
Peptide bond formation:
Polypeptide leaves the tRNA in the P site and attaches to
the aa on the tRNA in the A site
 The ribosome catalyzes the bond formation between 2 aa
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Translocation
The P site tRNA leaves the ribosome
 The tRNA carrying the polypeptide moves from the A to the
P site
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TRANSLATION: THE PROCESS
Divided into three phases
 3. Termination
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Elongation continues until
Ribosomes reaches a stop codon
 The completed polypeptide is freed
 The ribosome splits into its subunits
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MUTATIONS
A mutation is any change in the nucleotide sequence of DNA.
Mutations can change the amino acids in a protein.
Mutations can involve:
 Large regions of a chromosome
 Just a single nucleotidepair, as occurs in sickle cell anemia
 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
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Mutations may result from:
 Errors in DNA replication
 Physical or chemical agents called mutagens
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Although mutations are often harmful, they are the source of genetic
diversity, which is necessary for evolution by natural selection.
VIRUSES AND OTHER NONCELLULAR
INFECTIOUS AGENTS
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Viruses exhibit some, but not all, characteristics of
living organisms. Viruses:
Possess genetic material in the form of nucleic acids
 Are not cellular and cannot reproduce on their own.
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BACTERIOPHAGES
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Bacteriophages, or phages, are viruses that attack
bacteria.
 Phages have two reproductive cycles.
(1) In the lytic cycle:
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Many copies of the phage are made within the
bacterial cell, and then
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The bacterium lyses (breaks open)
(2) In the lysogenic cycle:
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The phage DNA inserts into the bacterial chromosome
and
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The bacterium reproduces normally, copying the
phage at each cell division
PLANT VIRUSES
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Viruses that infect plants can:
Stunt growth
 Diminish plant yields
 Spread throughout the entire plant
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Viral plant diseases:
Have no cure
 Are best prevented by producing plants that resist viral
infection
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ANIMAL VIRUSES
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Viruses that infect animals are:
Common causes of disease
 May have RNA or DNA genomes
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Some animal viruses steal a bit of host cell
membrane as a protective envelope.
The reproductive cycle of an enveloped RNA virus can be
broken into seven steps.
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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.
The behavior of HIV nucleic acid in an infected cell can
be broken into six steps.
 AIDS (acquired immune deficiency syndrome) is:
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Caused by HIV infection and
 Treated with drugs that interfere with the reproduction of
the virus
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VIROIDS AND PRIONS
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Two classes of pathogens are smaller than viruses:
Viroids are small circular RNA molecules that do not
encode proteins
 Prions are misfolded proteins that somehow convert
normal proteins to the misfolded prion version
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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
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EVOLUTION CONNECTION: EMERGING
VIRUSES
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Emerging viruses are viruses that have:
Appeared suddenly or
 Have only recently come to the attention of science
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Avian flu:
Infects birds
 Infected 18 people in 1997
 Since has spread to Europe and Africa infecting 300 people
and killing 200 of them
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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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