Chapter 10 Structure and Function of DNA
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Transcript Chapter 10 Structure and Function of DNA
CHAPTER 10
THE STRUCTURE AND FUNCTION
OF DNA
CM Lamberty
General Biology
BIOLOGY AND SOCIETY
Flu 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.
DNA STRUCTURE AND REPLICATION
Molecular Biology
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
Thymine, T
Cytosine, C
Adenine, A
Guanine, G
Uracil instead of thymine in RNA
WATSON AND CRICK (AND FRANKLIN)
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
originally AA or CC but that would cause bulges
Became apparent for double ring with single ring
Hydrogen bonding to link pairs
dasd
ROPE LADDER ANALOGY
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.
DNA REPLICATION
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
DNA and RNA are nucleic acids
Consist of chemical unit called nucleotides
Nucleotides joined by sugar-phosphate backbone
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
DNA REPLICATION
DNA can be damaged by UV light
DNA polymerase:
Are enzymes
Make the covalent bond b/t nucleotides of a new DNA
strand
Are invovled in repairing damaged DNA
DNA repliation in eukaryotes:
Begins at specific sties on a double helix
Proceeds in both directions
FLOW OF GENETIC INFORMATION
DNA functions as the inherited directions for a
cell or organism
How are the directions carried out?
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:
Transcription, the transfer of genetic information
from DNA into an RNA molecule
Translation, the transfer of information from RNA
into a protein
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
FROM NUCLEOTIDES TO AMINO ACIDS
Genetic information in DNA is
Transcribed into RNA then
Translated into polypeptides
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
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?
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
61 code for amino acids
3 are stop codons, indicating the end of the
polypeptide
TRANSCRIPTION:
FROM
DNA TO RNA
Transcription
Makes RNA from a DNA template
Uses a process that resembles DNA replication
Substitutes uracil (U) for thymine (T)
RNA nucleotides are linked by RNA polymerase
Initiation
The “start transcribing” signal is a nucleotide
sequence called a promoter
The 1st phase of transcription is initiation in which
RNA polymerase attaches to the promoter
RNA synthesis begins
TRANSCRIPTION:
FROM
DNA TO RNA
RNA Elongation
2nd phase
RNA grows longer
RNA strand peels away form the DNA template
Termination
3rd phase
RNA polymerases reaches a sequence of DNA bases called
terminator
Polymerases detaches from RNA
The DNA strands rejoin
THE PROCESSING OF EUKARYOTIC RNA
After transcription
Eukaryotic cells process RNA
Prokaryotic cells do not
RNA processing include
Adding a cap and tail
Removing introns
Splicing exons together to form messenger RNA (mRNA)
TRANSLATION: THE PLAYERS
Translation is the conversion from the nucleic
acid language to the protein language
Translation requires
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
RIBOSOMES
Organelles that
Coordinate the functions of mRNA and tRNA
Are made of two protein strands
Contain ribosomal RNA (rRNA)
A fully assembled ribosome holds tRNA and
mRNA for using in translation
TRANSLATION: THE PROCESS
Divided into three phases
1. Initiation
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
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
TRANSLATION: THE PROCESS
Divided into three phases
2. Elongation
Codon recognation:
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
Translocation
The P site tRNA leaves the ribosome
The tRNA carrying the polypeptide moves from the A to the
P site
TRANSLATION: THE PROCESS
Divided into three phases
3. Termination
Elongation continues until
Ribosomes reaches a stop codon
The completed polypeptide is freed
The ribosome splits into its subunits
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
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.
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
Are not cellular and cannot reproduce on their own.
BACTERIOPHAGES
Bacteriophages, or phages, are viruses that attack
bacteria.
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
PLANT VIRUSES
Viruses that infect plants can:
Stunt growth
Diminish plant yields
Spread throughout the entire plant
Viral plant diseases:
Have no cure
Are best prevented by producing plants that resist viral
infection
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.
The reproductive cycle of an enveloped RNA virus can be
broken into seven steps.
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:
Caused by HIV infection and
Treated with drugs that interfere with the reproduction of
the virus
VIROIDS AND PRIONS
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
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
EVOLUTION CONNECTION: EMERGING
VIRUSES
Emerging viruses are viruses that have:
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