CHAPTER 10
Molecular Biology of the Gene
Overview:
•DNA & RNA Structure
•DNA replication
•DNA-> RNA-> Protein
•Viruses
Saboteurs Inside Our Cells
• The invasion and damage
of cells by the herpesvirus
can be compared to the
actions of a saboteur intent
on taking over a factory
– The herpesvirus hijacks the
host cell’s molecules and
organelles to produce new
copies of the virus; the
virus remains permanently
latent in the body.
THE STRUCTURE OF THE GENETIC
MATERIAL
Experiments showed that DNA is the genetic material
• The Hershey-Chase
experiment showed
that certain viruses
reprogram host cells
to produce more
viruses by injecting
their DNA
THE STRUCTURE AND REPLICATION OF DNA
• DNA
– Was known as 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: Polymers of Nucleotides
• DNA and RNA are nucleic acids
– They consist of chemical units called nucleotides
– The nucleotides are joined by a sugar-phosphate
backbone
DNA and RNA are polymers of nucleotides
• DNA is a nucleic acid, made of long chains of
nucleotides
• The four nucleotides found in DNA
– Differ in their nitrogenous bases
– Are thymine (T), cytosine (C), adenine (A), and
guanine (G)
• RNA has uracil (U) in place of thymine
• RNA is also a nucleic acid
– RNA has a slightly different sugar
– RNA has U instead of T
DNA is a double-stranded helix
• James Watson and Francis Crick worked out the
three-dimensional structure of DNA, based on
work by Rosalind Franklin
• The structure of DNA consists of two
polynucleotide strands wrapped around each
other in a double helix
1 chocolate coat,
Blind (PRA)
• The model of DNA is like a rope ladder twisted into a spiral
• Hydrogen bonds between bases hold the strands
together
– Each base pairs with a complementary partner
– A pairs with T
– G pairs with C
• Detailed representations of DNA
– Notice that the bases pair in a complementary
fashion
DNA REPLICATION
DNA replication depends on specific base pairing
• In DNA replication, the strands separate
– Enzymes use each strand as a template to assemble
the new strands
• Untwisting and replication of DNA
• DNA can be damaged by ultraviolet light
– The enzymes and proteins involved in replication
can repair the damage
• DNA replication
– Begins at
specific sites
on a double
helix
– Proceeds in
both
directions
• Each strand of the
double helix is
oriented in the
opposite direction
3
• How DNA
daughter
strands are
synthesized
• The
daughter
strands are
identical to
the parent
molecule
THE FLOW OF GENETIC INFORMATION
FROM DNA TO RNA TO PROTEIN
• DNA functions as the inherited directions for a
cell or organism
– How are these directions carried out?
How an Organism’s DNA Genotype
Produces Its Phenotype
• An organism’s genotype, its genetic makeup is
the sequence of nucleotide bases in DNA
– The phenotype is the organism’s specific traits
• DNA specifies
the synthesis of
proteins in two
stages
– Transcription
– Translation
• The one gene–one protein hypothesis states that
the function of an individual gene is to dictate
the production of a specific protein
From Nucleotide Sequence to Amino
Acid Sequence: An Overview
• The information, or “language,” in DNA is
ultimately translated into the language of
polypeptides
• What is the
language of
nucleic acids?
– In DNA, it is
the linear
sequence of
nucleotide
bases
• The “words” of the DNA “language” are triplets of bases called codons
– The codons in a gene specify the amino acid sequence of a polypeptide
• 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 correspondence between the
nucleotides of an RNA molecule and the amino
acids of a polypeptide?
• Triplets of bases
– Specify all the amino acids
– Are called codons
The Genetic Code
• The genetic code is the set of rules relating
nucleotide sequence to amino acid sequence
Transcription produces genetic messages in the
form of RNA
Transcription: From DNA to RNA
• In transcription
– Genetic information is transferred from DNA to
RNA
– An RNA molecule is transcribed from a DNA
template
Initiation of Transcription
• The “start transcribing” signal is a nucleotide
sequence called a promoter
• The first phase of transcription is initiation
– RNA polymerase attaches to the promoter
– RNA synthesis begins
RNA Elongation
• The second phase of transcription is elongation
– The RNA grows longer
Termination of Transcription
• The third phase of transcription is termination
– RNA polymerase reaches a sequence of DNA bases
called a terminator
• In transcription, the
DNA helix unzips
– RNA nucleotides line
up along one strand of
the DNA following the
base-pairing rules
– The single-stranded
messenger RNA peels
away and the DNA
strands rejoin
The Processing of Eukaryotic RNA
• The eukaryotic cell processes the RNA after
transcription
• RNA processing
includes
– Adding a cap
and tail
– Removing
introns
(Noncoding
segments)
– Splicing exons
together
Translation:
• Translation
– Is the conversion from the nucleic acid language to
the protein language
Transfer RNA molecules serve as interpreters
during translation
• In the cytoplasm, a
ribosome attaches to
the mRNA and
translates its
message into a
polypeptide
• The process is aided
by transfer RNAs
• Each tRNA molecule has a triplet anticodon on
one end and an amino acid attachment site on the
other
Transfer RNA (tRNA)
• tRNA
– Acts as a
molecular
interpreter
– Carries amino
acids
– Matches amino
acids with codons
in mRNA using
anticodons
Ribosomes
• Ribosomes
– Are organelles
that actually
make
polypeptides
– Are made up of
two protein
subunits
– Contain
ribosomal RNA
(rRNA)
• A fully
assembled
ribosome
holds tRNA
and mRNA for
use in
translation
Translation: The Process
• Translation is divided into three phases
– Initiation
– Elongation
– Termination
Initiation
• The first phase brings together
– The mRNA
– The first amino acid with its attached tRNA
– The two subunits of the ribosome
• An mRNA molecule has a cap and tail that help
it bind to the ribosome
Elongation
• Step 1, codon recognition
– The anticodon of an incoming tRNA pairs with the
mRNA codon
• Step 2, peptide bond formation
– The ribosome catalyzes bond formation between
amino acids
• Step 3, translocation
– A tRNA leaves the P site of the ribosome
– The ribosome moves down the mRNA
Termination
• Elongation continues until the ribosome reaches a
stop codon
Review: DNA RNA Protein
• The flow of genetic information in a cell
• In eukaryotic cells
– Transcription occurs in the nucleus
– Translation occurs in the cytoplasm
Mutations
• A mutation
– Is any change in the nucleotide sequence of DNA
These are caused by errors in DNA replication or by mutagens Physical or chemical agents
Types of Mutations
• Mutations within a gene
– Can be
divided into
two general
categories
– Can result
in changes
in the amino
acids in
proteins
• Insertions and deletions
– Can have
disastrous
effects
– Change the
reading
frame of the
genetic
message
VIRUSES: GENES IN PACKAGES
• Viruses sit on the fence between life and nonlife
– They exhibit
some but not all
characteristics
of living
organisms
Bacteriophages
• Bacteriophages, or phages
– Attack bacteria
Bacterial
cell
Connection: Many viruses cause disease in
animals
• Many viruses
have RNA,
rather than
DNA, as their
genetic material
– Example: flu
viruses
Plant Viruses
• Viruses that infect plants
– Can stunt growth
and diminish
plant yields
– Can spread
throughout the
entire plant
• Most plant viruses have RNA
– Example: tobacco mosaic disease
• Genetic engineering methods
– Have been used to create virus-resistant plants
Connection: Emerging viruses
threaten human health
• The deadly Ebola virus
causes hemorrhagic
fever
– Each virus is an
enveloped thread of
protein-coated RNA
• Hantavirus is another
enveloped RNA virus
The AIDS virus makes DNA on an RNA
template
• HIV is a retrovirus
• HIV is a retrovirus
– A retrovirus is an
RNA virus that
reproduces by
means of a DNA
molecule
– It copies its RNA
to DNA using
reverse
transcriptase
• Inside a cell,
HIV uses its
RNA as a
template for
making DNA to
insert into the
host
chromosome
• AIDS is
– Acquired immune
deficiency
syndrome
– The disease caused
by HIV infection
– Treated with the
drug AZT
• Virus studies help establish molecular
genetics
• Molecular genetics helps us
understand viruses
– such as HIV, seen here attacking a
white blood cell
• How do new viruses arise?
– Mutation of
existing
viruses
– Spread to
new host
species