Molecular Biology
What Is DNA and How Does It
Work?
DNA Structure Must Be Compatible
with Its Four Roles
• DNA makes copies of itself.
– Occurs during S phase of the cell cycle before
mitosis or meiosis.
• DNA encloses information.
– Information that gives rise to discernible traits
in organisms.
DNA Structure Must Be Compatible
with Its Four Roles
• DNA controls cells and tells them what to
do.
– Determines function of the cell.
• DNA changes by mutation.
– Structure must be able to change.
Building Blocks of DNA
• Nucleotides
– Three components:
• Five-carbon sugar
• Phosphate group
• Nitrogen-containing
base
Building Blocks of DNA
• Four nitrogenous bases in DNA
–
–
–
–
Adenine
Thymine
Guanine
Cytosine
Structure of DNA
• Maurice Wilkins and
Rosalind Franklin
– Attempted to
determine structure of
DNA.
– Discovered DNA was
a helix.
Chargaff’s Ratios
• 1950
– Erwin Chargaff
• Observed that the four nitrogenous bases
conformed to a rule:
– Amount of Adenine = Amount of Thymine
– Amount of Cytosine = Amount of Guanine
• Served as a clue to help Watson and Crick
determine DNA structure!
Watson and Crick
• Early 1950s
– They were young scientists
at Cavendish Laboratory in
Cambridge, England.
• Using Chargaff’s ratios
and Franklin’s data,
Watson and Crick
determine DNA structure
is a double helix
DNA Double Helix
• Consists of two strands of
nucleotides.
• Nucleotides bonded
together with covalent
bonds.
– Adenine hydrogen bonds
with Thymine.
– Cytosine hydrogen bonds
with Guanine.
• Structure was compatible
with four roles of DNA
How Does DNA Copy Itself?
• DNA replication
– Precedes cell division.
– Process:
• DNA strands separate
• New complementary
base pairs are added
forming a new strand
– Result: two double
helices.
• Each containing one old
strand of DNA and one
new strand of DNA
Meselson and Stahl
• Proved the mechanism of DNA replication.
– Called semiconservative mechanism.
• Grew bacteria in medium containing
various radioactive nitrogen isotopes.
– Separated DNA by density using a dense,
viscous sugar solution.
How is the information in DNA
expressed?
• Genome
– Information to make proteins stored in all of
the DNA of a single set of chromosomes.
• Gene: blueprint for the synthesis of a protein.
• Proteins
– Polymers made of amino acids connected
end-to-end
• Similar to beads on a string.
How is the information in DNA
expressed?
• Chromosomes containing DNA contained
in nucleus.
• DNA codes for the construction of proteins
using an intermediary molecule:
– Ribonucleic acid or RNA.
• Decoding information in DNA requires two
processes:
– Transcription.
– Translation.
DNA vs. RNA
• RNA:
– Contains the sugar
ribose.
– Contains adenine, uracil,
cytosine and guanine.
– Single helix
• DNA:
– Contains deoxyribose.
– Contains adenine,
thymine, cytosine and
guanine.
– double helix.
DNA vs. RNA
• RNA:
– Smaller, mobile.
– Degrades easily.
– Travels form nucleus to cytoplasm.
• DNA:
– Larger, immobile.
– Lasts the life of cell.
– Resides in nucleus.
Types of RNA
• Messenger RNA
– Carries genetic
information from DNA
in nucleus to
cytoplasm.
• Information is used to
synthesize a protein.
– Codon: three
nucleotide sequence
that codes for one
amino acid.
Types of RNA
• Transfer RNA
– Functions as the
“interpreter”
– Transfer amino acids to the
sites where the information
in the mRNA is being used
to make a protein
– Anticodon: three
nucleotide sequence that is
complementary to a
particular codon in mRNA
Types of RNA
• Ribosomal RNA
– Combine with proteins
to form ribosomes
• Ribosomes
– Site of translation
– Large subunit
– Small subunit
Protein Synthesis
• Two processes:
– Transcription
• Occurs in the nucleus
• Produces RNA
– Translation
• Occurs in the cytoplasm
• Produces proteins
Transcription
Translation
• To line up the
appropriate amino
acids in the proper
order requires:
– mRNA
– tRNA
– Ribosomes
Translation
Translation
• Codon (mRNA) must
be complementary to
the anticodon (tRNA).
• Translation continues
until ribosome
encounters a stop
codon.
Genetic Code
• Three nucleotides in mRNA (codon) code
for one amino acid.
• Some sequences serve as starting points.
• AUG codes for the amino acid methionine which
also indicates to start translation.
• Some sequences do not have
complementary tRNA.
– Indicate to the ribosome to stop translation.
Genetic Code
What Makes Cells Different From
Each Other?
• Due to the information in the DNA, a cell
could manufacture 50,000 different
proteins, but it doesn’t.
• The proteins a cell produces influences its
function.
– Example: red blood cells and hemoglobin
Gene Expression
• Some genes are always transcribed and
translated.
– Others can be turned on or off by
environmental signals
• Gene expression is highly regulated.
Gene Expression in Prokaryotes
• Jacob and Monod
– Studied digestion of
lactose in bacteria.
– Discovered the lac
operon.
• Prokaryotes regulate
gene expression at
the level of
transcription
Gene Expression in Eukaryotes
• Regulated at the level of transcription.
– Transcription requires transcription factors.
• They recognize and bind to DNA sequences called
regulatory sequences
• Transcription factors can increase or decrease the
rate of transcription
• Longevity of RNA molecule also influences
gene expression.
How Does DNA Change Over
Time?
• Mutations: a permanent change in the
genetic material of a cell or organism.
– Can be inherited.
– Can involve whole chromosomes or changes
in DNA sequences.
Whole Chromosome Mutations
• Polyploid: organism or cell containing
three or more sets of chromosomes.
– Occurs due to a cell division error.
– Frequently seen in plants, rare in animals.
– Can have advantageous results.
Whole Chromosome Mutations
• Nondisjunction:
instances when
paired chromosomes
fail to separate during
mitosis or meiosis
– Can result in an
aneuploid: individual
whose chromosome
number is greater or
less than normal
Whole Chromosome Mutations
• Down’s Syndrome
– Due to nondisjunction
with chromosome 21.
– Characterized by
mental retardation,
distinctive facial
features.
Whole Chromosome Mutations
• Transposons:
– Variety of DNA sequences that can randomly
insert themselves by transposition in various
non-homologous regions on chromosomes
and other DNA.
– Can generate new gene combinations
– Can also induce genetic errors
Mutations Involving Single DNA
Nucleotides
• Point Mutations:
– Change in a single nucleotide base pair.
– Example: sickle cell anemia.
Mutations Involving Single DNA
Nucleotides
• Frame-shift mutation:
– A change in the reading frame resulting from
an insertion or deletion of nucleotides in the
DNA sequence for a protein.
– Extremely harmful.
Normal:
JOE ATE THE HOT DOG
After deletion:
JEA THE OTD OG