DNA, RNA, and Protein Synthesis
How do our genes work?
• “How do our genes determine
our physical characteristics?
• These questions and others led
scientists to develop
experiments to find answers
• Frederick Griffith, 1928, tried to
determine how bacteria make
people sick.
• Specifically, he studied bacteria
that produce pneumonia, a
serious lung disease
Griffith and Transformation
Video
Griffith Experiment
• Griffith called this process
transformation because
one strain of bacteria (the
harmless strain) had
changed permanently into
another (the diseasecausing strain).
• Griffith hypothesized that a
factor must contain
information that could
change harmless bacteria
into disease-causing ones.
Avery and DNA
• In 1944 Canadian biologist Oswald
Avery repeated Griffith’s experiment
• Made an extract from the heat-killed
bacteria
• Treated the extract with an enzyme
that destroyed proteins, lipids,
carbs, and RNA, leaving only DNA:
transformation still occurred
• They destroyed the DNA:
transformation ceased
• Concluded that DNA stored the
genetic information that is
transmitted
Hershey-Chase Experiment
• Alfred Hershey and Marsha
Chase studied viruses –
nonliving particles smaller
than cells
• Viruses contain DNA
surrounded by protein coat
• They experiment on
bacteriophages – viruses that
infect bacteria
• If they could determine which
part of the virus infected the
bacteria, they could conclude
whether genes were made of
protein or DNA
The Hershey-Chase Experiment
• If 35S was found in the bacteria, it would mean
that the viruses’ protein had been injected into
the bacteria.
Bacteriophage with
sulfur-35 in protein coat
Phage infects
bacterium
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No radioactivity inside
bacterium
The Hershey-Chase Experiment
• If 32P was found in the bacteria, then it was the DNA that
had been injected.
Bacteriophage with
phosphorus-32 in DNA
Phage infects
bacterium
Radioactivity inside
bacterium
• Nearly all of the radioactivity in the cell was from
phosphorous. Hershey-Chase concluded that DNA was the
source of genetic information, not proteins
Structure of DNA
• DNA is made up of units called nucleotides
• Each nucleotide is made up of three
components:
– 5-carbon sugar called deoxyribose
– A phosphate group
– A nitrogenous base
• Four kinds of nitrogenous base in DNA:
– Adenine and Guanine are purines (2 rings)
– Cytosine and Thymine are pyrimidines (1 ring)
• There are
four kinds
of bases in
in DNA:
– adenine
– guanine
– cytosine
– thymine
The Components and
Structure of DNA
DNA structure
• Backbone of DNA strand is formed by sugar
and phosphate groups of each nucleotide
• The nucleotides can be joined in any order
• Erwin Chargaff discovered that:
– The percentages of guanine [G] and cytosine [C]
bases are almost equal in any sample of DNA.
– The percentages of adenine [A] and thymine [T]
bases are almost equal in any sample of DNA.
• Therefore, this is called Chargaff’s Rule
The Components and
Structure of DNA
• X-Ray Evidence
– Rosalind Franklin used X-ray
diffraction to get
information about the
structure of DNA.
– She aimed an X-ray beam at
concentrated DNA samples
and recorded the scattering
pattern of the X-rays on film.
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The Double Helix
• Using clues from Franklin’s pattern, James Watson
and Francis Crick built a model that explained how
DNA carried information and could be copied.
• Watson and Crick's model of DNA was a double helix,
in which two strands were wound around each other.
• Watson and Crick discovered that hydrogen bonds
can form only between certain base pairs—adenine
and thymine, and guanine and cytosine.
• This principle is called base pairing.
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• DNA Double Helix
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• Blame it on the DNA
• Stanford Video
DNA and Chromosomes
• DNA and Chromosomes
• In prokaryotic cells, DNA is located in the cytoplasm.
• Most prokaryotes have a single DNA molecule
containing nearly all of the cell’s genetic information.
DNA and Chromosomes
• Many eukaryotes have 1000 times the amount
of DNA as prokaryotes.
• Eukaryotic DNA is located in the cell nucleus
inside chromosomes.
• The number of chromosomes varies widely
from one species to the next.
DNA and Chromosomes
• Chromosome Structure
– Eukaryotic chromosomes contain DNA and
protein, tightly packed together to form
chromatin.
– Chromatin consists of DNA tightly coiled around
proteins called histones.
– DNA and histone molecules form nucleosomes.
– Nucleosomes pack together, forming a thick fiber.
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DNA and Chromosomes
– Eukaryotic Chromosome Structure
Chromosome
Nucleosome
DNA
double
helix
Coils
Supercoils
Histones
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DNA Replication
• Each strand of the DNA double helix has all the
information needed to reconstruct the other half
by the mechanism of base pairing.
• In most prokaryotes, DNA replication begins at a
single point and continues in two directions
• In eukaryotic chromosomes, DNA replication
occurs at hundreds of places.
• Replication proceeds in both directions until each
chromosome is completely copied.
• The sites where separation and replication occur
are called replication forks.
DNA Replication
• Before a cell divides, it duplicates its DNA in a
process called replication.
• Replication ensures that each resulting cell
will have a complete set of DNA.
• During DNA replication, the DNA molecule separates
into two strands, then produces two new
complementary strands following the rules of base
pairing.
• Each strand of the double helix of DNA serves as a
template for the new strand.
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New Strand
Original strand
Nitrogen Bases
Growth
Growth
Replication Fork
Replication Fork
DNA Polymerase
Active Art
DNA Replication
• DNA replication is carried out by enzymes
(Helicase) that “unzip” a molecule of DNA.
• Hydrogen bonds between base pairs are
broken and the two strands of DNA unwind.
• The principal enzyme involved in DNA
replication is DNA polymerase.
• DNA polymerase joins individual nucleotides
to produce a DNA molecule and then
“proofreads” each new DNA strand.
Genes
• Genes are coded DNA instructions that control
the production of proteins.
• Genetic messages can be decoded by copying
part of the nucleotide sequence from DNA
into RNA.
• RNA contains coded information for making
proteins.
RNA
• RNA consists of a long chain of
nucleotides.
• Each nucleotide is made up of a
5-carbon sugar, a phosphate
group, and a nitrogenous base.
• There are three main differences
between RNA and DNA:
– The sugar in RNA is ribose instead
of deoxyribose.
– RNA is generally single-stranded.
– RNA contains uracil in place of
thymine.
Types of RNA
• There are three
main types of RNA:
– messenger RNA
– ribosomal RNA
– transfer RNA
• Messenger RNA (mRNA) carries copies of
instructions for assembling amino acids into
proteins.
Ribosome
Ribosomal RNA
• Ribosomes are made up of proteins and
ribosomal RNA (rRNA).
Amino acid
Transfer RNA
• During protein construction, transfer RNA (tRNA)
transfers each amino acid to the ribosome.
Transcription
• RNA molecules are produced by copying part
of a nucleotide sequence of DNA into a
complementary sequence in RNA. This process
is called transcription.
• Transcription requires the enzyme RNA
polymerase.
• During transcription, RNA polymerase binds to
DNA and separates the DNA strands.
Transcription
• RNA polymerase then uses one strand of DNA as a template from
which nucleotides are assembled into a strand of RNA.
• RNA polymerase binds only to regions of DNA known as promoters.
• Promoters are signals in DNA that indicate to the enzyme where to
bind to make RNA
Video
RNA Editing
• The DNA of eukaryotic genes contains
sequences of nucleotides, called introns, that
are not involved in coding for proteins.
• The DNA sequences that code for proteins are
called exons.
• When RNA molecules are formed, introns and
exons are copied from DNA.
RNA Editing
• The introns are cut out
of RNA molecules.
• The exons are then
spliced together to
form mRNA.
• The genetic code is the
“language” of mRNA
instructions.
• The code is written
using four “letters”
(the bases: A, U, C, and
G).
Exon Intron
DNA
Pre-mRNA
mRNA
Cap
Tail
The Genetic Code
• A codon consists of three consecutive nucleotides on
mRNA that specify a particular amino acid.
The Genetic Code
• Each codon specifies a particular amino acid.
• Some amino acids can be specified by more
than one codon.
• There is one codon AUG that can either specify
the amino acid methionine or serve as a “start”
codon for protein synthesis.
• There are three “stop” codons that do not code
for any amino acid.
• These “stop” codons signify the end of a
polypeptide.
The Genetic Code
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Translation
• Translation is the decoding of an mRNA message
into a polypeptide chain (protein).
• Translation takes place on ribosomes.
• During translation, the cell uses information
from messenger RNA to produce proteins.
Translation
• Messenger RNA is transcribed in the nucleus, and
then enters the cytoplasm where it attaches to a
ribosome.
Nucleus
mRNA
Video
Translation
• Translation begins when an mRNA molecule
attaches to a ribosome.
• As each codon of the mRNA molecule moves
through the ribosome, the proper amino acid
is brought into the ribosome by tRNA.
• In the ribosome, the amino acid is transferred
to the growing polypeptide chain.
Translation
• Each tRNA molecule carries only one kind of
amino acid.
• In addition to an amino acid, each tRNA
molecule has three unpaired bases.
• These bases, called the anticodon, are
complementary to one mRNA codon.
• The ribosome binds new tRNA molecules and
amino acids as it moves along the mRNA.
Phenylalanine
Methionine
Ribosome
mRNA
Start codon
tRNA
Lysine
Translation
• Protein Synthesis
Lysine
tRNA
Translation direction
mRNA
Ribosome
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Translation
• The process continues until the ribosome reaches a stop
codon.
Polypeptide
Ribosome
tRNA
Video
mRNA
The Roles of RNA and DNA
• The cell uses the DNA “master plan” to prepare
RNA “blueprints.” The DNA stays in the nucleus.
• The RNA molecules go to the protein building
sites in the cytoplasm—the ribosomes.
• Genes contain instructions for assembling
proteins.
• Many proteins are enzymes, which catalyze and
regulate chemical reactions.
• Proteins are each specifically designed to build or
operate a component of a living cell.
Codon
• The sequence of
bases in DNA is
used as a template
for mRNA.
• The codons of
mRNA specify the
sequence of amino
acids in a protein.
Codon
Codon
Single strand of DNA
Codon
Codon
Codon
mRNA
Alanine
Arginine
Leucine
Amino acids within a
polypeptide
Central Dogma
• DNA  RNA  Protein
• Mutations – changes in the genetic material
12-4 Mutations
Kinds of Mutations
• Mutations that produce changes in a single gene
are known as gene mutations.
• Mutations that produce changes in whole
chromosomes are known as chromosomal
mutations.
• Gene mutations involving a change in one or a
few nucleotides are known as point mutations
because they occur at a single point in the DNA
sequence.
• Point mutations include substitutions, insertions,
and deletions.
• Substitutions
usually affect no
more than a
single amino
acid.
Kinds of Mutations
• The effects of insertions or deletions are more
dramatic.
• The addition or deletion of a nucleotide causes a
shift in the grouping of codons.
• Changes like these are called frameshift
mutations.
• Frameshift mutations may change every amino
acid that follows the point of the mutation.
• Frameshift mutations can alter a protein so much
that it is unable to perform its normal functions.
• In an insertion, an
extra base is
inserted into a base
sequence.
• Video
• In a deletion, the loss of a single base is
deleted and the reading frame is shifted.
ChromosomalMutations
• Chromosomal mutations involve changes in
the number or structure of chromosomes.
• Chromosomal mutations include deletions,
duplications, inversions, and translocations.
• Deletions involve the loss of all or part of a
chromosome.
• Duplications produce extra copies of parts of a
chromosome.
Video
• Inversions reverse the direction of parts of
chromosomes.
• Translocations occurs when part of one chromosome
breaks off and attaches to another.
Video
Significance of Mutations
• Many mutations have little or no effect on gene
expression.
• Some mutations are the cause of genetic disorders.
• Beneficial mutations may produce proteins with new or
altered activities that can be useful.
• Polyploidy is the condition in which an organism has
extra sets of chromosomes.
• Example: Durum wheat is 3N (3 times N, where N equals
number of chromosomes; we are 2N)
• Mutations Video
• Abby & Brittany Hensel Video
Gene Regulation
Gene Regulation: An Example
• E. coli provides an example of how gene
expression can be regulated.
• An operon is a group of genes that operate
together.
• In E. coli, these genes must be turned on so
the bacterium can use lactose as food.
• Therefore, they are called the lac operon.
Operons
• The lac genes are turned off by repressors and
turned on by the presence of lactose.
• On one side of the operon's three genes are two
regulatory regions.
– In the promoter (P) region, RNA polymerase binds
and then begins transcription.
• The other region is the operator (O).
• When the lac repressor binds to the O region,
transcription is not possible.
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• When lactose is added, sugar binds to the
repressor proteins.
• The repressor protein changes shape and falls off
the operator and transcription is made possible.
Gene Regulation: An Example
• Many genes are regulated by repressor
proteins.
• Some genes use proteins that speed
transcription.
• Sometimes regulation occurs at the level of
protein synthesis.
Eukaryotic Gene Regulation
• Operons are generally not found in
eukaryotes.
• Most eukaryotic genes are controlled
individually and have regulatory sequences that
are much more complex than those of the lac
operon.
Eukaryotic Gene Regulation
• Many eukaryotic genes have a sequence
called the TATA box.
TATA
box
Upstream
enhancer
Promoter
sequences
Introns
Exons
Direction of transcription
Eukaryotic Gene Regulation
• The TATA box seems to help position RNA
polymerase.
Upstream
enhancer
TATA
box
Promoter
sequences
Introns
Exons
Direction of transcription
Eukaryotic Gene Regulation
• Eukaryotic promoters are usually found just before the
TATA box, and consist of short DNA sequences.
Upstream
enhancer
TATA
box
Promoter
sequences
Introns
Exons
Direction of transcription
Eukaryotic Gene Regulation
• Genes are regulated in a variety of ways by
enhancer sequences.
• Many proteins can bind to different enhancer
sequences.
• Some DNA-binding proteins enhance
transcription by:
• opening up tightly packed chromatin
• helping to attract RNA polymerase
• blocking access to genes
Development and Differentiation
• As cells grow and divide, they undergo
differentiation, meaning they become
specialized in structure and function.
• Hox genes control the differentiation of cells
and tissues in the embryo.
• Careful control of expression in hox genes is
essential for normal development.
• All hox genes are descended from the genes of
common ancestors.
Hox Genes
• In fruit flies, a series of
hox genes determines the
basic structure of the fly’s
body.
•Mice have very similar
genes on four different
chromosomes.
•The color bars along the
mouse’s back show the
approximate body area
affected by genes of the
corresponding colors.