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DNA and Genes
Unit 4
Chapter 11
DNA structure
 DNA controls
cellular activity
because it regulates
the production of
proteins.
 DNA is the blueprint
for proteins that are
necessary for
cellular metabolism.
Why are proteins so important?
 Some proteins become important structures,
such as the filaments in muscle tissue.
• Other proteins, such as enzymes, control
chemical reactions that perform key life
functions—breaking down glucose molecules
in cellular respiration, digesting food, or
making spindle fibers during mitosis.
Who discovered that DNA is the
blueprint for life?
 In 1952 Alfred Hershey and Martha Chase
performed an experiment using radioactively
labeled viruses that infect bacteria.
 Because viruses are protein and DNA only,
they figured out that viral DNA (not viral
protein) could force the bacteria to make new
viruses.
 This was evidence that DNA can determine
cell activity.
DNA is a polymer.
 Polymer: chemical structure made of
repeating units
 DNA is made of repeating nucleotide units.
 DNA nucleotides always have a phosphate
group, deoxyribose sugar, and a nitrogen
base.
DNA
nucleotide
Four DNA nitrogenous bases
 A nitrogenous base is a carbon ring with nitrogen atoms
and determines the name of the nucleotide.
 In DNA, there are four possible nitrogenous bases:
adenine (A), guanine (G), cytosine (C), and thymine (T).
Adenine (A)
Guanine (G)
Purines
Cytosine (C)
Thymine (T)
Pyrimidines
The structure of nucleotides
 Nucleotides join together to form long chains,
with the phosphate group of one nucleotide
bonding to the deoxyribose sugar of an
adjacent nucleotide.
 The phosphate groups and deoxyribose
molecules form the backbone of the chain,
and the nitrogenous bases stick out like the
teeth of a zipper.
DNA is a double helix and looks
like a twisted ladder.
 The outer parts are the
sugar-phosphate
backbone.
 Two nitrogen bases of
the nucleotides face
inward and form the
rungs of the helix
ladder.
 Adenine always binds
to thymine.
 Cytosine always binds
to guanine..
Who discovered the double helix
structure?
 In 1953, Watson and
Crick proposed that
DNA is made of two
chains of nucleotides
held together by
nitrogenous bases and
twisted together.
 They used Rosalind
Franklin’s X-ray
crystallography work to
figure this out.
The importance of comparing DNA
nucleotide sequences
 Each species has its
own unique DNA
sequence.
 The more closely
related two individuals
are, the more likely they
will share the same
DNA nucleotide
sequence.
 Comparing DNA base
pairs of two species will
show their evolutionary
history.
DNA replication – making copies of
the DNA code
 Necessary before a cell
undergoes mitosis, occurs in
interphase
Click on image to play video.
Copying DNA
How does DNA code for proteins?
 The sequence of nucleotides in each gene
contains information for assembling the string
of amino acids that make up a single protein.
 The ribosomes required to make proteins
cannot read DNA. (it’s like a foreign language)
 Therefore, for DNA to code for proteins, an
RNA molecule must be made.
 Ribosomes can only read RNA.
RNA is another nucleic acid,
nucleotide polymer.
 RNA differs from DNA
structure in three ways.
 Single stranded
instead of double
stranded
 Ribose sugar instead
of deoxyribose
 Uracil instead of
thymine nitrogen
base
Ribose sugar
Three types of RNA
 Messenger RNA: carries the DNA code
(message) to the ribosomes
 Ribosomal RNA: makes up the ribosomes
that reads the mRNA to build the correct
amino acid sequence
 Transfer RNA: brings the amino acids to the
ribosome
Transfer RNA
Click on the image to play the video.
Transcription
 The process of building an RNA strand from
the DNA template
In eukaryotes,
this occurs inside
the nucleus.
In prokaryotes,
this occurs in the
cytoplasm.
Click on image to play the video.
Transcription
mRNA processing in eukaryotes
 Since much of the DNA code is useless or
codes for multiple proteins, the unnecessary
portions of DNA that were coded into mRNA
must be removed.
 The useless portions of RNA (introns) are
removed. The coding portions (exons) are
linked together to make the final mRNA.
mRNA codes for amino acids.
 Three mRNA nucleotides code for one amino
acid, but more than one combination codes
for the same amino acid.
Translation
 The process of ribosomes reading the mRNA
code to properly make an amino acid chain
that is folded into a usable protein
Translation
 The ribosome binds to AUG, the starting code
(codon). The ribosome directs the methionine
tRNA to bring transfer the methionine (met)
amino acid.
Translation
1. The ribosome read the next codon and
directs the appropriate tRNA to transfer the
amino acid.
Translation process
1. The ribosome joins the amino acids together
and continues this process until the codon
indicates stop.
What happens if there is a mistake
(mutation) in the DNA code?
 Possibly proteins won’t be made or are made
improperly.
 If the mutations occur in the gametes, the offspring’s
DNA will be affected positively, negatively, or
neutrally.
 What can cause a mutation?





Replication error
Transcription error
Cell division error
Chemical agents (mutagens)
Spontaneous changes
Point mutation
 A point mutation is a change in a single base
pair in DNA.
 A change in a single nitrogenous base can
change the entire structure of a protein
because a change in a single amino acid can
affect the shape of the protein.
Point mutations
 May change the amino acid code if the
mutations occurs in the right place in the
code.
mRNA
Normal
Protein
Stop
Replace G with A
mRNA
Point
mutation Protein
Stop
Frameshift mutations
 Losing a single nucleotide base
 This mutation would cause nearly every
amino acid in the protein after the deletion to
be changed
Deletion of U
mRNA
Protein
Changes to the chromosome
 When a part of a chromosome is left out, a
deletion occurs
A B C D E
F G H
A B C E
F G H
Deletion
 When part of a chromatid breaks off and
attaches to its sister chromatid, an insertion
occurs.
A B C D E
F G H
A B C B C D E
Insertion
F G H
Changes to the chromosome
 When part of a chromosome breaks off and
reattaches backwards, an inversion occurs.
A B C D E F G H
A D C B E FGH
Inversion
 When part of one chromosome breaks off
and is added to a different chromosome, a
translocation occurs.
AB C D E F GH
WX Y Z
W X AB C DE F GH
Translocation
Y
Z
Repairing DNA
 Enzymes proofread the DNA and replace
incorrect nucleotides with correct nucleotides.
 The greater the exposure to a mutagen such
as UV light, the more likely is the chance that
a mistake will not be corrected.