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CONCEPTS IN BIOLOGY
TWELFTH EDITION
Enger • Ross • Bailey
CHAPTER 8
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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8.1 DNA and the importance of
proteins


Four common types of macromolecules (p. 144)
- lipids, carbohydrates, proteins, and nucleic acids
Proteins play a crucial role in the life of a cell. (p. 144)
–
–

Microtubules, intermediate filaments and microfilaments
maintain the shape of the cell.
Enzymes catalyze important reactions.
The recipes for proteins are found (p. 144)
–
–
in the cell’s DNA.
DNA is organized into genes.

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Each gene is a recipe for a different protein.
8.2 DNA structure and function

DNA accomplishes two things:
–
–

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Passes genetic information to the next generation
Controls the synthesis of proteins
DNA is able to accomplish these things
because of its unique structure.
DNA structure


DNA is a nucleic acid.
Nucleic acids
–
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Large polymers made of
nucleotides
 A sugar molecule
– Deoxyribose for
DNA
– Ribose for RNA
 A phosphate group
 A nitrogenous base
– Adenine
– Guanine
– Cytosine
– Thymine
DNA structure

DNA is double stranded.
–
–
–
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The two paired strands of
DNA form a helix,
with the sugars and
phosphates on the
outside
and nitrogenous bases in
the inside of the helix
DNA structure

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DNA is double
stranded.
– Held together by
hydrogen bonds
between the
bases
– A-T, G-C
Base pairing aids DNA replication

DNA replication
–
–
7
is the process by which DNA is copied.
 This is done before cell division.
 Provides the daughter cells with a copy
of the genetic information
relies on the base-pairing rules and many
enzymes.
Base pairing aids DNA replication
–
Is accomplished by DNA polymerase and other enzymes
1. Helicases (as enzymes) binds to DNA and forms a replication
bubble. And, helicases separate the two strands. (Fig. 8.3a &
8.3b)
2. DNA polymerase builds new DNA strands that will pair with
each old DNA strand. (Fig. 8.3c & 8.3d)
– Where there is an A on the old strand, polymerase will
add a T to the new strand.
3. In eukaryotic cells, replication process starts at the same time
in several places along the DNA molecule. As the points of
DNA replication meet each other, they combine and a new
strand of DNA is formed (Fig. 8.3 e)


When DNA polymerase finishes a segment of new DNA, it
checks its work and corrects mistakes if they happen.
DNA replication is highly accurate and is only one error made
for every 2X10 (order 9) nucleotides. (error-free)
DNA replication (Fig. 8.3)
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Repairing genetic information

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If a mistake is made
when building the new
strand
– The old strand still
has the correct
information.
– This information can
be used to correct
the new strand. (Fig.
8.4)
Fig. 8.4
The DNA code



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The order of bases in the DNA molecules is the
genetic information that codes for proteins.
– The sequence of ATGC forms words that are like
a recipe for proteins.
Each word contains three base letters.
– ATGC are the four letters that are used to make
the words.
– Each three-letter word codes for a specific amino
acid.
The order of amino acids in the protein is determined
by the order of nucleotides in the gene.
HOW SCIENCE WORKS 8.1 (p. 148)
8.3 RNA structure and function

RNA vs DNA
–
–
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RNA has ribose sugar
(DNA has deoxyribose).
RNA contains the bases
 Adenine
 Guanine
 Cytosine
 Uracil (DNA has
thymine)
DNA vs RNA
–
–
–
–
–
–
RNA’s, like DNA’s, base sequence carries
information.
RNA is made in the nucleus and transported to
the cytoplasm (DNA stays in the nucleus).
The protein coding information in RNA
 Comes from DNA
Like DNA replication, RNA synthesis follows the
base-pairing rules (A-U; G-C).
RNA is single-stranded.
Three types of RNA participate in protein
synthesis
 mRNA (nessenger)
 tRNA (transfer)
 rRNA (ribosomal)
8.4 Protein synthesis


The sequence of nucleotides in a gene dictates
the order of amino acids in a protein.
Before a protein can be made
–
The information in DNA must be copied into RNA.
 This
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process is called transcription.
Then the information in the RNA can be used to
make the protein.
– This is called translation.
Transcription

During transcription
–
–
DNA is used as a template to make RNA.
Accomplished by RNA polymerase


The process of transcription
–
–
–
Begins in the nucleus
RNA polymerase separates the two strands of
DNA.
Only one of the two strands will be used to create
the RNA.
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Follows the base-pairing rules

The coding strand
The other DNA strand is called the non-coding strand.
Transcription of an RNA molecule
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The process of transcription
–
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Only a segment of the DNA strand will be used to create
each RNA.
 These segments are called genes.
 Each gene starts with a promoter.
– The RNA polymerase binds to the promoter to start
building an RNA strand.
 Each gene ends with a terminator sequence.
– The RNA polymerase will stop transcribing at the
terminator sequence.
Translation

Three types of RNA participate in translation.
–
–

Codons are sets of three nucleotides that
code for specific amino acids.
–
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mRNA carries the recipe for making the protein.
tRNA and rRNA are used to read the recipe and
build the amino acid chain.
tRNA reads the codons and brings the correct
amino acids.
The genetic code
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Translation

Ribosomes are organelles that build proteins.
–
–
–
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rRNA is found in ribosomes.
mRNA is read on ribosomes.
Ribosomes are found in two places in the cell.
 Free-floating in the cytoplasm
 Bound to the endoplasmic reticulum
Translation initiation

Translation begins when
–
–
–
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The small ribosomal subunit binds to the
beginning of the mRNA.
At this point, a tRNA brings the first amino acid.
 The anticodon in the tRNA matches with a
codon on the mRNA.
 Each tRNA carries a specific amino acid based
on its anticodon.
The first tRNA binds to the start codon, AUG.
 This tRNA carries a methionine.
Then the large ribosomal subunit joins the
complex.
Initiation
Translation elongation



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The next tRNA binds with the next codon on
the mRNA.
The ribosome adds this amino acid to the
growing polypeptide.
The ribosome then moves down to the next
codon.
The process repeats itself.
Elongation
Translation termination

Elongation continues until the ribosome
encounters a stop codon.
–

A release factor binds to the stop codon.
–
–
–
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UAA, UAG, UGA are stop codons.
This causes the ribosome to release the
polypeptide.
The ribosomal subunits separate and release the
mRNA.
The mRNA can be translated again by another
ribosome.
Termination
Figure 8.10a
Summary of protein synthesis
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The genetic code is nearly universal



The process of making proteins from the
information in DNA is used by nearly all cells.
Nearly all organisms studied to date use the
same genetic code.
Because of this, we are able to use bacteria
as factories to make massive amounts of
proteins.
–
Insulin, growth factor, etc.
However, HIV is not like this, called
retrovirus. (see p. 156)
8.5 Control of protein synthesis


Gene expression is how the cell makes a
protein from the information in a gene.
Cell types are different from one another
because they express different sets of genes.
–


Cells control gene expression in response to
different environmental conditions.
Cells can alter gene expression
–
36
Therefore, have different sets of proteins
–
Controls the quantity of a protein
Controls the quality of a protein
Control of protein quantity

Cells can regulate how much of a given
protein is made by
–

controlling how much mRNA is available for
translation.
Cells do this in a number of ways:
–
Regulating how tightly the chromatin is coiled in a
certain region

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The more tightly the chromatin is coiled, the less likely a
gene in that region will be transcribed.
Eukaryotic genome packaging
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OUTLOOKS 8.2
Telomeres
Control of protein quantity
–
By increasing or decreasing the rate of
transcription of the gene by enhancer and silencer
regions on the DNA.


–
Through the binding of transcription factors,

–
these proteins bind to the promoter and facilitates RNA
polymerase binding and transcription.
By limiting the amount of time the mRNA exists in
the cytoplasm,
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Activation of enhancer regions increases transcription.
Activation of silencer regions decreases transcription.
some mRNA molecules are more stable and will exist
longer in the cytoplasm, yielding more protein.
Scientific American (Chinese)
No. 76, June, p. 56-69, 2008
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43
Transcription factors
44
Control of protein quality

Eukaryotic cells can use one gene to make
more than one protein.
–

After transcription, the introns must be cut
out and the coding regions, called exons,
must be put back together.
–
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In eukaryotic genes, non-coding sequences
called introns, are scattered throughout the
sequence.
This is called splicing.
Transcription of mRNA in eukaryotic
cells
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Alternative splicing

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Different
combinations of
exons from a
single gene can be
joined to build a
number of different
mRNAs for a
number of different
proteins.
8.6 Mutations and protein synthesis





48
A mutation is any change in the DNA sequence of
an organism.
Can be caused by mistakes in DNA replication
Can be caused by external factors
– Carcinogens, radiation, drugs, viral infections
Only mutations in coding regions of gene will change
the proteins themselves.
On occasion, the changes that occur because of
mutations can be helpful
Point mutations


A change in a single nucleotide of the DNA
sequence
A silent mutation does not cause a change in the
amino acid sequence. (see Fig. 8.16)
–

A nonsense mutation changes a codon to a stop
codon.
–
–

This causes the ribosome to stop translation prematurely.
CAA (Gln) to UAA
A missense mutation causes a change in the type of
amino acid added to a polypeptide.
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UUU to UUC; both code for Phe
–
This may change the way in which a protein functions.
UUU to GUU; Phe to Val change
Point mutations
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Sickle cell anemia

Results from a missense mutation in the
gene for hemoglobin
–
–
–
–
GAA to GUA
Glutamic acid to valine change (see p. 158)
Causes the hemoglobin protein to change shape
The molecules stick together in low oxygen
conditions. (see p. 159)


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
Get stuck in blood vessels
Break apart easily, leading to anemia
Causes weakness, brain damage, painful joints, etc.
Normal and sickled red blood cells
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Insertions and Deletions



An insertion mutation occurs when one or
more nucleotides is added to the normal
DNA sequence.
A deletion mutation occurs when one or more
nucleotides is removed from the normal DNA
sequence.
Insertions and deletions cause a frameshift.
–
–
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Ribosomes will read the wrong set of three
nucleotides.
Changes the amino acid sequence dramatically
Changes the function of the protein dramatically
Frameshift
54

Insertions cause a frameshift.
–
Some viruses, such as HIV, can insert
their genetic code into the DNA of their
host organism
–
Genital warts was caused by the human
papillomavirus (HPV), i.e., retroviruses,
the insertion mutations can transform
normal cells into cancerous ones (see Fig.
8.19)
Fig. 8.19 HPV
Chromosomal aberrations

Involves a major change in DNA at the level
of the chromosome
–
–
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–

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Inversions occur when a chromosome breaks,
and the broken piece becomes reattached in the
wrong orientation.
A translocation occurs when the broken segment
becomes integrated into a different chromosome.
A duplication occurs when a segment of a
chromosome is replicated and attached to the
original segment in sequence.
A deletion occurs when a broken piece is lost or
destroyed.
All of these effect many genes, thus many
proteins.
Street Drugs (LSD, see p. 160)