Download Chapter 21 Nucleic Acids and Protein Synthesis

Chapter 21 Nucleic Acids and
Protein Synthesis
21.1
Components of Nucleic Acids
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1
Nucleic Acids
Nucleic acids are
Molecules that store information for cellular growth
and reproduction.
Deoxyribonucleic acid (DNA) and ribonucleic acid
(RNA).
Large molecules consisting of long chains of
monomers called nucleotides.
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Nucleic Acids
The nucleic acids DNA and RNA
consist of monomers called
nucleotides that consist of a
Pentose sugar.
Nitrogen-containing base.
Phosphate.
nucleotide
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3
Nitrogen Bases
The nitrogen bases in
DNA and RNA are
Pyrimidines C, T, and U
Purines A and G.
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Nitrogen-Containing Bases in
DNA and RNA
DNA contains the nitrogen bases
Cytosine (C)
Guanine (G)
same in both DNA and RNA
Adenine (A)
Thymine (T)
different in DNA than in RNA
RNA contains the nitrogen bases
Cytosine (C)
Guanine (G)
same in both DNA and RNA
Adenine (A)
Uracil (U)
different in DNA than in RNA
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Pentose Sugars
The pentose (five-carbon) sugar
In RNA is ribose.
In DNA is deoxyribose with no O atom on carbon 2′.
Has carbon atoms numbered with primes to
distinguish them from the atoms in nitrogen bases.
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Nucleotides
A nucleotide
Is a nucleoside that
forms a phosphate ester
with the C5′ –OH group
of a sugar (ribose or
deoxyribose).
Is named using the
name of the nucleoside
followed by
5′-monophosphate.
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Formation of a Nucleotide
A nucleotide forms when the −OH on C5′ of a sugar
bonds to phosphoric acid.
NH2
NH2
N
N
O
O- P OH
O-
5’
+
O
HO CH2
O
OH
deoxycytidine and phosphate
N
5’ O
O
O- P O CH2
O
-
N
O
OH
deoxycytidine monophosphate (dCMP)
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Names of Nucleosides and
Nucleotides
TABLE 21.1
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Chapter 21 Nucleic Acids and
Protein Synthesis
21.10
Viruses
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Viruses
Viruses
Are small particles of DNA or RNA that require a
host cell to replicate.
Cause a viral infection when the DNA or RNA
enters a host cell.
Are synthesized in the host cell from the viral
RNA produced by viral DNA.
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Viruses
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Reverse Transcription
In reverse transcription
A retrovirus, which contains viral RNA, but no
viral DNA, enters a cell.
The viral RNA uses reverse transcriptase to
produce a viral DNA strand.
The viral DNA strand forms a complementary
DNA strand.
The new DNA uses the nucleotides and enzymes
in the host cell to synthesize new virus particles.
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Reverse Transcription
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HIV Virus and AIDS
The HIV-1 virus
HIV virus
Is a retrovirus that
infects T4
lymphocyte cells.
Decreases the T4
level making the
immune system
unable to destroy
harmful organisms.
Causes pneumonia
and skin cancer
associated with
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AIDS.
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AIDS Treatment
One type of AIDS treatment prevents reverse
transcription of the viral DNA.
When altered nucleosides such as AZT and ddI are
incorporated into viral DNA, the virus is unable to
replicate.
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AIDS Treatment
Azidothymine (AZT)
Dideoxyinosine (ddI)
O
H
H3C
HO CH2
O
N
N
O
H
N
O HO CH
2
N
O
H
H
H
H
N3
H
H
H
N
N
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AIDS Treatment
Another type of AIDS treatment involves protease
inhibitors such as saquinavir, indinavir, and ritonavir.
Protease inhibitors modify the active site of the
protease enzyme, which prevents the synthesis of
viral proteins.
Inhibited by
AZT, ddI
reverse
transcriptase
Viral RNA
Viral DNA
Inhibited by
protease inhibitors
protease
Viral proteins
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Chapter 21 Nucleic Acids and
Protein Synthesis
21.2
Primary Structure of Nucleic Acids
21.3
DNA Double Helix
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Primary Structure of Nucleic Acids
In the primary structure of nucleic acids,
Nucleotides are joined by phosphodiester bonds.
The 3’-OH group of the sugar in one nucleotide forms
an ester bond to the phosphate group on the
5’-carbon of the sugar of the next nucleotide.
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Primary Structure of Nucleic Acids
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21
Structure of Nucleic Acids
A nucleic acid polymer
Has a free 5’-phosphate
group at one end and a free
3’-OH group at the other
end.
Is read from the free 5’-end
using the letters of the
bases.
This section is read as:
5’—A—C—G—T—3’.
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Example of RNA
The primary
structure of RNA
Is a single
strand of
nucleotides.
Consists of the
bases A, C, G,
and U linked by
3’-5’ ester
bonds between
ribose and
phosphate.
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23
Example of DNA
In the primary
structure of DNA,
A, C, G, and T
are linked by 3’-5’
ester bonds
between
deoxyribose and
phosphate.
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Double Helix of DNA
The DNA structure is a
double helix that
Consists of two strands of
nucleotides that form a double
helix structure like a spiral
stair case.
Has hydrogen bonds between
the bases A–T and G–C.
Has bases along one strand
that complement the bases
along the other.
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Complementary Base Pairs
DNA contains complementary base pairs in which
Adenine is always linked by two hydrogen bonds
to thymine (A−T).
Guanine is always linked by three hydrogen bonds
to cytosine (G−C).
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DNA Double Helix Structure
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Learning Check
Write the complementary base sequence for the
matching strand in the following DNA section:
5’—A—G—T—C—C —A—A—T—C—3’
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Solution
Write the complementary base sequence for the
matching strand in the following DNA section:
5’—A—G—T—C—C—A—A—T—C—3’
3’—T—C—A—G—G—T—T—A—G—5’
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Chapter 21 Nucleic Acids and
Protein Synthesis
21.4
DNA Replication
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DNA Replication
DNA replication involves
Unwinding the DNA.
Pairing the bases in
each strand with new
bases to form new
complementary strands.
Producing two new
DNA strands that
exactly duplicate the
original DNA.
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Hydrolysis Energy
Energy from the
hydrolysis of
each nucleoside
triphosphate
adding to the
complementary
strand is used to
form the
phosphodiester
bond.
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Direction of Replication
During DNA replication,
An enzyme helicase unwinds the parent DNA at
several sections.
At each open DNA section called a replication fork,
DNA polymerase catalyzes the formation of
5’-3’ester bonds of the leading strand.
The lagging strand growing in the 3’-5’ direction is
synthesized in short sections called Okazaki
fragments.
The Okazaki fragments are joined by DNA ligase to
give a single 3’-5’ DNA strand.
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Direction of Replication
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Chapter 21 Nucleic Acids and
Protein Synthesis
21.5
RNA and Transcription
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RNA
RNA
Transmits information from DNA to make proteins.
Has several types
Messenger RNA (mRNA) carries genetic
information from DNA to the ribosomes.
Transfer RNA (tRNA) brings amino acids to the
ribosome to make the protein.
Ribosomal RNA (rRNA) makes up 2/3 of
ribosomes where protein synthesis takes
place.
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Types of RNA
TABLE 21.3
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tRNA
Each tRNA
Has a triplet called an
anticodon that
complements a codon
on mRNA.
Bonds to a specific
amino acid at the
acceptor stem.
anticodon
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Protein Synthesis
Protein synthesis involves
• Transcription
mRNA is formed from a gene on a DNA strand.
• Translation
tRNA molecules bring amino acids to mRNA to
build a protein.
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Transcription: Synthesis of mRNA
In transcription
A section of DNA containing the gene unwinds.
One strand of DNA is copied starting at the initiation
point, which has the sequence TATAAA.
A mRNA is synthesized using complementary base
pairing with uracil(U) replacing thymine(T).
The newly formed mRNA moves out of the nucleus
to ribosomes in the cytoplasm.
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RNA Polymerase
During transcription,
RNA polymerase moves along the DNA template
in the 3’-5’direction to synthesize the
corresponding mRNA.
The mRNA is released at the termination point.
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Protein Synthesis: Transcription
transcription
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mRNA Processing: Exons and
Introns
The DNA of eukaryotes contains exons that code
for proteins along with introns that do not.
The initial mRNA called a pre-RNA includes the
noncoding introns.
While in the nucleus, the introns are removed from
the pre-RNA.
The exons that remain are joined to form the
mRNA that leaves the nucleus with the information
for the synthesis of protein.
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Removing Introns from PremRNA
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Regulation of Transcription
Transcription is regulated by
A specific mRNA synthesized when the cell
requires a particular protein.
Feedback control, in which the end products
speed up or slow the synthesis of mRNA.
Enzyme induction, in which high levels of a
reactant induce the transcription process to
provide the necessary enzymes for that reactant.
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Lactose Operon Turned Off/On
RNA Polymerase
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Chapter 21 Nucleic Acids and
Protein Synthesis
21.6
The Genetic Code
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Genetic Code
The genetic code
Is a sequence of amino acids in a mRNA that
determine the amino acid order for the protein.
Consists of sets of three bases (triplet) along the
mRNA called codons.
Has a different codon for all 20 amino acids
needed to build a protein.
Contains certain codons that signal the “start”
and “end” of a polypeptide chain.
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The Genetic Code: mRNA Codons
TABLE 21.4
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Codons and Amino Acids
Suppose we want to determine the amino acids coded
for in the following section of a mRNA.
5’—CCU —AGC—GGA—CUU—3’
According to the genetic code, the amino acids for these
codons are
CCU = Proline AGC = Serine
GGA = Glycine
CUU = Leucine
The mRNA section codes for the amino acid sequence of
Pro—Ser—Gly—Leu
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Chapter 21 Nucleic Acids and
Protein Synthesis
21.7
Protein Synthesis: Translation
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tRNA Activation
The activation of tRNA
Occurs when a synthetase
uses energy of ATP
hydrolysis to attach an
amino acid to a specific
tRNA.
Prepares each tRNA to
use a triplet called an
anticodon to complement a
codon on mRNA.
Anticodon
52
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Initiation of Protein Synthesis
For the initiation of protein synthesis
A mRNA attaches to a ribosome.
The start codon (AUG) binds to a tRNA with
methionine.
The second codon attaches to a tRNA with the
next amino acid.
A peptide bond forms between the adjacent amino
acids at the first and second codons.
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Translocation
During translocation
The first tRNA detaches from the ribosome.
The ribosome shifts to the adjacent codon on the
mRNA.
A new tRNA/amino acid attaches to the open
binding site.
A peptide bond forms and that tRNA detaches.
The ribosome shifts down the mRNA to read the
next codon.
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Protein Synthesis
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translation
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Termination of Protein Synthesis
In the terminiation of protein synthesis
After a polypeptide with all the amino acids for a
protein is synthesized.
The ribosome reaches a “stop” codon: UGA, UAA,
or UAG.
There is no tRNA with an anticodon for the “stop”
codons.
Protein synthesis ends.
The polypeptide detaches from the ribosome.
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Summary of
Transcription/Translation
The normal DNA sequence produces a mRNA that
provides instructions for the correct series of amino
acids in a protein.
Correct order
57
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Chapter 21 Nucleic Acids and
Protein Synthesis
21.8
Genetic Mutations
21.9
Recombinant DNA
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Mutations
A mutation
Alters the nucleotide sequence in DNA.
Results from mutagens such as radiation and
chemicals.
Produces one or more incorrect codons in the
corresponding mRNA.
Produces a protein that incorporates one or more
incorrect amino acids.
Causes genetic diseases that produce defective
proteins and enzymes.
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Normal DNA Sequence
The normal DNA sequence produces a mRNA that
provides instructions for the correct series of amino
acids in a protein.
Correct order
60
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Mutation: Substitution
In a substitution mutation,
A different base substitutes for the proper base in DNA.
There is a change in a codon in the mRNA.
The wrong amino acid may be placed in the polypeptide.
Incorrect order
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Wrong amino acid
Mutation: Frame Shift
In a frame shift mutation,
An extra base adds to or is deleted from the normal
DNA sequence.
All the codons in mRNA and amino acids are incorrect
from the base change.
62
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Incorrect amino acids
Genetic Diseases
TABLE 21.6
63
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Recombinant DNA
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Products of Recombinant DNA
TABLE 21.7
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DNA Fingerprinting
In DNA fingerprinting (Southern transfer):
Restriction enzymes cut a DNA sample into smaller
fragments (RFLPs).
The fragments are sorted by size.
A radioactive isotope that adheres to certain base
sequences in the fragments produces a pattern on Xray film, which is the “fingerprint”.
The “fingerprint” is unique to each individual DNA.
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Polymerase Chain Reaction
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