Download Nucleic Acids and Protein Synthesis

Chapter 17 Nucleic Acids and
Protein Synthesis
1
17.1
Components of Nucleic Acids
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
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Nucleic Acids
2
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 found in the cell nucleus
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Nucleic Acids
3
The nucleic acids DNA and RNA
consist of monomers called
nucleotides that consist of a
• pentose sugar
• base
• phosphate
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nucleotide
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Bases
4
The bases in DNA and RNA are
• pyrimidines C, T, and U
• purines A and G
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Bases in DNA and RNA
5
DNA contains the bases
• Cytosine (C)
• Guanine (G)
same in both DNA and RNA
• Adenine (A)
• Thymine (T)
different in DNA than in RNA
RNA contains the bases
• Cytosine (C)
• Guanine (G)
same in both DNA and RNA
• Adenine (A)
• Uracil (U)
different in RNA than in DNA
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Pentose Sugars
6
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 the bases
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Nucleosides
7
A nucleoside
• is a combination of a sugar and base
• has a base linked by a glycosidic bond to C1′ of a sugar
(ribose or deoxyribose)
• is named by changing the base ending to -osine for purines
and -idine for pyrimidines
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Nucleosides
8
A base forms an N-glycosidic bond with a ribose or
deoxyribose sugar to form a nucleoside.
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Nucleotides
9
A nucleotide
• is formed when the phosphate group on C5′ in the
nucleoside group bonds to −OH on C5′ of a ribose or
deoxyribose sugar
• is named using the name of the nucleoside, followed by
5′-monophosphate
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Nucleotides
10
The nucleotides of RNA are identical to those of DNA,
except in DNA the sugar is deoxyribose and deoxythymidine
replaces uridine.
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Components in DNA and RNA
11
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Naming Nucleosides and Nucleotides
12
Nucleosides
• that contain a purine end with osine
• that contain a pyrimidine end with idine
• of DNA add deoxy to the beginning of their name
Corresponding nucleotides in RNA and DNA are named by
adding -5'-monophosphate.
Abbreviations for bases A, G, C, U, and T are often used in
respective nucleosides and nucleotides.
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Nucleosides and Nucleotides
in DNA, RNA
13
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Learning Check
14
Give the name and abbreviation for the following and list
its base and sugar.
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Solution
15
Give the name and abbreviation for the following and list
its base and sugar.
The name of this nucleotide is
Guanosine-59-monophosphate (GMP)
Deoxyguanosine-59-monophosphate (dGMP)
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Chapter 17 Nucleic Acids and
Protein Synthesis
16
17.5
Protein Synthesis
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Initiation of Protein Synthesis
17
For the initiation of protein synthesis,
• an mRNA attaches to a ribosome
• the start codon (AUG) in mRNA forms hydrogen bonds to
methionine on tRNA
• 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
During chain elongation, the ribosome moves along the mRNA
from codon to codon, attaching new amino acids to the growing
polypeptide chain.
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Initiation of Protein Synthesis
18
An activated tRNA
with anticodon AGU
bonds to serine at the
acceptor stem.
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Translocation
19
Once the peptide bond is formed, the initial tRNA detaches from
the ribosome, which shifts to the next available codon, a process
called 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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Peptide Formation
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Peptide chain starts to form
Met
Met
Ser
Ser
Anticodons UAC
AGA
AGA
tRNA
• ••
AUG
• ••
• • •
UCU CUC
Ribosome
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UCU
Leu
GAG
•• •
CUC UUU
Ribosome shifts
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Protein Synthesis: Translation
21
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Termination
22
In the termination step,
• all the amino acids are linked
• the ribosome reaches a “stop” codon: UGA, UAA, or UAG
• there is no tRNA with an anticodon for the “stop” codons
• the polypeptide detaches from the ribosome
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Termination
23
Once the polypeptide is released,
• the R groups of the amino acids in the new polypeptide can
form hydrogen bonds to give the secondary structures of α
helices, β-pleated sheets, or triple helices
• chains form cross-links such as salt bridges and disulfide
bonds to produce tertiary and quaternary structures, which
makes it a biologically active protein
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Learning Check
24
Assign each of the following terms to a definition or concept.
activation
initiation
translocation
termination
A.
B.
C.
D.
Ribosomes move along mRNA, adding amino acids to a
growing peptide chain.
A completed peptide chain is released.
A tRNA attaches to its specific amino acid.
A tRNA binds to the AUG codon of the mRNA on the
ribosome.
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Solution
25
Assign each of the following terms to a definition or concept.
activation
initiation
translocation
termination
A.
B.
C.
D.
Ribosomes move along mRNA, adding amino acids to a
growing peptide chain.
translocation
A completed peptide chain is released.
termination
A tRNA attaches to its specific amino acid.
activation
A tRNA binds to the AUG codon of the mRNA on the
ribosome.
initiation
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Summary of Protein Synthesis
26
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Complementary Sequences in DNA,
mRNA, tRNA, and Peptides
27
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Learning Check
28
The following section of DNA is used to build mRNA for a
protein.
—GAA—CCC—TTT—
A. What is the corresponding mRNA sequence?
B. What are the anticodons on the tRNAs?
C. What is the amino acid order in the peptide?
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
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Solution
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A. What is the corresponding mRNA sequence?
—CUU—GGG—AAA—
B. What are the anticodons for the tRNAs?
GAA for CUU; CCC for GGG; UUU for AAA
C. What is the amino acid order in the peptide?
—CUU—GGG—AAA—
Leu — Gly — Lys
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
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Learning Check
30
Place the following statements in order of their occurrence in
protein synthesis.
A. mRNA attaches to a ribosome.
B. The ribosome moves along mRNA to add amino acids to
the growing peptide chain.
C. A completed polypeptide is released.
D. A tRNA brings an amino acid to its codon on mRNA.
E. DNA produces mRNA.
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Solution
31
Place the following statements in order of their occurrence in
protein synthesis.
E. DNA produces mRNA.
A. mRNA attaches to a ribosome.
D. A tRNA brings an amino acid to its codon on mRNA.
B. The ribosome moves along mRNA to add amino acids to
the growing peptide chain.
C. A completed polypeptide is released.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
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Chapter 17 Nucleic Acids and
Protein Synthesis
32
17.6 Genetic Mutations
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Mutations
33
A mutation, or change in the nucleotide sequence of
DNA, can
• result from mutagens such as radiation and chemicals
• produce one or more incorrect codons in mRNA
• produce a protein containing one or more incorrect
amino acids
• produce defective proteins and enzymes
• cause genetic diseases
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Types of Mutations
34
•
•
•
•
A substitution or point mutation is the replacement of
one base in the template strand of DNA with another.
If a substitution or point mutation changes the nucleotide, a
different amino acid may be inserted into the polypeptide.
If this produces no change in the amino acid sequence, it is
called a silent mutation.
A frameshift mutation is the insertion of a single
nucleotide into the sequence resulting in a change to all
subsequent codons, leading to a new amino acid sequence.
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Normal DNA and Protein Synthesis
35
The normal DNA
sequence produces a
mRNA that provides
instructions for the
correct series of amino
acids in a protein.
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Mutation: Substitution
36
Substitution
• of a base in DNA
changes a codon in
the mRNA
• of a different codon
leads to the placement
of an incorrect amino
acid in the polypeptide
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Frameshift Mutation
37
In a frameshift 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
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Effect of Mutations
38
When a mutation causes a change in the amino acid sequence
the structure of the resulting protein may be severely altered,
causing loss of its biological activity.
Altered enzymes cannot catalyze reactions, and possible toxins
may accumulate in the body and may be lethal.
When this condition is hereditary, it is called a genetic disease.
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Examples of Genetic Diseases
39
Galactosemia
Cystic fibrosis
Down syndrome
Muscular dystrophy
Huntington’s disease
Sickle-cell anemia
Hemophilia
Tay-Sachs disease
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Learning Check
40
Identify each type of mutation as a substitution or frameshift.
A. Cytosine (C) enters the DNA sequence.
B. One adenosine is removed from the DNA sequence.
C. A base sequence of TGA in DNA changes to TAA.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
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Solution
41
Identify each type of mutation as a substitution or frameshift.
A. Cytosine (C) enters the DNA sequence.
frameshift
B. One adenosine is removed from the DNA sequence.
frameshift
C. A base sequence of TGA in DNA changes to TAA.
substitution
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Chapter 17 Nucleic Acids and
Protein Synthesis
42
17.7
Viruses
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Viruses
43
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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Some Diseases Caused by Viruses
44
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
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Viruses
45
FIGURE 17.17 After a
virus attaches to the
host cell, it injects its
viral DNA and uses the
host cell’s amino acids
to synthesize viral
protein. It uses the host
cell’s nucleic acids,
enzymes, and
ribosomes to make
viral RNA. When the
cell bursts, the new
viruses are released to
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Introduction to
General, Organic, and Biological Chemistry, Eleventh Edition
infect
other
cells.
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Reverse Transcription
46
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
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
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Reverse Transcription
47
FIGURE 17.18 After a
retrovirus injects its
viral RNA into a cell, it
forms a DNA strand by
reverse
transcription. The DNA
forms a doublestranded DNA called a
provirus, which joins
the host cell DNA.
When the cell
replicates, the provirus
produces the viral RNA
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needed
to produce more
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HIV Virus and AIDS
48
The HIV-1 virus
• is a retrovirus that infects
T4 lymphocyte cells
• decreases the T4 level
and the immune system
fails to destroy harmful
organisms
• causes pneumonia and
skin cancer associated
with AIDS
HIV virus
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AIDS Treatment
49
Treatment for AIDS is based on attacking HIV-1 at different points
in its life cycle, such as prevention of reverse transcription of the
viral DNA and protein synthesis.
For example, AZT, similar to thymidine, mimics the structures of
the nucleosides used for DNA synthesis, which inhibit the reverse
transcriptase enzyme.
Lexiva is a protease inhibitor that prevents protein synthesis used
by viruses to make more copies.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
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AIDS Treatment
50
Azidothymine (AZT)
Lexiva
Insert structure AZT
pg 625.
Insert structure Lexiva
pg 625.
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Learning Check
51
Match the following terms with the descriptions below:
virus
retrovirus
protease inhibitor
reverse transcription
A. a virus containing RNA
B. small particles requiring host cells to replicate
C. a substance that prevents the synthesis of viral proteins
D. using viral RNA to synthesize viral DNA
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Solution
52
A. a virus containing RNA
retrovirus
B. small particles requiring host cells to replicate
virus
C. a substance that prevents the synthesis of viral proteins
protease inhibitor
D. using viral RNA to synthesize viral DNA
reverse transcriptase
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Concept Map: Nucleic Acids and Protein
Synthesis
53
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