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Chapter 17
Nucleotides, Nucleic Acids, and
Heredity
Genes, Exons, and Introns
Gene: A segment of DNA that carries a base sequence that directs
the synthesis of a particular protein, tRNA, or mRNA.
• There are many genes in one DNA molecule.
• In bacteria, the gene is continuous.
• In higher organisms, the gene is discontinuous.
Exon: A section of DNA that, when transcribed, codes for a
protein or RNA.
Intron: A section of DNA that does not code for anything
functional.
Genes, Exons, and Introns
•
Figure 17.12 The properties of mRNA molecules in
prokaryotes cells during transcription and translation.
Genes, Exons, and Introns
•
Figure 17.12 The properties of mRNA in eukaryotic cells
during transcription and translation
Replication of DNA
The DNA in the chromosomes carries out two functions:
• (1) It reproduces itself. This process is called replication.
• (2) It supplies the information necessary to make all the
RNA and proteins in the body, including enzymes.
Replication begins at a point in the DNA called the origin of
replication or a replication fork.
Replication of DNA
Figure 17.13 General features of the replication of DNA.
The two strands of the DNA double helix are shown
separating at the replication fork.
Replication of DNA
The replication of DNA occurs in number of distinct steps.
1. Opening up of the superstructure of the chromosomes.
One key step is this process is acetylation-deacetylation of
lysine residues on histones. This reaction eliminates some
of the positive charges on histones and weakens the
strength of the DNA-histone interaction.
Replication of DNA
2. Relaxation of Higher-Order Structures of DNA.
Tropoisomerases (also called gyrases) temporarily
introduce either single-or double strand breaks in DNA.
Once the supercoiling is relaxed, the broken strands are
joined together and the tropoisomerase diffuses from the
location of the replication fork.
3. Replication of DNA molecules starts with the unwinding
of the double helix which can occur at either end or in
the middle. Special unwinding proteins called helicases,
attach themselves to one DNA strand and cause the
separation of the double helix.
Replication of DNA
4. Primers/Primases
Primers are short—4 to 15 nucleotides long—RNA
oligonucloetides synthesized from ribonucleoside
triphosphates. They are needed to initiate the primasecatalyzed synthesis of both daughter strands.
5. DNA Polymerase
Once the two strands are separated at the replication fork,
the DNA nucleotides must be lined up. In the absence of
DNA polymerases, this alignment is extremely slow. The
enzyme enables complementary base pairing with high
specificity. While bases are being hydrogen bonded to their
partners, polymerases join the nucleotide backbones.
Replication of DNA
Along the lagging strand 3’—>5”, the enzymes can
synthesize only short fragments, because the only way
they can work is from 5’ to 3’. These resulting short
fragments consist of about 200 nucleotides each, named
Okazaki fragments after their discoverer.
6. Ligation
The Okazaki fragments and any nicks remaining are
eventually joined by DNA ligase.
How Do We Amplify DNA?


To study DNA for basic and applied scientific purposes, we
must have enough of it to work with.
Millions of copies of selected DNA fragments can be made
within a few hours with high precision by a technique called
polymerase chain reaction (PCR).
◦ To use PCR, the sequence of a gene to be copied or at least a
sequenced segment bordering the desired DNA must be
known.
◦ In such a case, two primers that are complementary to the
ends of the gene or to the bordering DNA can be
synthesized. The primers are polynucleotides consisting of
12 to 16 nucleotides. When added to the target DNA
segment, they hybridize with the end of each strand of the
gene.
How Do We Amplify DNA?
•
Figure 17.16 Polymerase chain reaction (PCR).
Oligonucleotides complementary to a given DNA sequence
prime the synthesis of only that sequence.
How Do We Amplify DNA?
•
Figure 17.16
Polymerase chain
reaction (PCR).
Oligonucleotides
complementary to a
given DNA sequence
prime the synthesis of
only that sequence.
How Do We Amplify DNA
A polymerase extends the primers in each direction as
individual nucleotides are assembled and connected on
the template DNA. In this way two copies are created.
The two-step process is repeated (cycle 2) when the
primers are hybridized with new strands and the
primers extended again. At this point, four new copies
have been created. The process is continued, and in 25
cycles, 225 or some 33 million copies can be made.
This process is practical because of the discovery of
heat-resistant polymerases isolated from bacteria that
live in hot thermal vents on the sea floor. A
temperature of 95°C is required to unwind the double
helix to hybridize the primer to the target DNA.
How Is DNA repaired?
•
•
•
•
The viability of cells depends on DNA repair enzymes that
can detect, recognize, and remove mutations from DNA.
Externally, UV radiation or highly reactive oxidizing agents,
such as superoxide, may damage a base.
Errors in copying or internal chemical reactions, such as
deamination of a base,can create damage internally
Deamination of cytosine turns it into uracil, which creates a
mismatch. The former C-G base pair becomes a U-G mispair
that must be removed.
One of the most common base repair prepare means is called
BER, base excision repair (Figure 17.15).
How Is DNA repaired?


The BER pathway contains two parts:
1. A specific DNA glycolase (1) recognizes the damaged base
and catalyzes the hydrolysis of the -glycosidic bond between
the uracil base base and the deoxyribose, then releases the
damaged base completing the excision. The sugar-phosphate
backbone is still intact. At the AP site (apurinic or
apyrimidinic site) created in this way, (2) the backbone is
cleaved by a second enzyme, endonuclease. A third enzyme,
exonuclease (3), liberates the sugar-phosphate unit of the
damaged site.
How Is DNA repaired?

2.
In the synthesis step, the enzyme DNA polymerase (4)
inserts the correct nucleotide, cytidine, and the enzyme DNA
ligase seals (5) the backbone to complete the repair.
DNA Fingerprinting