Download 26.6 Replication of DNA

25.6 Replication of DNA
• DNA replication begins in the nucleus with
partial unwinding of the double helix; this
process involves enzymes known as helicases.
• The unwinding occurs simultaneously in many
specific locations known as origins of
replication. The DNA strands separate,
exposing the bases. These branch points,
called replication forks, provide a “bubble”
into which the replication process can begin.
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• (a) DNA unwinds,
exposing single
strands.
• (b) Single-stranded
DNA is exposed at
numerous
replication forks as
DNA unwinds.
• (c) DNA polymerase
enzymes facilitate
copying of the
single-stranded
DNA.
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• DNA polymerase
catalyzes the reaction
between the 5’
phosphate on an
incoming nucleotide
and the free 3’ –OH
on the growing
polynucleotide.
• The template strand
can only be read in
the 3’ to 5’ direction,
and the new DNA
strand can grow only
in the 5’ to 3’
direction.
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• Only the leading
strand grows
continuously from
5’ to 3’ towards
the fork.
• The lagging strand is
replicated
from 5’ to 3’
in short
segments
called Okazaki
fragments.
• These short
sections are joined
later by DNA ligase.
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Two identical copies of the DNA double helix are
produced during replication. In each new double
helix, one strand is the template and the other is the
newly synthesized strand. We describe the result as
semiconservative replication (one of the two strands
is conserved).
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25.7 Structure and Function of RNA
• Ribosomal RNAs: Outside the nucleus but within
the cytoplasm of a cell are the ribosomes, small
granular organelles where protein synthesis takes
place. Each ribosome is a complex consisting of
about 60% ribosomal RNA (rRNA) and 40%
protein, with a total molecular weight of
approximately 5,000,000 amu.
• The transfer RNAs (tRNA) are smaller RNAs that
deliver amino acids one by one to protein chains
growing at ribosomes. Each tRNA carries only one
amino acid.
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The messenger RNAs (mRNA) carry information
transcribed from DNA. They are formed in the cell
nucleus and transported out to the ribosomes,
where proteins will be synthesized. These
polynucleotides carry the same code for proteins
as does the DNA.
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25.8 Transcription: RNA Synthesis
• Only one of the two DNA strands is transcribed
during RNA synthesis. The DNA strand that is
transcribed is the template strand; its
complement in the original helix is the
informational strand.
• The mRNA molecule is complementary to the
template strand, which makes it an exact RNAduplicate of the DNA informational strand, with
the exception that a U replaces each T in the DNA
strand.
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• The transcription process begins when RNA
polymerase recognizes a control segment in DNA
that precedes the nucleotides to be transcribed.
• The sequence of nucleic acid code that
corresponds to a complete protein is known as a
gene.
• The RNA polymerase moves down the DNA
segment to be transcribed, adding
complementary nucleotides one by one to the
growing RNA strand as it goes.
• Transcription ends when the RNA polymerase
reaches a codon triplet that signals the end of the
sequence to be copied.
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• Some of these bases, however, do not code
for genes. It turns out that genes occupy only
about 10% of the base pairs in DNA
• The code for a gene is contained in one or
more small sections of DNA called an exon.
• The code for a given gene may be interrupted
by a sequence of bases called an intron.
Introns are sections of DNA that do not code
for any part of the protein to be synthesized.
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• The initial mRNA strand contains both exons
and introns, and is known as heterogeneous
nuclear RNA (or hnRNA).
• In the final mRNA molecule released from the
nucleus, the intron sections have been cut out
and the remaining pieces are spliced together
through the action of a structure known as a
spliceosome.
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25.9 The Genetic Code
• Codon: A sequence of three ribonucleotides in the
messenger RNA chain that codes for a specific amino
acid; also a three-nucleotide sequence that is a stop
codon and stops translation.
• Genetic code: The sequence of nucleotides, coded in
triplets (codons) in mRNA, that determines the
sequence of amino acids in protein synthesis.
• Of the 64 possible three-base combinations in RNA,
61 code for specific amino acids and 3 code for chain
termination.
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25.10 Translation: Transfer RNA
and Protein Synthesis
• Overview of protein synthesis:
The codons of mature mRNA
are translated in the ribosomes,
where tRNAs deliver amino
acids to be assembled into
proteins (polypeptides).
• The three stages in protein
synthesis are initiation,
elongation, and termination.
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• Structure of tRNA.
(a) The cloverleaf
shaped tRNA
contains an
anticodon triplet and
a covalently bonded
amino acid at its end.
• (b) A computergenerated model
of the serine tRNA
molecule.
• (c) The 3-D shape
of a tRNA molecule.
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• Initiation: Protein synthesis begins when an mRNA,
the first tRNA, and the small subunit of a ribosome
come together.
• The first codon on the end of mRNA, an AUG, acts as
a “start” signal for the translation machinery and
codes for a methionine carrying tRNA.
• Initiation is completed when the large ribosomal
subunit joins the small one and the methioninebearing tRNA occupies one of the two binding sites
on the united ribosome.
• If it is not needed, the methionine from chain
initiation is removed by post-translational
modification before the new protein goes to work.
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• The three
elongation steps
now repeat:
• The next tRNA
binds to the
ribosome.
• Peptide bond
formation
attaches the new
amino acid to the
chain and the first
tRNA is released.
• Ribosome
position shifts to
free the second
binding site for
new tRNA.
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Termination: A “stop” codon signals the end of
translation. An enzyme called a releasing factor then
catalyzes cleavage of the polypeptide chain from the
last tRNA. The tRNA and mRNA molecules are
released from the ribosome, and the two ribosome
subunits again separate.
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Chapter Summary
• Nucleic acids are polymers of nucleotides. Each
nucleotide contains a sugar, a base, and a phosphate
group. A nucleoside contains a sugar and a base, but
not the phosphate group. Nucleotides are connected
by phosphate diester linkages between the 3’ –OH
group of one and the 5’ phosphate group of the next.
• DNA consists of two polynucleotide strands twisted
together in a double helix. The sugar–phosphate
backbones are on the outside, and the bases are in
the center of the helix. The bases are complementary,
opposite every T is an A, opposite every G is a C. The
base pairs are connected by hydrogen bonds.
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