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Replication, transcription,
translation and expression of
nucleic acid
Central dogma of molecular biology
Describe the flow of genetic information from DNA through RNA
and eventually to protein
replication
transcription
DNA
RNA
translation
PROTEIN
Solid arrow indicate types of information transfers that occur in cells. DNA
directs its own replication to produce new DNA molecule; DNA is transcribes
into RNA; RNA is translated into protein. The dashed lines represent
information transfers that occur in certain organisms.
Information Flow
DNA
RNA
Protein
Replication: DNA duplicates itself
Transcription: RNA is made on a DNA
template
Translation: Protein is synthesized
from AAs and the three RNAs.
Reverse Transcription: RNA directs
synthesis of DNA
RNA replication: RNA replicates itself
DNA replication
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DNA replication is an anabolic polymerization
process, that allows a cell to pass copies of its
genome to its descendants.
Must occur before every cell division
After two strands of DNA separate, each serves as
template for the synthesis of a complementary strand.
Biologists say that DNA replication is
semiconservative replication because each daughter
DNA molecule is composed of one original strand
and one new strand.
PRINCIPAL OF DNA REPLICATION
DNA REPLICATION PROCESS
c) Synthesis of lagging strand
Initial Processes in DNA Replication
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DNA replication begins at a specific sequence of
nucleotides called an origin.
First, a cell removes chromosomal proteins, exposing
the DNA helix.
Next, an enzyme called DNA helicase locally
"unzips/unwind" the DNA molecule by breaking the
hydrogen bonds between complementary nucleotide
bases, which exposes the bases in a replication fork.
Other protein molecules stabilize the single strands so
that they do not rejoin while replication proceeds
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After helicase untwists and separates the strands, a molecule of an
enzyme called DNA polymerase III binds to each strand.
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DNA polymerases III replicate DNA in only one direction - 5' to
3' - like a jeweler stringing pearls to make a necklace, adding
them one at a time, always moving from one end of the string to
the other.
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Because the two original (template) strands are antiparallel cells
synthesize new strands in two different ways:
1) One new strand, called the leading strand, is synthesized
continuously as a single long chain of nucleotides.
2)The other new strand, called the lagging strand, is synthesized
in short segments that are later joined.
Synthesis of the Leading Strand
A cell synthesizes a leading strand toward the replication fork
in the following series of five steps
1) An enzyme called primase synthesizes a short RNA molecule
that is complementary to the template DNA strand. This RNA
primer provides the 3' hydroxyl group required by DNA
polymerase.
2) Triphosphate deoxyribonucleotides form hydrogen bonds with
their complements in the parental strand. Adenine nucleotides
bind to thymine nucleotides, and guanine nucleotides bind to
cytosine nucleotides.
3) Using the energy in the high-energy bonds of the triphosphate
deoxyribonucleotides, DNA polymerase III covalently joins
them one at a time by dehydration synthesis to the leading
strand.
4) DNA polymerase III also performs a proofreading function. About
1 out of every 100,000 nucleotides is mismatched with its
template; for instance, a guanine might become incorrectly paired
with a thymine.
DNA polymerase III recognizes most such errors and removes the
incorrect nucleotides before proceeding with synthesis. This role,
known as the proofreading exonuclease function, acts like the
delete key on a keyboard, removing the most recent error.
Because of this proofreading exonuclease function, only about
one error remains for every ten billion (1010) base pairs replicated.
5) Another DNA polymerase - DNA polymerase I - replaces the RNA
primer with DNA. Note that researchers named DNA polymerase
enzymes in the order of their discovery, not the order of their
actions.
Synthesis of the Lagging Strand
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The steps in the synthesis of a lagging strand are as
follows :
As with the leading strand, primase synthesizes RNA
primers.
Nucleotides pair up with their complements in the
template-adenine with thyamine, and cytosine with
guanine.
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DNA polymerase III joins neighboring nucleotides and
proofreads. In contrast to synthesis of the leading strand,
however, the lagging strand is synthesized in discontinuous
segments called Okazaki fragments. Each Okazaki fragment
requires a new RNA primer and consists of 1000 to 2000
nucleotides.
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DNA polymerase I replaces the RNA primers of Okazaki
fragments with DNA and further proofreads the daughter
strand.
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DNA ligase seals the gaps between adjacent Okazaki
fragments to form a continuous DNA strand.
Transcription
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TRANSCRIPTION is the synthesis of RNA
under the direction of DNA
DNA strand provide a template for assembling
a sequence of RNA nucleotides
The resulting RNA molecule is the transcript
of the gene’s protein-building instruction
Called mRNA (messenger RNA) – carry
genetic message from DNA
TRANSCRIPTION
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Cells transcribe four main types of RNA from DNA :
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RNA primer molecules for DNA polymerase to use during
DNA replication
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messenger RNA (mRNA) molecules, which carry genetic
information from chromosomes to ribosomes
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ribosomal RNA (rRNA) molecules, which combine with
ribosomal polypeptides to form ribosomes-the organelles that
synthesize polypeptides
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transfer RNA (tRNA) molecules, which deliver amino acids to
the ribosomes
The stages of transcription
1) Initiation
2) Chain elongation
3) termination
Initiation of Transcription
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RNA polymerases - the enzymes that synthesize RNA
RNA polymerase bind to specific nucleotide sequences
called promoter - include the transcription startpoint (the
nucleotides where RNA synthesis begin)
Initiation of Transcription
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Once it bind to the promoter sequence, RNA
polymerase unwinds and unzips the DNA molecule in
the promoter region
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After unzip, RNA polymerase initiate RNA synthesis
at the promoter on the template strand
Elongation of the RNA Transcript
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As RNA polymerase moves along the DNA, it continues to
untwist the double helix for pairing with RNA nucleotides
The enzyme add nucleotides to the 3’ end of the growing RNA
molecule as it continues along the double helix
Elongation of the RNA Transcript
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In the wake of transcription, the DNA strands
re-form the double helix and the new RNA
molecule peels away from its DNA template
Termination of Transcription
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Transcription proceeds until shortly after the RNA
polymerase transcribes a DNA sequence called a
terminator
Terminator = sequence of nucleotides along the
DNA, that signal the end of transcription unit
After the RNA is released, the polymerase
dissociate from the DNA
TRANSLATION
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Translation is the process whereby
ribosomes use the genetic information of
nucleotide sequences to synthesize
polypeptides composed of specific amino
acid sequences.
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In translation process, cell interprets a
genetic message and builds a protein
Message = is a series of codons along an
mRNA molecule
Interpreter = transfer RNA (tRNA)
tRNA = transfer amino acids from
cytoplasm’s amino acid pool to ribosome
The ribosome adds each amino acid
brought to it by tRNA to the growing end of
a polypeptide chain
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As a tRNA molecule arrives at a ribosome,
it bears a specific amino acid at one end.
At the other end is a nucleotide triplet
called an anticodon, which binds according
to base-pairing rules to a complementary
codon on mRNA.
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How do ribosomes interpret the nucleotide
sequence of mRNA to determine the
correct order in which to assemble amino
acids?
The genetic code
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Is a coding dictionary that specifies a
meaning for a base sequence
the genetic code define as triplets of mRNA
nucleotides called codons that code for
specific amino acids.
64 possible arrangements - more than
enough to specify 21 amino acids.
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61 codons specify amino acids and 3 codons
-UAA, UAG, and UGA-to stop translating
UGA codes for the 21st amino acid,
selenocysteine.
Codon AUG also has a dual function, acting as
both a start signal and coding for an amino acid
– methionine.
AUG = start codon
Mutations of Genes:
Types of mutation
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Mutations range from large changes in an
organism's genome, such as the loss or gain of an
entire chromosome, to the most common type of
mutation - point mutations - in which just one
nucleotide base pair is affected.
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Mutations include base pair insertions,
deletions, and substitutions.
Effects of Mutations
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Some base-pair substitutions
produce silent mutations
because the substitution does
not change the amino acid
sequence because of the
redundancy of the genetic
code.
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For example, when the DNA
triplet AAA " is changed to
AAG, the mRNA codon will
be changed from UUU to
UUC; however, because both
codons specify the amino acid
phenylalanine, there is no
change in the phenotype - the
mutation is silent because it
affects the genotype only.
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Of greater concern are substitutions that
change a codon for one amino acid into a
codon for a different amino acid.
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A change in a nucleotide sequence
resulting in a codon that specifies a
different amino acid is called a missense
mutation; what gets transcribed and
translated makes sense, but not the right
sense.
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The effect of missense mutations depends
on where in the protein the different
amino acid occurs.
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When the different amino is in a critical
region of a protein, the protein becomes
nonfunctional; however, when the
different amino acid is in a less important
region, the mutation has no adverse effect.
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A third type of mutation
occurs when a base-pair
substitution changes an
amino acid codon into a
stop codon.
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This is called a nonsense
mutation. Nearly all
nonsense mutations result
in nonfunctional proteins.
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Frameshift mutations
(that is, insertions or
deletions) typically
result in drastic
missense and nonsense
mutations, except when
the insertion or deletion
is very close to the end
of a gene