Replication, Transcription, and Translation
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Before a cell can divide, the DNA in the
nucleus of the cell must be duplicated.
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Since the DNA molecule consists of two
complimentary stands, if those two
strands separate and the right conditions
are present, two new stands that are the
compliments of the originals will be
produced.
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Each new DNA molecule will consist of
one old stand, and a new complimentary
strand.
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The gray strands in the figure to the right
are new strands in the process of being
assembled.
Assembling the New Bases
• The term semiconservative replication means that in the new DNA
molecule there is one old and one new strand.
• This is seen in the figure below.
DNA Replication
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Since the DNA molecule is very large, there must be a way to copy it faster
than just unwinding from one end to the other!
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This happens when the DNA molecule separates at many sites, forming
thousands of replication bubbles. This allows parts of the DNA message to
be replicated simultaneously in many locations.
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DNA polymerase adds new nucleotides , while DNA ligase joints the DNA
segments together.
Why the fuss about DNA replication??
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As you will note when you read the textbook (if you haven’t already!) the
process of DNA replication involves a number of enzymes and proteins, and
it a bit more complicated than seen in the previous slide.
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The important idea is that an exact duplication of the DNA message is
required, so that each new cell in the body has the same set of genetic
instructions as the cells that preceded it.
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This also insures that every new generation of individuals has the same
genetic information as his/her parents.
DNA carries information that can be used to construct the
proteins which form structures and regulate the body’s activities.
• Protein synthesis involves two processes: transcription and
translation.
• In transcription the DNA message is converted into an RNA
molecule.
• In translation the RNA message is used to assemble amino acids
into a protein chain.
times, they are a changing
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For many years biologists referred to the one gene-one enzyme hypothesis.
It was believed that each gene controlled the production of a single protein.
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This was changed to the one gene-one protein hypothesis because many
proteins are structural proteins, not enzymes.
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Since some proteins consist of several polypeptide chains that are linked
together, the hypothesis was changed again. This time one gene-one
polypeptide seemed more accurate.
•
As a result of the Human Genome Project, the one gene-one polypeptide
hypothesis has had to be changed again! We now know that a gene can
produce more than one polypeptide depending upon how the information in
the gene is read. More about this later!
The genetic code
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The genetic code is written in the
sequence of the 4 bases of DNA:
A, T, C, and G.
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Three bases read in sequence
specify one of the 20 amino acids
found in protein molecules.
•
A codon is the 3-base sequence
for an amino acid.
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The message in the DNA is
transcribed into an RNA molecule,
and then translated into a
polypeptide
The Genetic Code II
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There are 64 (4X4X4) possible
triplet codes, but only 20 amino
acids.
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As seen in the table, more than 1
triplet may code for the same
amino acid. This is no problem, as
long as no triplet can code for
more than one amino acid.
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Note that several codons can also
act as start (AUG) or stop (UAA)
signals.
Why do we need RNA too?
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There are three types of RNA produced in the nucleus: mRNA, tRNA, rRNA.
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Messenger RNA (mRNA) is a copy of the DNA that codes for a polypeptide.
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When the two DNA strands of a gene separate, one of the strands is
transcribed into an RNA molecule with the aid of the enzyme RNA
polymerase.
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The RNA strand elongates until it reaches a termination signal (a sequence
of bases in the DNA strand). At this time the RNA molecule is released from
the DNA, allowing the DNA strands to reunite.
•
After production the RNA molecules leave the nucleus and enter the
cytoplasm.
Cleaning up the Message
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When the genetic message is
copied to make mRNA, the
message contains unwanted base
sequences.
The ‘junk’ sequences (called
introns) are removed from the
message and the remaining
sequences (exons) are linked
together to produce a sequence of
codons that will translate into a
polypeptide.
This process occurs before the
message leaves the nucleus.
Where oh where can the amino acids be?
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A second type of RNA is transfer
RNA, whose function is to attach
to a specific amino acid and bring
that amino acids to the site where
polypeptides are being
constructed.
This RNA strand is twisted and
bonded into the shape seen on the
right.
One end of the molecule attached
to a specific amino acid.
The other end has an exposed
sequence of 3-bases. These are
called the anticodon.
How many kinds of tRNA must
there be?
You must know your base pairs!!
• If you said 20 types of tRNA you are wrong!
• There must be a different tRNA molecule for each of the possible
triplets. This means 64 anticodons.
The anticodons of the tRNAs each have a complimentary codon in
the mRNA. For example the codon AUG would be the compliment
of the anticodon UAC.
The role of Ribosomes
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The third type of RNA is risosomal
RNA (rRNA).
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Ribosomes are the ‘decoding’ units
of the cell.
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Each ribosome consists of two
subunits, and is an assemblage of
rRNA and proteins.
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Ribosomes have binding sites for
both tRNA and mRNA molecules.
Reading the Message
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An mRNA molecule attaches to a
ribosome.
As the ribosome moves along the
mRNA, 3-base codons are
exposed one at a time.
A tRNA with an anticodon that is
complimentary to the codon of the
mRNA temporarily bonds with the
mRNA.
The ribosome positions the
molecules so that this bonding
occurs.
As the ribosome continues its
journey along the mRNA additional
tRNAs bring their a.a. to the site of
peptide synthesis.
Elongation of the chain
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As new amino acids are brought to
the ribosome, the growing peptide
chain is attached to the new amino
acid by a peptide bond.
Elongation of the chain continues
until a stop codon is encountered.
At that point the peptide chain is
released from the tRNA.
A single mRNA can be read
repeatedly to make many copies of
a polypeptide.
Once a tRNA gives up its amino
acid it can return to the cytoplasm
and attach to another of its
specified amino acid.
A Summary of the flow of Genetic Information
in a Cell
• Information is stored in the
triplet codes (codons) of DNA
nucleotides.
• This information is transcribed
into 3 types of RNA.
• mRNA carries the information
to assemble a polypeptide.
• In the nucleus, introns are
removed and the remaining
exons spliced together to
make a functional mRNA
strand.
• tRNA molecules attach to
specific amino acids.
• rRNA and proteins form
ribosomes.
• mRNA attaches to a ribosome
and the message is decoded
when the anticodon of a tRNA
is bonded to a mRNA codon.
• Subsequent amino acids are
attached to the growing
peptide chain until a stop
codon is reach and the chain
is terminated.
• A summary of these events
can be seen in the next slide.
Mutation: When the Code is Miscopied
• A mutation occurs when the
code doesn’t copy correctly,
and a protein is formed that
doesn’t function.
• If a base is substituted or
deleted, the triplet(s) are
different and so is the protein
formed.
• Mutations can also involved
inversion or deletion of larger
sections of the message.
• Substances that trigger
mutations are called
mutagens and can be physical
or chemical in nature.