Replication
(not part of transcription/translation)
• Before a cell can divide, the DNA in
the nucleus of the cell must be
duplicated.
• 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.
• Each new DNA molecule will consist
of one old stand, and a new
complimentary strand.
The gray strands in the
figure 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
• 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!
• 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.
• DNA polymerase adds new nucleotides , while DNA
ligase joins the DNA segments together.
Why the fuss about DNA replication??
• The process of DNA replication involves a number of
enzymes and proteins, and it a bit more complicated than
seen in the previous slide.
• 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.
• This also insures that every new generation of individuals
has the same genetic information as his/her parents.
TRANSCRIPTION & TRANSLATION
(Protein Synthesis)
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.
The genetic code
• The genetic code is written in
the sequence of the 4 bases of
DNA: A, T, C, and G.
• 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.
• The message in the DNA is
transcribed into an RNA
molecule, and then translated
into a polypeptide
The Genetic Code II
• There are 64 (4X4X4)
possible triplet codes, but
only 20 amino acids.
• 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.
• Note that several codons
can also act as start (AUG)
or stop (UAA) signals.
Why do we need RNA too?
• There are three types of RNA produced in the nucleus:
mRNA, tRNA, rRNA.
• Messenger RNA (mRNA) copies the DNA that codes for a
polypeptide. - transcription
• The mRNA continues until it reaches a termination (STOP)
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. (with the code or codons)
TRANSCRIPTION
You must know your base pairs!!
• 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.
TRANSLATION
• A second type of RNA is
transfer RNA (tRNA), whose
function is to attach to a specific
amino acid and bring that amino
acids to the site where
polypeptides are being
constructed.
• 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?
The role of Ribosomes
• The third type of RNA is
ribosomal RNA (rRNA).
• Ribosomes are the
‘decoding’ units of the
cell.
• Each ribosome consists of
two subunits
• Ribosomes have binding
sites for both tRNA and
mRNA molecules.
Building
Site
Foreman
(ribosome)
Reading the Message
• 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.
• As the ribosome continues
its journey along the
mRNA additional tRNAs
bring their a.a. to the site
of peptide synthesis.
TRANSFER RNA
The Anti-Codons
Elongation of the chain
• As new amino acids are
brought to the ribosome,
the growing peptide chain
is attached to the new
amino acid by a peptide
bond.
• The chain continues until a
stop codon is encountered.
• Once a tRNA gives up its
amino acid it can return to
the cytoplasm and attach
to another of its specified
amino acid.
Completed Protein
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.