Replication, Transcription, and Translation

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Replication, Transcription, and 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 to the right are
  new strands in the process of being assembled.

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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.

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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 joints the DNA segments together.

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Why the fuss about DNA replication??

• As you will note when you read the textbook (if
  you havent 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.
• 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.

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DNA carries information that can be used to
construct the proteins which form structures and
regulate the bodys 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.

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times, they are a changing

• 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.
• This was changed to the one gene-one protein
  hypothesis because many proteins are structural
  proteins, not enzymes.
• 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!

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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

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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.

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Why do we need RNA too?

• There are three types of RNA produced in the
  nucleus mRNA, tRNA, rRNA.
• Messenger RNA (mRNA) is a copy of the DNA that
  codes for a polypeptide.
• 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.
• 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.

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Cleaning up the Message

• 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.

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Where oh where can the amino acids be?

• 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?

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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.

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The role of Ribosomes

• The third type of RNA is risosomal RNA (rRNA).
• Ribosomes are the decoding units of the cell.
• Each ribosome consists of two subunits, and is an
  assemblage of rRNA and proteins.
• Ribosomes have binding sites for both tRNA and
  mRNA molecules.

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Reading the Message

• 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.

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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.
• 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.

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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.

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Mutation When the Code is Miscopied

• A mutation occurs when the code doesnt copy
  correctly, and a protein is formed that doesnt
  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.