The Molecular Structure of DNA

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The Molecular Structure of DNA
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Watson and Crick Starting Information

• Macromolecule
• Polymer - many recurring subunits
• Polynucleotide - subunits are nucleotides
  composed of A, T, C, G, deoxyribose sugar,
  phosphate
• Sugar backbone - 5-3phosphodiester linkage
• Hydrogen bonds - in solution A and T bond and C
  and G bond

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Contribution of Maurice Wilkens and Rosalind
Franklin
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Watson and Crick Model

• Bases inside, sugar-phosphate backbone outside
• Antiparallel chains in opposite 5 - 3
  direction
• A - T and G - C hydrogen bond pairing
• Complimentary base pairing
• 3 ATGGCTATACGA 5
• 5 TACCGATATGCT 3

Hydrogen bonds
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Adenine - Thymine Bonding
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Cytosine - Guanine Bonding
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What is a nucleotide ?

• The three major components of a nucleotide are
• a 5 - carbon deoxyribose sugar
• a phosphate group
• a nitrogen containing base
• - adenine, thymine, cytosine, guanine

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A Simplistic View of the DNA Molecule Note the
sugar-phosphate backbone and the nitrogenous base
hydrogen bonding.
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The Structure of DNA
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The DNA StructureVideo Store
This page cannot be sent to you for use due to
the lack of Quicktime Movie Viewer on the school
computers.The site is at Umass and is a molecular
genetics site. The address is http//rna.micro.um
ass.edu/molgent/
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DNA Replication

• It is the duplication of the DNA strand.
• It starts at nucleotide sequences called origins
• Binding proteins stabilize the split molecule.
• It is semiconservative.
• Occurs in a 5 - 3 direction.
• Continuous on one strand and not the other.
• Okazaki fragments form on the noncontinuous
  strand.

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The Enzymes of Replication

• DNA helicase unwinds the DNA strand
• Primase identifies where the replication begins.
• DNA polymerase facilitates the joining of the
  bases
• DNA ligase seals joints between continuous
  strands

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What can go wrong ?

• deletions
• substitutions
• additions
• all can lead to frame shift mutations

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Chromosome Formation

• Each chromosome is one strand of DNA
• Each cell has about 1 meter of DNA
• Eukaryotic DNA has proteins bound to it called
  histones
• Some histones form spools for the DNA called
  nucleosomes.
• Histones cause the coiling of the DNA in the
  chromosome.
• Non gene areas are supported by scaffolding
  proteins.

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The DNA ReplicationVideo Store
This page cannot be sent to you for use due to
the lack of Quicktime Movie Viewer on the school
computers.The site is at Umass and is a molecular
genetics site. The address is http//rna.micro.um
ass.edu/molgent/
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PROTEIN SYNTHESISThe Process Overview 1View
the Process 2
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TRANSCRIPTION OF RNA

• the information in DNA is transcribed to an
  intermediary molecule called messenger RNA
• One side of the DNA strand acts as a template for
  the formation of the RNA. (5-3)
• Promoter sequences are found where the
  transcription is to begin. RNA polymerase
  attaches to the promoter to initiate
  transcription.
• The mRNA 3 letter codes are called codons.

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• The polymerase facilitates the joining of the
  bases to the DNA template.
• Uracil replaces thymine as a base in RNA.
• It occurs at a rate of 60 bases per second.
• The polymerase reaches a terminator sequence
  that identifies the end of the transcription. The
  RNA is then released.
• More than one polymerase can work on a strand of
  DNA allowing for greater production.
• Transcription Video
• Capping of mRNA

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TRANSLATION

• It is the translating of the RNA sequence into
  protein.
• This involves mRNA, tRNA and rRNA
• tRNA is the interpreter between the nucleic acid
  language and the protein. It is about 80
  nucleotides in length and at one end has a
  triplet code called an ANTICODON. Enzymes bond
  specific amino acids to the anticodons according
  to the genetic code.

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The Role of the Ribosome

• Ribosomes are comprised of proteins and
  rRNA.
• They coordinate the coupling of tRNA and mRNA.
• The P site holds the tRNA that is attached to
  the growing polypeptide chain. The A site holds
  the tRNA carrying the next amino acid to be
  connected to the chain.
• It catalyzes the formation of the peptide bond
  between amino acids.

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Another View of The Code
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The Synthesis Process

• It is made up of three steps initiation,
  elongation and termination.
• Initiation - proteins called initiation factors
  identify where the frame reading is to begin. A
  special initiator tRNA (UAC) carrying the amino
  acid methionine will bond to a small ribosomal
  subunit. A larger subunit will combine placing
  the initiator tRNA in the P site of the ribosome.

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• ELONGATION - This is the lengthening of the
  polypeptide. The mRNA of the A site forms H bonds
  with the anticodon of an incoming tRNA. A peptide
  bond forms between the P site amino acid and the
  A site amino acid. The P site tRNA leaves and the
  A site tRNA moves into the P site. The poly
  peptide begins to grow. Another tRNA will move
  into the A site and eventually be moved to the P
  site

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• TERMINATION - The elongation continues until a
  termination codon is reached. These do not code
  for amino acids. They act as a signal for water
  to add to the end of the polypeptide chain in
  order to bring about termination. The ribosome
  then splits again into two subunits.
• The folding and shaping of the protein occurs
  during the translation process.
• Translation Video

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Protein Destination

• Proteins destined for use in the cytoplasm use
  free floating ribosomes and are then released.
• Proteins destined to be released from the cell
  are formed by ribosmes attached to the ER. A
  signal substance at the beginning of the
  polypeptide tells the ribosome to attach to the
  ER and the protein is released into the ER to be
  released from the cell.

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Introns and Exons

• Introns are non-coding parts of the DNA chain.
• Exons are coding parts of the DNA chain.
• Introns must be removed from the DNA sequence
  following transcription. A large complex
  spliceosome controls the cutting out of the
  introns and the splicing of the exons together to
  form one continuous strand of mRNA. View the
  process.

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What is an Operon?

• A series of genes that function in sequence.
• Lac operon was the first identified.
• Numerous structural genes form one mRNA.
• A promoter containing an operator sequence is
  found at the beginning of the operon. The
  operator is the on/off switch.
• Repressors will attach to the operator to turn
  off the sequence. They must combine with an
  inducer to activate the gene sequence.
• View the process.

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