Nucleic Acid Structure and DNA Replication

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Nucleic Acid Structure and DNA Replication

• Brooker Chapter 11

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Nucleic Acids

• Nucleic acids transmit and store information
  crucial to heredity
• Two classes
• Deoxyribonucleic acids (DNAs)
• Genetic information stored in DNA
• Has all information for making individual
• Ribonucleic acids (RNAs)
• RNA transmits information from DNA into cellular
  structure
• tRNA (transfer), mRNA (messenger), rRNA
  (ribosomal)
• Some RNAs are catalytic (ribozymes)

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Nucleic Acid Subunit Structure

• The subunit of a Nucleic acid is a Nucleotide
• A nucleotide has three parts
• base
• sugar
• phosphate group

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Nucleotide Structures Bases

• Nucleic acids are made up of nucleotide subunits
• All nucleotides contain nitrogenous bases. There
  are two types
• Pyrimidines (small, single ring)
• Purines (large, double ring)

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NucleotideStructures
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Nucleic Acid Structure

• Nucleotide subunits are linked via the sugar
  phosphate groups.
• Phosphodiester bonds hold phosphate and sugars
  together
• Nitrogenous bases hang off to the side.

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Nucleic Acids
DNA Deoxyribonucleic Acid

• Bases AGCT
• Sugar deoxyribose
• Phosphate

PO4
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RNA Ribonucleic Acid
PO4
Messenger RNA mRNA
Transfer RNA tRNA
Ribosomal RNA rRNA

• Ribonucleic Acid
• Bases AGCU
• Sugar Ribose
• Phosphate

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Hydrogen Bonding Holds DNA Strands Together

• Adenosine - A
• binds Thymine - T
• Guanosine - G
• binds Cytosine -C
• A-T 2 H bonds
• G-C 3 H bonds
• G-C stronger

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• Nitrogenous bases face into the middle
• Charged phosphates are on the outside
• The 2 strands run in opposite directions

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DNA Structure

• Phosphates are attached between the 5' and 3'
  carbons of the sugars
• DNA has 5 and 3 phosphates exposed on each
  strand.
• On the next slide strands are of opposite
  polarity antiparallel strands are hydrogen
  bonded via A pairing with T and C with G.

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DNA versus RNA
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How Does DNA Replicate?

• 1957 Matthew Meselsohn and Fred Stahl
• DNA replication is semiconservative
• One old strand kept with each of the new
  molecules one old paired with one new strand
• The mechanism for DNA replication is basic to
  many Biotechnologies today.

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DNA replication is semi-conservative

• In semiconservative replication, each strand
  would give rise to a new strand

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How Does DNA Replicate?

• New nucleotides are added at the existing 3 end
  ONLY
• The new strand undergoes base pairing with the
  mother strand.

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• Semi-conservative replication
• Parental strand
• Daughter strand

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How Does Replication Start?

• The replication proteins bind at the origin of
  replication, which is identified by a particular
  base sequence

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How Does Replication Start?

• 2. Helicase unwinds the DNA, which is held open
  with helix-destabilizing proteins. Replication
  starts in the replication fork.

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Replication Proceeds on Two Strands

• RNA primers are first laid down in each case
  (yellow) by a primase

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Replication Proceeds on Two Strands

• DNA polymerase adds nucleotides at the 3 end,
  but the new strands elongate in opposite
  directions
• The leading strand elongates into the fork
• The lagging strand elongates away from the fork
• Elongation proceeds smoothly on the leading strand

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Ligating the Strands

• The gaps between new DNA strands is closed in by
  DNA ligase

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Summary of DNA Replication

• 1. The replication proteins bind at the origin of
  replication
• 2. Helicase unwinds the DNA
• 3. RNA primers are laid down
• 4. DNA polymerase adds nucleotides at the 3 end
• 5. The gaps between new DNA strands is closed in
  by DNA ligase

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DNA Packaging Chromosome Structure

• DNA is tightly wrapped up into chromosomes WHY?
• Protection from environment
• There is a lot of genetic information to package
  in a tiny nucleus
• Bacterial DNA would be about 1.1-1.5 mm long,
  even though the cell is maybe only 1-3 microns
  long!
• Eukaryotic DNA would be about 1 meter!

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DNA Levels of Organization

• Nucleotides
• Chromatin is relaxed DNA
• A gene is a specific sequence of nucleotides
• Double helix
• Chromosomes are condensed DNA
• DNA is wrapped around nucleosomes

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Nucleosomes

• DNA is wrapped around basic () charged
  Nucleosomes
• Nucleosomes are made of histone proteins
• DNA linkerstrands hold nucleosomes together
• Nucleosomes are structural, but also aid with
  regulation of DNA replication and expression
• String wrapped around a series of styrofoam balls

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Histone-Depleted DNA

• In this image, the DNA has been denatured by high
  salt treatment

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Eukaryotic Chromosomes

• The genetic material is held on chromosomes.
• During cell division, chromosomes must be
  precisely duplicated.
• Eukaryotes can have one to many chromosomes.
• Usually linear arrangement of nuclear genes
  circular only in mitochondria and chloroplasts
• Recent work from the Human Genome Project
  indicates 30,000 genes (fewer than expected)

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• What is a gene?
• A gene is a nucleotide sequence that carries the
  information needed to produce a specific RNA or
  protein product