Molecular Biology of the Gene

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Molecular Biology of the Gene

• Chapter 10

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Molecular Biology

• DNA and how it serves as the molecular basis of
  heredity
• Structure of DNA
• How it replicates
• How DNA controls the cell
• DNA and protein synthesis
• How DNA can change

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Viruses

• Share some characteristics of living organisms
• Genetic material in the form of nucleic acids
• Not considered alive because it is not cellular
  and cannot reproduce on its own
• Basically a nucleic acid wrapped in a protein
  coat
• Host provides most of the tools and raw material
  for viral multiplication

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Viruses

• Fools the cell into taking it into nucleus
• Can be dormant in the cell or replicate
• Uses the cells own molecules and organelles to
  replicate the virus
• Virus production eventually bursts the cell to
  release viruses

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Viruses and DNA Structure

• By the 1940s, scientists knew that eukaryotic
  cells had proteins and DNA
• Thought proteins were the material of genes
• It was not until experiments like the
  Hershey-Chase experiment were performed that
  scientists were convinced otherwise
• Used radioactive isotopes to track which type of
  molecule was transferred to infected cells in
  order to replicate the virus

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Bacteriophages

• Bacterial phages or phages (feed on bacteria)
• Consist solely of DNA and protein

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Hershey-Chase Experiment
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DNA

• Deoxyribonucleic acid
• DNA is a polymer or polynucleotide, made of long
  chains of nucleotides
• Phosphate group, nitrogenous base and a sugar
• Sugar, phosphate backbone

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DNA

• DNA has four kinds of bases, A, T, C, and G
• Nucleotides joined by covalent bonds
• O- is shared between phosphate group of one and
  sugar of another

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RNA

• RNA is also a nucleic acid
• RNA has a slightly different sugar
• Ribose
• RNA has U instead of T
• A, U, G, C

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DNAs Double-Stranded Helix

• James Watson and Francis Crick worked out the
  three-dimensional structure of DNA
• Saw that t he 3D structure was a helix with
  nitrogenous bases stacked one on top of another
• Also decided that it had to be a double helix
  based on the diameter of the helix
• Worked together to deduce a double helix that
  would conform to the chemical properties of DNA

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DNA Structure

• The structure of DNA consists of two
  polynucleotide strands wrapped around each other
  in a double helix

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Structure of DNA

• Hydrogen bonds between bases hold the strands
  together
• Each base pairs with a complementary partner
• A pairs with T
• G pairs with C

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Structure of DNA

• Base-pairing effects the side-by-side
  combinations of nucleotides, but there are no
  restrictions on the sequence of nucleotides up
  and down the strand
• By showing the structure of DNA we can see how
  the genetic information must be encoded in the
  nucleotide sequence
• Also explains some aspects of genetic inheritance

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DNA Replication

• In DNA replication, the strands separate
• Enzymes use each strand as a template to assemble
  the new strands
• Free nucleotides are attached to make two strands

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DNA Replication

• The DNA has to unwrap and untwist in order to
  replicate
• It also must copy both strands at the same time
  in order to prevent the unzipped strands from
  reattaching
• Nucleotides are added at a very rapid rate

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DNA Replication

• DNA replication begins at specific sites
• Origins of replication
• Starts in both directions
• DNA strands open up a bubble as Daughter
  Strands are formed
• Eventually bubbles merge

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DNA Replication

• Each strand of the double helix is oriented in
  the opposite direction
• This is important because the enzymes that add
  the nucleotides only work in one direction
• 5 ---gt 3 end

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DNA Replication

• DNA polymerases work from 5 ?3 and add
  nucleotides to the daughter strands in pieces
• After the polymerases add the nucleotides an
  enzyme called a DNA ligase then ties the strand
  pieces together to form a continuous strand

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How do our genes become our inherited traits?

• An organisms genotype is the heritable
  information contained in the DNA
• The phenotype is the organisms specific traits
• The molecular basis of the phenotype lies in the
  proteins of various functions
• DNA specifies the synthesis of proteins, each
  gene codes for a specific protein

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Protein synthesis from DNA

• The gene does not directly build the protein, it
  is like the blue-print of a plan
• Other molecules do the actual building
• DNA in the nucleus of the cell makes RNA, this is
  called transcription
• The RNA then exits the nucleus and goes into the
  cytoplasm where it is read and made into a
  protein, this is called translation

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Protein synthesis from DNA
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The Language of DNA

• DNA and RNA are large polymers of single monomer
  nucleic acids
• These nucleotides differ only in their
  nitrogenous bases
• The words of the DNA language are triplets of
  nitrogenous bases called codons
• The codons in a gene specify the amino acid
  sequence of a polypeptide
• Series of amino acids make proteins

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The Language of DNA

• Virtually all organisms share the same genetic
  code
• Not all of the codons code for an amino acid,
  some code instructions for translation

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Transcription of RNA

• In transcription, the DNA helix unzips
• RNA nucleotides line up along one strand of the
  DNA following the base-pairing rules
• The single-stranded messenger RNA peels away and
  the DNA strands rejoin

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Messenger RNA

• The kind of RNA that encodes amino acids is
  called messenger RNA
• Noncoding segments called introns are spliced out
• A cap and a tail are added to the ends
• Transported out of the cytoplasm to be translated
  into proteins

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Transfer RNA

• In the cytoplasm, a ribosome attaches to the mRNA
  and translates its message into a polypeptide
• The process is aided by transfer RNAs
• AAs are already there
• tRNAs match AAs to the proper codon
• Structure has specific sites for recognition and
  attachment

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Ribosomes Build Polypeptides

• Coordinate the functions of the mRNA and the tRNA
• Actually make polypeptides
• The assembly line for reading mRNAs and
  assembling proteins from AAs

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Translation

• Caps help bind the mRNA to the ribosome
• Sspecific start codon signals the tRNA to start
  building the polypeptide
• tRNAs bring in the next AAs to empty site,
  strand is attached to new AA, and old tRNA is
  dropped off

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Translation

• The mRNA moves a codon at a time relative to the
  ribosome
• A tRNA pairs with each codon, adding an amino
  acid to the growing polypeptide
• A stop codon stops translation and releases the
  polypeptide and mRNA is released

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Genetic Mutations

• Any change in the nucleotide sequence of DNA is
  called a mutation
• Alternate forms of alleles may be mutations
• May be a change in a large sequence of DNA or in
  a single nucleotide pair
• The change of a single DNA nucleotide causes
  sickle-cell disease

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Types of Mutations
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Virus

• Enter into the cell via fusing with the plasma
  membrane
• Inject their RNA
• RNA either replicates itself
• Or synthesizes the protein that it codes for
• New viral proteins are made
• Proteins surround the newly formed RNA and make a
  new virus that is shipped out of the cell

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The AIDS Virus

• Enters the cell by means of fusing with the
  plasma membrane and injecting its RNA into the
  cytoplasm
• Retrovirus- when the RNA enters the cell it gets
  transcribed into DNA
• Inserts itself into the nuclear genome
• Now can act just like a normal plant cell gene
  and make its own proteins