Viruses, Jumping Genes and Other Unusual Genes

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Viruses, Jumping Genes and Other Unusual Genes

• Chapter 12

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Mobile Genes

• Move around on chromosomes
• Move from one chromosome to another
• Move from cell to cell

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• Transposons jumping genes code for the enzyme
  transposase.
• A simple transposon codes for transposase and
  nothing else
• Can break DNA
• Delete other genes
• Disrupt another gene

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A complex transposon

• Two transposons, close together, may carry
  another piece of DNA between them.
• THEDOGHOWAREYOUSAWTHECAT
• Can attach pieces of one chromosome to another
• Can duplicate sections of DNA
• Overall effect is to increase genetic variation

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Integrated replication

• Because transposons are part of the DNA, they are
  only copied when the DNA is copied they are not
  copied apart from the DNA of the cell.

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Plasmids

• Are small, circular pieces of DNA in bacterial or
  yeast cells that contain 3 to 300 genes.
• Most plasmids exist separate from the chromosome
  of the cell.
• Usually replicated when DNA is copied, but some
  can reproduce at other times autonomous
  replication

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• A cell can have as many as 1,000 copies of a
  plasmid and a cell may have more than one
  plasmid.
• Plasmids can travel from one bacterial cell to
  another when bacteria undergo a sex-like process
  called conjugation.

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Bacterial conjugation connected by a pilus
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• Plasmids contain genes that benefit the host.
• Antibiotic resistance
• Resistance to heavy metal poisoning
• Resistance to the toxins of other bacteria

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Viruses

• A virus contains either DNA or RNA
• Surrounded by a protein coat called a capsid.
• A few viruses make a viral coat out of the
  infected cells plasma membrane and some viral
  proteins membrane envelope
• Viruses are smaller than cells but larger than
  single protiens.
• Two basic forms
• Long helix
• Icosahedron, a 20 sided shape

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• Viral replication must take place inside a cell,
  bacterial or eukaryotic.
• The virus attaches to the cell receptor
  molecules
• The viral nucleic acid enters the cell
• The cell synthesizes proteins specified by the
  viral genes
• The cell replicates the viral DNA or RNA
• Tumor viruses integrate into host DNA
• Some come in and out of host DNA episomes
• In host DNA provirus or prophage
• RNA plus-stranded, minus-stranded, reverse
  transcriptase
• The new viral proteins and nucleic acids are
  assembled into viruses
• The new viruses are released from the cell.

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Viral replication

1. The virus attaches to the cell
2. The viral nucleic acid enters the cell
3. The cell synthesizes proteins specified by the
   viral genes
4. The cell replicates the viral DNA or RNA
5. The new viral proteins and nucleic acids are
   assembled into viruses
6. The new viruses are released from the cell.

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• Lysis -When viruses cause the host cell to break
  open and release virus particles
• Lytic cycle
• Provirus lysogenic cycle

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• Even though viruses are not alive they are
  still subject to natural selection, and they do
  evolve or change.
• Restriction enzymes enzymes made by bacteria to
  cut up viral DNA
• Cuts at target sites particular sequences of
  bases
• E coli protects itself by modifying its DNA
• Viruses eliminated the sites for restriction
  enzymes
• Viruses are efficient and contain no junk DNA

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Where did viruses come from?

• May originally have been transposons
• Conferred a benefit
• Became able to replicate autonomously
• Acquired genes to form a protein coat

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• Mitochondria and chloroplasts contain their own
  DNA
• Up to 1 of DNA in a cell may be mitochondrial
  DNA
• Mitochondrial DNA is circular
• In plants, chloroplast DNA may make up 15 of the
  cells DNA
• Also have complete systems for transcription and
  translation that resemble bacteria
• Support endosymbiotic theory