128 ❯ STEP 4. Review the Knowledge You Need to Score High
Generalized Transduction Imagine that a phage virus infects and takes over a bacterial
cell that contains a functional gene for resistance to penicillin. Occasionally during the
creation of new phage viruses, pieces of host DNA instead of viral DNA are accidentally
put into a phage. When the cell lyses, expelling the newly formed viral particles, the phage
containing the host DNA may latch onto another cell, injecting the host DNA from one
cell into another bacterial cell. If the phage attaches to a cell that contains a nonfunctional
gene for resistance to penicillin, the effects of this transduction process can be observed.
After injecting the host DNA containing the functional penicillin resistance gene, crossover
could occur between the comparable gene regions, switching the nonfunctional gene with
the functional gene. This would create a new cell that is resistant to penicillin.Specialized Transduction This type of transduction involves a virus that is in the
lysogenic cycle, resting quietly along with the other DNA of the host cell. Occasionally
when a lysogenic virus switches cycles and becomes lytic, it may bring with it a piece of the
host DNA as it pulls out of the host chromosome. Imagine that the host DNA it brought
with it contains a functional gene for resistance to penicillin. This virus, now in the lytic
cycle, will produce numerous copies of new viral offspring that contain this resistance gene
from the host cell. If the new phage offspring attaches to a cell that is not penicillin resistant
and injects its DNA and crossover occurs, specialized transduction will have occurred.Conjugation
This is the raciest of the genetic recombinations that we will cover... the bacterial version
of sex. It is the transfer of DNA between two bacterial cells connected by appendages called
sex pili.Movement of DNA between two cells occurs across a cytoplasmic connection
between the two cells and requires the presence of an F plasmid,which contains the genes
necessary for the production of a sex pilus.Genetic Engineering
DNA technology is advancing at a rapid rate, and you need to have a basic understanding
of the most common laboratory techniques for the AP Biology exam.
Restriction enzymesare enzymes that cut DNA at specific nucleotide sequences.
When added to a solution containing DNA, the enzymes cut the DNA wherever the
enzyme’s particular sequence appears. This creates DNA fragments with single-stranded
ends called “sticky-ends,” which find and reconnect with other DNA fragments contain-
ing the same ends (with the assistance of DNA ligase). Sticky ends allow DNA pieces from
different sources to be connected, creating recombinant DNA.Another concept important
to genetic engineering is the vector,which moves DNA from one source to another.
Plasmids can be removed from bacterial cells and used as vectors by cutting the DNA of
interest and the DNA of the plasmid with the same restriction enzyme to create DNA withFigure 11.8 A phage.