Biology Notes Chapter 9 Genetic Recombination Pages 181-
Why It Matters
Mutations in genetics are what causes the variation seen in
life, it is important and makes organisms "sexual" (inserting for
variation) rather than "asexual" (clones)
GeneticRecombination:Literally cutting and pasting DNA
backbones into new combinations. Allows "jumping" genes to
move, insert some viruses into the chromosome of their hosts,
underlies the spread of antibiotic resistance among bacteria
and archaea and is at the heart of meiosis in eukaryotic
9.1 Mechanism of Genetic Recombination
There are two requirements for genetic recombination, 1. two
homologous chromosomes or regions, 2. a mechanism for
bringing the DNA molecules close together and the enzymes
that "cut", "exchange", and "paste" the DNA back together.
Homologous:two things that appear the same but aren't when
looked at microscopically, the two chromosome regions are
homologous because they are vary similar but different for the
sake of variation.
Once the homologous regions of DNA are paired, enzymes
break the covalent bond in each of the four sugar-phosphate
backbones, the free ends of each backbone are then
exchanged and reattached to those of the other DNA
molecules. Cutting and pasting 4 DNA backbones results in 1
Bases of DNA molecules are held together by relatively weak
hydrogen bonds. 9.2 Genetic Recombination in Bacteria
Genetic Recombination was for a long time only associated
with meiosis in sexually reproducing eukaryotes. However
genetic recombination can be found in bacteria and other non
sexual organisms. It can bring two bacteria together and the
two DNA molecules can recombine and make different
9.2A Genetic Recombination Occurs in E. coli
In 1946, at Yale University, Lederberg and Tatum set out to
determine if genetic recombination occurs in bacteria to see if
they had a kind of sexuality in their reproduction process.
They used E. coli.
Bacteria are grown in a minimalmediumcontaining water, an
organic carbon source such as glucose, and a selection of
inorganic salts, including one that provides nitrogen. This can
be a liquid or made into a gel by adding agar.
Clones: in this sense scientists clone bacteria to form a culture
in the agar gel, this collection is called a colony.
Lederberg and Tatum mixed together to types of bacteria, one
that could grow with biotin and methionine in the medium, the
other could only grow with leucine, threonine, and thiamine.
They were identified as bio+met+leu-thr-thi- and bio-met-
leu+thr+thi+ as it follows genetic notation. They were placed in
a medium and colonies grew, therefore there must have been
a form of DNA recombination to allow for all 5 genes to be
9.2B Bacterial Conjugation Brings DNA of Two Cells into
Close Proximity DNA recombination occurs between bacteria cells by a matter
of forming a long tubular structure called the sexpilus, forming
a cytoplasmic bridge, and continuing to conjugation.
Conjugation is where a copy part of the DNA of one cell
moves to the other through the sex pilus. DNA recombination
Conjugation is initiated by a bacterial cell that contains a small
circle of DNA, called a plasmid, in addition to the main circular
chromosomal DNA. The plasmid that initiates fertility, is known
as the F Factor. The F Factor carries several genes as well as
a replication origin that permits a copy to be passed on to
each daughter cell during the usual process of bacterial cell
division. VerticalInheritanceis from one generation to the next.
However, during conjugation, the F factor also has the ability
to be copied and passed directly from the donor cell to the
recipient cell, this is HorizontalInheritance. Donor cells are
called F+ cells because they contain the F factor and are able
to mate with recipients but not other donor cellos. Recipients
are called F- cells due to the lack of the F factor. The F factor
carries about 20 genes, several for the sex pills (F pilus).
During conjugation F plasmid replicates using a special DNA
replication called rollingcircle.
There is a site called the origin of transfer on the F plasmid,
then there is a break in just one strand of the double helix of
this site. It is then pulled and the remaining strand, still in a
circle,e "rolls" like a spool of tape. DNA synthesis fills in the
complementary bases to ensure that the F facto is double
stranded in both donor and the recipient cells. Once the F
factor has been transferred and replicated, it circularizes again. /although the recipient becomes F+, no chromosomal
DNA is transferred between cells in this process, that is, no
genetic recombination occurs between the DNA of two
different cells in such a mating.
When transferring the F factor into another bacteria, it
becomes part of the whole chromosome, and these are
known as Hfr cells (High frequency recombination). Hfr cells
can promote genetic recombination between two different
bacteria cells. This is as the section of the chromosome that
includes the F factor integrated, where the origin of transfer is
right in the middle of the F factor. So that when the bacteria
makes its sex pili with the F- cell, the F factor (half because it
starts in the middle) enters the cytoplasmic bridge and
recombines with the host cell. The chromosome is fragile and
generally breaks, leaving only several genes recombined,
coining this cell a PartialDiploidas it contains two of some
genes. The extra genes are lost, and the cell is then able to
reproduce normally, producing new combinations of alleles.
There are two scientists responsible for genetic mapping
discovery Jacob and Wollman, from Paris. Thye mated Hfr
and F cells that differed in a number of alleles. At regular
intervals after conjugation commenced, they removed some of
the cells and broke them apart and separated and analyzed.
They found that the longer they allowed cells to conjugate, the
greater number of donor genes that entered the recipient.
They were able to map relative positions of genes in the E.
9.2C Transformation and Transduction Provide Additional Sources of DNA for Recombination
The discovery of conjugation and genetic recombination in E.
coli showed that genetic recombination is not restricted to
Also, DNA can transfer by two other mechanisms that direct
one way and create partial diploids in which recombination
can occur between alleles in the homologous DNA regions.
This DNA used in the mechanisms are obtained from dead
Transformation: is the process by which bacteria pick up pieces
of DNA that are released into the environment as other cells
-Fred Griffith, in 1928, observed in his experiment with the
virus pneumonia and mice, he used two strains of
Streptococcus pneumoniae, one that was virulent and had a
polysaccharide capsule, and the other nonvirulent, one did
not. He saw that a mixture of previously heated and dead
virulent strains and living nonvirulent strains still caused a
dangerous virus. Clearly the dead cells released DNA that
was transformed to other hosts. This is a demonstration of
transformation for DNA recombination where the introduction
of the normal allele for capsule formation is combined to the
DNA structure and expresses the formation, making the cell
In 1944 Oswald Avery found out that the substance that
affected Griffiths experiments was DNA.
Transduction:DNA is transferred from donor to recipient cells
inside the head of an infecting bacterial virus, called a
Transduction begins when new phages are assembled within an infected bacterial cell. They sometimes incorporate
fragments of the host cell DNA along with, or instead of, the
viral DNA. After the host bursts, the new phages infect other
hosts with either bacterial DNA and/or viral DNA. This new
DNA makes the cell partial diploid also and allows for DNA
recombination to take place.
GeneralizedTransduction: all donor genes are equally likely to be
Some virulentbacteriophagesundergo generalized transduction
as they kill their host cells during each cycle of infection. The
phage develops around the virulent DNA, and the bacterial
chromosome is degraded and used for synthesis of new
phage chromosomes. However, sometimes a fragment of the
host DNA is packaged within a new phage by mistake. This is
called a transducingphageand delivers DNA linearly. This host
cell will survive, incoming DNA may then pair, and recombine
with the host chromosome.
SpecializedTransduction: Lambda is a temperatebacteriophage.
That is, when a lambda first infects a new host cell, it first
determines whether it is damaged/healthy/good enough. The
lambda chromosome then lines up with a small region of
homology on the bacterial chromosome, and a phage-coated
enzyme catalyzes a single recombination event (four DNA
backbones). This phage is integrated into the host can called
a prophage. (Similar to the F factor) The prophage is replicated
and passed to daughter cells as long as conditions remain
favourable. However, if there is damage the prophage
activates several genes, releases itself from the
chromeosome by a recombination event, and proceeds to
manufacture new phages. They are released when the host bursts as a result of lyticgrowth.
A mistake can occur in specialized transduction when the
prophage is excised or partially excised from the host. As a
result, this bacterial DNA is packaged into new phages and
carried to recipient cells. Since the transducing phage is
defective, having left some of its genes behind in the host, it
does not kill the new host. Generally only bacterial genes that
lay close to the integration site of the phage will undergo a
9.3 Genetic Recombination in Eukaryotes: Meiosis