Genetic recombination occurs through meiosis to generate genetic variation which allows organisms to adapt to environmental changes over time. During meiosis, homologous chromosomes pair up and may undergo crossing over, where segments are exchanged between non-sister chromatids. This leads to new combinations of alleles in the offspring. There are three main theories that describe the mechanisms underlying genetic recombination: breakage and reunion, breakage and copying, and complete copy choice. Topological manipulation of DNA through processes like supercoiling is important for allowing the strands to separate during recombination, and DNA topoisomerases introduce negative supercoils to relax the DNA strands.

1. Mechanism of Genetic
Recombination
Dr. Mausam Ghosh
Genetic Recombination – Part I
BTH-102 Unit 3

2. ?? Why Genetic Recombination ??
• Adaptation along with the environmental changes to
maintain DNA sequence stability in the cells generation
after generation
• Environment changes with time – so why not genetic
sequences?
• For long-term survival genetic variation is necessary
• This occurs through the ability of DNA to undergo
genetic rearrangements or reshuffling resulting in a little
to massive change in the genetic combinations
• Thus, recombination is the process of formation of new
recombinant chromosome by combining the genetic
material from two organisms.
• The new recombinants show changes in phenotypic
characters.

3. ?? How Genetic Recombination Occurs??
• Most of the eukaryotes show a complete sexual life cycle
including meiosis or otherwise reduction division
• Meiosis is an important event that generates new allelic
combinations by recombination.
• possible through chromosomal exchange resulting from
crossing over between the two homologous chromosomes
• More precisely crossing over between two non-sister
chromatid containing identical gene sequences.
• Formation of chiasma between two
non-sister chromatid
• a chiasma (pl. chiasmata) is the point
of contact of two chromatids
• Any “X” like configuration formed in
the biological system is called chiasma
• Synapsis of homologous duplexes BTH-102 Unit 4

4. Mechanism of Recombination
• Basically, there are three theories that explain the
mechanism of recombination viz.,
• breakage and reunion,
• breakage and copying
• complete copy choice

5. Breakage and reunion
• Two homologous duplex of chromosome laying in paired form breaks
between the gene loci a and b, and a+ and b+.
• The broken segments re-join crosswise and yield recombinants
containing a and b+ segment, and a+ and b segment.
• This type of recombination does not require the synthesis of new
DNA.
BTH-102 Unit 4

6. Breakage and copying
• One helix of paired homologous chromosome (ab
and a+ b+) breaks between a and b.
• Segment b is replaced by a newly synthesized
segment copied from b+ and attached to a section.
• Thus the recombinants contain and ab+ and
a+ b+

7. Complete copy choice
• a portion of one parental strand of homologous chromosome acts as
template for the synthesis of a copy of its DNA molecule.
• The process of copying shifts to the other parental strand.
• Thus, the recombinants contain some genetic information of one
parental strand and some of the other strand
• This mode was proposed by Belling in 1931

8. Topological manipulation of DNA
• Topological manipulation of DNA
is a central aspect of all its
functional activities like
recombination, replication, and
transcription as well as of the
organization of higher-order
structure
• All synthetic activities involving
double-stranded DNA require the
strands to separate
• the strands are intertwined thus do
not simply lie side by side
• Their separation therefore requires
the strands to rotate about each
other in space
BTH-102 Unit 3

9. Topological manipulation of DNA
• Supercoiling - DNA actually behaves as a closed structure without free ends,
thus when the double helix get coiled about itself in space it forms
supercoiling
• The supercoiling creates a tension in the double helix that changes its
structure
• Positive supercoil - when the DNA is twisted in space in the same sense as
the strands are wound around one another, causes the double helix to be
more tightly wound, it is called positive supercoil
• Negative supercoil - when the DNA is twisted in space in the opposite sense
from the internal winding of the strands, causes the double helix to be less
tightly wound
• Twisting number (T) is a property of the double helical structure itself,
representing the rotation of one strand about the other. It represents the
total number of turns of the duplex. It is determined by the number of base
pairs per turn.
• Writhing number (W) represents the turning of the axis of the duplex in
space. For a relaxed molecule, W = 0
• Linking number is Writhing number (W) + Twisting number (T)

10. Topological isomers of DNA
and
DNA Topoisomerase
• Topological isomers: DNA molecules of
identical sequence may have different
linking numbers, due to different degrees
of supercoiling. Molecules of DNA that
are the same except for their linking
numbers are called topological isomers
• DNA Topoisomerase: It is an enzyme that
introduce negative supercoil and relaxes
the DNA strand
• Types of DNA topoisomerase:
• DNA Topoisomerase Type I and
• DNA Topoisomerase Type II

11. DNA Topoisomerase Type I
• The enzyme binds to a
region in which duplex
DNA becomes separated
into its single strands
• Then it breaks one strand,
pulls the other strand
through the gap, and
finally seals the gap.

12. DNA Topoisomerase Type II
• Type II DNA topoisomerases
generally relax both negative
and positive supercoils.
• The reaction requires ATP
• One ATP is hydrolysed for
each catalytic event.
• The reaction is mediated by
making a double-stranded
break in one DNA duplex, and
passing another duplex region
through it.

13. Thank you
References:
1. Gardner/Simmons/Snustad. (2006). Principal of Genetics. 8th Edition
2. Klug, W.S.,Cummings. (2003). Concepts of genetics, 7th Edition