Recombination in prokaryotic

1. 1
ASSIGNMENT PRESENTATION ON
Recombination mechanism in Prokaryotes and Eukaryotes
GP 508 : CELL BIOLOGY AND MOLECULAR GENETICS

2. What is Recombination ?
• The formation of new combination of genes,
either naturally OR by crossing over OR
independent assortment OR in the laboratory
by direct manipulation of genetic material.

3. • Genetic Recombination - Transfer of DNA from
one organism (donor) to another recipient. The
transferred donor DNA may then be integrated
into the recipient's nucleoid by various
mechanisms.
• Homologous Recombination - Homologous DNA
sequences having nearly the same nucleotide
sequences are exchanged by means of Rec A
proteins. This involves breakage and reunion of
paired DNA segments.

4. Mechanism of recombination in
Prokaryotes
A. Transformation
B. Transduction
C. Conjugation

5. A.Transformation
Transformation first discovered by Griffith in
1928 in S.pneumonia.
• Genetic recombination in which a DNA
fragment from a dead, degraded bacterium
enters a competent recipient bacterium and it is
exchanged for a piece of the recipient's DNA.

6. Griffith Experiment

7. 1. A donor bacterium dies and is degraded
2. A fragment of DNA from the dead donor
bacterium binds to DNA binding proteins on the
cell wall of a competent, living recipient
bacterium
3. The Rec A protein promotes genetic
exchange between a fragment of the
donor's DNA and the recipient's DNA
4. Exchange is complete
The 4 steps in Transformation

8. TRANSDUCTION
8
Transfer of DNA from one bacterial strain into
another by a virus and the subsequent
recombination between the chromosomes of
recipient cells and the introduced DNA is known as
transduction.
N. Zinder and J. Lederberg in 1952.
Transduction may be either (1) Generalised
(2) Specialized.

9. What are Bacteriophages?
Bacteriophage (phage) are obligate
intracellular parasites that multiply inside
bacteria by making use of some or all of the
host biosynthetic machinery (i.e., viruses
that infect bacteria.

10. • Genetic recombination in which a DNA
fragment is transferred from one bacterium to
another by a bacteriophage.
Structure of T4 bacteriophage Contraction of the tail sheath of T4

11. • There are two types of transduction:
– Generalized Transduction: A DNA fragment is
transferred from one bacterium to another by a lytic
bacteriophage that is now carrying donor bacterial DNA due to
an error in maturation during the lytic life cycle.
– Specialized Transduction: A DNA fragment is transferred
from one bacterium to another by a temperate bacteriophage
that is now carrying donor bacterial DNA due to an error in
spontaneous induction during the lysogenic life cycle.

12. A.Seven steps in Generalised Transduction
1. A lytic bacteriophage adsorbs to a susceptible
bacterium.
2. The bacteriophage genome enters the bacterium.
The genome directs the bacterium's metabolic
machinery to manufacture bacteriophage
components and enzymes

13. 4. The bacteriophages are released.
5. The bacteriophage carrying the donor bacterium's
DNA adsorbs to a recipient bacterium
3. Occasionally, a bacteriophage head or capsid
assembles around a fragment of donor bacterium's
nucleoid or around a plasmid instead of a phage
genome by mistake.

14. 6. The bacteriophage inserts the donor
bacterium's DNA it is carrying into the
recipient bacterium.
7. The donor bacterium's DNA is exchanged for
some of the recipient's DNA.

15. B. Six steps in Specialised Transduction
1. A temperate bacteriophage adsorbs to a
susceptible bacterium and injects its
genome .
2. The bacteriophage inserts its genome into
the bacterium's nucleoid to become a
prophage.

16. 3. Occasionally during spontaneous induction, a
small piece of the donor bacterium's DNA is
picked up as part of the phage's genome in place
of some of the phage DNA which remains in the
bacterium's nucleoid.
4. As the bacteriophage replicates, the segment
of bacterial DNA replicates as part of the
phage's genome. Every phage now carries that
segment of bacterial DNA.

17. 5. The bacteriophage adsorbs to a recipient
bacterium and injects its genome.
6. The bacteriophage genome carrying the
donor bacterial DNA inserts into the
recipient bacterium's nucleoid.

18. CONJUGATION
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• Conjugation in bacteria was discovered by J.
Lederberg and E. Tatum in 1946; they were
awarded the Nobel Prize in 1956.
• During Conjugation, DNA is transferred from the
donor cell (sometimes called Male) to the
recipient cell (also called Female or F-) through
a conjugation bridge formed between them.
• Since F factor can integrate into the bacterial
chromosome as well as remain independent of it,
it is called Episome.

19. 19

20. Mechanism of recombination in Eukaryotes
• It is mainly through homologous recombination.
• There are two models for homologous recombination
1. The Holliday Model
2. The Double-Stranded Break Repair Model
(More accurately describes recombination
processes)

21. Holliday model for homologous recombination
• It was given by Robin Holliday in 1964.
• The Holliday model is recombination between
two homologous double stranded molecules.

23. Double-strand break and repair model

24. CASE STUDY

25. Fig.: The formation and resolution of Holliday junctions. (a) Recombination
between two duplex DNA molecules is initiated by a region of single-stranded
DNA. (b) Homologous pairing and the reciprocal exchange of strands leads to the
formation of a Holliday junction (N, E, W and S indicate strand orientation). (c)
Nucleolytic cleavage of the symmetrical junction in one of the two possible
orientations leads to recombinant molecules of either the patch (ci) or splice (cii)
type.

26. CASE STUDY
26
Annu. Rev. Biochem. 2008. 77:229–57
First published online as a Review in Advance on February 14, 2008

27. Figure 1
Pathways of DNA double-strand break repair by
homologous recombination. Double-strand breaks
(DSBs) can be repaired by distinctive homologous
recombination (HR) pathways, such as synthesis-
dependent strand annealing (SDSA) and double-strand
break repair (DSBR).
(a) After DSB formation, the DNA ends are resected
to yield 3 single-strand DNA (ssDNA) overhangs,
which become the substrate for the HR protein
machinery to execute strand invasion of a partner
chromosome. After a successful homology search,
strand invasion occurs to form a nascent D-loop
structure. DNA synthesis then ensues.
(b) In the SDSA pathway, the D loop is unwound and
the freed ssDNA strand anneals with the
complementary ssDNA strand that is associated
with the other DSB end. The reaction is completed
by gap-filling DNA synthesis and ligation. Only
noncrossover products are formed.
(c) Alternatively, the second DSB end can be
captured to form an intermediate that harbors two
Holliday junctions (HJ)s, accompanied by gap-
filling DNA synthesis and ligation. The resolution
of HJs by a specialized endonuclease can result in
either noncrossover (black triangles) or crossover
products (gray triangles).

28. REFERENCES:
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 B. D. Singh (2009), Genetics, Second edition, Kalyani
publishers, New Delhi, India.
 Filippo J. S., Sung P. and Klein H. (2008). Mechanism of
Eukaryotic Homologous Recombination, Annual Review of
Biochemistry. 77:229-257
 https://www.biologydiscussion.com/genetics/recombination/reco
mbination-defination-mechanism-and-types-microbiology/65220
 Pranav Kumar and Usha Mina (2017), Life science –
fundamentals and Practice – II, Sixth edition, New Delhi, India.
 Robert G. L. and Gary G. S. (1992). Genetic analysis of
homologous recombination in prokaryotes . Current opinion in
Genetics and developement. 2:683-690.

29. THANK YOU…
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