CROSSING OVER AND ITS SIGNIFICANCE

1. UNIVERSITY OF ALLAHABAD
TOPIC - CROSSING OVER AND ITS SIGNIFICANCE
SUBJECT-CYTOGENETICS,MOLECULAR GENETICS AND PLANT BREEDING
SUBMITED TO SUBMITED BY
Dr. GIRJESH KUMAR DEEPAK KUMAR MAURYA
M .SC 1ST SEM. (Ag.Botany)
( Department of botany ) ENROLL. NO. U1875119
DEPARTMENT OF BOTANY

2. CROSSING OVER
AND
ITS SIGNIFICANCE

3. Crossing-over
• Crossing-over takes place during pachytene sub-stage of
prophase I of meiosis,but visible at diplotene sub-stage .
• The term crossing over was coined by Morgan.
• It is another name for recombination or physical exchange
of equal pieces of adjacent non-sister chromatids.
• During the process of crossing-over one of the paired
chromosome arms exchanged physically at one or more
locations.
• The two homologous chromosomes are connected at a
certain point called chiasma. When crossing-over occurs
chromatids break at chiasma and reattached to a different
homologous chromosome .
• The chromatids resulting from the interchange of segments
are known as the cross over recombinants and the
chromatids that remain intact are called noncrossover
parental chromatids.

5. Characteristics of Crossing over
• Occurs at two levels, at gross chromosomal level
and at DNA level.
• Occurs between non-sister chromatids of
homologous chromosomes.
• Exchange is normally reciprocal but sometimes
unequal.
• Frequency of crossing over is closely related to
physical distance between genes located on
chromosomes
• Crossing over is a crucial process that generates
genetic difference within a population .

6. The major steps in meiotic
crossing over
• 1) Synapsis-The pairing phenomenon of homologous
chromosomes in presence of syneptonemal complex(DNA and
Protein).
• 2) Duplication of chromosome-Homologous
chromosome gets duplicated to form chromatid.
• 3) Crossing over –The exchange of chromosomal
segments between two non-sister chromatids
• 4) Terminalisation-Sepration of non homologous
chromosomes after crossing over

7. Mechanism of crossing over
• Crossing over, leading to recombination of linked
genes, is due to interchange of sections of
homologous chromosomes.
• At meiosis, the homologous chromosomes come
together and pair, or synapse, during prophase.
• The pairing is remarkably precise and is evidently
brought about by mutual attraction of the parts of
the chromosomes that are similar or homologous
because they contain allellic genes.

8. The enzymes involved in crossing over
• 1) Recombinase is the major enzyme regulating
recombination event
• 2)Endonuclease
• 3)Ligase enzyme
• Endonuclease is responsible for breakage of 2
non-sister chromatids at corresponding sites.
• This is followed by the exchange of segments and
finally the exchanged segments are joined or the
gap is filled by ligase enzyme.

9. Types of crossing over
1. Somatic crossing over-
Pairing of homologous chromosomes occurs in germinal cells
but some times in somatic cells. Somatic crossing over is
reported in Drosophila by Curt Stern (1935).somatic crossing
over occurs rarely.
2.Meiotic crossing over or germinal crossing over-
• This type of crossing over takes place in germinal cells during
gametogenesis.
• Kinds of Crossing Over
• Single crossing over (only one chiasmata is form)
• Double crossing over (two chiasmata are form)
• Multiple crossing over . (more than two chiasmata are
formed)

10. Factors Affecting the Crossing Over
• 1. Distance between the genes: The frequency of crossing over between
the two genes is positively associated with the distance between their
location in the chromosome. Crossing over between the two genes would
increase with an increase in distance between them.
• 2. sex: The frequency of recombination is markedly influenced by the sex
of heterozygotes for linked genes. In general, the heterogametic sex shows
relatively lower recombination frequencies than the homogametic sex of
the same species. Eg: No crossing over occurs between linked genes in
Drosophila males and females of silkworm.
• 3. Age of female: The frequency of crossing over shows a progressive
decline with the advancing age of Drosophila females.
• 4. Temperature: In Drosophila, the lowest frequency of crossing over is
observed when females are cultured at 220C. The frequency of
recombination tends to increase both at the lower and higher
temperatures than 220C.
• 5. Nutrition: The frequency of crossing over in Drosophila is affected by
the presence of metallic ions Eg: Ca+2 and Mg+2 in its food. Higher the
amount, lower will be the crossing over frequency and vice-versa.

11. • 6. Chemicals: Treatment of Drosophila females with certain
antibiotics like mitomycin D and actinomycin D and certain
alkylating agents such as ethylmethane sulphonate promotes
crossing over.
• 7. Radiations: An increase in frequency of crossing over is observed
when Drosophila females are irradiated with x-rays and γ-rays.
• 8. Plasmagenes: In some species, plasma genes reduce the
frequency of crossing over. Eg: The Tifton male sterile cytoplasm
reduces the frequency of crossing over in bajra.
• 9. Genotype: Many genes are known to affect the occurrence as
well as the rate of crossing over. For example C3G gene of
Drosophila located in chromosome 3 prevents crossing over when
present in homozygous state while it promotes crossing over in the
heterozygous state.
• 10.Chromosomal aberrations: In Drosophila, some chromosomal
aberrations Eg: paracentric inversions, reduce recombination
between the genes located with in the inverted segment.
• 11.Distance from centromere: Centromere tends to suppress
recombination.Therefore genes located in the vicinity of
centromeres show a relatively lower frequency of crossing over
than those located away from them.

12. Significance of crossing over
• Crossing over providesa strong proof in favour of
linear arrangement of genes on the chromosomes.
• Recombinations or new gene combinations are
produced due to crossing over,which change genetic
pool by changing of gene frequency and this provide
a way of evolution(micro evolution)
• Crossing over generates genetic difference within a
population .
• Linkage map and genetic maps are constructed on
the basis of crossing over

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