Crossing over

1. Crossing Over
Genetic Recombination

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Overview
• Who, What, When, Where, Why
• Basics
• Example
• Summary

3. Who Does Crossing-Over
Effect?
Eukaryotes
Limited in Prokaryotes

4. What is Crossing Over?
The exchange of chromosomal
segments between two non-
sister chromatids

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Ref: http://gnn.tigr.org/whats_a_genome/Chp3_2.shtml
Crossing Over

6. When Does it Happen?
During Prophase I of Meiosis
Remember the differences between
Mitosis and Meiosis?
Meiosis increases genetic diversity in a species
Mitosis creates genetically identical daughter cells

7. Where does Crossing Over
Occur?
Genetic swapping occurs
between paired homologous
chromosomes in our sex cells—
The Egg and Sperm

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Egg and Sperm
• Chromosomes pair
• Chromosomes may swap genetic material
• New genetic material has been acquired
from the other homologue
• NOTE: Genes that have a tendency to
remain together during crossing over are
said to be linked—we will talk about this
concept during Crossing Over Basics

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Homologous Chromosomes Exchanging DNA
by Crossing Over
From: http://www.ultranet.com/~jkimball/BiologyPages/M/Meiosis.html#crossing_over

10. Why Does Crossing Over
Occur?
To provide genetic variation
during meiosis

11. Its Why You and I Don’t
Look Alike
Crossing Over ensures a
combination of the maternal and
paternal genes we inherited
BOTTOM LINE

12. The History of Crossing
Over

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Linkage
• Gregor Mendel, 1823-1884
– Patterns of Inheritance
• Carl Correns, 1900s
– Gene Linkage
• Alfred Sturtevant
http://www.oeaw.ac.at/biblio/Archiv/Mendel/mendel.html
http://www.library.villanova.edu/html2/blueprints/feb01index.html
http://zygote.swarthmore.edu/cleave1.html

14. Crossing Over Basics

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Crossing Over Basics
• Occurs at One or More Points Along
Adjacent Homologues during Synapsis
• Points contact each other
• DNA is Exchanged
http://waynesword.palomar.edu/images/cross3.jpg

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Ref: Access Excellence
http://www.accessexcellence.org/AB/GG/crossing.html

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Crossing Over Basics
• Gene Mapping
– Tracking crossing over helps determine where
genes are located on the chromosome
– Genes that are far apart have a GREATER
chance of crossing over
– Genes that are closer have a LESS LIKELY
chance of crossing over
• Genes that stay together are said to be LINKED
• One gene can be identified as a MARKER that can
infer the presence of the other gene
– This can be used in identifying disease predisposition

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Crossing Over Basics
• Gene Mapping
– Tracking crossing over helps determine
where genes are located on the
chromosome
– Genes that are far apart have a
GREATER chance of crossing over
– Genes that are closer have a LESS
LIKELY chance of crossing over

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Genes get shuffled when
chromosomes exchange pieces
Watch an animation of crossing over with an explanation
of how the concept was discovered at
http://www.dnaftb.org/dnaftb/11/concept/index.html
This web site was produced by the
Dolan DNA Learning Center, Cold Spring Harbor Laboratory

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Summary
• Who, What, When, Where, Why
• History
• Basics
• Example
• Summary
From: http://www.ultranet.com/~jkimball/BiologyPages/M/Meiosis.html#crossing_over