Crossing OverCrossing OverGenetic RecombinationGenetic Recombination
http://library.thinkquest.org/19037/genome3.html
Jack Cassidy, Jessica Matthews, Keith Murphy, Mike Nickelsburg
Original 2002Modified by M.Elizabethwww.marric.us/HS_4_crossing_over
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Overview• Who, What, When, Where, Why• Basics• Example• Summary
Who Does Crossing-Over Effect?
EukaryotesLimited in Prokaryotes
What is Crossing Over?
The exchange of chromosomal segments between two non- sister
chromatids
Original 2002Modified by M.Elizabethwww.marric.us/HS_4_crossing_over
5Ref: http://gnn.tigr.org/whats_a_genome/Chp3_2.shtml
Crossing Over
When Does it Happen?
During Prophase I of MeiosisRemember the differences between
Mitosis and Meiosis?
Meiosis increases genetic diversity in a speciesMitosis creates genetically identical daughter cells
Where does Crossing Over Occur?
Genetic swapping occurs between paired homologous chromosomes in our sex cells
—The Egg and Sperm
Original 2002Modified by M.Elizabethwww.marric.us/HS_4_crossing_over
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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
Original 2002Modified by M.Elizabethwww.marric.us/HS_4_crossing_over
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Homologous Chromosomes Exchanging DNA by Crossing Over
From: http://www.ultranet.com/~jkimball/BiologyPages/M/Meiosis.html#crossing_over
Why Does Crossing Over Occur?
To provide genetic variation during meiosis
Its Why You and I Don’t Look Alike
Crossing Over ensures a combination of the maternal
and paternal genes we inherited
BOTTOM LINE
The History of Crossing Over
Original 2002Modified by M.Elizabethwww.marric.us/HS_4_crossing_over
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Linkage• Gregor Mendel, 1823-1884
– Patterns of Inheritance• Carl Correns, 1900s
– Botonist – cites Mendel and Darwin: cytoplamic inheritance
• Alfred Sturtevant, 1891-1970. Family horse farm and pedigree study: found traits that travel together = linkage
http://www.oeaw.ac.at/biblio/Archiv/Mendel/mendel.htmlhttp://www.library.villanova.edu/html2/blueprints/feb01index.htmlhttp://zygote.swarthmore.edu/cleave1.html
Crossing Over Basics
Original 2002Modified by M.Elizabethwww.marric.us/HS_4_crossing_over
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Crossing Over Basics• Occurs at One or More Points Along
Adjacent Homologues during Synapsis
• Points contact each other • DNA is Exchangedhttp://waynesword.palomar.edu/images/cross3.jpg
Original 2002Modified by M.Elizabethwww.marric.us/HS_4_crossing_over
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Ref: Access Excellencehttp://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
Original 2002Modified by M.Elizabethwww.marric.us/HS_4_crossing_over
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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
Original 2002Modified by M.Elizabethwww.marric.us/HS_4_crossing_over
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Genes get shuffled when chromosomes exchange pieces
Watch an animation of crossing over with an explanationof how the concept was discovered at
http://www.dnaftb.org/dnaftb/11/concept/index.html
This web site was produced by theDolan DNA Learning Center, Cold Spring Harbor Laboratory
Original 2002Modified by M.Elizabethwww.marric.us/HS_4_crossing_over
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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