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

2

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

8

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

9

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

13

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.html

http://www.library.villanova.edu/html2/blueprints/feb01index.html

http://zygote.swarthmore.edu/cleave1.html

Crossing Over Basics

Original 2002Modified by M.Elizabethwww.marric.us/HS_4_crossing_over

15

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

Original 2002Modified by M.Elizabethwww.marric.us/HS_4_crossing_over

16

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

Original 2002Modified by M.Elizabethwww.marric.us/HS_4_crossing_over

17

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

18

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

19

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

20

Summary

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

From: http://www.ultranet.com/~jkimball/BiologyPages/M/Meiosis.html#crossing_over