ch4 crossing over
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4.0 Crossing over
Prepared by Pratheep SandrasaigaranLecturer at Manipal International University
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By the end of this chapter you should be able to:
• Overview of Mitosis and Meiosis• Crossing over
• Holliday model
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4.1 Overview of Mitosis and Meiosis
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Mitosis
• Two genetically identical diploid (2n) daughter cells are produced.
• Mitosis : 2n 2n
• Distinct phases:• Prophase• Metaphase• Anaphase• Telophase• Cytokinesis*
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2n
2n 2n
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Prophase to metaphase• Prophase:
a. The pairs of chromatin threads coil up and condense to make chromosomes (2 sister chromatids) joined by centromeres. (2)
b. The centrioles move apart. (3)c. A network of mitotic spindle develops
between them. (3)d. As the spindle grows, it pushes the
centrioles to opposite ends of the cell. (3)e. The cell nucleus breaks apart. (4)
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Diagram adopted fromThe Facts On File Illustrated Guide to the Human Body: Cells and Genetics
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Prophase to metaphase
• Metaphase:a. The chromosomes line up in the middle of
the cell on the spindle. (5)
• What happen at (1)?
• Before mitosis begins, a pair of centrioles and DNA duplicates. (1)
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Diagram adopted fromThe Facts On File Illustrated Guide to the Human Body: Cells and Genetics
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Diagram adopted fromThe Facts On File Illustrated Guide to the Human Body: Cells and Genetics
Anaphase to cytokinesis
• Anaphasea. The chromosomes split apart at their
centromeres. (6) b. Each chromatid is now a daughter
chromosome. (6) c. The spindle fibers pulling each set of
chromosomes toward either end of the cell. (7)
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Diagram adopted fromThe Facts On File Illustrated Guide to the Human Body: Cells and Genetics
Anaphase to cytokinesis
• Telophasea. The chromosomes in each half of the cell
uncoil and become chromatin threads again.(8)
b. The spindle disappears and nuclear membranes form around each mass of chromatin.(9)
• Cytokinesisa. The cell splits into two identical daughter
cells. (10)
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Meiosis• Meiosis is the process by which reproductive or sex
cells are produced.
• Egg and sperm cells have only 23 chromosomes—half the normal amount.
• The zygote has the correct number of 46 chromosomes.
• The parent cell shown in example here has only four chromosomes, rather than the actual 46 found in human body cells
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Before meiosis begins• The centriole duplicates itself, so that there are two just
before cell division. (1)
• The chromosomes in the nucleus duplicate themselves, too.
• First meiotic division: This division is split into four phases• Prophase 1• Metaphase 1• Anaphase 1• Telophase1
• The division ends with cytokinesis
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Diagram adopted fromThe Facts On File Illustrated Guide to the Human Body: Cells and Genetics
Prophase I• The chromosomes pair up, with two chromatids
joined at the centromeres. (2)
• Groups of four chromatids are called tetrads. (2)
• The chromatids in each tetrad swap genetic material (crossover). (3-4)
• It ensures that each sex cell has a unique mix of genes. (3-4)
• Meanwhile, the nuclear membrane breaks down and a spindle forms between the two centrioles. (3-4)
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Diagram adopted fromThe Facts On File Illustrated Guide to the Human Body: Cells and Genetics
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Metaphase 1 to cytokinesis
• Metaphase I: The tetrads line up in the middle of the spindle. (6)
• Anaphase I: The spindle fibers retract and pull the chromosomes toward opposite ends of the cell. (7)
• This splits up the tetrads, but the chromatids remain in their original pairs. (7)
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Diagram adopted fromThe Facts On File Illustrated Guide to the Human Body: Cells and Genetics
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Metaphase 1 to cytokinesis
• Telophase I: The chromatids remain joined as they move to opposite ends. (8)
• Nuclear membranes reform around each of the chromosomes and the spindle disappears. (9)
• Cytokinesis: The cytoplasm divides. This is called cytokinesis. (10)
• Two daughter cells result. Each of these cells has the genetic material of one pair of chromatids from each tetrad. (10)
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Second meiotic division
• This second division halves the chromosomes in each cell.
• It is similar to mitosis, except that the chromosomes are not duplicated first.
• Both the cells divide, resulting in four cells with two chromosomes each, half the original number. Diagram adopted from The Facts On File Illustrated
Guide to the Human Body: Cells and Genetics
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Define the following:
•Gene•Allele•Locus•Chromatin•Chromosome•Sister chromatids•Centromere•Centrosome
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Crossing over
• The process of crossing over (recombination) between homologs depends on the breakage and rejoining of the DNA strands.
• This results in the exchange of genetic information between DNA molecules to increase genetic diversity.
• The genetic exchange may happen between any two homologous double stranded DNA molecules• Virus chromosomes • Bacterial chromosomes • Eukaryotic homologs during meiosis
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4.2.1 Crossing overHolliday model
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Holliday model• By Robin Holliday and Harold L. K. Whitehouse in 1964
• How to defines Holliday model?
I. Start with the formation of heteroduplex DNA; the creation of a cross bridge
II. Cross bridge migrates along the two heteroduplex strands (branch migration)
III. Resolution or splicing, of the intermediate structure to yield different types of recombinant molecules.*
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Holliday model
• Two homologous double helices (duplexes) are shown.
• Each pair represents a chromatid and the two pairs represent two non-sister chromatids.
• The helices are aligned so that the bottom strand of the first helix has the same polarity as the top strand of the second helix.
Endonuclease cut
Diagram adopted from: An Introduction to Genetic Analysis. 7th edition
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Holliday model
• Two parallel or two antiparallel strands are cut by endonuclease.
• The free ends become associated with the complementary strands in the homologous double helix.
Diagram adopted from: An Introduction to Genetic Analysis. 7th edition
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Holliday model• Ligation creates hybrid duplexes
called heteroduplex DNA molecules, held together by a cross-bridge structure.
• The position of this cross bridge can then move down the chromosome by a process referred to as branch migration.
• This occurs as a result of a zipper-like action as hydrogen bonds are broken and then re-formed between complementary bases of each duplex.
ligationDiagram adopted from: An Introduction to Genetic Analysis. 7th edition
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Holliday model
• Resolution can occur in one of two ways
Diagram adopted from: An Introduction to Genetic Analysis. 7th edition
Two non-recombinant chromosome Recombinant chromosome
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I. Enzymatic cleavage and the creation of heteroduplex DNA.
• From figure (A), we can see that two homologous double helices are aligned
• They have been rotated so that the bottom strand of the first helix has the same polarity as the top strand of the second helix (5 → 3 in this case) or ′ ′vice versa.
• Then a nuclease (endonuclease) cleaves the two strands that have the same polarity (b).
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I. Enzymatic cleavage and the creation of heteroduplex DNA.
• The free ends leave their original complementary strands and undergo hydrogen bonding with the complementary strands in the homologous double helix (C).
• Ligation produces the structure shown in Figure D.
• This partially heteroduplex double helix is a crucial intermediate in recombination, and has been termed the Holliday structure.
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II. Branch migration.
• The Holliday structure creates a cross bridge, or branch, that can move, or migrate, along the heteroduplex (D and E).
• This phenomenon of branch migration is a distinctive property of the Holliday structure. Diagram adopted from: An Introduction to Genetic
Analysis. 7th edition
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III. Resolution of the Holliday structure.
Two non-recombinant chromosome VS recombinant chromosome
Diagram adopted from: An Introduction to Genetic Analysis. 7th edition
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Two Non-recombinant chromosome
• If the cleavage/ nick made by endonuclease ‘here’ to break the entangled DNA.
• Two non-recombinant chromosome
• Having short hetero-duplex regions.
Two non-recombinant chromosome Diagram adopted from: An Introduction to
Genetic Analysis. 7th edition
Diagram adopted from: Concepts of Genetics, Klug, Cummings, Spencer, Palladino, 2012
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Recombinant chromosome
Diagram adopted from: Concepts of Genetics, Klug, Cummings, Spencer, Palladino, 2012
Isomerization
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Recombinant chromosome
Endonucleasenicking
Diagram adopted from: Concepts of Genetics, Klug, Cummings, Spencer, Palladino, 2012
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Recombinant chromosome
Ligation
Diagram adopted from: An Introduction to Genetic Analysis. 7th edition
Diagram adopted from: Concepts of Genetics, Klug, Cummings, Spencer, Palladino, 2012
• Having short hetero-duplex regions.• The model presented involved single-
stranded breaks but there are other recombination models that involved double-stranded breaks.
• Meselson-Radding model.
https://www.youtube.com/watch?v=zTlgi1NzUhs
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Holliday model can be briefly define in three steps, list them?
I. Start with the formation of heteroduplex DNA; the creation of a cross bridge
II. Cross bridge migrates along the two heteroduplex strands (branch migration)
III. The subsequent resolution, or splicing, of the intermediate structure to yield different types of recombinant molecules.
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Diagram adopted from: An Introduction to Genetic Analysis. 7th edition
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I. Name the resultant of heteroduplex DNA
II. Why such recombinants occur? Briefly explain the mechanism.
III. What are the enzymes involved in this process?