Integrity and accuracy of genetic information

6.5 Recombination at the DNA Level 3 VNU-University of Science - DNThai New combinations of alleles are created by two types of events in meiosis:  Independent assortment – each pair

Chapter 6: Part 2

CHAPTER OUTLINE:

6.4 DNA Replication (cont)

6.5 Recombination at the DNA Level

Chapter 6 of the textbook: Genetics: From Genes

to Genomes, 4th edition (2011), Hartwell H et al

Integrity and accuracy of genetic information must be preserved

2

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Each organism ensures the informational fidelity of its DNA in

three important ways:

• Redundancy Either strand of the double helix can specify the

sequence of the other

• Precision of cellular replication machinery

– DNA polymerase I and III have proofreading ability (more about this in Chapter 7)

• Enzymes that repair chemical damage to DNA The cell has

an array of enzymes devoted to the repair of nearly every

imaginable type of chemical damage

6.5 Recombination at the DNA Level

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New combinations of alleles are created by two types of events in meiosis:

 Independent assortment – each pair of homologous

chromosomes segregates freely from the other (Chapter 4)

Creates new allele combinations for unlinked genes

 Crossing over – two homologous chromosomes exchange

portions of DNA (Chapter 5)

Creates new allele combinations for linked genes

Ensures proper chromosome segregation during meiosis

• Mistakes can result in nondisjunction

DNA molecules break and rejoin during

recombination: The experimental evidence

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M Meselson and J Weigle, co-infected E coli with radio-labeled phage

Bacteriophage lambda with genetic markers grown on E coli in

media with heavy (13C and 15N) or light (12C and 14N) isotopes

Separated phage DNA on

CsCl density gradient

Genetic recombinants had

DNA with hybrid densities

Fig 6.22

5

Heteroduplex regions occur at sites

of genetic exchange

Two strands of DNA don't break

and rejoin at the same location

• Breakpoints on each strand can

be 100s-1000s bp apart

Heteroduplex – region of DNA

between breakpoints

• One strand is maternal and other

is paternal

• Strands can have mismatches

Fig 6.23

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• DNA repair enzymes

eliminate mismatches

• Either allele can be converted

• Gene conversion – deviations

from expected 2:2

segregation, e.g 3:1 or 1:3

• In yeast, gene conversion

occurs 50:50 with and

without crossing over of

flanking markers

Mismatches in

heteroduplexes

can be repaired

VNU-University of Science - DNThai Fig 6.23c 6

• Experimental observations that led to development of a model

of recombination

• Tetrad analysis in yeast showed that only two of the four

chromatids are recombinant

• Recombination occurs only between homologous regions and

is highly accurate

• Crossover sites often associated with heteroduplex regions

• Gene conversion can occur in absence of crossing over

– Not all recombination leads to crossovers

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Crossing-over at the molecular level: A

model

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• Homologous chromosomes break, exchange DNA, and rejoin

• Breakage and repair creates reciprocal products of

recombination

• Recombination events can occur anywhere along the DNA

• Precision in the exchange (no gain or loss of nucleotide pairs)

prevents mutations from occurring

• Gene conversion can give rise to an unequal yield of two

different alleles

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Double-strand-break repair model

of meiotic recombination

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• Step 1: Double-strand break formation

• Step 2: Resection

• Step 3: First strand invasion

• Step 4: Formation of a double Holliday junction

• Step 5: Branch migration

• Step 6: The Holliday intermediate

• Step 7: Alternative resolutions

• Step 8: Probability of crossover occurring

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A Model of Recombination at the

Molecular Level with 8 steps

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Step 1:

Double-strand break formation

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Dmc1 breaks phosphodiester bonds of both strands of one chromatid

 Spo11 in yeast is homologous to Dmc1 of multicellular

eukaryotes

Part of Fig 6.24

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Step 2: Resection

VNU-University of Science - DNThai Part of Fig 6.24

5' ends of each broken strand are degraded to create 3’

single-stranded tails

One single-strand tail invades a non-sister chromatid and forms

stable heteroduplex

Displacement loop (D-loop) from invaded chromatid is stabilized

by single-strand binding protein

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Step 3: First strand invasion

VNU-University of Science - DNThai Part of Fig 6.24

• D-loop enlarged by new DNA synthesis at 3'-end of invading

strand

• New DNA synthesis fills in gap in bottom strand using

displaced strand as template

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Step 4: Formation of double Holliday

junctions

VNU-University of Science - DNThai Part of Fig 6.24

Heteroduplex region of both DNA molecules is lengthened

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Step 5: Branch migration

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Part of Fig 6.24

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Step 6: The Holliday intermediate

VNU-University of Science - DNThai Part of Fig 6.24

Cutting of Holliday junctions by endonucleases in either vertical

or horizontal plane is equally likely

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Step 7: Alternative resolutions

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• Non-crossover occurs when both junctions are resolved in

same plane

• Crossover occurs with the two junctions are resolved in

different planes

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Step 8: Probability of crossover

occurring

VNU-University of Science - DNThai Part of Fig 6.24

1 DNA is the nearly universal genetic material This fact was

demonstrated by experiments showing that DNA causes the

transformation of bacteria and is the agent of virus production

in phage-infected bacteria

2 According to the Watson-Crick model, proposed in 1953 and

confirmed in the succeeding decades, the DNA molecule is a double helix composed of two antiparallel strands of

nucleotides; each nucleotide consists of one of four

nitrogenous bases (A, T, G, or C), a deoxyribose sugar, and a phosphate An A on one strand can only pair with a T on the other, and a G can only pair with a C

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Essential concepts

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3 DNA carries digital information in the sequence of its bases,

which may follow one another in any order Because of the

restriction on base pairing, the information in either strand of

a double helix defines the information that must exist in the

opposite strand The two strands are considered

complementary

4 The DNA molecule reproduces by semiconservative

replication In this type of replication, the two DNA strands

separate, and the cellular machinery then synthesizes a

complementary strand orf each By producing exact copies of the base sequence information in DNA, semiconservative

replication allows life to reproduce itself

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Essential concepts

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• Recombination arises from a highly accurate cellular

mechanism that includes the base pairing of homologous

strands of nonsister chromatids Recombination generates new

combinations of alleles in sexually reproducing organisms

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Essential concepts

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