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Ch. 16: The molecular basis of inheritance

• Describe how DNA was identified as the genetic material

• Understand DNA structure and function

• Explain the semi‐conservative nature of DNA replication

• Describe the steps of DNA replication and dentifythe key players involved

• Describe the basic process of DNA repair

• Define the DNA end replication problem

What is the genetic material?

Streptococcus pneumoniae

protective capsule

Some heritable substance transformed the R cells into S cells

DNA is the genetic material

What is the heritable genetic material?

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• Nucleotides = building blocks of nucleic acids

– A,T,C & G

• 1950’s: new lines of evidence

– Chargraff’s principle:  # A = #T, #C = #G

– Rosalind Franklin and Maurice Wilkins: X‐ray diffraction

• 1953:  Watson & Crick:  DNA double helix

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ChromosomesChromosomal

DNA molecules

Centromere

Chromosomearm

Chromosome duplication(including DNA replication)and condensation

Sisterchromatids

Separation of sisterchromatids intotwo chromosomes

1

2

3

How does DNA replicate?

(a) Parent molecule (b) Separation ofstrands

(c)“Daughter” DNA molecules,each consisting of oneparental strand and onenew strand

A

A

A

A

A

A

A

A

A

A

A

A

T

T

T

T

T

T

T

T

T

T

T

T

C

C

C

C

C

C

C

C

G

G

G

G

G

G

G

G

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DNA replication: 3 possibilities

Overview: DNA replication

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Key players in replication

Initiates DNA synthesis

Adds RNA primer

Stabilize unwound parental strands

Unwinds parental strands

Relieves the strain associated with unwinding

Origin of replication

Overview

Leadingstrand

Leadingstrand

Laggingstrand

Lagging strand

Overall directionsof replication

12

Leadingstrand

Laggingstrand

Overview

Origin of replication Laggingstrand

Leadingstrand

Primer

Overall directionsof replication

Origin of replication

RNA primer

Sliding clamp

DNA polymerase IIIParental DNA

35

5

33

5

3

5

3

5

3

5

Continuous replication

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Templatestrand

RNA primerfor fragment 1

Okazakifragment 1

RNA primerfor fragment 2

Okazakifragment 2

Overall direction of replication

3

3

3

3

3

3

3

3

3

3

3

3

5

5

5

5

5

55

55

55

5

2

2

21

1

1

1

1 DNA polymerase III

DNA polymerase I

Overview

Leadingstrand

Origin of replication Lagging

strand

LeadingstrandLagging

strand Overall directionsof replicationLeading strand

Primer

DNA pol III

DNA pol I

Lagging strand

DNA ligase5

5

5

33

3 34

2 1

Figure 16.18

Parental DNA

DNA pol III

Leading strand

Connectingprotein

Helicase

Lagging strandDNA pol III

Laggingstrandtemplate

5

5

5

5

5

5

3 3

33

3

3

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Nuclease

DNA polymerase

DNA ligase

5

5

5

5

5

5

5

5

3

3

3

3

3

3

3

3

DNA repair

End replication problem

“Telomeres”

DNA double helix(2 nm in diameter)

DNA, the double helix

Nucleosome(10 nm in diameter)

Histones

Histones

Histonetail

H1

Nucleosomes, or “beads ona string” (10-nm fiber)

Chromatin packing

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30-nm fiber

30-nm fiber

Loops Scaffold

300-nm fiber

Chromatid(700 nm)

Replicatedchromosome(1,400 nm)

Looped domains(300-nm fiber) Metaphase

chromosome