Chapter 14: You should know: Mendel’s two laws of inheritance –The law of segregation –The law...

Chapter 14: You should know:• Mendel’s two laws of inheritance

– The law of segregation– The law of independent assortment

• You should know how to do mono-hybrid and di-hybrid crosses and the associated vocabulary

• Understand types of interaction that are more complicated than those witnessed by Mendel– Co-dominance, incomplete dominance, multiple alleles,

complex traits

Chapter 15:

You should know:

• The relationship between Mendelian inheritance and chromosomes– As indicated by Morgan’s work

• Patterns of inheritance with linked genes– Sex-linked– Linked traits

The molecular basis of inheritance

Our plan:

• The importance of DNA

• The evolution of our knowledge about DNA

• DNA replication

• DNA repair

The importance of DNA

• DNA is the information storage molecule of life

• The continuity of life is based on DNA• Since Watson and Crick published their

model of DNA structure in 1953, it has become the most celebrated molecule of our time

The search for the genetic material

Since Mendel’s work was published (1850’s) biologists have generated a lot of knowledge about DNA


Morgan’s contributions (~1910)• Morgan’s group noticed that genes are

located on chromosomes




•Strands of DNA coiled around proteins




•Strands of DNA coiled around proteins•DNA and protein became the two candidates for storing genetic material

–Proteins had more support initially due to their complexity–Nucleic acids seemed too uniform


Hershey and Chase (1952)• Used a bacteriophage (virus that infects

bacteria) to study DNA

Bacterial cell

Phage head

Tail sheath

DNA


Hershey and Chase (1952)• Used a bacteriophage (virus that infects

bacteria) to study DNA

Bacterial cell

Phage head

Tail sheath

DNA

•Viruses contain DNA in a protein coat•They insert their DNA into host cells to replicate


Hershey and Chase (1952)• Grew phages in media labeled with

radioactive isotopes of phosphorous or sulfur• P was incorporated into viral DNA• S was incorporated into viral proteins• Detected the radioactive DNA in the bacterial

cell• Concluded DNA is the genetic material

Phage

DNA

Bacterial cell

Radioactive protein

Radioactive DNA

Batch 1: radioactive sulfur (35S)

Batch 2: radioactive phosphorus (32P)

Empty protein shell

Phage DNA

Centrifuge

Centrifuge

Pellet

Pellet (bacterial cells and contents)

Radioactivity (phage protein) in liquid

Radioactivity (phage DNA) in pellet


Hershey and Chase (1952)

Chargaff (1950)• The structure of nucleotides

was known

–Three phosphate groups, a 5-C sugar, and a nitrogenous base–Four types of nitrogenous bases

Purines (A and G)Pyrimidines (C and T)



was known• It was also known that DNA

was polymer of nucleotides




was polymer of nucleotides• Chargaff determined that there

were patterns in the amounts of nitrogenous bases– Base composition differed

between species (further evidence for its role as genetic material)

– Amount of A=T– Amount of G=C




was polymer of nucleotides• Chargaff determined that there

were patterns in the amounts of nitrogenous bases– Base composition differed

between species (further evidence for its role as genetic material)

– Amount of A=T– Amount of G=C


By this time most scientists agreed DNA was the genetic material. The new challenge was to determine the structure.

Determining the structure of DNA

Franklin (1950’s)• Used x-ray diffraction to

generate images of DNA

• She concluded DNA was a very long and thin molecule

• Sugar and phosphate groups were on the outside

• There was a repeating pattern every 0.34 and 3.4 nanometers


Watson and Crick (1953)• Made a model that conformed to

Franklin’s images



Franklin’s images• Concluded DNA was double helix

with a sugar-phosphate backbone




with a sugar-phosphate backbone• Nitrogenous bases paired in the

middle (A to T and G to C)

•Maintains the uniform width of the helix•Triple hydrogen bonds between G and C •Double hydrogen bonds between A and T





middle (A to T and G to C)• Bases were paired across the helix

every 0.34 nanometers and the helix turned every 3.4 nanometers







• Anti-parallel molecule (two-way street)







• Anti-parallel molecule (two-way street)

• The linear sequence of bases can be varied (yielding genetic diversity)

DNA Replication

When does DNA replicate?Why is this important?

DNA Replication

“It has not escaped our notice that the specific pairing we have postulated immediately suggest a possible copying mechanism for genetic material” Watson and Crick (1953)

DNA Replication

“It has not escaped our notice that the specific pairing we have postulated immediately suggest a possible copying mechanism for genetic material” Watson and Crick (1953)

•Each strands has the information to code for the other•Complimentary strands serve as a template for the construction of new strands•New nucleotides line up on the template and are linked together

DNA Replication

•Replication begins at origins of replication (AT-rich regions)

•One in proks•Many in euks

•Proteins recognize the sequence and begin separating the strands

•Replication proceeds in both directions until entire DNA molecule is copied

Origin of replication Parental (template) strand

Daughter (new) strand

Replication fork

Replication bubble

Two daughter DNA molecules

(a) Origins of replication in E. coli

Origin of replication Double-stranded DNA molecule

Parental (template) strandDaughter (new) strand

Bubble Replication fork


(b) Origins of replication in eukaryotes

0.5 µm

0.25 µm

Double-strandedDNA molecule

DNA Replication

•Replication begins at origins of replication (AT-rich regions)

•One in proks•Many in euks

•Proteins recognize the sequence and begin separating the strands

•Replication proceeds in both directions until entire DNA molecule is copied

Origin of replication Parental (template) strand

Daughter (new) strand

Replication fork

Replication bubble


(a) Origins of replication in E. coli

Origin of replication Double-stranded DNA molecule

Parental (template) strandDaughter (new) strand

Bubble Replication fork


(b) Origins of replication in eukaryotes

0.5 µm

0.25 µm

Double-strandedDNA molecule

How does replication proceed?

DNA Replication•Helicase unwinds the helix at the replication forks•Topoisomerase helps relieve strain from untwisting the helix

Topoisomerase

Helicase

PrimaseSingle-strand binding proteins

RNA primer

55

5 3

3

3

DNA Replication•Helicase unwinds the helix at the replication forks•Topoisomerase helps relieve strain from untwisting the helix•Strands are stabilized by proteins

Topoisomerase

Helicase


RNA primer

55

5 3

3

3

DNA Replication•Helicase unwinds the helix at the replication forks•Topoisomerase helps relieve strain from untwisting the helix•Strands are stabilized by proteins•Two separated strands are available as a templates

Topoisomerase

Helicase


RNA primer

55

5 3

3

3

DNA Replication•Helicase unwinds the helix at the replication forks•Topoisomerase helps relieve strain from untwisting the helix•Strands are stabilized by proteins•Two separated strands are available as a templates•Primase adds complimentary RNA nucleotides as part of a short primer

Topoisomerase

Helicase


RNA primer

55

5 3

3

3

DNA Replication

Topoisomerase

Helicase


RNA primer

55

5 3

3

3

•Helicase unwinds the helix at the replication forks•Topoisomerase helps relieve strain from untwisting the helix•Strands are stabilized by proteins•Two separated strands are available as a templates•Primase adds complimentary RNA nucleotides as part of a short primer•Nucleotides are added to the 3’ end of the primer

DNA Replication•New strands only grow in the 5’ to 3’ direction

•This creates a leading and lagging strand

•Leading strand:•Elongating toward the fork•One primer is required

Leading strand

OverviewOrigin of replication

Lagging strand

Leading strandLagging strand

Primer

Overall directions of replication Origin of replication

RNA primer

“Sliding clamp”DNA poll IIIParental DNA

5

3

3

3

3

5

5

5

5

5


•Lagging strand:•Elongating away from the fork

Overview

Origin of replication

Leading strand

Leading strand

Lagging strand

Lagging strand

Overall directions of replication

Template strand

RNA primer

Okazaki fragment

Overall direction of replication

12

3

2

1

1

1

1

2

2

51

3

3

3

3

3

3

3

3

3

5

5

5

5

5

5

5

5

5

5

53

3


•Lagging strand:•Elongating away from the fork•Requires numerous primers

Overview


Leading strand

Leading strand

Lagging strand

Lagging strand


Template strand

RNA primer

Okazaki fragment


12

3

2

1

1

1

1

2

2

51

3

3

3

3

3

3

3

3

3

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5

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5

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5

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3


•Lagging strand:•Elongating away from the fork•Requires numerous primers•DNA polymerase adds complimentary nucleotides until it reaches another primer

Overview


Leading strand

Leading strand

Lagging strand

Lagging strand


Template strand

RNA primer

Okazaki fragment


12

3

2

1

1

1

1

2

2

51

3

3

3

3

3

3

3

3

3

5

5

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5

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5

5

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5

53

3


•Lagging strand:•Elongating away from the fork•Requires numerous primers•DNA polymerase adds complimentary nucleotides until it reaches another primer•Synthesized in fragments

Overview


Leading strand

Leading strand

Lagging strand

Lagging strand


Template strand

RNA primer

Okazaki fragment


12

3

2

1

1

1

1

2

2

51

3

3

3

3

3

3

3

3

3

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5

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5

5

5

53

3


•Lagging strand:•Elongating away from the fork•Requires numerous primers•DNA polymerase adds complimentary nucleotides until it reaches another primer•Synthesized in fragments •Primers are replaced with DNA (by DNA polymerase)•Fragments are joined by DNA ligase

Overview


Leading strand

Leading strand

Lagging strand

Lagging strand


Template strand

RNA primer

Okazaki fragment


12

3

2

1

1

1

1

2

2

51

3

3

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3

3

3

3

3

3

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5

5

5

5

5

5

53

3

DNA proofreading and repair

DNA proofreading and repair• It is important that replication is done correctly• DNA repair mechanisms fix mismatched bases

before the DNA is replicated again• Errors become mutations

– Mutations may lead to defects (a misspelled word that does not mean anything)

– Mutations can be advantageous (the invention of a new word that has great meaning)

DNA proofreading and repair• DNA polymerase

proofreads as it adds bases

• Mismatched bases are cut my nucleases

• The resulting gap is filled by DNA polymerase and sealed by DNA ligase

Nuclease

DNA polymerase

DNA ligase

You should understand:

• The importance of DNA

• The evolution of our knowledge about DNA

• DNA replication

• DNA repair

Chapter 14: You should know: Mendel’s two laws of inheritance –The law of segregation –The law...

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Transcript of Chapter 14: You should know: Mendel’s two laws of inheritance –The law of segregation –The law...