DNA: The Genetic MaterialChapter 14

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The Genetic Material

Griffith’s conclusion:- information specifying virulence passed

from the dead S strain cells into the live R strain cells

- Griffith called the transfer of this information transformation

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The Genetic Material

Avery, MacLeod, & McCarty, 1944repeated Griffith’s experiment using purified

cell extracts and discovered:- removal of all protein from the transforming material did not destroy its ability to transform R strain cells- DNA-digesting enzymes destroyed all transforming ability- the transforming material is DNA

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The Genetic Material

Hershey & Chase, 1952- investigated bacteriophages: viruses that

infect bacteria- the bacteriophage was composed of only

DNA and protein- they wanted to determine which of these

molecules is the genetic material that is injected into the bacteria

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DNA Structure

Determining the 3-dimmensional structure of DNA involved the work of a few scientists:

– Erwin Chargaff determined that • amount of adenine = amount of thymine• amount of cytosine = amount of guanine

This is known as Chargaff’s Rules

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DNA Structure

Rosalind Franklin and Maurice Wilkins– Franklin performed X-ray diffraction

studies to identify the 3-D structure– discovered that DNA is helical– discovered that the molecule has a

diameter of 2nm and makes a complete turn of the helix every 3.4 nm

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DNA Structure

James Watson and Francis Crick, 1953– deduced the structure of DNA using

evidence from Chargaff, Franklin, and others

– proposed a double helix structure

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DNA Structure

The double helix consists of:– 2 sugar-phosphate backbones– nitrogenous bases toward the interior of

the molecule– bases form hydrogen bonds with

complementary bases on the opposite sugar-phosphate backbone

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DNA Structure

The two strands of nucleotides are antiparallel to each other– one is oriented 5’ to 3’, the other 3’ to 5’

The two strands wrap around each other to create the helical shape of the molecule.

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DNA Replication

Matthew Meselson & Franklin Stahl, 1958investigated the process of DNA replicationconsidered 3 possible mechanisms:

– conservative model– semiconservative model– dispersive model

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DNA Replication

Bacterial cells were grown in a heavy isotope of nitrogen, 15N

all the DNA incorporated 15N cells were switched to media containing

lighter 14NDNA was extracted from the cells at various

time intervals

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DNA Replication

The DNA from different time points was analyzed for ratio of 15N to 14N it contained

After 1 round of DNA replication, the DNA consisted of a 14N-15N hybrid molecule

After 2 rounds of replication, the DNA contained 2 types of molecules:– half the DNA was 14N-15N hybrid– half the DNA was composed of 14N

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DNA Replication

Meselson and Stahl concluded that the mechanism of DNA replication is the semiconservative model.

Each strand of DNA acts as a template for the synthesis of a new strand.

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Prokaryotic DNA Replication

The chromosome of a prokaryote is a circular molecule of DNA.

Replication begins at one origin of replication and proceeds in both directions around the chromosome.

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Prokaryotic DNA Replication

The double helix is unwound by the enzyme helicase

DNA polymerase III (pol III) is the main polymerase responsible for the majority of DNA synthesis

DNA polymerase III adds nucleotides to the 3’ end of the daughter strand of DNA

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Eukaryotic DNA Replication

The larger size and complex packaging of eukaryotic chromosomes means they must be replicated from multiple origins of replication.

The enzymes of eukaryotic DNA replication are more complex than those of prokaryotic cells.

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Eukaryotic DNA Replication

Synthesizing the ends of the chromosomes is difficult because of the lack of a primer.

With each round of DNA replication, the linear eukaryotic chromosome becomes shorter.

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Eukaryotic DNA Replication

telomeres – repeated DNA sequence on the ends of eukaryotic chromosomes– produced by telomerase

telomerase contains an RNA region that is used as a template so a DNA primer can be produced

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DNA Repair

- DNA-damaging agents- repair mechanisms- specific vs. nonspecific mechanisms

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DNA Repair

Mistakes during DNA replication can lead to changes in the DNA sequence and DNA damage.

DNA can also be damaged by chemical or physical agents called mutagens.

Repair mechanisms may be used to correct these problems.

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DNA Repair

DNA repair mechanisms can be:– specific – targeting a particular type of

DNA damage• photorepair of thymine dimers

– non-specific – able to repair many different kinds of DNA damage• excision repair to correct damaged

or mismatched nitrogenous bases

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