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DNA: The Genetic Material. Chapter 14. 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. The Genetic Material.

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The genetic material
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


The genetic material1
The Genetic Material

Avery, MacLeod, & McCarty, 1944

repeated 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


The genetic material2
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


Dna structure
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


Dna structure1
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


Dna structure2
DNA Structure

James Watson and Francis Crick, 1953

  • deduced the structure of DNA using evidence from Chargaff, Franklin, and others

  • proposed a double helixstructure


Dna structure3
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


Dna structure4
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.


Dna replication
DNA Replication

Matthew Meselson & Franklin Stahl, 1958

investigated the process of DNA replication

considered 3 possible mechanisms:

  • conservative model

  • semiconservative model

  • dispersive model


Dna replication1
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 14N

DNA was extracted from the cells at various time intervals


Dna replication2
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


Dna replication3
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.


Prokaryotic dna replication
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.


Prokaryotic dna replication1
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


Eukaryotic dna replication
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.


Eukaryotic dna replication1
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.


Eukaryotic dna replication2
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


Dna repair
DNA Repair

- DNA-damaging agents

- repair mechanisms

- specific vs. nonspecific mechanisms


Dna repair1
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.


Dna repair2
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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