PROPERTIES OF THE GENETIC MATERIAL
- Contains information
- Can be replicated and transmitted
- Controls cellular processes and thus determines phenotype
- Can be source of new variation
- All of these characteristics are associated with the molecule deoxyribonucleic acid
DNA IS THE MOLECULE OF HEREDITY
- Inherited traits are determined by the elements of heredity (genes) that are transmitted
from parents to offspring in reproduction.
- As you shall see, genes are made up of the molecule deoxyribonucleic acid (DNA).
- DNA was discovered by Friedrich Miescher in 1869.
- In the 1920s, microscopic studies with special stains showed that DNA is present in
chromosomes.
- In the late 1920s Griffiths showed that the genetic material of some cells could be
changed by adding external molecules
- In 1944, Avery, McLeod, and McCarty provided the first evidence that DNA is the
genetic material.
- Friedrich Miescher had identified nuclein as a component of white blood cells in pus,
and later in the sperm of salmon, and showed it was made of carbon, oxygen, nitrogen,
hydrogen and phosphorus.
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- In the middle of the nineteenth century, Walther Flemming identified a fibrous structure
in the nuclei of cells which he called chromatin.
Fundamentally, DNA did not seem to be chemically complex enough to be the carrier of the
genetic information, as it was found to basically made up of 4 elements, the nucleotides
adenine, guanine, cytosine and thymine. This was in contrast to proteins that were made up of
in the order of 20 different amino acids. For a long time a predominant theory was that DNA
was a boring repetitive polymer of tetra-nucleotides
Careful work from Chargaff in the 1940’s established that the ratios of nucleotides did not fit
a perfect 1:1:1:1 pattern, and in fact differed among organisms. However, he did find that the
amount of A equalled T and G equalled C, even if the A+T/G+C pattern could show big
differences among different cells.
Chemical analyses therefore were not showing anything spectacular for DNA, but a set of
functional studies pointed to its key role in heredity
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