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NEL Beyond Mendel 643

nucleus. To further test his hypothesis, he conducted additional experiments in which

he transplanted stalks from a species ofAcetabulariawith a flowerlike cap onto the foot

of another species with a disk-shaped cap. The caps that eventually developed on the

transplanted stalks were all disk-shaped. Hammerling concluded that the instructions

needed to build these new caps were very likely in the nucleus in the foot of the cell and

not elsewhere.

Hammerling’s results encouraged scientists to concentrate their search for the mate-

rial of hereditary material on the nucleus and its contents. Proteins and DNA are present

in the nucleus in large quantities, but DNA was initially thought to be too simple a mate-

rial to account for the great variety seen in cells and cell processes, while proteins were

already known to play a significant role in metabolic functions. However, work by British

biologist Frederick Griffith on Streptococcus pneumoniae, in 1928, laid the foundation for

later research. Canadian-born scientists Oswald Avery and Colin MacLeod,

along with their American teammate Maclyn McCarty, built upon this work over a

14-year period culminating in 1944, and came to the conclusion that DNA was indeed

the molecular material of heredity.

Section19.3

Figure 2
Hammerling’s experiment strongly suggested that the hereditary material is located
in the nucleus.

One Man’s Castle

Is Another Man’s Lab

Friedrich Miescher’s discovery

took place in the vaults of an old

castle that had been converted to

a laboratory. You can hear

Miescher describe the process he

used to isolate nuclein in an

animation found by accessing the

Nelson Web site.

DID YOUKNOW??

DNA’s Homes

DNA does not just reside in the

nucleus. A small amount of DNA

is also found in chloroplasts and

mitochondria. The size of the

genome varies depending on the

species. Plants tend to have a

larger mitochondrial genome

compared with that of animals.

DID YOUKNOW??

http://www.science.nelson.com GO

Experiment 1

Experiment 2

cap

stalk

cap removed

new cap

regeneration

nucleus

nucleus

foot

foot removed

no foot

regeneration