Biology 12

7.1 Isolating the Material of Heredity

218 MHR • Unit 3 Molecular Genetics

In 1865, the Austrian monk Gregor Mendel

presented the results of his research on patterns of

inheritance in garden peas to the Natural Science

Society in Brunn, Austria. He proposed a number

of hypotheses that challenged much of the thinking

of his day about heredity. He argued, for example,

that the maternal and paternal gametes contributed

equally to the development of the offspring. He

also held that the information contributed by each

parent was not blended but rather passed on to the

offspring as discrete bits of information or “factors

of inheritance.” He went on to state that, while two

factors will exist for any one visible trait, one of

them (known as the recessive factor) might not

be expressed.

Mendel’s findings on the transmission of

hereditary information were not widely recognized

at the time. This was partly due to the strong

divisions that existed among scientific disciplines

then, which meant that the work of a botanist was

not likely to be noticed by zoologists or by medical

doctors. The apparently fixed nature of Mendelian

factors of inheritance also seemed to be at odds

with the newly emerging theory of evolution. Over

the next few decades, however, scientists began to

recognize the many similarities among cellular

processes in bacterial, plant, animal, and human

cells (including the processes you studied in Unit 1).

They also found that Mendel’s principles were

consistent with the idea that species change and

evolve over time. Today, Mendel’s work is

recognized as the foundation of modern genetics.

Only four years after Mendel’s presentation in

Brunn, and less than 300 km away, the young

Swiss physician and scientist Friedrich Miescher

isolated a substance he called “nuclein” from the

nuclei of white blood cells. Miescher, shown in

Figure 7.1, determined that nuclein was made up

of an acidic portion (which he termed “nucleic

acid”) and an alkaline portion (which was later

shown to be protein). Shortly thereafter Miescher

turned to the study of chemical properties of other

cellular structures. Almost a century passed before

scientists established the connection between the

nucleic acid isolated by Miescher and Mendel’s

factors of inheritance.

Figure 7.1Friedrich Miescher was 25 years old when he

isolated nucleic acids from the nuclei of white blood cells

in 1869. He was working in a hospital treating wounded

soldiers, and he was able to collect white blood cells from

their bandages.

The Components of Nucleic Acids

Following the work of Miescher, Phoebus Levene

studied nucleic acid in more detail. During a career

that stretched from the early 1900s to the 1930s,

Levene isolated two types of nucleic acids that

could be distinguished by the different sugars

involved in their composition. One acid contained

the five-carbon sugar ribose, so Levene called it

“ribose nucleic acid” (ribonucleic acidor RNA).

The other acid contained a previously unknown

five-carbon sugar molecule. Since this sugar was

similar in structure to ribose but lacked one oxygen

molecule, Levene called it deoxyribose. He went on

EXPECTATIONS

Explain the roles of evidence, theories, and paradigms in the development

of scientific knowledge about genetics.

Demonstrate an understanding of the process of discovery that led to the

identification of DNA as the material of heredity.

Interpret the findings of key experiments that contributed to this process.