Maharashtra Board Class 12 Biology Chapter 4 Molecular Basis of Inheritance PDF Download

Molecular Basis of Inheritance

4.1 The Discovery Of DNA

Modern understanding of DNA has evolved from the discovery of nucleic acid to the development of the double-helix model. In 1869, Friedrich Miescher began working with white blood cells which are the major component of pus from infections. He collected a lot of pus from bandages at the local hospital. He used a salt solution to wash the pus off the bandages. When he added a weak alkaline solution to the cells, the cells lysed and nuclei precipitated out of the solution. From the cell nuclei, he isolated a unique chemical substance to which he called nuclein. Chemically, nuclein has high phosphorus content. Moreover it showed acidic properties. Hence it was named as nucleic acid.

By the early 1900s, we knew that Miescher's nuclein was a mix (mixture) of proteins and nucleic acids. There are two kinds of nucleic acids. DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

Teacher's Note

Friedrich Miescher discovered nuclein from pus in a hospital. This was the first discovery of DNA. Just like your teacher discovers what you know in class, scientists discover new things in laboratories.

Exam Trick

Remember: Nuclein = found in nucleus = has phosphorus = called nucleic acid. Think of the word "nucleus" to remember nuclein.

Points to Remember

Friedrich Miescher discovered nuclein in 1869.
Nuclein was found in white blood cells from pus.
Nuclein had high phosphorus content.
Nuclein showed acidic properties.
Nuclein was a mix of proteins and nucleic acids.

4.2 The Genetic Material Is A DNA

By the early 1900s, geneticist knew that genes control the inheritance of traits, that genes are located on chromosome and that chemically chromosomes are mainly composed of DNA and proteins. Initially, most geneticists thought that protein are large, complex molecules and store information needed to govern cell metabolism. Hence it was assumed that proteins caused the variations observed within species.

On the other hand DNA thought to be small, simple molecule whose composition varied little among species. Over the time, these ideas about DNA were shown to be wrong. In fact DNA molecules are large and vary tremendously within and among species. Variations in the DNA molecules are different than the variation in shape, electrical charge and function shown by proteins so it is not surprising that most researchers initially favored proteins as the genetic material.

Over a period of roughly 25 years (1928-1952), geneticists became convinced that DNA and not protein, was the genetic material. Let us study three important contributions that helped cause this shift of opinion.

Griffith's Experiments

In 1928, a British medical officer Frederick Griffith performed an experiment on bacterium Streptococcus pneumoniae that causes pneumonia in humans and other mammals. Griffith used two strains or two genetic varieties of Streptococcus to find a cure for pneumonia, which was a common cause of death at that time. The two strains used were:

i. Virulent, smooth, pathogenic and encapsulated S type.

ii. Non-virulent, rough, non-pathogenic and non-capsulated R type.

Griffith conducted four experiments on these bacteria. First, when he injected bacteria of strain R to mice, the mice survived because it did not develop pneumonia. Second, when he injected bacteria of strain S to mice, the mice developed pneumonia and died. In the third experiment, he injected heat-killed strain S bacteria to mice, once again the mice survived. In fourth experiment, he mixed heat-killed S bacteria with live bacteria of strain R and injected to mice. The mice died and Griffith recovered large numbers of live strain S bacteria from the blood of the dead mice.

Teacher's Note

Griffith's experiments showed that dead bacteria could change live bacteria. Like when your friend teaches you something, the knowledge passes from him to you and changes how you think.

Exam Trick

Remember: Heat-killed S + Live R = Mice die. This means something from dead S changed live R into S. That something is DNA.

Points to Remember

Griffith worked with Streptococcus pneumoniae bacteria.
S strain was deadly and killed the mice.
R strain was harmless and mice survived.
Heat-killed S alone did not kill mice.
Heat-killed S mixed with R killed the mice, meaning R became deadly.

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