Who proposed the cell theory
A. Schleiden and Schwann
B. Watson and Crick
C. Mendel and Morgan
D. Robert Hooke

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Hint: The cell theory deals with the origin of a cell. According to these scientists, ‘ All living beings are made up of cells and it is the basic structural and functional unit of life’. These scientists discovered this cell theory in 1838.

Complete answer:
Schleiden, a German botanist, and Schwann, a British zoologist, led to the development of the cell theory or cell doctrine. Schleiden examined a large number of plant tissues and came to the conclusion that the ultimate unit of all plant tissue is these cells. Similarly, Schwann also studied different types of animal tissues including the development of embryos. He later concluded that animal cells lack cell walls but are covered by a membrane and that this is the only difference between a plant and animal cell, otherwise, they are similar. Thet compared their findings and put out a joint paper in 1839. Though they were not the first ones to state that the organisms are made up of cells, they were the first ones to piece together their observations and conclusion in the form of a theory.

Additional Information:
Schleiden and Schwan did not know the exact mode of formation of new cells. This was discovered by Rober Virchow in his famous theory of 1855 ‘ Omnis cellula e cellula’ which means that new cells are developed from the pre-existing cells.
Watson and Crick are the scientists who were the first ones to deduce the structure of DNA. Mendel, the father of modern genetics, has developed the rules of inheritance known as Mendel’s laws. Thomas H Morgan reported the phenomenon ‘linkage’ in Drosophila. They both are important contributors in the field of Genetics. Robert Hooke introduced the term ‘cell’ while examining the dead cells of cork tissues in trees.
So, the correct answer is ‘Schleiden and Schwann’.

Note: Cell theory has now been modified to include the following observation:
-All living beings are made up of one or more than one cell.
-All cells have primarily the same chemical composition.
-The hereditary information of the cell is in the form of DNA which is passed down from cell to cell.
-Energy flow occurs between the cells
-Every cell contains a protoplasm with a nucleus and organelles. It is surrounded by a protective membrane.