Schleiden and Schwann in 1838-39, formulated the cell theory, which originally contained following two statements

(i) All living beings are made up of cells and products formed by the cells.

(ii) Cells are the structural and functional units of life.

The cell theory stated by Schleiden and Schwann failed to explain the question of origin of cells. A major expansion of the cell theory was expressed by Virchow in 1855, in his statement 'Omnis cellula e cellula' (all cells arise from pre-existing cells). This concept, was the actual idea of Nagelli (1846), which later on was elaborated by Virchow, along with considerable evidences in its support. The work of Nagelli and Virchow established cell division as the central phenomenon in the continuity of life.

Thus, the modern cell theory is based on two facts

(i) all living organisms are composed of cells and products of cell.

(ii) cells are the bosic structural and functional unites of life.

(iii) all cells arise from pre-existing cells.

Viruses are exception to cell theory as they are not composed of cell. They consist of a nucleic acid (DNA or RNA) surrounded by a protein sheath and are incapable of independent existence, self regulation and self reproduction.

Difference between Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER) are as below

Chemical composition of plasma membrane

Lipids are the major components of the cell membrane as they form the continuous structural frame of the cell membrane. Lipids such as phospholipids, glycolipids, and steroids are found in membranes.

The lipid molecule possess both polar hydrophilic (water loving) and non-polar hydrophobic (water repelling) ends. The hydrophilic region is in the form of a head, while the hydrophobic part contains two tails of fatty acid. Hydrophobic tail is present towards the centre of the membrane. This structures results is the formation of lipid bilayer known as unit membrane/biological membrane/cell membrane.

A plasmid is usually a circular (sometime linear), double strandad DNA, that can replicate itself autonomously. These are found in the cytoplasm of the bacterial cell, plasmids normally remain separated from the chromosome, but sometimes may integrate into it temporarily and replicate with it incidentally.

Role of Plasmids in Bacteria

Plasmids are the extrachromosomal circular, independently replicating unit besides nucleoid in the bacterial cell. Plasmids are used to transfer the information from one cell to another, i.e., transfer of important genes (e.g., they may confer resistance of particular, antibiotics to their bacterial cells), enabl to metabolise a nutrient, which normally a bacteria is unable to. It also helps in conjugation of bacteria. These days, plasmids are used in a variety of recombination experiments, as cloning vectors. A such example of plasmid vector is given below in figure.

Histones are the basic group of globular proteins that have a high content of basic amino acis, i.e., arginine and lysine. Histones forms the part of the chromosomal material in eukaryotic cells.

There are five types of histone proteins $\mathrm{H}_1, \mathrm{H}_2 \mathrm{~A}, \mathrm{H}_2 \mathrm{~B}, \mathrm{H}_3$ and $\mathrm{H}_4$. Four of them $\left(\mathrm{H}_2 \mathrm{~A}, \mathrm{H}_2 \mathrm{~B}, \mathrm{H}_3\right.$ and $\left.\mathrm{H}_4\right)$ occur in pairs to produce histone octamer called Nubody or core of nucleosome.

Functions Histones bear positively charged ends, that attaract negetively charged strands of DNA. Histones provides a medium around which DNA winds, and they play a vital role in gene regulation, as they act as gene repressor. They form the chromatin material and provide stability as are not coagulated by heat.