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.

All organisms are made up of cells, which further organises themselves into tissues, organs and organ systems. Thus, forming the building blocks of organisms cells also the property of totipotency, capable of developing into a new organism. Besides, forming the structural unit they perform different specialised functions in the same way as each organ or system carries out in an organism. Thus, exhibiting division of labour i.e., cell organelles are specific in their functions.

	Structure	Function	Diagram
1.	Cell membrane all cell possess a phospholipid based cell membrane.	The cell membrane is selectively permeable, i.e., only selected material can pass through it.
2.	Cytoplasm It is a watery solution containing controlled concentration of organic and inorganic compounds.	It functions as a site for metabolism and provide energy and material for growth and reproduction.
3.	Nucleus It essentially consists of DNA, the nuclear matrix or the nucleoplasm containing nucleolus and chromatin.	It serves to store and transmit information to direct the synthetic activities of the entire cell. In also transfers the genetic information required for growth and reproduction.
4.	Golgi apparatus (Golgi complex) They constitute of many flat disc shaped sacs or cistermae of $0.5 \propto \mathrm{~m}-1.0 \propto \mathrm{~m}$ diameter. These stacks are arranged parallel to each other.	These are mainly involved in packaging the materials to be delivered either to intracellular targets or secreted outside the cells.
5.	The Endoplasmic Reticulum (ER) ER is often an extensive 3- dimensional network of intra cellular membranes formed by three elements- cisternae, tubules and vesicles	The ER bearing ribosomes on it surface is called rough endoplasmic reticulum (CRER). These are involved in protein synthesis and secretion. The ER without ribosomes are called smooth endoplasmic reticulum involved in synthesis of lipids like steroidal hormones.
6.	Mitochondria Mitochondria is a double membrane bound structure with outer and inner membrane dividing its lumen into two compartments, i.e., outer membrane forming of organelle and inner membrane forming number of infoldings called the cristae.	Mitochondria are the sites for aerobic respiration. It is known as power house of the cell since producing cellular energy in the form of ATP.
7.	Lysosomes These are membrane bound vesicular structures, formed by the process of packaging in the Golgi apparatus.	These are rich in many types of hydrolytic enzymes (hydrolases lipases, proteases, carbohydrates).
8.	Vacuoles The vacuole is the membrane bound space found in the cytoplasm, it contain water, sap, excretory product and other material not useful for the cell. The vacuole is bound by single membrane called tonoplast.	In plants the tonoplast facililates the transport of a number of ions and other materials against concentration gradients into the vacuole.

All these factors, i.e., structural and functional attributes it to be called a living cell.

(a) Rudolf Virchow (1855) first explained that the cells divide and new cells are formed from the pre-existing cells (omnis cellula e cellula). Virchow was the first to demonstrate that the cell theory applies to diseased tissue as well as to healthy tissue. He also engaged his research in the fields of archeology and anthropology.

(b) Schleiden and Schwann observed the cells and cell membranes for the first time. They proposed the cell theory which explains that the animal and plant cells are composed of cells and product of cells.

Theodore Schwann (1839) studied different types of animal cells and reported that cells had a thin outer layer which is called as plasma membrane. Schleiden also concluded on plant studies, i.e., the presence of cell wall is a unique character of plant cells. Thus, they proposed that plants and animals are composed of cells and their products.

Yes, extragenomic DNA is present in both prokaryotes and eukaryotes. In case of eukaryotes, extragenomic DNA is present in two organelles, e.g., plastids and mitochondria.

Extrachromosomal DNA in Prokaryotes In case of prokaryotic cells, the extragenomic DNA is present in the form of plasmids. The plasmid are circular DNA molecules, which confer certain unique phenotypic characters to the bacteria.

One such character is antibiotic resistance to bacteria. The plasmid DNA is also used to monitor bacterial transformation with foreign DNA. Bacteria transformation is the method of sexual reproduction in bacteria.

Extrachromosomal DNA in Eukaryotes Mitochondrial DNA (mtDNA) is usually circular. Mitochondrial DNA spans about 16,500 DNA building blocks (base pairs) representing a small fraction of the total DNA in cells.

mtDNA contains 37 genes, all of which are essential for normal mitochondrial functions. Thirteen of these genes provides instruction for making enzymes involved in oxidative phosporylation.

Mitochondrial genes are among the estimated 20000-25000 total genes in the human genome.

Plastids These are small bodies found free in most plant cells and are of three types leucoplast, chromoplast and chloroplast. These are double membrane bound structures for trapping radiation and storage purpose. It contains small, double stranded circular DNA molecules and ribosomes essential in synthesising certain proteins. As they both can synthesise proteins and replicate on their own they are known as semi-autonomous organelles.

In animals and plants, plasma membrane structure and function are correlated. A cell membrane consists of protein, lipid and carbohydrates.

The protein component present in membrane acts as a solute channels allowing the flow of minerals, hormones and cellular information from one organelle to another or from one cell to another.

In case of plants where minerals are absorbed actively from the soil. The plasma membrane possess proteinaceous carrier. The oligosaccharides attached to membrane serve as recognition centres and help in recognising foreign entities before allowing them entry into the cell.

The lipid content of plasma membrane in plant and animal cells are arranged with their hydrophilic polar head directed outwards and non-polar hydrophobic tails directed inwards providing fluidity to the membrane. Glycocalyx present in cell membrane also helps in cellular attachment.