what is DNA????

 

DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is calledmitochondrial DNA or mtDNA).

The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.

DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.

aNiMaL HiStology

   

   

Epithelial Tissue

Epithelial tissue, or epithelium, has the following general characteristics:

• Epithelium consists of closely packed, flattened cells that make up the inside or outside lining of body areas. There is little intercellular material.
• The tissue is avascular, meaning without blood vessels. Nutrient and waste exchange occurs through neighboring connective tissues by diffusion.
• The upper surface of epithelium is free, or exposed to the outside of the body or to an internal body cavity. The basal surface rests on connective tissue. A thin, extracellular layer called the basement membrane forms between the epithelial and connective tissue.

There are two kinds of epithelial tissues:

• Covering and lining epithelium covers the outside surfaces of the body and lines internal organs.
• Glandular epithelium secretes hormones or other products.

Epithelium that covers or lines

Epithelial tissues that cover or line surfaces are classified by cell shape and by the number of cell layers. The following terms are used to describe these features.

Cell shape:

• Squamous cells are flat. The nucleus, located near the upper surface, gives these cells the appearance of a fried egg.
• Cuboidal cells are cube- or hexagon-shaped with a central, round nucleus. These cells produce secretions (sweat, for example) or absorb substances such as digested food.
• Columnar cells are tall with an oval nucleus near the basement membrane. These thick cells serve to protect underlying tissues or may function to absorb substances. Some have microvilli, minute surface extensions, to increase surface area for absorbing substances, while others may have cilia that help move substances over their surface (such as mucus through the respiratory tract).
• Transitional cells range from flat to tall cells that can extend or compress in response to body movement.

Number of cell layers:

• Simple epithelium describes a single layer of cells.
• Stratified epithelium describes epithelium consisting of multiple layers.
• Pseudostratified epithelium describes a single layer of cells of different sizes, giving the appearance of being multilayered.