RNA

RNA (Ribo Nucleic Acid)

One of the two main types of nucleic acid (the other being DNA), which functions in cellular protein synthesis in all living cells and replaces DNA as the carrier of genetic information in some viruses.

Like DNA, it consists of strands of repeating nucleotides joined in chainlike fashion, but the strands are single (except in certain viruses), and it has the nucleotide uracil (U) where DNA has thymine (T). Messenger RNA (mRNA), a single strand copied from a DNA strand that acts as its template, carries the message of the genetic code from DNA (in chromosomes) to the site of protein synthesis (on ribosomes). Ribosomal RNA (rRNA), part of the building blocks of ribosomes, participates in protein synthesis. Transfer RNA (tRNA), the smallest type, has fewer than 100 nucleotide units (mRNA and rRNA contain thousands). Each nucleotide triplet on mRNA specifies which amino acid comes next on the protein being synthesized, and a tRNA molecule with that triplet's complement on its protruding end brings the specified amino acid to the site of synthesis to be linked into the protein. Various minor types of RNA also exist; at least some act as catalysts (ribozymes), a function long ascribed only to proteins.


Source: Britannica encyclcopedia

Choromosome

Choromosome
Microscopic, threadlike part of a cell that carries hereditary information in the form of genes.

Art:During the first stages of cell division, the recognizable double-stranded chromosome is formed by …

Choromosome

 * During the first stages of cell division, the recognizable double-stranded chromosome is formed by …

The structure and location of chromosomes differentiate prokaryotic cells from eukaryotic cells (see prokaryote, eukaryote). Every species has a characteristic number of chromosomes; humans have 23 pairs (22 pairs of autosomal, or nonsex, chromosomes and one pair of sex chromosomes). Human chromosomes consist primarily of DNA. During cell division (see meiosis, mitosis), chromosomes are distributed evenly among daughter cells. In sexually reproducing organisms, the number of chromosomes in somatic (nonsex) cells is diploid, while gametes or sex cells (egg and sperm) produced by meiosis are haploid (see ploidy). Fertilization restores the diploid set of chromosomes in the zygote.

Source: Britannica encyclcopedia

Lysosome

Lysosome
Membrane-enclosed organelle found in all eukaryotic cells (see eukaryote) that is responsible for the cell's digestion of macromolecules, old cell parts, and microorganisms.

Lysosomes contain a wide variety of enzymes that break down macromolecules such as nucleic acids, proteins, and polysaccharides. Many of the products of lysosomal digestion, including amino acids and nucleotides, are recycled back to the cell for use in synthesizing new cellular components.

Source: Britannica encyclcopedia

Endoplasmic reticulam

Endoplasmic reticulam

Endoplasmic reticulam
Membrane system within the cytoplasm of a eukaryotic cell (see eukaryote), important in the synthesis of proteins and lipids.

The ER usually makes up more than half the membrane of the cell and is continuous with the outer membrane of the nuclear envelope (see nucleus). There are two distinct regions of ER: the rough ER, or RER (so called because of the protein-synthesizing ribosomes attached to it), and the smooth ER (SER), which is not associated with ribosomes and is involved in the synthesis of lipids and the detoxification of some toxic chemicals.
Source: Britannica encyclcopedia

DNA

DNA
or deoxyribonucleic acid
One of two types of nucleic acid (the other is RNA); a complex organic compound found in all living cells and many viruses.
Art:DNA double helix. A. Molecular model of DNA. The molecules include (1) hydrogen, (2) oxygen (3) …

DNA

* DNA double helix. A. Molecular model of DNA. The molecules include (1) hydrogen, (2) oxygen (3) …
It is the chemical substance of genes. Its structure, with two strands wound around each other in a double helix to resemble a twisted ladder, was first described (1953) by Francis Crick and James D. Watson. Each strand is a long chain (polymer) of repeating nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T). The two strands contain complementary information: A forms hydrogen bonds (see hydrogen bonding) only with T, C only with G. When DNA is copied in the cell, the strands separate and each serves as a template for assembling a new complementary strand; this is the key to stable heredity. DNA in cells is organized into dense protein-DNA complexes (see nucleoprotein) called chromosomes. In eukaryotes these are in the nucleus, and DNA also occurs in mitochondria and chloroplasts (if any). Prokaryotes have a single circular chromosome in the cytoplasm. Some prokaryotes and a few eukaryotes have DNA outside the chromosomes in plasmids. See also Rosalind Franklin; genetic engineering; mutation; Maurice Wilkins.

Source: Britannica encyclcopedia

Polysaccharide

Polysaccharide
Any of a large class of long-chain sugars composed of monosaccharides.

Polysaccharide

Because the chains may be unbranched or branched and the monosaccharides may be of one, two, or occasionally more kinds, polysaccharides can be categorized in various ways. Cellulose, starch, glycogen, and dextran are all polysaccharides of glucose, with different configurations. Pectins are composed of a galactose derivative, chitin of a glucose derivative. Connective tissues, joint fluid, and cartilage contain two-component polysaccharides, including heparin. See also oligosaccharide.

Source: Britannica encyclcopedia

Nucleic acid

Nucleic acids
Any of the organic compounds making up the genetic material of living cells.

Nucleic acids

Nucleic acids direct the course of protein synthesis, thereby regulating all cell activities. Their transmission from one generation to the next is the basis of heredity. The two main types, DNA and RNA, are composed of similar materials but differ in structure and function. Both are long chains of repeating nucleotides. The sequence of purines and pyrimidines (bases)—adenine (A), guanine (G), cytosine (C), and either thymine (T; in DNA) or uracil (U; in RNA)—in the nucleotides, in groups of three (triplets, or codons), constitutes the genetic code.

Source: Britannica encyclcopedia

ATP

ATP
in full adenosine triphosphate

Organic compound, substrate in many enzyme-catalyzed reactions (see catalysis) in the cells of animals, plants, and microorganisms.

ATP's chemical bonds (see bonding) store a large amount of chemical energy. ATP therefore functions as the carrier of chemical energy from energy-yielding oxidation (see oxidation-reduction) of food to energy-demanding cellular processes. Three such processes of metabolism are sources of ATP and stored energy: fermentation, the tricarboxylic acid cycle, and cellular respiration (also called oxidative phosphorylation). All form ATP from adenosine monophosphate (AMP) or adenosine diphosphate (ADP) and inorganic phosphate. When the reaction goes in the other direction, ATP is broken down to ADP or AMP and phosphate, and the released energy is used to perform chemical, electrical, or osmotic work for the cell.



Source: Britannica encyclcopedia

Antioxidant

Antioxidant

Any of various compounds added to certain foods, natural and synthetic rubbers, gasolines, and other products to retard autoxidation (combination with oxygen in the air at room temperature) and its effects.

Aromatic compounds such as aromatic amines, phenols, and aminophenols delay loss of elasticity in rubber and gummy deposits in gasoline. Preservatives such as tocopherol (vitamin E), propyl gallate, butylated hydroxytoluene (BHT), and butylated hydroxyanisole (BHA) prevent rancidity in fats, oils, and fatty foods. In the body, antioxidants such as vitamins C and E and selenium may reduce oxidation caused by free radicals.

Source:Britannica

encyclopedia

ALGAE (BIOLOGY)

ALGAE (BIOLOGY)

Members of a group of mostly aquatic, photosynthetic organisms (see photosynthesis) that defy precise definition.

They range in size from the microscopic flagellate Micromonas to giant kelp that reaches 200 ft (60 m) in length. Algae provide much of Earth's oxygen, serve as the food base for almost all aquatic life, and provide foods and industrial products, including petroleum products. Their photosynthetic pigments are more varied than those of plants, and their cells have features not found among plants and animals. The classification of algae is changing rapidly because new taxonomic information is being discovered. Algae were formerly classified into three major groups—the red, brown, and green seaweeds—based on the pigment molecules in their chloroplasts. Many more than three groups are now recognized, each sharing a common set of pigment types. Algae are not closely related to each other in an evolutionary sense. Specific groups can be distinguished from protozoans  and fungi (see fungus) only by the presence of chloroplasts and by their ability to carry out photosynthesis; these specific groups thus have a closer evolutionary relationship with the protozoa or fungi than with other algae. Algae are common on “slimy” rocks in streams (see diatoms) and as green sheens on pools and ponds. Use of algae is perhaps as old as humankind; many species are eaten by coastal societies.

SOURCE: BRITANNICA ENCYCLOPEDIA

ALFALFA (CATTLE FOOD-)

ALFALFA (CATTLE FOOD-)

Perennial, clover-like legume (Medicago sativa).

It is widely grown primarily for hay, pasturage, and silage. It is known for its tolerance of drought, heat, and cold, and for its improvement of soil by nitrification (see nitrifying bacteria) due to bacteria associated with its roots. The plant, which grows 1–3 ft (30–90 cm) tall, develops numerous stems that arise from a much-branched crown at soil level, each bearing many three-leaved leaflets. Its long primary root—as long as 50 ft (15 m) in some plants—accounts for its unusual ability to tolerate drought. Its remarkable capacity for regeneration of dense growths of new stems and leaves following cutting makes possible as many as 13 crops of hay in one growing season. Alfalfa hay is very nutritious and palatable, high in protein, minerals, and vitamins.

Source: Britannica encyclopedia

Albumin

ALBUMIN

Any of a diverse class of proteins historically defined by their ability to dissolve in water and in a half-saturated (see saturation) solution of ammonium sulfate.

They are readily coagulated by heating. Examples include serum albumin, a major component of plasma; α-lactalbumin, found in milk; ovalbumin, which makes up about half the proteins of egg white; and co albumin, another egg-white protein. Ovalbumin is used commercially in the food, wine, adhesives, paper coatings, pharmaceutical, and other industries and in research.

Source: Britannica encyclopedia

ALBINISM

ALBINISM

Absence of the pigment melanin in the eyes, skin, hair, scales, or feathers.

It arises from a genetic defect and occurs in humans and other vertebrates. Because they lack the pigments that normally provide protective coloration and screen against the sun's ultraviolet rays, albino animals rarely survive in the wild. Humans have long intentionally bred certain albino animals (e.g., rabbits) for their appearance. In humans with generalized, or total, albinism, the affected person has milk-white skin and hair; the iris of the eye appears pink, the pupil red. Vision abnormalities such as astigmatism, nystagmus (rapid involuntary oscillation of the eye), and photophobia (extreme sensitivity to light) are common. Generalized albinism occurs throughout the world in about one in 20,000 persons.

SOURCE: Britannica encyclopedia

ABO BLOOD GROUP SYSTEM

ABO BLOOD GROUP SYSTEM

Classification of human blood according to whether red blood cells (erythrocytes) have or lack the inherited antigens called A (including A1 and A2) and B on their surface.

Blood can be type O (lacking both), type A (having only A), type B (having only B), or type AB (having both). The ABO antigens make certain blood types incompatible for transfusion. They are developed well before birth and remain through life. The frequencies of blood groups vary among different racial groups and in different geographic areas. Certain diseases are rarer in persons with particular blood groups.

SOURCE: BRITANNICA ENCYCLOPEDIA