Wednesday, November 8, 2017

Properties of the genetic material

Genetic material is the substance that carries the biological information regarding the  structural, functional, developmental and behavioural properties of organisms. It also serves  as the agent that transfers or transmits biological information from parent to progeny. In most organisms, DNA is the genetic material. But, in some viruses, RNA serves as the  genetic material.
Properties of the genetic material                       
Some of properties are the following:
(i)Genetic material should be present in every cell.
(ii) Ability to store and transmit biological information in a stable form.
(iii) Ability to replicate with high fidelity to produce identical functional copies.
(iv) Ability to distribute its copies equally from parent cells to daughter cells with extreme  accuracy and minimal error.
(v) High physical and chemical stability to prevent the loss of information and also to  ensure genetic constancy in organisms.
(vi) Potentiality to generate variations (through mutation, recombination and minor errors in replication and distribution) in order to promote genetic diversity.
(vii) Ability to act and express itself for controlling the inheritance of characters specified by it.

Sunday, November 5, 2017

Experiments of Avery MacLeod and McCarthy

Griffith could not understand the cause of bacterial transformation and that is first of all identified by Oswald Avery, Colin MacLeod and Maclyn McCarty (1944). They partially purified the transforming principle from the cell extract (i.e., cell free extract of S-III bacteria) and demonstrated that it was DNA. These workers modified the known schemes for isolating DNA and prepared samples of DNA from S-III bacteria. They added this DNA to a live R bacterial culture; after a period of time they placed a sample of S-III containing R-II bacterial culture on an agar surface and allowed it to grow to form colonies. Some of the colonies (about 1 in 104) that grew were S-III typeTo show that this was a permanent genetic change, they dispersed many of the newly formed S-III colonies and placed them on a second agar surface. The resulting colonies were again S-III type. If an R-II colony arising from the original mixture was dispersed, only R-II bacteria grew in subsequent generations. Hence, the R-II colonies retained the R-II character, whereas the transformed S-III colonies bred true as S-III.Further, because S-III and R-II colonies differed by a polysaccharide coat around each S-III
bacterium, the ability of purified polysaccharide to transform was also tested, but no transformation
was observed.
Avery, MacLeod and McCarthy repeated Griffith's expts in vitro in a much refined way. Culture of live IIR cells produced typical IIR cell colonies, while a culture of heat-killed III s cell or a a culture of the DNA isolated from IIIS cells produced no colony. At the same time. of IIR cells, mixed either with heat-killed IIIS cells or with the DNA isolated from lllS on a medium containing antibodies for IIR cells (Ab IIR), produced some colonies of III s cells. AbIlR was used for inactivating some llR cells so that the number of IIR cells may not exceed the number of IIIS cells. These findings reveal that DNA can be the transforming Since DNA preparations often contain traces of RNA and proteins. this conclusion is not beyond doubt. In order to establish beyond doubt that DNA alone is the transforming principle, Avery and associates conducted two separate experiments, using the DNA isolated from lllS cells.In one of them, the DNA isolated from lllS cells was treated with the enzyme RNAse (to digest RNA if any), and in the other with the enzyme protease (to digest protein), before it was mixed with live llR cells. ln both these expts, some IllS cell colonies were fomied. This clearly shows  that RNA and Proteins are not responsible for the transformation of III R cells to lllS cells.In another expt DNA was treated with the enzyme DNAase before it was mixed with live IIR cells.This did not yield a III S colony.This confirms that DNA is the transforming principle.

The Transformation Experiments

In 1928, Frederick Griffith encountered a phenomenon now known as genetic transformation. Colonies of virulent strain (pathogenic) of pneumonia causing bacterium, Streptococcus pneumonia grown on nutrient agar, have a smooth (S) glistering appearance owing to the presence of a type specific, polysaccharide (a polymer of glucose and glucuronic acid) capsule. The avirulent (non-pathogenic) strains, on the other hand, lack this capsule and they produce dull, rough(R) colonies. Smooth (S) and rough (R) characters are directly related to the presence or absence of the capsule and this trait is known to be genetically determined. Both S and R forms occur in several types and are designated as S-I, S-II, S-III, etc., and R-I, R-II, R-III, etc., respectively. All these subtypes of S and R bacteria differ with each other in the type of antigens, they produce. The kind of antigen produced is likewise genetically determined. Smooth (S) forms sometimes mutate to rough (R) forms, but this change has not been found reversible. In the course of his work,
Griffith injected laboratory mice with injected with virulent S-III pneumococci, the mice suffered from pneumonia and died.He then  injected laboratory mice with live R-II pneumococci; the mice suffered no illness because R-II pneumococci was avirulent. However,  when he injected the heat killed S-III bacteria in mice, they did not suffer from pneumonia. But, when the mice were injected with the mixture of living avirulent R-II and heat killed S-III virulent, the unexpected symptoms of pneumonia appeared and high mortality resulted in them. By postmorteming the dead mice, it was found that their heart blood had both R-II and S-III pneumococci. From these results, Griffith concluded that the presence of the heat-killed S-III bacteria must have caused a transformation of the living R-II bacteria, so as to restore to them the capacity for capsule formation they had earlier lost by gene mutation. This was called “Griffith effect” or more popularly “bacterial trans-formation”.

Sunday, September 17, 2017

Hemoglobin structure and function

Hemoglobin (Hb) is the iron containing chromoprotein forming 95% of dry weight of RBC .Average hemoglobin (Hb) content in blood is 14 to 16 g/dL. In adult males it is 15 g/dL and in adult females it is  14.5 g/dL .Felix Hope Seyler in 1862 isolated pure Hemoglobin.Christian Bohr in 1904 discovered that hemoglobin is the transporter of oxygen.In 1912 Kutster established the structure of hemoglobin.Hans Fischer synthesized heme in laboratory in 1920 (Nobel prize, 1930).In 1945, Linus Pauling (Nobel prize, 1954) described abnormal hemoglobins.Max Perutz (Nobel Prize, 1962) studied the 3D structure of Hemoglobin.
Hemoglobin is a globular heme protein in vertebrate red blood cells and in the plasma of many invertebrates that carries oxygen and carbon dioxide; heme group binds oxygen and carbon dioxide and as well as imparts red color to the blood; also spelt as hemoglobin.Red colored conjugated protein (made up of heme and Globin) present inside the RBCNormal Hb% in adult male is 14 to 16 gm.Approximately 6.25 gm of Hb are synthesized and destroyed every day.Heme structure does not vary from species to species.It is the basic protein globin that varies in amino acid composition and sequence in different species.Globin is rich in Histidine and lysine.
Structure of Hemoglobin: Hemoglobin is a conjugated protein. It consists of a
protein combined with an ironcontaining pigment. The protein part is globin and the iron containing pigment is heme. Heme also present in the structure of myoglobin ie oxygenbinding pigment in muscles and neuroglobin ie oxygenbinding pigment in brain. Heme is iron porphyrin compound.Iron is present in ferrous (Fe2+) form. It is in unstable or loose form. In some abnormal conditions,the iron is converted into ferric (Fe3+) state, which is a stable form.The pigment part of heme is called porphyrin. It is formed by four pyrrole rings (tetrapyrrole) called, I, II, III and IV. The pyrrole rings are attached to one another by methane (CH4) bridges.The iron is attached to ‘N’ of each pyrrole ring and ‘N’ of globin molecule.Globin contains four polypeptide chains. Among the four polypeptide chains, two are βchains and two are α-chain /α-chain is made up of 141 aminoacids β-chain is made up of 146 aminoacids.
Varieties of normal human Hb are
HemoglobinA1 (two α-chains and β-chains)
HemoglobinF (two α-chains and ¥-chains)
HemoglobinA2 (two α-chains and delta-chains)
Embyonic Hemoglobin (two α-chains and €-chains)
Hemoglobin-A3 (Altered from Hb-A found in old red cells)
HemoglobinA1C (Glycosylated Hb, present in concentration of 3-5% of total Hb). In diabetes mellitus it is increased to 6 to 15%.
1  Hemoglobin as oxygen carrier:The main function of hemoglobin is to carry oxygen from the lungs to all the tissues of the body. This is due to the affinity of hemoglobin for oxygen. When hemoglobin comes in contact with oxygen, it combines with it and form oxy-hemoglobin. This is a week bond. When blood reaches to tissues, where oxygen is deficient, the bond is broken and oxygen diffuses out to tissues.
2.Hemoglobin as carbon dioxide carrier:Some of carbon dioxide is transported from tissues to lungs through hemoglobin. Although the majority of it is transported via plasma but still it carries some of CO2 to lungs.
3.Give  color of blood:The red color of blood is due to hemoglobin. When red blood cells are separated from the blood, the red color disappears. This means that the red color of blood is due to red blood cells. Hence the name red blood cells is given to it. And as we know that hemoglobin is present inside red blood cells, therefore it gives red coloration to RBCs
4.Buffering action:Hemoglobin also acts as a buffer.Buffer means to resist change in pH.Blood has 7.4 pH and it remains in the narrow range.Because, if it changes the life of the person may be endangered.Therefore, hemoglobin plays very important role in keeping the pH of blood constant.
5.Erythrocyte metabolism:Hemoglobin plays an important role in the modulation of erythrocyte metabolism. 
6.Interaction with drugs:Not only for oxygen, but hemoglobin act a very important role the transport of various drugs to their site of action.
7. Physiological active catabolites:Hemoglobin is a source of various physiological active catabolites.

Pulmonary surfactant

Surfactant is a surface acting material or agent that is responsible for lowering the surface tension of a fluid. Surfactant that lines the epithelium of the alveoli in lungs known as pulmonary surfactant and it decreases the surface tension on the alveolar membrane.
Pulmonary surfactant is secreted by two types of cells:
1. Type II alveolar epithelial cells in the lungs, which are called surfactant secreting alveolar cells with microvilli on their alveolar surface.
2. Clara cells(bronchiolar exocrinecells.), which are situated in the bronchioles.

Surfactant is a lipoprotein complex formed by lipids especially phospholipids, proteins and ions.
1. Phospholipids: Phospholipids form about 75% of the surfactant. Major phospholipid present in
the surfactant is dipalmitoylphosphatidylcholine (DPPC).
2. Other lipids: Other lipid substances of surfactant are triglycerides and phosphatidylglycerol (PG).
3. Proteins: Proteins of the surfactant are called specific surfactant proteins. There are four main surfactant proteins, called SPA, SPB, SPC and SPD.SPA and SPD are hydrophilic, while SPB
 and SPC are hydrophobic. Surfactant proteins are vital components of surfactant and the surfactant
becomes inactive in the absence of proteins.
4. Ions: Ions present in the surfactant are mostly calcium ions.
Functions of surfactant
1. Surfactant reduces the surface tension in the alveoli of lungs and prevents collapsing tendency of lungs.Phospholipid molecule in the surfactant has two portions. One portion of the molecule is hydrophilic. This portion dissolves in water and lines the alveoli. Other portion is hydrophobic and it is directed towards the alveolar air. This surface of the phospholipid along with other portion spreads over
the alveoli and reduces the surface tension. SPB and SPC play active role in this process.
2. Surfactant is responsible for stabilization of the alveoli, which is necessary to withstand the collapsing tendency.
3. It plays an important role in the inflation of lungs after birth. In fetus, the secretion of surfactant begins after the 3rd month.
4. Another important function of surfactant is its role in defense within the lungs against infection and inflammation. Hydrophilic proteins SPA and SPD destroy the bacteria and viruses by means of opsonization. These two proteins also control the formation of inflammatory mediators.

Thursday, August 10, 2017

Structure of Human Digestive System

Digestion is the process by which complex food materials are broken down to simple absorbable molecules, so that they can be directly used by thecells of the body. The digestive system is the organ system that processes food, extracts nutrients from it, and eliminates the residue. The study of the digestive tract and the diagnosis and treatment of its disorders is called gastroenterology.
General structure  of Alimentary Canal
The digestive system has two anatomical subdivisions, the digestivetract (alimentary canal) and the accessory organs. The digestive tract is a tube extending from mouth to anus, measuring about 9 meters in length. It includes the oral cavity, pharynx, oesophagus, stomach, small intestine and large intestine. Of these, the stomach and intestines constitute the gastrointestinal (GI) tract. The accessory organs are the teeth, tongue, salivary glands, liver, gall bladder and pancreas.
Histology of gut:
Most of the regions of the digestive tract have the same basic structural plan , with a wall composed ofthe following tissue layers,  in order from outer  to inner surface:
1.Serosa (serous coat): The serosa is composed of a thin layer of areolar tissue covered by a
simple squamous mesothelium. Serosa is present from about the lower 3 to 4 cm of the oesophagusand ends at the sigmoid colon. The other regions are covered by a fibrous connective tissue called the adventitia.The oesophagus, stomach and intestines have a nervous network called the enteric nervous system, which regulates the motility of the digestive tract. The stomach and intestines are enfolded and suspended from the body wall by extensions of the peritoneum which form the mesenteries.
2.Muscularis externa: It comprises smooth  muscle layers; an inner layer of circular muscles and an outer layer of longitudinal muscles. Contractions of this layer help in the thorough mixing of food with digestive enzymes in the lumen of the gut  .
3. Submucosa: It is a thicker layer of loose connective tissue containing blood vessels, lymphatic vessels, nerve plexus, and in some places, glands that secrete mucus.
Mucosa: This is the innermost layer and consists of an inner layer of epithelium, a loose connective tissue layer called lamina propria, and a thin layer of smooth muscle called the muscularis mucosae. The epithelium is simple columnar in most areas but stratified squamous type from oral cavity to oesophagus and in the lower anal canal.
It receive food and is guarded by lips and it leads to the oral or buccal cavity. The sides of the buccal cavity are formed by the cheeks.There are the upper and iower jaws bearing teeth, 32 in number. There are two incisors, one canine, two premolars and three molars on each half of the jaw to cut, tear and grind the food.