Life Cycle of HIV

The HIV infects and eliminates the important cells of immune system that results in the weakness of the body to defend itself from infections.

The simplified life cycle of HIV may be summarised in the following 4 steps:

1. Attachment: When HIV is introduced into the human blood stream, the virus circulates throughout the entire body but infects only CD4+ cells (special kind of WBCs). The HIV recognizes a specific kind of target cells because every kind of cell in the human body has specific cell-surface glycoprotein markers that serve to identify them. Each HIV possesses a glycoprotein called gp120 on its surface that precisely fits a cell surface marker protein called CD4 on the surfaces of immune system cells called macrophages and T-cells. Macrophages are infected first.
2. Entry into Macrophages: HIV requires a second macrophage receptor called CCR5. After gp120 binds to CD4, it grows to bind to CCR5. It forms gp120-CD4 complex into the cell through the cell membrane.
3. Replication: HIV has an enzyme called reverse transcriptase that synthesizes a double stranded DNA complementary to the virus RNA. This double stranded DNA directs the host cell machinery to produce many copies of the virus. Over a period of years HIV synthesises large number of viruses.
4. Entry into T-cells: The HIV constantly replicates and mutates. It causes the gp120 protein to change. Now it requires a different second receptor known as CXCR4 which occurs on the surface of T-lymphocytes CD4+ cells. Soon the body's T-lymphocytes becomes infected with HIV. New viruses rupture the cell membrane, killing the infected T-cells. It blocks the immune response leading to the onset of AIDS.

Circulatory Pathways Among Vertebrates

All vertebrate possess a muscular heart. They show 3 types of Circulatory Pathways : Single circulation, Incomplete double circulation and Double circulation.

frogs'heart 

fish's heart

1. Single Circulation: Fishes show the single circulation pathway. Their hearts are 2-chambered with an atrium and a ventricle. The heart pumps out only deoxygenated blood to the gills for oxygenation. The oxygenated blood is supplied to all body parts and the deoxygenated blood from all the parts is returned to the heart.  
2. Incomplete Double Circulation: Amphibians and the  Reptiles excluding crocodiles show the incomplete double circulation. The is 3 chambered with 2 atria and a ventricle. The left atrium receives oxygenated blood while the right atrium receives deoxygenated blood. But in single ventricle the oxygenated blood and deoxygenated blood get mixed up and that is pumped to all parts of the body.                   3. Double Circulation: Aves and Mammals show the double circulation. The heart is 4 chambered with 2 atria and 2 ventricles. The oxygenated and deoxygenated blood are completely partitioned from each other. The left auricle receives oxygenated blood, that passes into the left ventricle which pumps  it to all parts of the body. The right atrium receives deoxygenated blood from all the parts of the body that passes into the right ventricle which pumps it to the lungs for oxygenation. 

Human Male Reproductive System

Human male reproductive system consists of the following main organs:

1. Primary Sex Organs
2. Secondary Sex Organs
3. External Genitalia

1. Primary Sex Organ: The organs which produce sex cells or gametes and sex hormones are called Primary Sex Organs. Testes is the PSO of Human male. It is 2 in number. The testes are located outside the abdominal cavity within a pouch called Scrotum. This position is called extra-abdominal position. Scrotum provides a temperatures 2-2.5 degree C lower than the body temperature. It is due to the fact that formation of sperms (=spermatogenesis) requires low temperature.  

structure of Testis

Each testis is oval in shape. It measures about 5 cm in length, 2-3 cm width and about 3 cm in thickness. The  testis is covered by a dense covering called tunica albuginea. Internally the testis is divided into about 250 compartments called testicular lobules. Each lobule contains 1-3 highly coiled seminiferous tubules. Seminiferous  tubules are the structural and functional units of testis. Sperms are produced in the testis. The seminiferous tubule is internally lined bu germinal epithelium. The germinal epithelium consists of 2 types of cell : Sertoli cell and Male germ cell (Spermatogonia). Sertoli cells provide nutrition and shape to the developing germ cells and spermatozoa. The spermatogonia undergo cell divisions to form spermatozoa. The spaces between two seminiferous tubules have Leydig cells or Interstitial cells which synthesize and secrete the male sex hormones called androgens or Testosterone. 

2. Secondary Sex Organs: The organs which conduct gametes and sex hormones are called SSO. Duct system and Accessory glands are SSO. 

Duct System: The seminiferous tubules connect and form network called rete testis. From the rete testis arise 10-20 fine tubes called vasa efferentia. They open into the epididmis. The epididmis is a single convoluted tubule that is located along the posterior surface of the testis. The epididmis continues as the vas deferens or sperm duct. It loop over the urinary bladder and receives a duct from the seminal vesicle to form ejaculatory duct that runs through the prostate glands. The urethra receives the ducts of prostate gland and the Bulbourethral (Cowper's) gland and runs through the penis to its external opening called urethral meatus. 

Accessory glands: The accessory glands of male reproductive system include a pair of seminal vesicles, a prostate gland and a pair of Cowper's gland. Their secretions are collectively called as seminal plasma. Seminal plasma is rich in fructose, calcium and certain enzymes. It provides nutrition and a medium for the spermatozoa to swim in the female reproductive tract. The seminal plasma along with the spermatozoa is called semen. 

3. External Genitalia : Penis is the external genitalia in human males. It is made of special erectile  tissue that helps in erection of the penis to facilitate insemination. The enlarged tips of the penis is called glans penis. It is covered by a loose fold of skin  called fore skin or perpuce.

Blood Groups

There are several blood groups. The important blood group system is ABO blood grouping system. The other important blood grouping is Rh-grouping. In both cases, the criterion is the presence or absence of certain antigens on the surface of erythrocytes (RBCs). 

The presence or absence of antigen A and/ or antigen B on the surface of RBCs is the basis of ABO Blood grouping. The following table gives the characteristics of the different blood groups:

..................................................................................................................................................................

Blood Group     Antigen on RBC     Antibody in Plasma     Can be donated to group(s)

..................................................................................................................................................................

A                                     A                                  anti-B                               A,O

B                                     B                                  anti-A                               B, O

AB                                A, B                                 nil                               A,B,AB, O

O                                    nil                             anti-A, anti-B                       O

................................................................................................................................................................

ABO Blood grouping was discovered by Landsteiner.

O group is called 'Universal Donar' as such individual can donate blood to persons of any blood group. It is due to the absence of antigen on RBC.

AB group is called 'Universal Acceptor' as such individual can take blood of any group. This is because, there is no antibody in this blood group to react with antigens of the donor.

Rh-Grouping:

Rh-antigen is a protein discovered in Rhesus monkeys. Rh-antigen is also present on the surface of RBCs of human beings. About 80% of human individuals have Rh-antigen and they are called Rh-positive. Others, who lack this antigen are called  Rh-negative.

If Rh-positive blood is transfused into an Rh-negative individual, specific anti-Rh-antibodies are formed in the blood of the recipient. If the Rh-negative woman bears an Rh-positive foetus, at the  time of delivery of the first child, when foetal blood comes in contact with the mother's blood, anti-Rh antibodies are formed by the mother's blood. These antibodies  remain in the blood of mother for a long period. If a second Rh-positive foetus is borne by the mother, the anti-Rh-antibodies in the mother's blood enter the foetal circulation and destroy  the RBCs, it results in jaundice and severe anemia in the new born. Such a condition is known as erythroblastosis foetalis.

Blood - 2

RBC : They are also called Erythrocytes. They are many in number. They are biconcave and disc-like corpuscles. Their diameter is about 7-8 μm. Their number is about 5.0 million/cubic mm. They do not have nucleus. They do not have mitochondria or endoplasmic reticulum. They are formed in the red bone marrow. Their life span is about 120 days. The spleen is known as the graveyard of RBC. The RBCs contain haemoglobin (Hb) pigment which is a red colored substance. The haemoglobin contains iron. An adult person has about 12-16 g of Hb in 100 ml of blood. RBC also contains carbonic anhydrase.

WBC : They are also called Leucocytes. They are large. They are colourless due to the absence of any pigments. Their number is about 7000/cubic mm. They have a nucleus centrally. Their life span is about 24 hours. Leucocytes are broadly classified into granulocytes and agranulocytes.

Granulocytes: They are the cells with granular cytoplasm and polymorphic nucleus. They are further grouped into 3 categories:

1. Neutrophils : They do not stain with acidic or basic dyes. The nucleus is 5-7 lobed. They are phagocytic and provide protection against infection. They constitute about 62% of leucocytes.
2. Basophils : They are stained by acidic dyes like Eosin. The nucleus is bilobed. They are slightly phagocytic and help in detoxification. They constitute about 2-3 % of leucocytes.
3. Eosinophils : They are stained with basic dyes like Methylene Blue. The nucleus is 2 or 3 lobed. They play an important role in inflammatory reactions. They constitute about 0.5-1% of leucocytes.

Agranulocytes: They are the cells that have no granular cytoplasm. Their nucleus is kidney shaped. Their colour is pale-blue. They are of the following 2 types:

1. Monocytes : They are large cells. Their diameter is about 10-18 μm. Nucleus is kidney shaped. They are phagocytic and motile. They form 6-8% of leucocytes.
2. Lymphocytes: They may be small (7-10 μm in diameter) or large (10-14 μm in diameter). The nucleus is large and round. They help the immune system of the body. They form about 25-30% of leucocytes.

Thrombocytes :  They are the cell fragments formed by megakaryocytes in the bone marrow. Their number ranges between 1,50,000 and 3,50,000 cubic mm. They release blood clotting substances. 

Blood - 1

Blood is a fluid connective tissue. It consists of two main components : Blood Plasma and Formed Elements (or, Corpuscles).

Blood Plasma: It is the fluid part of the blood. It is a viscous fluid and a straw colored fluid. It constitutes about 55% of the total blood. It is made of water, mineral ions  and proteins. It contains about 90% of water and 6-8% of proteins. The reaming portion is made of mineral ions such as Na, Ca, Mg, HCO3, Cl, PO4 etc. The major proteins of blood plasma are :

Fibronogens that are essential for blood clotting.

Globulins that  are essential for defense mechanism.

Albumins that help to maintain the osmotic balance.

Other organic compounds of plasma are glucose, amino acids, lipids, hormones, vitamins etc.

Formed elements constitute about 45% of the blood. There are three types of corpuscles :

1. RBC (Red Blood Corpuscles) or Erythrocytes
2. WBC (White Blood Corpuscles) or Leucocytes
3. Thrombocytes or Platelets

Evolution : Meaning & Definition

Evolution is derived from Latin word evolvere (e= from/ out & volvere=to roll) and it means the act of unfolding or to unroll or to revel hidden potentialities. Evolution is the doctrine according to which higher forms of life gradually arisen out of lower.


The term EVOLUTION was first used by Charles Bonnet (according to Calow, 1983). However according to Savage (1969), the term evolution was first used by Herbert Spencer.


Evolution has been defined as a gradual orderly change from one condition to another. As per Charles Darwin's view, evolution is descent with modification or continuity of life with constant modification i.e. closely related species resembling one another because of their inheritance and differing from one another because of the hereditary differences accumulated during the separation of their ancestors. 


Evolution is a dynamic process by which related populations diverge from one another, giving rise to new species or higher groups. (Dodson & Dodson, 1976)


Evolution in its simplest understanding means change - orderly change between generations. 


Evolution means the transformation of species.


The doctrine of evolution assumes that the present day complex animals and plants have been developed in the course of long ages by a process of gradual change in the earlier simpler forms of life. However, not all kinds of biological change can be addressed as evolution. 


Developmental change within the life of an organism should not be addressed  as evolution in strict sense and the definition refers to descent with modification. The concept of evolution should not include the idea that evolution is always progressive leading invariably from simpler to more complex forms of life. Evolutionary modification in living thing depends on external environmental change and on random genetic innovation. Thus, the form of future change cannot be predictable.


I think that evolution is the love-affair between Nature and Energy.

Evolution : Synopsis I

This post is giving you synopsis of evolution taken from NCERT class XII Chapter 7. It is very useful for revision purpose. It is also helpful in making Biology notes.

• Evolutionary Biology is the study of history of life forms on earth.
• What exactly is evolution?
• Stellar distances are measured in light years.
• when we see stars we apparently are peeping into the past.
• The universe is very old – almost 20 billion years old.
• Huge clusters of galaxies comprise the universe.
• Galaxies contain stars and clouds of gas and dust.
• The Big Bang theory attempts to explain to us the origin of universe. It talks of a singular huge explosion unimaginable in physical terms. The universe expanded and hence, the temperature came down. Hydrogen and Helium formed sometime later. The gases condensed under gravitation and formed the galaxies of the present day universe.
• In the solar system of the milky way galaxy, earth was supposed to have been formed about 4.5 billion years back. There was no atmosphere on early earth. Water vapour, methane, carbondioxide and ammonia released from molten mass covered the surface. The UV rays from the sun brokeup water into Hydrogen and Oxygen and the lighter H2 escaped. Oxygen combined with ammonia and methane to form water, CO2 and others. The ozone layer was formed. As it cooled, the water vapor fell as rain, to fill all the depressions and form oceans.
• Life appeared 500 million years after the formation of earth, i.e., almost four billion years back.
• Did life come from outerspace?
• Some scientists believe that it came from outside. Early Greek thinkers thought units of life called spores were transferred to different planets including earth. ‘Panspermia’ is still a favourite idea for some astronomers.
• For a long time it was also believed that life came out of decaying and rotting matter like straw, mud, etc.This was the theory of spontaneous generation.
• Louis Pasteur by careful experimentation demonstrated that life comes only from pre-existing life. He showed that in pre-sterilised flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose from ‘killed yeast’. Spontaneous generation theory was dismissed once and for all. However, this did not answer how the first life form came on earth.
• Oparin of Russia and Haldane of England proposed that the first form of life could have come from pre-existing non-living organic molecules (e.g. RNA, protein, etc.) and that formation of life was preceded by chemical evolution, i.e., formation of diverse organic molecules from inorganic constituents. The conditions on earth were – high temperature, volcanic storms, reducing atmosphere containing CH4, NH3, etc. In 1953, S.L. Miller, an American scientist created similar conditions in a laboratory scale. He created electric discharge in a closed flask containing CH4, H2, NH3 and water vapaur at 8000C. He observed formation of amino acids. In similar experiments others observed, formation of sugars, nitrogen bases, pigment and fats. Analysis of meteorite content also revealed similar compounds indicating that similar processes are occurring elsewhere in space. With this limited evidence, the first part of the conjectured story, i.e., chemical evolution was more or less accepted.
• We have no idea about how the first self replicating metabolic capsule of life arose. The first non-cellular forms of life could have originated 3 billion years back. They would have been giant molecules (RNA, Protein, Polysaccharides, etc.). These capsules reproduced their molecules perhaps. The first cellular form of life did not possibly originate till about 2000 million years ago. These were probably single-cells. All life forms were in water environment only. This version of a biogenesis, i.e., the first form of life arose slowly through evolutionary forces from non-living molecules is accepted by majority. However, once formed, how the first cellular forms of life could have evolved into the complex biodiversity of today is the fascinating story.

 thanks!