Current Affairs 12th Class

Embryonic development involves following dynamic changes and identifiable process. (1) Gametogenesis : It involve the formation of haploid sex cells or gametes called sperms and ova from diploid primary germ cells called gametogonia present in the reproductive organs called gonads (testes and ovary). It is of two types (i) Spermatogenesis : Formation of sperm. (ii) Oogenesis : Formation of ova (2) Fertilization : It involve the fusion of haploid male and female gametes to form diploid zygote. The fusion of gametic pronuclei is called Karyogamy while the mixing of two sets of chromosomes of two gametes is called amphimixis. (3) Cleavage : It includes the rapid mitotic division of the zygote to form a single layered hollow spherical larva called blastula and its formation is called blastulation. (4) Implantation : The process of attachment of the blastocyst (mammalian blastula) on the endometrium of the uterus is called implantation. (5) Gastrulation : It includes the mass and orderly migration of the organ specific areas from the surface of blastula to their predetermined position which finally produces a 3 layered gastrula larva. It is with 3 primary layers. (6) Organogenesis : It includes the formation of specific organs system from three primary germ layers of gastrula and also includes the morphogenesis and differentiation.   Important Tips   Historical background of Embryonic Development:

• George Newport: Observed fertilization of frog’s egg.
• Oscar Hertwig: Described the fusion of sperm and egg nuclei in sea urchin.
• Prevost and Dumas: Reported cleavage of frog’s egg.
• Swammerdam: Observed the first cleavage of frog in 1738.
• Spallanzani: Detailed process of cleavage of frog’s egg.
• Von Bear: Proposed recapitulation theory.
• Ernst Haeckel: Modified recapitulation theory to “Biogenetic law”. It states “Ontogeny repeats phylogeny.”
• Spemann and Mangold: Reported embryonic induction on newt and gave concept of primary organizers.
• Pander: Formation of three germinal layers in chick embryo.
• Oviparity: Fertilization may be external or internal but development always outside the female body and inside the egg e.g. most of non-chordates, fishes, amphibians and reptiles; all birds and prototherians.
• Ovoviviparity: Fetilization always internal. Development also inside the uterus and baby is born but there is no placenta formation so egg is yolky e.g. rattele snake, Dog fish
• Viviparity: Fertilization and development always inside the body. Placenta is formed and female gives birth to young one e.g. most of mammals.

(i) Definition: Placenta is defined as a temporary intimate mechanical and physiological connection between foetal and maternal tissues for the nutrition, respiration and excretion of the foetus.   (ii) Structure: Human placenta consist of chorion only. Hence, it is called a chorionic placenta. Allantois remains small. The allantoic blood vessels, however, extend to vascularize it. A large number of branching villi from the vascular chorion penetrate the corresponding pits, the crypts, formed in the uterine wall. The latter becomes very thick and highly vascular to receive the villi. The intimate connection established between the foetal membrane and the uterine wall is known as the placenta. The placenta has two parts: the part contributed by the foetus, i.e., chorionic villi, is called the foetal placenta; and the part shared by the mother, i.e., part of uterine wall, is termed the maternal placenta. The chorionic villi receive blood from the embryo by umbilical artery and return it by umbilical vein. These blood vessels are derived from the allantois and run between the foetus and the uterine wall in the tough umbilical cord covered with cells derived from the amnion and chorion. The choroinic villi come to lie in uterine lacunae that receive blood from the uterine arteriole and return it by uterine venule. The cells forming the wall of chorionic villi bear microvilli which increase their surface area for quick and adequate exchange of materials by diffusion, active transport and pinocytosis.     The placenta is fully formed by the end of the third month and it lasts throughout pregnancy. When complete, it is a reddish – brown disc. In the placenta, the foetal blood comes very close to the maternal blood, and this permits the exchange of materials between the two. Food (glucose, amino acids, simple proteins, lipids), water, mineral salts, vitamins, hormones, antibodies and oxygen pass from the maternal blood into the foetal blood, and foetal metabolic wastes, such as carbon dioxide and urea, also water and hormones, pass into the maternal blood. The placenta, thus, serves as the nutritive, respiratory and excretory organ of the foetus. The continuous uptake of oxygen by foetal blood is ensured by the difference in affinity for oxygen between foetal and maternal haemoglobin.   The maternal and foetal blood are not in direct contact in the placenta, because (i) the two may be incompatible; (ii) the pressure of maternal blood is far too high for the foetal blood vessels; and (iii) there must be a check on the passage of harmful materials (blood proteins, germs) into the foetal blood.   (iii) Functions (1) Placenta helps in the nutrition of the embryo as the nutrients like amino acids, monosugars, vitamins, etc. pass from the maternal blood into foetal blood through placenta. (2) It also helps in respiration of the embryo as \[{{O}_{2}}\] of the maternal more...

Reproduction : Reproduction is the ability of living organism to produce a new generation of living individuals similar to themselves. Basic features of reproduction : All organisms reproduce. Modes of reproduction vary in different organisms. However, all modes have certain common basic features. These are (1) Replication of DNA. This is the molecular basis of reproduction. (2) Cell division, only mitotic, or both mitotic and meiotic. This is cytological basis of reproduction. (3) Formation of reproductive bodies or units. (4) Development of reproductive bodies into offspring. Types of reproduction : These are of two main types (1) Asexual (Non-gametic)                           (2) Sexual (gametic) (1) Asexual reproduction Definition : Production of offspring by a single parent without the formation and fusion of gametes is called asexual reproduction. The young one receives all its genes from one parent. Asexual reproduction is also known as agamogenesis or agamogeny. It involves only mitotic cell divisions, and also termed somatogenic reproduction. Asexual reproduction produces identical offspring commonly referred to as a clone. Today, the scientists have been able to produce clones of multicellular animals (e.g., boar calf names as Frosty, and Finn Dorset lamb named as the famous Dolly) artificially in the laboratory. Occurrence : Asexual reproduction occurs in protozoans and some lower animals such as sponges, coelentrates, certain worms and tunicates. It is absent among the higher non-vertibrates and all vertibrates. Types : Asexual reproduction takes place in five principal ways : (i) Binary fission : Binary fission is the division of the parent into two small, nearly equalized daughter individuals. During binary fission nuclear divisions or karyokinesis, always followed by division of cytoplasm or cytokinesis. Examples - Protozoans (Amoeba, Euglena etc.) Bacteria and Planarians. Modes of binary fission : In Binary fission, the nucleus divides first and the cytoplasm next. Subsequently, the mother cell splits into two equal sized daughter halves or cells. There are three modes of binary fission. (a) Simple binary fission : If the plane of cytoplasmic division passes through any direction, the fission is called simple fission. Example - Amoeba.     (b) Transverse binary fission : If the plane of cytoplasmic division coincides with the transverse axis of the individual, the fission is termed transverse binary division. Example - Paramecium and Planaria.     (c) Longitudinal binary fission : If the plane of cytoplasmic division concides with the longitudinal axis of the individual. This kind of fission is designated as longitudinal binary fission. Example Euglena and vorticella.   Binary fission involves mitosis only and consequently, the resultant offspring's are genetically identical to the parent and each other. (ii) Multiple fission : Multiple fission is the division of the parent into many small daughter individuals simultaneously. Examples – Multiple fission occurs in many protozoans such as Plasmodium, more...

Definition : Ageing is the slow deterioration in the structure and function of body cells tissues and organs of an animal and starts after the adulthood. Gerontology : The field of developmental biology that deal with the process and problems of ageing is known as gerontology – (Gr. geron = old man; logos = discourse). The scientists involved in the science of ageing are called gerontologist. Life cycle and life span : In all metazoan animals, the life cycle includes two developmental period; embryonic period (pre-natal developmental period) which extends from zygote to offspring till hatching or birth, and post embryonic period (post-natal developmental period)- which includes growth, adulthood, reproduction, ageing. Thus, the life cycle comprises five main events : birth, growth, maturity, old age and death, that follow in the sequence named. Maximum life span is the maximum number of years survived by any member of a species, while average life span is the number of years survived by members of a population. Life expectancy is the age at which half the population still survives.   Life span in different organisms

S.No.	Animals name	Life span
1.	Mayfly	24 hours
2.	Silk moth	2-3 days
3.	Mouse	3-5 years
4.	more... Catgeory: 12th Class Death Definition : Death may be defined as the permanent cessation of all the vital function in an organism. Characteristics (1) It is the last event in the degenerative processes of ageing. (2) Death of an organism involves the death of the body cells. But all the cells of the body do not die at the same rate e.g., ciliated cells lining the respiratory tract of mammals continue to beat their cilia for a long time even after animal's death. Brain cells of body are last to die. (3) There is no natural death in the protozoans e.g., Amoeba. (4) Death involves widespread cell breakdown and cell death. (5) It usually occurs due to lack of oxygen supply to body tissues. Causes of death : Causes of death are many. These can be separated into following main categories : (1) The weakening of the body tissues and of vital organs like heart, lungs, liver, kidneys, etc. which cause physiological and metabolic disorders of permanent nature leading to death. Death, in some cases, occurs due to sudden stoppage of the circulation of blood, food and oxygen to heart and brain leading to immediate death. (2) The immune system (A system that provide resistance against disease - causing microbes) of the body is gradually impaired with advancing age. This increases the chances of infection in old age. Many old persons die of infectious diseases. (3) Sudden blockage in the circulation of blood to heart, lungs and brain. This causes instantaneous death. Brain or cerebral death : In the presence of cardiac activity, the permanent loss of cerebral functions, manifested clinically by absence of responses to external stimuli, lack of breath, and absence of cerebral reflexes is called brain death. The only truth of life : The death is an inevitable reality of life, and should be gladly accepted. It is a biological necessity for the maintenance of the balance of nature. Old organisms must make room for new ones. Habit that influence life span : Life-style habits can influence life span considerably. Although a healthful diet does not guarantee immortality, regular exercise and avoiding of alcohol, smoking and drugs, contentment and freedom from stress can make a person's last years more pleasant. Significance : Death is an essential and inescapable biological phenomenon which helps in maintaining ecological balance or homeostasis in nature. It prevents overcrowding of the members of a specific species and justifies the ‘continuity of life’ on earth. Catgeory: 12th Class Growth Meaning and definition of growth : Growth is an important properties of all living organisms. All organisms grow from a young stage to an adult stage. Growth is a permanent increase in dimensions of the body and its parts. It results from the addition to the body tissues. Growth at different levels (1) Molecular level : At molecular level, the growth involves synthesis of new molecules and their aggregation into organelles and storage products in the cells. (2) Cellular level : At the cellular level, the growth involves. (i) Cell expansion (hypertrophy) : Increase in the size of the cells due to addition of new cell material, called protoplasm. (ii) Cell division (hyperplasia) : Increase in the number of cells by cell division. (iii) Cell differentiation : Specialisation of cells for specific roles, in its broad sense, growth includes. (iv) Matrix formation : Addition of intercellular materials, termed apoplasmatic substances, secreted by the cells between them. The apoplasmatic substances include the matrix of connective tissues and intercellular fluid. (3) Individual level : At individual level, the growth is the visible increase in the body, dimension, size volume and weight. Increase in weight will show that the growth has taken place. Physiological condition for growth : A variety of chemical reaction occur all the time in the living organisms. These are collectively referred to as metabolism. Metabolism has two phases building up phases or anabolism and break down phase or catabolism. Variation in the rates of the metabolic phases result in three types of growth. (1) Positive growth :  Anabolism normally out weighs catabolism and this brings about growth during the growing period of the organism and maintain the body thereafter. This is called positive growth. (2) Zero growth : If the anabolic and catabolic processes are balanced, there is no addition to the bulk of the body and no increase in body size. This is referred to as zero growth. (3) Negative growth : If catabolism occurs at a faster rate than anabolism, as happens in fasting the organism gradually becomes weak and may finally die. At this time, first the food reserves (glycogen, fat) and then body's own protein are used as sources of energy to run the body machine. This depletes the living material, causing negative growth. Due to this the reserve food and living material decrease in amount and is called degrowth. States of growth : Two states of growth are : (1) Pre-functional state of growth : It is the early embryonic stage during which the developmental processes transform a zygote into an embryo. During this growth period the organ rudiments are established but are not functional. (2) Functional state of growth : It is the late embryonic and post embryonic developmental stage during which organ rudiments become functional and organogenesis begins.   Differences between Embryonic growth and Post-embryonic growth more... Catgeory: 12th Class Repair and Regeneration Definition : It is that post-embryonic morphogenetic phenomenon which when temporarily stimulated brings about repair of the damaged cells/Tissues, or replacement or redevelopment of severed body parts or reconstruction of whole body from a small body fragment. Capacity for regeneration : Among animals, power of regeneration was first discovered in Hydra by Trambley, in 1740. The capacity of repeated regeneration, though, present throughout the animal kingdom, but to varying degree. It is more marked in the lower animal than in the higher animals. Among invertebrates, protozoans, sponges and coelenterates, the regeneration capacity is very high. In higher animals, regenerative ability is much greater in the embryonic and larval stages than in the adult. In man, it is restricted to healing of injured tissues such as skin, muscles, bones, blood vessels and nerves; the lost organs cannot be regenerated. Types of regeneration : Regeneration is of two main type - Reparative and Restorative. (1) Reparative regeneration : In this, multicellular organism has the power only to repair certain damaged cells of the body. It is a common phenomenon observed in both invertebrates as well as the vertebrates. (2) Restorative regeneration : In this, a multicellular organism can redevelop the severed body parts or the whole body can be formed from a body segment. It is very common in invertebrates. It may occur by epimorphosis or morphallaxis. The power of restorative regeneration varies in different groups of organisms e.g.,  (i) Autotomy power in some animals, some part of the body is broken off the body on being threatened by the enemy or predator. This phenomenon of self mutilation of body is called autotomy. The lost part may be tail, limb, viscera or arm e.g.,, Crabs break of their leg on approaching the enemy. Lizards throw off their tail. Holothurians (Echinoderm) throw off their internal viscera (respiratory tree etc.). It is called Evisceration. Starfish (Echinoderm) can regenerate the whole arm. Autotomy is a special adaptation for escaping the danger of attack by enemy or predator. (ii) The climax of regeneration in which whole body can be developed from a body fragment is found in Hydra among the coelenterates; Scypha among the sponges and Planaria among flat worms. Mechanism of regeneration : T.H. Morgan recognized two primary mechanism of regeneration in animals. (1) Morphallaxis : It is the reconstruction of an entire animal from a small fragment by reorganizing the existing cells. The regenerated animal is far smaller than the original one after the completion of the process. It grows to attain the normal size. e.g., Hydra (2) Epimorphosis : It replaces a lost organ of the body by proliferating new cells from the surface of the injured part. Regeneration of an appendage in an arthropod, arm in a starfish, limb in a salamander and tail in a lizard occurs in this manner. Regeneration of a limb of a newt or salamander : Newt/salamander has very high power of regenerating their lost limb by the process of restorative regeneration. It more... Catgeory: 12th Class Biotic Or Ecological Succession Community are never stable but keep on changing. This relatively definite sequence of communities over a period of time in the same area is called ecological succession. Types : Succession is of two types : (1) Primary succession : It includes changes which occur when living things become established on a previously uninhabited area such as a newly exposed sea floor, lake sediments or sand dunes. (2) Secondary succession : It occurs where early communities have been damaged, leaving a few organisms and considerable organic matter. These remnant species, along with some new ones, regenerate a new community. According to another classification, the succession is of following types : (1) Autogenic succession : When the succession has begun, the vegetation itself is responsible for replacing itself by changing existing environmental conditions. (2) Allogenic succession : When in succession other conditions (not vegetation itself) are responsible for replacing communities, then it is called allogenic succession. Depending upon the substratum, succession are divided into three types : (1) Hydrosere or Hydrarch : Succession beginning in fresh water, e.g., ponds, lakes, streams, etc. (2) Xerosere or Xerarch : Succession beginning in dry conditions. It is further classified into three types : (i) Lithosere : When succession starts on bare rocks. (ii) Psammosere : When succession begins on sand. (iii) Halosere : When succession starts in saline conditions. (3) Mesarch : When succession begins in mesic conditions. General processes of succession Succession is completed in following steps : (1) Nudation : Formation of bare area without any form of life is called nudation. The cause of nudation may be climatic, topographic or biotic. (2) Invasion : Successful establishment of a species in this bare area is called invasion. It has three steps : (i) Migration                       (ii) Ecesis or establishment                          (iii) Aggregation (3) Competetion and coaction : Competetion may be interspecific or intraspecific. Intraspecific competetion is called coaction. (4) Reaction : Modification of environment due to organisms in it is called reaction. (5) Stabilization : The terminal community becomes stabilized in the prevailing conditions. Biotic succession on bare rock (Lithosere, Xerosere). The sequence of successional stages that occur on bare rocks is called lithosere. Because the bare rock is deficient in water, the lithosere is also called xerosere. The various seral stages are as follows : (1) Lichen stage : Bare rock is invaded first by crustose lichens (e.g., Graphis). They corrode the rock at places causing foliose lichens to invade, eliminate crustose lichens and creating conditions for invasion by mosses. In tropics, blue green algae are pioneers instead of lichens. (2) Moss stage : Mosses are of larger size, have gregareous habit and their rhizoids penetrate deeper in the rocks. They shade the lichens and hence replace the same. Mosses accumulate more soil and organic matter. (3) Annual grass stage : Annual hardy grasses and herbs invade the humus rich moss dominated rock surface, e.g., Aristida, Poa. Their roots cause fragmentation of more... 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