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Meselson and Stahl Experiment: Semiconservative DNA Replication ||Class 12 Biology

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Matthew Meselson and Franklin Stahl in 1958 perform an experiment on Escherichia coli (E. coli). Outline of the experiment are      1. E. coli was grown in a medium containing 15NH 4 Cl as the only nitrogen source for many generations. 15N got incorporated into newly synthesised DNA (and other nitrogen containing compounds). This heavy DNA molecule could be distinguished from the normal DNA by centrifugation in a cesium chloride (CsCl) density gradient.        2. They then transferred the cells into a medium with normal 14NH 4 Cl and took samples at various definite intervals as the cells multiplied and extracted the DNA that remained as double- stranded helixes. DNA samples were separated Independently on CsCL gradient to measure DNA densities.        3. The DNA obtained from the sample of the growth of one generation after the transfer from 15N to 14N medium, had a hybrid or intermediate density.       4. While the DNA obtained from the sample growth after another generation (second
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Oogenesis: Human Reproduction ||Class 12 Biology

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     The process of formation of a mature female gamete is called oogenesis. It is the production of eggs (ova) that starts in ovaries of female foetus (of 25 weeks). It begins before birth but complete only after fertilization.      It comprised of the following three main stages: 1. Multiplication Phase 2. Growth Phase 3. Maturation Phase 1. Multiplication Phase: Cells of germinal epithelium undergoes mitotic division producing undifferentiated germ cells called oogonia or egg mother cells within each foetus ovary. These cells start dividing and enter prophase-I of meiotic division and get primarily arrested at this stage, known as primary oocytes. 2. Growth Phase: Each primary oocyte then gets surrounded by a large of granulosa cells and called primary follicle. A large number of these follicles degenerate during the phase from birth to puberty. Therefore, at puberty only 60,000-80,000 primary follicles are left in each ovary. The primary follicles get surrounded by more layers of g
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Avery MacLeod McCarty Experiment: Molecular Basis of Inheritance ||Class 12 Biology

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Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44) worked to determine the biochemical nature of transforming principle in Griffith experiment. They purified biochemicals (proteins, RNA, DNA) from heat killed S-cells and discovered that DNA alone from S-bacteria caused R-bacteria to be transformed.      They also discovered that protease (protein digesting enzyme) and RNAses (RNA-digesting enzymes) did not affect transformation. Digestion with DNAse inhibit transformation, indicating that DNA caused transformation. They concluded that DNA is the hereditary material.
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Menstrual Cycle: Human Reproduction ||Class 12 Biology

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     The menstrual cycle is the regular natural change that occurs in the female reproductive system that makes pregnancy possible. It is known as the cyclical reproductive events in females of all primates, i.e. monkey, apes and human beings. The first menstruation begins at puberty and is called menarche. In human females, menstruation is repeated at an average interval of about 28-29 days. The menstrual cycle stops at around 50 years of age, termed as menopause. The cyclic menstruation is an indication of normal reproductive phase and continues between menarche and menopause. The menstrual cycle consist of following four phases:  1. Menstrual phase 2. Follicular or Proliferative phase 3. Ovulatory phase 4. Luteal phase 1. Menstrual phase: The cycle starts with this phase which last for about 4 days. It involves the following main events-      • Reduction of LH level causes regression of corpus luteum and a consequent fall in the progesterone level in the blood.       • The uterine l
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Immunity: Human Health and Diseases ||Class 12 Biology

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Immunity is the capacity of an organism to resist or defend itself from the development of a disease. Immunity involves both specific and non specific components. The non specific act as barriers or eliminators of a wide range of pathogens irrespective to their antigenic make-up. Other components of the Immune system adapt themselves to each new disease encountered and can generate pathogen specific Immunity.       Immunity is of two main types: 1. Innate Immunity 2. Acquired Immunity or Adaptive Immunity 1. Innate Immunity : It is type of Immunity which is present from the birth and is inherited from the parents. It is non-specific in nature.      Innate Immunity is of following main types of barriers- i) Physical barriers: It prevent entry of microorganisms in the body, e.g. skin, mucus coating of epithelium, lining the respiratory, gastrointestinal and urogenital tracts. ii) Physiological barriers: It prevent microbial growth in the body, e.g. acid in the stomach, saliva in the mout
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Drug and Alcohol Abuse: Human Health and Diseases ||Class 12 Biology

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      Drug : A drug is any substance that causes a change in an organisms physiology or psychology when consumed. Drug is given people in order to treat or prevent an illness or disease.       Drug abuse : Drug abuse is use of a drug in amounts or by methods which are harmful to the individual or other.        Alcohol : It has an anaesthetic effect on nervous system affecting cerebrum, cerebellum and other parts.      Effects of alcohol include peptic ulcer, gastric carcinoma, hepatitis, liver failure, euphoria etc.      The source of drugs are mainly folwering plants and some of the fungi etc. Drugs fall into following categories:       Opioids : These are the drugs which bind to the specific opioid receptors present in our central nervous system. It is obtained from unripe capsule of Papaver somniferum or Poppy plant. They may be natural, e.g. opium, morphine, codeine, semisynthetic, e.g. heroin, smack and synthetic, e.g. pethidine, methadone. These have narcotic, analgesic, sedative
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Common Infectious Diseases in Humans ||Class 12 Biology

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     Infections diseases are classified on the basis of pathogen. Some common diseases in human are as follows:  1. Bacterial diseases: i) Typhoid: It is caused by bacterium (Salmonella typhi) which enters the small intestine through food and water and migrate to other organs through blood.       Symptoms: High fever (39-40°C), weakness, stomach pain, constipation, headache, loss of appetite.      Widal test is done as a confirmation test for the typhoid ii) Pneumonia: It is caused by the bacterium Streptococcus pneumoniae and Haemophilus influenzae which infect alveoli of the lungs. The alveoli get filled with fluid causing decrease in respiratory efficiency of the lungs.      Pneumonia spreads by inhaling droplets or aerosol from infected individuals or even by sharing glasses and utensils with patients.      Symptoms: Fever, chills, cough, headache etc. 2. Viral Disease: i) Common cold: It occurs due to a group of viruses called rhinoviruses which infect the nose and respiratory pas
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Mendel's Laws of Inheritance: Definitions & Examples ||Class 12 Biology

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     Based on Mendel observation, he proposed the following three main laws- 1. Law of Dominance (First Law) 2. Law of Segregation (Second Law) 3. Law of Independent Assortment (Third Law) 1. Law of Dominance :       According to the Mendel's first law of Inheritance, when two alternative forms of a trait or character (genes) are present in an organism, only one factor expresses itself in F1-progeny and is called dominant, where the other that remains hidden or masked is called recessive.      The law is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1-generation and expression of both in the F2-generation. It also explains the reason for the proportion of 3:1 obtained in the F2-generation. 2. Law of Segregation :       According to the second law, Alleles do not show any blending (mixing of characters) and both the characters are recovered as such in the F2-generation, though one of these is not seen in the F1-generation.      D
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CO-DOMINANCE ||Class 12 Biology

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     In heterozygous condition both the alleles are expressed. Both alleles are expressed in dominant forms. The alleles which shows this property are called codominant alleles and the phenomenon is known as co-dominance.       In this condition, the offsprings shows resemblance to both the parents, eg- ABO blood groups in humans. ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells (RBCs) has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene. The gene (I) has three alleles: IA, IB and i. The alleles IA and IB produce a slightly different form of the sugar while allele i does not produce any sugar. Because humans are diploid organisms, each person possesses any two of the three I gene alleles. IA and IB are completely dominant over i, in other words when IA and i are present only IA expresses (because i does not produce any sugar) and when IB and i are present IB expresses. But when IA and IB are present t
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Incomplete Dominance ||Class 12 Biology

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     Incomplete Dominance is defined by Carl Correns. It is a phenomenon in which the F1 hybrid shows intermediate characters of the parental genes. In incomplete dominance, the phenotypic ratio deviates from the normal Mendel's monohybrid ratio. Characters are not blending in F1 generation because parental types are recovered in F2 generation.       Example: Flowers colour in Mirabilis jalapa or 4 o' clock plant and Snapdragon or Antirrhinnus sp.         In a cross between red flower (RR) and white flower (rr) plant, the F1 generation obtained was pink (Rr). Again, when the same F1 progeny was self-pollinated, the F2 resulted in the ratio 1:2:1, i.e. not 3:1.
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