Plant Bible

                                                            Cytological Evidence for Crossing Over   Crossing over is the exchange of chromatid blocks between chromosomes during meiosis. The crossing over occurs during the le stage of meiosis. This is proved by cytological evidences. 1. Stern's experiment. 2. Tetrad analysis 3. Creighton and Mc Clintock's Experiment   1. Stern's Experiment: Crossing over is the interchange of chromosomal parts between homologous chromosomes. This is cytologically proved by Stem Drosophila. The female Drosophila carries XX chromosome and the m Drosophila carries one X chromosome and one Y chromosome In a type of female Drosophila, the two X chromosomes different from each other. An X chromosome has a piece of Y chromosome attached to it. The other X chromosome has been broken into two unequal segments and it is shorter than the unbroken X chromosome. Thus, the two X chromosomes are structurally different from the normal X c

                                                                                       DNA REPAIR Definition: Restoring the damaged part of DNA is called DNA repair. DNA is a fragile molecule that can be easily damaged. It is prone to damage due to replication and mutagenic agents. The following damages are caused to DNA. I. Insertion of a wrong base during replication. Eg. Conversion of cytosine to uracil (found only in RNA). UV rays alter bases.  2. Loss of a purine base. 3. Alteration of bases. Eg. amino group (deamination) converts C to U. 4. Breakage of DNA strands. It may be single stranded break (SSB) or double stranded break (DSB). 5. Covalent linkages between bases. It may be on the same strand (intrastrand) or between strands (interstrand). 6. Mismatches of the normal bases. Eg. Incorporation of U (found only in RNA) instead of T. The following agents cause damage to DNA: 1. Radiation - UV rays, X rays, gamma rays, etc. 2. Highly reactive oxygen - radicals p

Crossing Over Definition: Crossing over is the interchange of chromosomal parts be tween non-sister chromatids of a homologous pair of chromosomes resulting in recombination of genes. Crossing over was discovered by Morgan. Crossing over is the interchange of chromosomal segments between the homologous chromosomes. It occurs during meiosis or gametogenesis. The crossing over occurs only between non-sister chromatids of the homologous chromosomes. The number of crossing over depends upon the length of the chromosome. The longer the length, the higher the percentage of crossing over. When the genes are located distantly, the chance for crossing over is higher. When the genes are closely located the chance for crossing over is lesser. Percentage of crossing over: The number of crossing over between two genes is represented by percentage of crossing over. The percentage of crossing over is directly proportional to the distance between two genes. The percentage of crossing over is t

 Krebs cycle or Citric acid cycle or TCA cycle   Two molecules of acetyl CoA formed from link reaction now enter into Krebs cycle. It is named after its discoverer, German Biochemist Sir Hans Adolf Krebs (1937). The enzymes necessary for TCA cycle are found in mitochondrial matrix except succinate dehydrogenase enzyme which is found in mitochondrial inner membrane. TCA cycle starts with condensation of acetyl CoA with oxaloacetate in the presence of water to yield citrate or citric acid. Therefore, it is also known as Citric Acid Cycle (CAC) or Tri Carboxylic Acid (TCA) cycle. It is followed by the action of different enzymes in cyclic manner. During the conversion of succinyl CoA to succinate by the enzyme succinyl CoA synthetase or succinate thiokinase, a molecule of ATP synthesis from substrate without entering the electron transport chain is called substrate level phosphorylation. In animals a molecule of GTP is synthesized from GDP+Pi. In a coupled reaction GTP is converted to G

                                               Theories on the mechanism of crossing over   There are many theories to explain the mechanism of crossing over   1. Chiasma Type Theory: This theory was proposed by Janssens. This theory suggested that prior to crossing over, the chromosomes of each bivalent geld plicated to form a tetrad. Crossing over occurs only between the non-sister chromatids of a tetrad. In the diplotene stage, the nor sister chromatids overlap with one another and form chiasma or point of contact. In the chiasma, the chromatids break and they rejoin with the mutual exchange of segments. The other two chromatids remain intact. This theory is known as chiasma type theory.   2. The Breakage First Theory:   This theory was proposed by Muller. According to this theory the non-sister chromatids of homologous chromosomes first break off without crossing over. The broken segments rejoin to form new combinations.   3. The Contact First Theory:   This theory

PROTEINS   PROTEINS: Proteins are the most abundant biological macromolecules, occurring in all cells. It is also the most versatile organic molecule of the living systems and occurs in great variety; thousands of different kinds, ranging in size from relatively small peptides to large polymers. Proteins are the polymers of amino acids covalently linked by the peptide bonds. The building blocks of proteins are the twenty naturally occurring amino acids. Thus, proteins are the polymers of amino acids. CONFORMATION OF PROTEIN: 1. Biuret test When 2 ml of test solution is added to an equal volume of 10% NaOH and one drop of 10% CuSO4 solution, a violet col­our formation indicates the presence of peptide linkage. 2. Ninhydrin test When 1 ml of Ninhydrin solu­tion is added to 1 ml protein solution and heated, the formation of a violet color indicates the presence of α-amino acids.   CLASSIFICATION OF PROTEIN : Based on the chemical nature, structure, shape, and solub

                                                                                       Fermentation Some organisms can respire in the absence of oxygen. This process is called fermentation or anaerobic respiration. There are three types of fermentation:   1. Alcoholic fermentation 2. Lactic acid fermentation 3. Mixed acid fermentation Alcoholic fermentation: The cells of roots in water logged soil respire by alcoholic fermentation because of lack of oxygen by converting pyruvic acid into ethyl alcohol and CO2. Many species of yeast (Saccharomyces) also respire anaerobically. This process takes place in two steps: Industrial uses of alcoholic fermentation: 1. In bakeries, it is used for preparing bread, cakes, biscuits.   2. In beverage industries for preparing wine and alcoholic drinks. 3. In producing vinegar and in tanning, curing of leather. 4. Ethanol is used to make gasohol (a fuel that is used for cars in Brazil). Lactic acid fermentation:   Some bacter

                                                                      Crossing Over in Drosophila   The phenomenon of crossing over is well illustrated in Drosophila melanogaster. When grey long Drosophila is crossed with a black vestigial, the F, hybrid is grey long. When female flies of F, generation are back crossed with double recessive male, four types of offspring are produced. Among the four types of offspring, two types - black vestigial and grey long- are just like the parents and they are known as parental combinations. Among the F1, about 41.5% of flies are like the dominant parent and another 41.5% are like the recessive parent. The other two types grey vestigial and black long are different from their parents genotypically and phenotypically, and they are known as non-parental combinations. The non-parental combinations of F, flies include 8.5% grey vestigial and 8.5% black long. The F, female hybrid produces four towards each other and come to lie side-by-side.