The Nucleus:

The nucleus is nerve or control centre of cell. It is functional genetic apparatus in eukaryotic cell and is liable for reproduction of cell in producing genetically similar daughter cells. Body comprises of head a stalk about 40mm in length, and base of root like rhizoids. Nucleus is situated at base of stalk. Hammering worked with two different kind namely A. mediterranea, whose head is cup-shaped, and A. creninata whose head is tentacled-like.

The Central belief of Molecular Biology:

Central belief of molecular biology describes that DNA codes for RNA, that codes for proteins. DNA is molecule of heredity which passes from parents to offspring. It has instructions for building RNA and proteins that make up structure of body and perform most of its functions. Inside cells of all living things, small molecular machines are continuously reading information in DNA and using it to construct proteins.

DNA is metabolically very stable, with its amount per cell in given species of higher organisms being rather constant and unchanged by changes in environment or nutrition. The rate of mutation in cell is highest at wavelength of maximum absorption of ultraviolet (UV) light by DNA (i.e. 260nm).

DNA Replication:

During so-called resting stage which cell suffers previous to cell division, DNA of chromosomes should be duplicated to make sure that each daughter cell receives full complement of genetic information in its nucleus. DNA occurs as the helical structure having two strands that are complementary to one another. Repeating monomer in nucleic acid is known as a NUCLEOTIDE, each and one base. The base is a ring-shaped molecule which has nitrogen and has basic properties. There are four different kinds of bases I DNA - tow purines, adenine and guanine, and two pyrimidines, thymine and cytosine. One double helical strands one purine is bounded to the pyrimidine through weak hydrogen bonds. It has been established guarnine binds to cytosine with three hydrogen bonds. (G=C) whereas adenine binds thymine with two hydrogen bonds. The bonds can be represented thus G = C, T = A.

Guanine never binds to thymine, and adenine to cytosine. As the first step in replication, relatively weak hydrogen bonds between base pairs of two DNA stands are broken by the enzyme, DNA polymerase. Before, synthesized nucleotide triphosphate units then move in, and form hydrogen bonds with exposed bases on both of old strands that act templates; a pyrophosphate is lost in process.

Other possible forms of replication are conservative and dispersive replications. In conservative type after new strands have developed on old strands they separate from templates and join together to form double helix while old stands too re-group.

1) Mechanism of Replication:

Interphase chromosomes are observable in light microscope when suitably stained. Under these situations, chromosomes illustrate regional differentiation in two distinct types of organization, termed heterochromatin and euchormatin. Heterochromatic regions are condensed and stain darkly; euchromatic regions are more dispersed and stain lightly. It replicate late in cell cycle and are thought to signify genetically inert chromatin. The process starts at numerous initiation points along duplex strands. Initiation starts generally in regions which in A - T base pairings, as A - T base pairs have lower energy bonds than G -C pairs. A protein complex having RNA and DNA polymerases first synthesize short segment of RNA (50 - 100 residies) co-joined to growing DNA strand. RNA primer has triphosphate residue for its 5' position and free 3' OH terminus to which new DNA joins; therefore growth of new DNA strand is in 5' → 3' direction. As DNA chains grow and approach each other three events should occur:

(i) Excision of RNA primer fragment

(ii) Filling in of remaining gaps with deoxyribonucleotide residues,

(iii) Fusion of DNA fragments by phosphate diester bond to form continuous DNA daughter strand.

2) Reverse Transcription:

When RNA tumor viruses infect the normal cell, they have their RNAs transcribed in DNA. This procedure of RNA serving as templates for DNA synthesis is called as reverse transcription and is mediated by the polymerase known as reverse transcriptase.

Mutation:

Mutation can be stated as abrupt, stable change of the gene that is expressed in some unusual phenotypic (visible traits) character, often as biochemical modification. In mutation there may be loss of ability to perform some specific biochemical function. Spontaneous mutation rates are influenced by variety of external factors like dietary mutagens, temperatures and radiation exposure, and internal factors like, fidelity of organism's own replication enzymes.

Physical and Chemical Mutagenesis:

Naturally mutations may occur by accident either by physical or chemical changes of bases in DNA or by shifting Condon reading frame by deletion, addition or modification of DNA base.

1) HNO₂ as a deaminating reagent:

Conversion of adenine to hypoxanthine will yield in incorrect pairing with cytosine; change of cytosine to uracil leads to adenine base pairing, while guanine to xanthine results in cytosine pairing which is normal.

2) Hydroxylamine:

This is very influential mutagen but only with isolated systems, as normal components of cell would readily scavenger the reagent. The reagent reacts purposely with cytosine.

3) Alkylating Reagents:

Alkylating reagents - dimethyl sulfate (DMS) and ethyl methane sulfonate (EMS) are particular for guanine. Reaction that follows methlation leads to formation of quaternary nitrogen that destabilizes deoxyribosis link and releases deoxyriboside. Loss of base can lead to replacement by any of four bases or even rupture of DNA chain.

These compounds and another alkylating reagent, Bchloroethylamine, a nitrogen mustad, are very toxic.

4) Methylating Reagents:

These are very mutagenic compounds and therefore are very unsafe to use unless very careful precautions are taken; they include

N - methyl - N' - Nitro - N- nitrosoguanidine

This reagent readily converts to diazomethane

The extremely effective methylating reagent. This compound utilized commonly for methylation of carboxylic acid and amino groups should be treated with great respect and care. Reagent methlates nucleic acids.

5) Un and X-Ray Radiation:

Mutagenic action by ultraviolet (UV) and X-=ray irradiation is extremely effective in inducing mutagenesis. X-rays possibly react with DNA by free radical mechanism to cause single stranded chain breaks in DNA chain. Ultraviolet irradiation with strong absorption at 260nm leads often to photochemical dimerization of two adjacent thymines, thymine-cytosine or two cytosines to dimmers.

DNA Repair Mechanisms:

X-ray and ultraviolet irradiations cause damage to cell DNA. X-radiation causes dimerization. All cells have machinery by which damage to DNA can be eliminated, rapidly repaired and original form of DNA double helix restored. After UV radiation, repair commences by nicking of damaged. DNA strand on 5' side of thymine dimmer by endonuclease. Oligonucleotides (i.e., short strips of nucleotides) having thymine dimmers are removed by 5' 3'. This enzyme also makes sure that nucleotide triphosphate, while losing pyrophosphate each, are fixed in gap, with complementary DNA strand serving template. X-ray radiation damage is readily fixed by use of DNA polymerase I to remove damaged strand, filling of gap, and then fusing gap by DNA ligase.

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