Tag: structure of chromosome, cell cycle and cell division

Spindle formation is mainly done by centrioles. It helps in separation of chromosomes during cell division. Cytochalasin B is a cell-permeable mycotoxin. It inhibits cytoplasmic division by blocking the formation of contractile microfilaments due to which cell is unable to maintain the desired number of chromosomes and leads to numerical abberations.

Two polynucleotide chains of DNA are wound around the same axis and are held together by complementary base pairing between nitrogenous bases of two strands in the same plane. Adenine of one strand forms two hydrogen bonds with thymine of other strand and guanine forms three hydrogen bonds with cytosine.  The two bases forming hydrogen bonds to joins the polynucleotide strands together are present in the same plane. Thus, the correct answer is A.

As per Watson and Crick standard base pairing rules, adenine forms double hydrogen bond with thymine and guanine forms triple hydrogen bond with cytosine. Hence, content of adenine is equal to content of thymine and content of guanine is equal to content of cytosine.

Nucleotides are organic molecules that serve as the monomers, or subunits, of nucleic acids like DNA and RNA. The building blocks of nucleic acids, nucleotides are composed of a nitrogenous base, a five-carbon sugar-ribose or deoxyribose, and at least one phosphate group. Thus, a nucleoside plus a phosphate group yields a nucleotide. The distance between two nucleotide pairs in DNA is 0.34nm.

Nucleosomes are fundamental organizational units of chromatin which appear as “beads-on-a-string” arrangement. Here beads represent complexes of histones and DNA. Each bead contains a core of histone octamer: two copies each of H2A, H2B, H3, and H4; out of 200 bp DNA, 146 bp DNA is bound tightly around the histone core and the remainder serve as linker DNA between nucleosomal beads. The H1 histone associates with the linker DNA to pack adjacent nucleosomes together and forms a compact 30-nm chromatin fiber. Thus, the correct answer is A.

RNA and DNA are genetic materials. Cytosine, Thymine, Guanine, and Uracil are all nitrogenous bases. They are present in RNA. Uracil is replaced by adenine in DNA. Arginine is an amino acid which is basic in nature. It is the building block of protein. 

Purines and pyrimidines, make up the two groups of nitrogenous base which plays on important role in structural backbone of DNA. Adenine and guanine are purines, while cytosine and thymine are pyrimidines. Purines pairs with pyrimidines, by formation of H-bonds. Adenine attaches to thymine, by two hydrogen bonds, while cytosine and guanine forms three hydrogen between them.

Genetic information is carried in the linear sequence of nucleotides in DNA. Each molecule of DNA is a double helix formed from two complementary strands of nucleotides held together by hydrogen bonds between G-C and A-T base pairs.

According to the Watson Crick double helical model of DNA, the diameter of DNA molecule is 20 angstrom. Hence, the distance between two DNA strand is 20 angstroms.
So, the correct answer is option A.

DNA is acidic. For its compaction, it requires basic (histone) proteins. The histone proteins, are positively charged and occupy the major grooves of DNA at an angle of 30° to helix axis.