Tag: zoology

A flame cell is a specialized excretory cell found in the simplest freshwater invertebrates, including flatworms(except the turbellarian order Acoela), rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system. Bundles of flame cells are called protonephridia. So the correct option is 'Flatworm rotifers, earthworm’.

In 1932 Palaeontologica Indica, Birbal Sahni added an account of the Bennettitalean plant that he named Williamsonia sewardi.

The components of the cell and the arrangement of the individual parts within the cell form the cellular organisation. There are five stages which include cell, tissue, organs, organ systems and organisms. The cellular organisation provides the complete about the working model of the cell and its components.

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.