Cell Biology

Phospholipids are most abundant lipids in plasma membrane. They are amphipathic molecules having a hydrophilic head group and hydrophobic fatty acid tails. Two types of phospholipids occur in bilayer: glycerophospholipids, in which the hydrophobic regions are composed of two fatty acids joined to glycerol and sphingophospholipids, in which a single fatty acid is joined to sphingosine.

As replication of DNA occurs in S-phase of cell cycle, DNA of both euchromatin and heterochromatin will undergo replication process and they take radioactive adenosine from the medium for the formation of complementary base pairs and hence, both chromatin will show Radioactive Adenosine. Heterochromatin is densely packed and deeply stained chromatin present in the periphery of nucleus responsible for regulation of gene expression due to position effect variegation. Euchromatin is loosely packed and lightly stained chromatin present in nucleus which contains almost 92% of human genome and undergoes transcription.

Endoplasmic reticulum (ER) is the largest intracellular compartment with an extensive array of interconnecting membrane tubules and cisternae that extend throughout the cell including the nuclear envelope. The outer membrane of nuclear envelope is continuous with that of the ER.

These are the actual processes of chromosomal packing of DNA within the nucleus. DNA double helix coils around Histone octamers to form nucleosomes and then, the nucleosomes undergo supercoiling to produce more thicker chromatids which are accommodated within the nucleus.

Galactosyl transferase (GT) belongs to the glycosyl transferases. In several tissues and cell lines, the enzyme is localised by immunocytochemistry to the two to three trans cisternae of the Golgi complex and thus, may be considered as a specific membrane component of this type of endomembrane. As a consequence, it is the most common Golgi "marker" enzyme in cell fractionation studies.

G-protein coupled receptors (GPCR) are responsible for signal transduction. GPCRs are linked to trimeric G proteins. G-proteins are GTPase switch proteins. These proteins are turned on or remained active when bound to GTP and turned off or inactivated when bound to GDP.

Core histone of nucleosome is a histone octamer consisting of 2 copies each of H2A, H2B, H3 and H4. All the core histones are rich in lysine and arginine (amino acids with basic side chains) and their positive charges can effectively neutralise the negatively charged DNA backbone.

Colchicine is an alkaloid prepared from the dried corns and seeds of Colchicum autumnale (autumn crocus). It inhibits microtubule polymerisation by binding to tubulin. During mitosis, use of colchicine inhibits spindle formation and the cell division remains arrested in its metaphase stage.

They prevent ribosomal association at the end of initiation step and cause misreading of the genetic code.

Integral membrane proteins have one or more segments that are embedded in the phospholipid bilayer. Most integral proteins contain residues with hydrophobic side chains that interact with fatty acyl groups of the membrane phospholipids by hydrophobic interactions and thus, anchoring the protein to the membrane.

Most of the cases receptor mediated endocytosis from plasma membrane involves clathrin-coated vesicles. Macromolecules bind to cell-surface receptors, accumulate in clathrin-coated pits and enter the cell as a receptor and ligand complexes in clathrin-coated vesicles. Clathrin consists of three copies each of heavy chain and light chain, forming a three-legged structure called a triskelion.

The fossil record shows that earliest eukaryotic cells evolved about 1.5 billion years ago. Details of the evolutionary path from prokaryotes to eukaryotes cannot be deduced from the fossil record alone, but morphological and biochemical comparison of modern organisms has suggested a reasonable sequence of events consistent with the fossil evidence.

The rate of cell division in early cleavage cells is very high. So in these cells, cell cycle consists of only S and M phases. During S phase, cells synthesise the necessary materials for division and then, they proceeds towards the mitotic division that is the M phase. It is actually a time saving procedure for producing huge numbers of cells in shorter time span during early cleavage.

Membrane potentials across cell membranes are a function of the electrolyte concentrations in the intracellular and extracellular solutions and of the selective permeabilities of the ions. Membrane potential in a nerve cell during resting stage is -60 mV, which means interior of the cell is negative with respect to the exterior. During depolarisation of nerve cells, the inside becomes positive due to the influx of Na⁺ ions.

Somatic cells are diploid and undergo mitotic division. So, the number of chromosomes remains same in the daughter cells.

Mitochondria are double membrane bound organelles. It has an outer membrane, an inner membrane and an inter-membrane space. The inner membrane is highly convoluted, forming folds called cristae.

Cell organelles are correctly matched with their functions. Mitochondria synthesise ATP by means of electron transport chain. SER is responsible for the synthesis of fatty acids whereas post translational modification of proteins occur in the lumens of RER. Lysosomes are responsible for autophagy, a process of self-digestion.

Proteins are synthesised by ribosomes associated with the cytoplasmic side of the membrane of rough endoplasmic reticulum (RER). Synthesised proteins enter into the membrane and lumen of RER by the process of co-translational translocation. Modification of proteins is also occurred in the lumen. Proteins destined to be secreted then follow the secretory pathway. On the other hand, SER is responsible for lipid biosynthesis, detoxification, etc.

Gamete cells undergo meiotic division and they have haploid number of chromosomes, i.e. half of the chromosomes present in the somatic cells. In diploid cells, number of chromosomes is 56. So, number of chromosomes in gamete cells is 56/2 = 28.

Mitochondrial DNA is not inherited paternally. It is inherited maternally. Mitochondria and their DNA molecules (mtDNA), are the genetic units of this method of inheritance. Mitochondrial genes, in the contrast to genes in the nucleus, have an exclusively maternal mode of inheritance in mammals.