THE HUMAN CELL

INTRODUCTION
The cell is the fundamental unit of life
The cell elemental composition shows that the cell is 90% water. Of the remaining molecules present the dry weight is approx. 50% protein,15% CHO,15% nucleic acid,10% lipid,10% miscellaneous.
Total approx composition by element: 60%H,25%O,12%C,5%N.

BASIC STRUCTURE

The Nucleus
The nucleus is separated from the cytoplasm by an envelope consisting of two membranes with the intermembranous space between. The membranes are phospholipid bilayer structure with nuclear pore complexes. Molecules are imported and exported through the pore complexes.
The chromosomal DNA is held in nucleus in a specific fashion
The DNA is packed into chromatin fibres by it’s association with an equal mass of histone
The nucleus also contains a wide variety of other non histone proteins employed in replication and repair of DNA and it’s transcription into messenger RNA
The DNA is a double helix composed of four deoxyribonucleotides (base + sugar+phosphate) polymerised in an unbranched manner.

A gene is a sequence of deoxyribonucleotides controlling hereditary information to be transcribed into RNA and translated for the synthesis of protein.
The human genome (the totality of all genes) is commonly assumed to contain 100,000 genes.
The nuclear contents communicate with the cytosol by means of openings in the nuclear envelope called nuclear pores.
The nucleolus is a factory in nucleus where the cell’s ribosomes are assembled. When ever cells are actively producing proteins, a prominent nucleolus is apparent. In rapidly growing cancer cells, large prominent nucleoli are a discriminating feature


The CytoplasmEndoplasmic Reticulum
Diffusely distributed throughout the cytoplasm
They are flattened sheets, sacs and tubes of membrane
The ER membrane is in structural continuity with the outer membrane of the nuclear envelope.
It specialises in the synthesis and transport of lipids and membrane proteins.
The RER occurs as flattened sheets and is studded on it’s outer surface with ribosomes engaged in protein synthesis.
The SER is generally more tubular and lacks attached ribosomes. It functions in metabolisms of lipids, steroid hormones, cholesterol and other related compounds.
The ribosomes occur as free poly ribosomes or attached to RER. They bind with MRNA and translate genetic code using aa provided by TRNA to produce unique aa sequence of protein.
The Golgi Apparatus
Named after the Italian histopathologist Camilo Golgi
Usually located in the vicinity of the nucleus
A system of stacked membrane bounded sacs involved in modifying, sorting and packaging macromolecules (proteins, CHO, lipids) for secretion or for delivery to other organelles.
Numerous small vessicles around the golgi apparatus are thought to carry material between the golgi apparatus and different compartments of the cell e.g. to lysosomes, secretory vessicles, or insertion into the plasma membranes.
The Mitochondria
Mitochondria are the power plant of all eucaryotic cells.
They produce energy by burning (oxidizing or converting) CHOS and fatty acids to CO2 and water. The product is large quantity of energy rich adenosine triphosphate, (ATP). ATP is used in synthesis, chemical reactions, active transport through membranes, muscle contractions and movement of cells during division etc.
Mitochondria are rods enveloped by two unit membranes. The inner leaflet is folded to form the cristae. This is usually shelf-like, therefore increasing the inner membranes surface.
They vary in size and move freely within the cytosol aggregating in sites with high energy demand.

The Lysosomes
These are membrane bounded vessicles that contain hydrolytic enzymes involved in intracellular digestion.
The lysosomal enzymes comprise more than 40 different acid hydrolases which are optimally active at pH of about 5.0. This may be a protective mechanism for the cell, should the lysosomal enzymes escape into the cytosol with a higher pH.
There are primary and secondary lysosomes.
Secondary lysosomes are formed by fusion of primary lysosome with another type of vacoule.
Absence or deficiency of lysosomal enzymes leads to large accumulations of incompletely degraded products e.g. the storage diseases, Hunter Hurler syndrome, an x- linked recessive disorder with accumulation mucopolysaccharides

Peroxisomes
Are membrane bounded vesicles containing oxidative enzymes that generate and destroy hydrogen peroxide.
The enzymes form a detoxification system which converts toxic compounds into H2O2 and then into water.

The Cytoskeleton
These are arrays of protein filaments within the cytosol that give the cell it’s shape and provide a basis for it’s movements.
These filaments are also responsible in intracellular guidance of transport, transporting material across the cell surface, sorting and partitioning of replicated chromosomes by the mitotic spindle.
Three main kinds of cytoskeletal filaments are microtubules, actin filaments and intermediate filaments. The centriole, paired cylinders serve as templates for formation of cilia and flagella.

The Plasma Membrane
It is the outer boundary of the cell.
PM is arranged as bimolecular layer of phospholipids with hydrophilic heads oriented towards extracellular and intracellular compartments.
Hydrophobic tails are directed towards the PM center.
It is a continous sheet in which various proteins are embedded.

The Cell CycleCell Division
The time interval between mitotic divisions, the life cycle of an individual cell is called the cell cycle. Alternatively it’s a sequence of biochemical or morphological events in the life of a cell which can be mapped on a time scale.
The cells of the body are divided into three groups of cells on the basis of their proliferative capacity vis the continously dividing cells (labile cells), quiescent (stable) cells and non dividing (permanent) cells.
The phases of cell cycle include the short mitotic phase (M phase) and the non dividing phase (interphase) which usually occupies most of the life cycle of the cell.
Within the interphase are the G1phase, the S phase, and the G2 phase.
G1 phase is the interval between the M phase and the beginning of S phase.
S phase is when nuclear DNA is replicated
G2 phase is the interval between the end of S phase and the beginning of M phase
Cells progress through the cell cycle at different rates
Most of the variability is observed in G1 or to a lesser extent in G2
Other cells may leave G1 phase, cease progression and enter a quiescent phase referred to as G0.

Cell Division
The nuclei of all cells of an individual contain the same set complement of DNA, a quantity called the genome. This is present in a fixed number of chromosomes, this number being specific to each specie.
The DNA is a large molecular wt polymer consisting of deoxyribonucleotides with a double stranded structure. Each deoxyribose unit is bound to a purine or pyrimidine base which in turn is linked to a complimentary base on the other strand. Thus linking the strands together.
The bases are four with adenine only linking to thymine and cytosine only linking to guanine.
The sequence of bases in either strand of the DNA forms the genetic code for the individual .The bases are read in groups of three called codons, each coding for one aa.
In human cell there are 43 chromosomes comprising 23 homologue pairs.
Somatic cell division occurs in two phases
Firstly the chromosomes duplicate in S phase and are distributed equally between two potential daughter cells. This process is known as mitosis. The dividing cell is cleaved into genetically identical daughter cells by cytoplasmic division or cytokinesis.
Abnormalities in mitosis and cytokinesis may result in formation of two daughter cells with unequal amounts of cytoplasm and formation of binucleate and multinucleate cells respectively.
Mitosis is divided into phases: prophase, prometaphase, metaphase, anaphase and telophase.
Cell division requires the presence of the mitotic apparatus, which comprises a spindle of longitudinally arranged microtubules extending between a pair of centrioles at each pole of the dividing cell

The Prophase: the chromosomes are condensed and clearly visible in the cell nucleus. Each chromosome has duplicated during the preceding S phase and consists of two sister chromatids. The mitotic spindle begins to form outside the nucleus. This is a bipolar structure composed of microtubules and associated proteins.

Prometaphase: starts with disintegration of nuclear envelope and the mitotic spindle moves into the nuclear area . The pair of centrioles in the centrosome has duplicated and migrate towards to the opposite poles of the cell with a spindle of microtubule forming between them. Each duplicated chromosome becomes attached at the kinetochore to the microtubules

Metaphase: the chromosomes have lined up in one place half way between the spindle poles( the metaphase plate)

Anaphase: the chromosomes have separated into their two sister chromatids which are moving to opposite poles.

Telophase: the chromosomes are decondensing to regain their interphase conformation. The nuclear envelope reassembles and the nucleoli again become apparent.

Cytokinesis
The cytoplasm divides by a process known as cleavage
The plane of cytoplasmic division is defined by position of spindle equator, thus producing two cells of equal size.