Page 39 - Textbook of Pathology, 6th Edition

together measuring about a metre in length in a human 23
diploid cell. Of these, there are 22 pairs (44 chromosomes) of
autosomes and one pair of sex chromosomes, either XX (female)
or XY (male). Each chromosome is composed of two
chromatids connected at the centromere to form ‘X’
configuration having variation in location of the centromere.
Depending upon the length of chromosomes and centromeric CHAPTER 3
location, 46 chromosomes are categorised into 7 groups from
A to G according to Denver classification (adopted at a meeting
in Denver, USA).
Chromosomes are composed of 3 components, each with
distinctive function. These are: deoxyribonucleic acid (DNA)
comprising about 20%, ribonucleic acid (RNA) about 10%,
and the remaining 70% consists of nuclear proteins that
include a number of basic proteins (histones), neutral
proteins, and acid proteins. DNA of the cell is largely
contained in the nucleus. The only other place in the cell that
contains small amount of DNA is mitochondria. Nuclear
DNA along with histone nuclear proteins form bead-like
structures called nucleosomes which are studded along the Cell Injury and Cellular Adaptations
coils of DNA. Nuclear DNA carries the genetic information
that is passed via RNA into the cytoplasm for manufacture
of proteins of similar composition. During cell division, one
half of DNA molecule acts as a template for the manufacture
of the other half by the enzyme, DNA polymerase, so that
the genetic characteristics are transmitted to the next progeny
of cells (replication). Figure 3.3 Diagrammatic structure of portion of helical structure of
The DNA molecule as proposed by Watson and Crick in DNA molecule.
1953 consists of two complementary polypeptide chains
forming a double helical strand which is wound spirally Cytosol and Organelles
around an axis composed of pentose sugar-phosphoric acid The cytosol or the cytoplasm is the gel-like ground substance
chains. The molecule is spirally twisted in a ladder-like in which the organelles (meaning little organs) of the cells are
pattern, the steps of which are composed of 4 nucleotide bases: suspended. These organelles are the site of major enzymatic
two purines (adenine and guanine, i.e. A and G) and two activities of the cell which are possibly mediated by enzymes
pyrimidines (cytosine and thymine, i.e. C and T); however, A in the cytosol. The major organelles are the cytoskeleton,
pairs specifically with T while G pairs with C (Fig. 3.3). The mitochondria, ribosomes, endoplasmic reticulum, Golgi
sequence of these nucleotide base pairs in the chain, apparatus, lysosomes, and microbodies or peroxisomes.
determines the information contained in the DNA molecule
or constitutes the genetic code. In April 2003, sequencing of 1. CYTOSKELETON. Microfilaments, intermediate
human genome was completed which revealed that 23 pairs filaments, and microtubules are responsible for maintaining
of chromosomes in the nucleus of each human cell contains cellular form and movement and are collectively referred to
approximately 3 billion base pairs, and each chromosome as cytoskeleton.
contains an estimated 30,000 genes in the human genome, i) Microfilaments are long filamentous structures having
which carry the instructions for making proteins. a diameter of 6-8 nm. They are composed of contractile
In the interphase nucleus (i.e. between mitosis), part of proteins, actin and myosin, and diverse materials like parts
the chromatin that remains relatively inert metabolically and of microtubules and ribonucleoprotein fibres. Bundles of
appears deeply basophilic due to condensation of microfilaments are especially prominent close to the plasma
chromosomes is called heterochromatin, while the part of membrane and form terminal web. Extension of these bundles
chromatin that is lightly stained (i.e. vesicular) due to of microfilaments along with part of plasma membrane on
dispersed chromatin is called euchromatin. For example, in the surface of the cell form microvilli which increase the
lymphocytes there is predominance of heterochromatin while absorptive surface of the cells.
the nucleus of a hepatocyte is mostly euchromatin.
ii) Intermediate filaments are filamentous structures, 10 nm
NUCLEOLUS. The nucleus may contain one or more in diameter, and are cytoplasmic constituent of a number of
rounded bodies called nucleoli. Nucleolus is the site of cell types. They are composed of proteins. There are
synthesis of ribosomal RNA. Nucleolus is composed of 5 principal types of intermediate filaments:
granules and fibrils representing newly synthesised a) Cytokeratin (found in epithelial cells).
ribosomal RNA. b) Desmin (found in skeletal, smooth and cardiac muscle).