The Cell, Genetics | Science & Technology for UPSC CSE PDF Download

The cytology or, cell Biology is a specialized biological science which deals with the study of cells for their morphology. The cell is the basic unit of organisation or structure of all living beings. In Animal Kingdom, following types of cells are found:

A cellular: They are the organisms which do not contain cellular components. They have genetic elements, DNA or RNA, and a protein wall. its examples are viruses.
Prokaryotic Cells: Having central nuclear components (DNA, RNA and nuclear proteins) without nuclear membrane, surrounded by cytoplasmic substances. There is no defined cell-organelles except, ribosomes which, too, is of prokaryotic type. Examples are Bacteria and Blue-green algae.

Eukaryotic Cells: These are true cells, occur in plants and animals. They have different shape, size and physiology but all the cells are typically composed of plasma membrane, cytoplasm and its organelles.

Chromosomes

Each chromosome during cell division has two chromatids. Each chromosome is characterized by the presence of a centromere, which can be easily recognized as a clear constricted zone. The centromere divides the chromosome into two segments called as chromosome arms. Another constriction found in one of the two arms of chromosome is called secondary constriction. The terminal regions on either side of chromosome are called telomeres. Detailed study of chromosome morphology shows a coiled filament throughout the length of a chromosome, called as chromonema. The chromonemata form the gene-bearing portions of chromosomes. The chromonemata are embedded in the matrix which is enclosed in health or pellicle. Genetic information is being carried by the chromosomes and a particular region on the chromosome which determines a particular character is a called a gene. A gene is a segment of a particular form of long chain polymeric molecule, called deoxyribose nucleic acid or DNA.

It is the DNA which acts as the primary carrier for all the information necessary for the synthesis of cellular components except in some viruses, where it is replaced by RNA.
DNA is a polymer of nucleotides and each nucleotide consists of a pentose sugar, a nitrogenous base and a phosphate group. The sugar is ribose in case of RNA and deoxyribose in case of DNA. 

Nitrogenous base: Two types of nitrogenous bases are found namely-(i) purines and (ii) pyrimidines.
Purines are again of two types viz. Adenine and Guanine.
 

In the same way pyrimidines are of two types viz. Cytosine and Thymine.
RNA is also a component of chromatin, secondary to DNA in importance of heredity. It is single strand structure. Its nitrogenois bases are similar to those of DNA, except that Thymina of DNA is replaced by vracil in RNA.RNA (Ribose Nucleic Acid)

Types of RNA

(1) Ribosomal RNA (r-RNA): These are found in ribosomes of cells. They contain 85-90 of total RNA. The main function of r-RNA is to attract and to provide large surface for spreading of m-RNA over ribosomes during translation process of protein synthesis.

(2) Messenger RNA (m-RNA): Genetic information is carried from nuclear DNA to cytoplasm by 5-10% of total RNA present in a cell. The molecular weight of m-RNA is 5,00,000 to 2,00,000. They are synthesized in nucleus during transcription from DNA.

(3) Transfer RNA (t-RNA): These are the smallest RNA composed of 75-80 nucleotides freely present in cytoplasm. 

Its function is to attract particular amino acid at the terminal end. These are also called soluble RNA (sRNA).

Cell Division

Mitosis

Mitosis is otherwise known as vegetative or somastic cell division. No change in the Chromosome no, takes place in the Division and one cell (mother cell) divides to form two daughter cells equal to the mother cell, quantitatively and qualitatively. In between two successive mitosis division there is a rest period called interphase. Mitosis itself can be divided into 4-phases i.e. prophase, meta-phase, anaphase and telophase. Disappear-ance of the nuclear membrane and nucleolus along with the doubling of chromosomes (each is called chromaid) takes place in prophase. Metaphase is characterised by spindle formation. Chromosomes join themselves to the equatorial plane of the spindle.
During anaphase, centromere divides longitudinally and sister chromotids move towards opposite pole. During telophase grouping of chromatids at each pole along with formation of new nuclear membrane and nucleolus takes place. All above incidents can be grouped under the heading karyokinesis (Division of nucleus) which is followed by cytokenesis (Division of cytoplasm) which divides the cell, either by cell plate formation or by furowing.

Meiosis 

Meiosis is otherwise called reproductive cells division because it is associated with all sexual reproduction, Chromosome no. becomes half in this division and this may take place at the time of gamete formation (in diploid organism) or after fertilization (in haploid organism).

Division includes (two) phases i.e., reduction division and equati-onal division. Thus 4-haploid daughter cells are produced. Both the phases are div-ided in the same way as prophase I & II, metaphase I & II and so on. Prophase I is again divided into 5-Substages, egLeptotene, Zy-gotene, Pachyt-ene, Diplotene and Dikinesis. Diplotene is characterised by close associ-ation of homol-ogous chromosomes, zygotone by synapsis, pachytene is marked by chia-sma formation and Crossing over and Diplotene is by termi-nalization and dikinesis-by the disappearance of nuclear membrane and nucleolus. In MetaphaseI spindle is in Anaphase I-homologous chromosomes move apart and in telophase I grouping of chromosomes take place at each pole. Hence, this is a reduction division in which the no. of chromosomes are reduced at each pole.

Genetics

Law of dominance

Mendel’s experiments show that when two plants with contrasting characters are crossed together, only one of their characters is expressed in the first generation.
Physical appearance of a character (trait) is called phenotype. The phenotype is determined by alternative forms of a single gene, called alleles.

Law of segregation

Mendel postulated that there are separate genes for each character and that these genes are transferred from parents to offspring through gametes. The genes can be dominant or recessive. There is no blending of allelic genes in cells or zygotes. They separate during the formation of gametes and different alleles go into different gametes.

Test Cross and Back Cross

The cross of an individual of unknown genotype to a completely recessive individual is known as test cross. Recessive individual will produce only one type of gametes.
In such a cross, the type of progeny will depend upon the types and frequencies of gametes produced by the parent of unknown genotype. The test crosses, therefore, help in determining the genotype of an individual. Back cross involves the crossing of progeny to one of the parents.

Law of Independent Assortment

In dihybrid crosses, Mendel proved that each allelic pair segregates or assorts independently of each other. Dihybrid cross yields four types of progeny, in F2 generation, in the ratio of 9:3:3:1.

Incomplete or Partial Dominance

In a cross between parents with contrasting characters, the partial dominance results in an inter-mediate phenotype. A cross between red-flowered (RR) and white-flowered (rr) Mirabilis jalapa shows only pink-flowered plants (Rr) in F₂ generation. The F₂ generation reveals phenotypic ratios of 1 (red): 2 (pink): 1 (white).

Co-dominance

This distinct genetic expression of both alleles is called co-dominance. For example, the MN blood groups in humans. In MN system, the persons are either of blood groups M,MN or N. Offsprings of two heterozygous MN parents appear in a ratio of 1 (M): 2 (MN): 1(N).

Pleiotropism

A multiple phenotypic effects that has in genes is called Pleiotropic. In fact all genes may be pleitropic, with their different effects not yet recognized. Even a pleitropic gene has only one primary function of producing one polypetide, which may produce more than one phenotypic effects.

Inheritance of Comb Shape in Poultry

Bateson and Hurst (1990) found a case of two genes influencing the same character in fowls. A cross between ‘rose-combed (RRpp) and ‘pea combed’ (rrPP) varieties yield only ‘walnu-combed’ (Rr Pp) birds in F1 generation. When the walnut-combed birds of F1 generation are bred together, the F2 generation shows rose, pea, walnut and single-combed (rrpp) birds.

Sex Determination in Man

In normal human beings (Homo sapiens), males are XY and females are XX. In man XX-XY type sex determining mechanism occurs but here the Y chromosome contains potent male determining genes which can almost completely overcome the feminizing action of the rest of the genotype. The evidence for it comes from the Barr experiment.
Murray Barr (1949) reported the deeply stained chromatin body in the most somatic cells of the normal females. This was called Barr body, so named after its discoverer. This was absent in male cells thus they are sex chromatin negative while the female cells are sex chromatin positive.

Human Sex Anomalies

Genetic Disorders

A change in the number of chromosomes or gene mutations often result in various kinds of disorders which are heritable. These are called genetic disorders, some examples of which are given:

Colour Blindness—Inability to distinguish red-green colours. It is inherited as a sex linked disease owing to a recessive gene.

Haemophilia—Profuse bleeding even from minor cuts as the blood does not clot because of a sex-linked recessive gene.

Phenylketonuria—Serious brain damage in infants caused by a recessive gene. The child inflicted with the disease is unable to metabolise phenylpyruvic acid which accumulates and damages the brain, producing an idiot.

Sickle Cell Anaemia—A condition caused by an abnormal haemoglobin molecule due to a recessive gene in homozygous condition resulting in sickling of the red blood cells. The heterozygous individuals (carriers) may also suffer at high altitudes due to low oxygen tension.

Thalassemia—Also called Cooley’s anaemia, it occurs mostly in children and is nearly fatal. This too occurs due to an abnormality of the haemoglobin, controlled by a recessive gene which in homozygous condition causes severe anaemia.

Kinefelter Syndrome (47, XXY)—They are males despite the presence of two X-chromosomes. External genitalia normal, testes small, sperms not produced, mentally retarded, arms longer than average, breast development common. They bare sex chromatin positive.

Turner’s Syndrome (45, X—They are sterile females, having short stature, shield shaped chest, slightly mentally retarded, breast absent, public hair reduced, genitalia infantile. They bare sex chromatin negative.

Down’s Syndrome (Mongolism) (2 = 47)—They show mental retardation, below average height, peculiarity in upper eyelid, sloping forehead, flattened nose, short hands, sexual maturity not attained, males sterile. Addition in 21st chromosome.

Edward’ s Syndrome (Trisomy- 18 )— They are characterised by multiple malformation, severe mental retardation, more pronounced in females, death occurs generally at 3-4 months of age. This is due to addition of extra 18th chromosome.

• Sex-Linked Inheritance

Human have X and Y chromosomes which are not entirely homologous. The genes that occur only on the X chromosome will be presented twice in females but once in males.
Genes located exclusively on the X chromosome are called Sex-linked, while the genes that occur only on the Y chromosome, can produce their effects only in males, these are called holandric genes.

Inheritance of X-linked genes in Man 

In man about 50 sex linked genes have been reported. The most important and common X-linked genes of man are:

Colour or Red-green blindness Haemophilia Anhidrotic ectoderma (non functional Sweat glands) Night blindness Myopia (Short sightedness) Juvenile Glaucoma (hardening of eye ball) White fork lock, etc.
These disease are associated with X-linked recessive genes and are most common in man. Human beings have 46 chromosomes (23 pairs) present in each somatic cell. Female has 22 pairs + XX while male has 22 pairs + XY chromosomes. Since female will produce only one types of gametes, the gametes from the male individual will determine the sex of the progeny.

Eugenics

The term Eugenics (Gr. eugenes = well born) was coined by F. Galton in 1885.

This is the science which deals with the application of the laws of genetics to the improvement of human race. More precisely the eugenics can be defined as a science of well born, improving the inborn qualities of race and obtaining the better heritage by judicious breeding. For the human betterment the eugenics can be applicable by adopting following two methods. By encouraging the marriages between desirable persons (positive eugenics). By discouraging the marriages between undesirable persons (negative eugenics).

Positive Eugenics:

It includes the following measures. Early marriage of those having desirable traits. Subsidizing the fit. Education. By avoiding germinal waste. Genetic counselling.
Improvement of environmental conditions. Promotion of genetic research.

Negative Eugenics:

Defective germplasm of the society can be eliminated by adopting following measures.
(a) Sexual separation of the defective persons.
(b) Sterilization.
(c) control of immigration.  
(d) Regulation of marriage.

Genes and Heredity.

Avery, Mceod, and McCarty (1944) categorically proved that DNA is the genetic material. This was further confirmed by Hershey and Chase (1952) using labelled DNA and proteins. The following year, Watson and Crick (1953) proposed that DNA was a doublestranded polymer of nucleotides arranged as a helix (double helix). Since then, much work has been done to determine the relationship between DNA and genes and also to find out how exactly genes control heredity. This has led to the finding that genes are only segments of DNA and they control heredity by controlling the synthesis of enzymes (in fact, all proteins).

As all the metabolic activities in a cell are control led by enzymes and that determines the differentiation of a cell into a particular type of tissue or organism, i.e., the heredity, the genes (DNA) are the ultimate controlling factors of heredity.

The Genetic Code

It has been proved that the nucleotide sequence in DNA determines the sequence of amino acids in a protein. This is known as the collinearity hypothesis, meaning that a certain length of DNA corresponds to (is collinear with) a sequence of amino acids. In fact, a particular sequence of nucleotides is the code for a specific amino acid.

Since there are only four types of nucleotides in a DNA molecule and amino acids in a protein are at least of 20 types, each amino acid is coded by a triplet of bases (nucleotides). This is known as the triplet code hypothesis and the code constitutes the Genetic code. Three of the codons, UAA, UAG, and UGA do not code for any amino acid and were, therefore, called nonsense condons.