The Cell, Genetics UPSC Notes | EduRev

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UPSC : The Cell, Genetics UPSC Notes | EduRev

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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.

The Cell

1. Plasma membrane —regu lation a nd con trol of materials passing through pinocytic intake and absorption.

2. Cytoplasm—suspension medium for organelles and inclusions, site of metabolic activities.

3. Membranous organelles

(i) Endoplasmic reticulum —c arr ies en zy me systems, provide for storage and conduction, surface for physico-chemical activities.

(ii) Golgi apparatus—linking of proteins with sugars, storage and transportation of secretions.

(iii) Mitochondria—store houses of ATP and seat of respiratory activity.

(iv) Lysosomes—digestion within the cell.

4. Other organelles

(i) Ribosomes-sites of protein synthesis.

(ii) Centrosome-takes part in cell-division.

5. Inclusions

(i) secretary granules (ii) food granules

(iii) pigment granules (iv) vaculoles

6. Nucleus

(i) Chromosomes—carriers of genetic information.

(ii) Nucleolus—controls the synthesis of some enzymes.

Deficiency Diseases

A. Protein










Children up to the age of five years, who are mainly on carbohydrate diet instead of mother’s milk suffer. Children become irritable, cease to grow, lose weight, skin pigmented, potbelly due to retention of water by the cells, mental retardation.

Infants below the age of one year suffer when breast feeding is replaced by less nutritive diet low in protein and calories. Muscle degeneration, thinning of limbs and abdominal wall, ribs prominent, skin pigmention and oedema absent.


NameSourceFunctionDeficiency Disease







7.Sodium &Potassium

Milk, milk products, green leafy vegetables, carrots, fish, etc.

Liver, meat, egg-yoke green vegetables, etc.

Fishes, algae, Iodised salt etc.

Cereals, pulses milk, etc.Cereals & leafy vegetables, etc.

Common salt. cereals, fruits, vegetables, etc. Banana etc.


Formation of Bone;

Formation of heomoglobin

Formation of thyroxin hormone Building of teeth and bones Regulates the functioning of muscles and nerves.

Activator of various enohymus. Working of stomach, nerves and various enaymes Maintain ionic and water heart balance of the body.

Rhermatism, Ostcomalacia


Goitre Weaking of bones & teet Improper functioning of muscles & nerves.Dehydration of body tissues Dehydration; in creased beat, Kidney damage.


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 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 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.

VitaminSourceFunctionDeficiency disease





(Cyanocobalamine) C
(Ascorbic acid)




Fish liver, kidney, egg, milk, butter, carrots, spinach, all green and yellow vegetables.

Cereals, wheat, cabbage, carrots, milk, spinach etc.

Liver, cheese, milk, egg, soyabean, green vegetables, etc.

Yeast, cereals, milk, egg, liver etc.

Liver, egg, milk, meat, fish, fruits etc.

Lemon, orange, grape, amla, tomato, apple, green vegetables etc.

Animal fats, butter, ghee, oil, milk, egg yolk etc.

Wheat, egg yolk, butter, milk, leafy vegetables etc.

Green vegetables, berry, tomato, soyabean etc

Health of the eyes, skin, and mucous membranes

Growth and carbohydrate metabolism.

Growth, health of skin and mouth, normal functioning of eyes.

Growth, and metabolism.

Growth and blood formation.
Development of teeth and gums.

Formation of bones and teeth.

Normal functioning of sex glands.

Normal blood clotting.

Night blindness, and xerophthalmia.

Beri-beri and loss of appetite.

Retarded growth, loss of hair, sourness of tongue and lips, premature aging.

Inflammation of the skin, mental disorders, loss of appetite.

Pernicious anaemia, slow growth. Scurvy.








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).


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).


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.

Facts To Be Remembered
  • Human Genome Project (HGP): The HGP is a $ 3 billion-15 year project underway in various US Universities to prepare a Human Genome Map of the 100,000 genes or so that make up the single person’s DNA. 
  • The first genetic map, called a physical map, is still in a preliminary stage.
  • Visible Human Project (VHP): The VHP launched by US Universities, envisages production of a 3-dimensional database of the human body. The project envisages to cut a perfect human body (both male and female) into the quarter of a millimetre series by a special milling machine. each section is them photographed, degitised and stored into an image processing computer. Its applications are tremendous, ranging from clinical medicines to dissections, surgery, neurosurgery etc.
  • The Human Genetic Diversity Project (HDGP): The HGDP envisaged by the US scientists proposes to construct a DNA library of all human populations sampled from peoples inhabiting all corners of the world. It will build a genetic encyclopedia of virtually every human race. The aim is to create a genealogical tree of the different peoples of the world so that human beings could know their origin as a species, their evolution and diversification.
  • P 53: P 53, a protein of extraordinary cellular importance were declared Molecule of the year 1993, an honour previously held by nitrous oxide and by bucky ball (buckminister fullerene or C60) before that.
  • Therapy for Brain Cancer: The treatment involves injecting genetically altered mouse cells directly into the tumour mass of each patient. The cells are designed to release infections retro-viral vectors. These vectors make the cancer cells vulnerable to attack by an anti-viral drug called ganciclovin.
  • Depo-Provera: Depo-provera is the trade name of depot medroxy progestrone acetate (DMPA), a synthetic injectable contraceptive hormone. It is similar to progesterone that naturally occurs in females and inhibits ovulation. When Depo-provera was launched in India by Max Pharma India Ltd. in alliance with the Upjohn Company, it created controversies and opposition by women. The opposition was based on the evidences of breast and cervical cancer as side effects in long term users of the drug.
  • Bauah Heart-21: Dr Dhani Ram Barua, a surgeon, has designed the world’s first fully implantable biological heart in his laboratory in Bombay. He has named the heart after him: Baruah heart 21. The figure stands for the 21st century. The heart is yet to be tested on an animal.
  • Dementia: Dementia refers to a decline in cognitive and intellectual abilities interfering with the social occupation and performance of an awake and alert individual. Cognitive abilities include memory, language, reasoning, judgement and planning. It results from the dysfunction cerebral part of the brain.
  • Alzheimer’s Disease: Over 10,000 Americans now die of this disease—an incurable type of Dementia—every year.
  • The disease catches the old. It is estimated that disease is going to sweep across India in a few years. As of now, those is no way out for there affected with this disease.
  • Oxygen can cause Cataract: Inside the body, while metabolism, oxygen splits into two fractions. The free radicals, not neutralised, are in excess they go on the rampage, killing bacteria, germs and viruses. If still in excess, they injure the inside (protein) lining of the eye lens which then become opaque, leading to contract, the major cause of blindness in India. They also cause wrinkles and early ageing.

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.

Genetical Terminology
  • Autosome—The chromosomes which are not associated with sex determination. Except X and Y chromosomes in human being, all are autosomes.
  • Back Cross—The cross between a progeny individual with any of its parents (in plants).
  • Bivalent—A pair of synapsed homologus chromosome (during mitosis).
  • Carrier—A heterozygous individual which carries an unexpressed recessive sex-linked gene in it.
  • Co-dominance—When both the alleles (dominant and recessive) are equally expressed in the hybrid, e.g. some times cross between red and white flowered plants produce pink flowers. Same type of phenomenon is also true for animals.
  • De-oxyribonucleic Acid (DNA)—It is a genetic material of many viruses, bacteria, plants and animals. It is found to be most stable biological molecule which contains encoded genetic informations.
  • Diploid—An individual or cell containing two complete haploid sets of chromosomes.
  • Dominant trait—When out of two contrasting characters only one express or appears in a generation, that trait is called dominant.
  • F1 or First Filial generation—The word filial is derived from the Latin word Filin, meaning the son. The first generation of a given cross is known as F1 generation.
  • Gamete—A sex-cell having haploid set of chromosomes and arising due to meiosis is known as gamete. The male gamete is known as pollen (Plants) or sperm (Animals) and female gamete is known as ovum or egg (common in plants as well as animals).
  • Gene—A particular segment of DNA molecule which determines heredity of a particular trait.
  • Genome-A complete set of chromosomes which is inherited as a unit from one parent.
  • Genotype—The genetic make up or genetic constitution of an individual.
  • Haploid (Monoploid)—An individual or cell containing a single complete set of chromosomes.
  • Haemophilia—A metabolic disorder characterised by free bleeding even from slight wounds because of the lack of clot-forming substance. It is linked to a gene occurring in sex chromosome.
  • Homozygote (Homozygous)—The organism having two similar genes for a particular character. They are genetically pure, compared to Heterozygote or Heterozygous.
  • Phenotype—The external appearance of an organism for a particular contrasting character is phenotype e.g., expressed in words as tallness or dwarfness, etc.
  • Phenocopies—When two different genotypes produce same phenotype due to different environments.
  • Sex Chromosomes—The chromosomes which do not occur in identical pairs and determine sex of an individual, e.g., X (female) and Y (male) chromosomes.
  • Sex linked genes—Genes occurring only on the sex chromosomes.
  • Test Cross—The cross of F1 hybrid with its recessive parent.
  • Wild Type—The commonly occurring phenotype among the members of a population.
  • Acquired Character—The alteration in the morphology or physiology of an organism in response to its environment.
  • They are usually not heritable.
  • Albino-The animal without pigmentation in skin, hairs and eyes.
  • Allele (Allelomorph)—Two or more alternatives forms of a gene or contrasting characters are called allele, e.g., gene for length in pea stem has two forms T for tallness and t for dwarfness.
  • 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.


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

Facts To Be Remembered
  • The feeding mechanism is of several types. If it is entirely plantlike (photosynthesis) it is called Holophytic. If it is entirely (Holo) animal like (zoic) that is devouring already existing food, it is called Holozoic.
  • The word Coprodaeum refers to the last part of the alimentary canal. The excreta (undigested matter) comes out of this part. So the organisms that take in such materials are called Coprophagous.
  • Herbivorous organisms are those that take for their food plant parts like leaves raw in contrast to vegetarian, which means that plant products are taken raw or after being cooked.
  • Autotroph:” This term means organisms that manufacture their own food. Troph means feeding. Auto means self.
  • The green plants which manufacture their own food are called autotrophs. All others which depend upon the plants directly or indirectly are called heterotrophs.
  • pH refers to acidic or alkaline nature of substances. Enzymes have their own optimum levels of pH. For example, pepsin requires an acidic medium. The pancreatic enzymes require in alkaline medium. Optimum means most suitable to the particular enzyme.
  • A slight increase in temperature will increase the activity of enzymes. But beyond 50°C all enzymes will be denatured.
  • A 10°C increase of temperature in the lower ranges will double the action of enzymes. This is called Q10.
  • Glands are of two types. The exocrine glands have ducts to carry their secretions and these secretions contain enzymes. On the other hand endocrine glands are ductless glands and they secrete mainly hormones.
  • Zymogens are precursors of enzymes. Many enzymes are produced in this form and later they are made into active forms by combining with proshetic group, for example, the pepsinogen is an inactive zymogen. It becomes active Pepsin in combination with dilute HCI.
  • Salivary glands of man are 3 pair = 6. They are—one pair of parotid glands, one pair of sub-maxillary or submandibular glands and one pair of sublingual glands.
  • Mumps: This is a swelling caused by inflammation of parotid glands placed in front of our ear region. The inflammation is caused by a virus.
  • The swallowing of food is involuntary. Once the food passes down the pharynx, the oesophagus takes the food by a wave-like peristaltic movement. This movement is created by involuntary muscles on the wall of oesophagus.
  • Pancreozymin is one of the hormones secreted by the wall of the duodenum when the food arrives there. This hormone stimulates pancreas to produce more enzymes so that the food can be digested properly.
  • The liver secretes bile. Bile has no enzyme at all. It is an alkaline substance which converts the acidic food coming from the stomach into alkaline substance so that pancreatic and intestinal enzymes can act on it.
  • Aquatic mammals breathe through their nostrils only. Though these mammals are living in water, their respiratory organs are the lungs adapted for breathing air. So the animals will have to come to the surface to take in air through their nostrils, (e.g.) whale, sea lion, seals etc.
  • The Diaphragm is a muscular wall separating the chest cavity from the abdominal cavity. It is present only in mammals.
  • Buffer substances: This is the bicarbonate of sodium. It helps to keep the pH constant. If more CO2 comes into the blood it will combine with water to form carbonic acid which might increase the acidity. But these buffers prevent it.
Differences between Animal and Plant Cells

Animal Cells

(A) Typically spherical in shape.

(B) Cell wall is absent.

(C) Vacuoles are numerous but smaller in size

(D) Nucleus and cytoplasm including cell organelles are usually centrally placed

(E) Centrioles, essential for cell-division, are present

(F) Chloroplasts are absent

Plant Cells

(A) Rectangular in shape

(B) Cell wall present

(C) Large central vacuole

(D) The Large central vacuole replaces nucleus and cytoplasm including cell organelles to the periphery of the cell

(E) Centriole is absent

(F) Chloroplasts are present


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.

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