Structure of DNA

By | February 8, 2019

Question 1 Give important features of the double helical model of DNA proposed by Watson and Crick?

Question 2 What is a gene?

Question 3 Explain XX-XY type of sex determination in human beings?

Question 4 Give examples where environmental factors govern the sex determination?

Question 5 Define the term homogametic and heterogametic?

Question 6 Mention few characteristics of genes?

Question 7 What does a nucleotide consist of in a DNA molecule?

Question 8 Why is DNA called a polynucleotide?

Question 9 How many different bases are found in a DNA molecule? Which bases pair together?

Question 10 Where are the genes located?What is the chemical nature of gene?


DNA is deoxyribonucleic acid. It was first isolated by the scientist Frederick Meisher from the nucleus of the pus cells in 1869.

DNA is the carrier of the genetic information from generation to generation. It transmits the hereditary in a coded language from parents to the offsprings.DNA is a macromolecule or polymer. It is made of very large number of nucleotide units and hence is termed polynucleotide.

Each nucleotide unit in a DNA molecule is made up of three components:

(1) Deoxyribose sugar: It is a pentose sugar.

(2) Nitrogenous base: Each nucleotide unit has a nitrogen containing base. In a DNA molecule, nitrogenous bases are of two types:

(a) Purines: The purines in a DNA molecule are – Adenine (A) and Guanine (G)

(b) Pyrimidines: The pyrimidines in a DNA molecule are – Cytosine (C) and Thymine (T)

(3) Phosphate group: The phosphate group contains one phosphorus atom and four specifically linked oxygen atoms.

Double Helical Model of DNA

Double Helical structure of DNA

J.D. Watson and F.H.C. Crick proposed the double helical model of DNA in 1953.

The important features of the double helical model are:

(1) DNA molecule is made up of two long polynucleotide strands forming a double helical structure just like a spiral staircase. Each helical turn of the DNA molecule is 3-4 nm in length in which ten nucleotide base pairs are present.

(2) Deoxyribose sugar and phosphate molecules are joined alternately to form the backbone of each polynucleotide strand. The nitrogenous base of each nucleotide is attached to the sugar molecule and projected towards the interior of the double helix.

(3) In the interior of double helix, the nitrogenous bases of two polynucleotide strands form a pair with the help of hydrogen bonds. Adenine (A) always pairs with thymine (T) and guanine (G) always pairs with cytosine (C).

The two polynucleotide strands of the DNA molecule are joined by hydrogen bonds between specific nitrogenous bases. Such a specific pairing of the bases of the opposite strands of the DNA molecule is called complementary pairing.

Adenine (A) and thymine (T) are complementary to each other. Similarly, guanine (G) and cytosine (C) are complementary to each other. The hydrogen bonding between the specific nitrogenous bases keeps the two strands to hold together.All the base pairs remain stacked between the two strands.


Mendel was the first scientist to visualise a gene as a unit of inheritance in 1866. He called it factor.

The word gene was, however, introduced by Johannsen in 1909.

Genes are segments of DNA on a chromosome occupying specific positions.

For example: 30,000-40,000 genes are present on 46 human chromosomes.

(1) Each gene has a specific sequence of nucleotides which determines its functional property.

(2) It is the genes which determine everything about us, say our appearance, our complexion, the shape and colour of our eyes, the shape of our nose and chin.

(3) Gene is a segment of DNA molecule which has coded information to form a particular protein in the cell. This protein can function either as a structural protein or as a functional protein (enzyme).

A gene is a hereditary unit, a segment of DNA having specific sequence of nucleotides that determines its specific biological function.It can maintain constancy from generation to generation but at times may also undergo sudden mutations (inheritable changes) to bring variations.

Sex Determination

In sexual reproduction male and female gametes fuse during fertilization to form zygote. This zygote then develops into the offspring (male or female).

The mechanism by which the sex of an individual is determined as it begins life, is called sex determination.

In diploid organisms having separate sexes, a specific pair of chromosomes in each diploid cell determines the sex of the individual. They are called sex chromosomes. All other chromosomes are termed autosomes as these have genes which control the somatic (body) characters.

In human beings, there are 46 chromosomes. Of these, 44 (22 pairs) are autosomes and 2 (one pair) are sex chromosomes. The sex chromosomes are of two types- X chromosome and Y chromosome.

Human sex determination

(1) A male individual contains one X chromosome and one Y chromosome i.e. XY. Male produces two different kinds of gametes (sperms), half of the gametes having X chromosome and other half having Y chromosomes. Therefore, male is called heterogametic.

(2) A female individual contains two similar X chromosomes i.e., XX . Female, therefore, produces same type of all gametes (ova or eggs). So, female is called homogametic.

The sex of the child is determined at the time of fertilization when male and female gametes fuse to form zygote.

(a) If a sperm (male gamete) carrying X chromosome fertilizes an egg or ovum (female gamete) carrying X chromosome, then the offspring will be a girl (female). This is because the offspring will have XX combination of sex chromosomes.

(b) If a sperm (male gamete) carrying Y chromosome fertilizes an egg or ovum (female gamete) which has X chromosome, then the offspring will be a boy (male). This is because the offspring will have XY combination of sex chromosomes.

The sex of the child (offspring) is determined by the type of sperm that fuses with ovum at the time of fertilization. Therefore there is 50% chance of a male child being born and a 50% chance of a female child being born.

This mechanism of sex determination is called XX-XY mechanism.

In grasshoppers and some other insects, the male has only one sex chromosome (XO) whereas the female has two homomorphic (of same size and shape) sex chromosomes (XX). This type of sex determination mechanism is called XX-XO mechanism.

Role of Environmental Factors in Sex Determination

Sex determination is sometimes regulated by environmental factors . In some reptiles, the temperature at which the fertilised egg is incubated before hatching, is important environmental factor for determining the sex of the offspring.

Two examples are cited below:

(1) First example is of a turtle . In this species, high incubation temperature results in development of female progeny.

(2) Second example is of a lizard. In this-species, high incubation temperature results in male progeny.

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