Hereditary characters are transferred from parents to offsprings by gene, hence they are said to be structural and functional units of heredity.

Organisms produced by asexual reproduction show minor variations.

The component which is in the nuclei of cells and carries the hereditary characteristics is called Chromosome.

Chromosomes are mainly made up of DNA.

Organisms produced through sexual reproduction show major variations.

1. The structure in the nucleus of cells that carries the hereditary characteristics is called the chromosome.
2. It is made up mainly of nucleic acids and proteins.
3. ‘Genes’ which contain the information about hereditary characteristics in coded form are located on chromosomes.
4. Each chromosome is made up of DNA and has a constricted region called the ‘Primary constriction’ or ‘Centromere’.
5. Types of chromosomes can be easily identified during cell division.
6. Depending upon the specific position of the centromere, there are four types of chromosomes:
1. Metacentric: The centromere is exactly at the mid-point in this chromosome. The arms of this chromosome are equal in length.
2. Sub-metacentric: The centromere is somewhere near the mid-point. One arm is slightly shorter than the other.
3. Acrocentric: The centromere is near one end of this chromosome. One arm is much smaller than other.
4. Telocentric: The centromere is right at the end of this chromosome. This chromosome consists of only one arm.
7. If the pair consists of similar chromosomes by shape and organization, they are called ‘homologous chromosomes’ and if they are not similar they are called ‘heterologous chromosomes’.
8. There is a pair of ‘sex chromosomes’ or ‘allosomes’ and all other chromosomes are called ‘autosomes’.

1. Chromosomes are mainly made up of DNA.
2. As per Watson and Crick’s model, two parallel threads of nucleotides are coiled around each other. This arrangement is called a ‘double helix’.
3. Each strand in the molecule of DNA is made up of many small molecules known as ‘nucleotide’.
4. There are four types of nitrogenous bases adenine, guanine, cytosine and thymine.
5. Adenine and guanine are called as ‘purines’ while cytosine and thymine are called ‘pyrimidines’’.
6. In the structure of the nucleotide, a molecule of a nitrogenous base and phosphoric acid are each joined to a molecule of sugar.
7. Nucleotides are arranged like a chain, in a molecule of DNA.
8. The two threads of the DNA molecule are made up of alternately joined molecules of sugar and phosphoric acid.
9. Each rung of the ladder is a pair of nitrogenous bases joined by hydrogen bonds.
10. Adenine always pairs with thymine and cytosine always pairs with guanine.

1. The sequence of genes in the DNA is unique to every person.
2. It is like fingerprint and is useful for identifying criminals.
3. DNA fingerprinting is useful in case of parental dispute.
4. It can prove very useful to study evolution.
5. It has great uses in anthropology.
6. We can also learn about the secrets of the past using DNA fingerprinting.

1. RNA is made up of ribose sugar, phosphate molecules and four types of nitrogenous bases adenine, guanine, cytosine and uracil.
2. The nucleotide of the chain of the RNA molecule is formed by combination of a ribose sugar, phosphate molecule and one of the nitrogenous bases.
3. Large numbers of nucleotides are bonded together to form the macromolecule of RNA.
4. There are three types of RNA. They are:
   1. Ribosomal RNA (rRNA): Ribosomal RNA (rRNA): The molecule of RNA which is a component of the ribosome organelle is called a ribosomal RNA. Ribosomes perform the function of protein synthesis.
   2. Messenger RNA (mRNA): Messenger RNA (mRNA): The RNA molecule that carries the information of protein synthesis from genes i.e. DNA chain in the cell nucleus to ribosomes in the cytoplasm which produce the proteins, is called messenger RNA.
   3. Transfer RNA (tRNA): Transfer RNA (tRNA): The RNA molecule that transports specific amino acids to the ribosomes during protein synthesis is called transfer RNA.

1. It is necessary for people to have their blood examined before marriage to detect any genetic disorders or diseases that could be passed on to offspring.
2. Sickle-cell anaemia is a hereditary disease.
3. It occurs due to changes in genes during conception.
4. If the father and mother are both affected by sickle-cell anaemia or if they are carriers of this disorder, their offspring are likely to suffer from this disease.
5. Hence, marriages between the persons who are carriers of or suffering from sickle-cell anaemia should be avoided.
6. If a parent is suffering from a dreadful disease like AIDS or Hepatitis, it is likely to be transmitted to the child.
7. Genetic abnormalities give rise to Turner syndrome in women and Klinefelter syndrome in men causing sterility.
8. Therefore, it is necessary to have blood examined before marriage.