Nucleus
* The nucleus is surrounded by a nuclear envelope or nuclear membrane.
* The nuclear envelope consists of the outer membrane and inner membrane.
* At a number of places, the two membrane fuse and give rise to minute pores called nuclear pores.
* Present inside the nuclear envelope is the nucleoplasm, the nuclear matrix that contains the nucleolus, a spherical structure that membrane.
* The nucleoplasm also contains chromatin, a network of highly extended and elaborate nucleo protein fibres.
* When a cell begins to divide, the chromatin condenses and coils to form rod like structures called chromosomes.
* A chromosome consists of a primary constriction or centromere that has disc shaped structures called kinetochores attached to its side.
* Based on the position of the centromere, chromosomes can be classified as metacentric, sub- metacentric, acrocentric and telocentric.
Structure of DNA
* Nucleic acids are made up of polynucleotide chains, which are formed by several nucleotides that make up the structure of the DNA when bonded together.
* The length of DNA is defined by the number of nucleotides or pairs of nucleotides present in the DNA.
* It was Friedrich Meischer who first identified the DNA as an acidic material present in the nucleus in 1869.
* In 1953, James Watson and Francis Crick proposed the revolutionary simple double helix model for the structure of DNA.
