Nucleus is surrounded by a double layered membrane with pores 70 nm size.These pores serve as transport channels for RNA to migrate to cytoplasm. Nucleus contains chromatin and one or more nucleoli. The nucleoli contain RNA, while, the chromatin has the DNA, which carries the genetic information. The structure of DNA shows two strands, which are folded into a double helical structure called chromosomes. The number of chromosomes present in the nucleus of human being is 46, which includes 22 pairs of autosomes and one pair of sex chromosomes.
DNA is about 10 to 20 nm in size and chemically made up of four nucleotides namely, adenosine, thymidine, guanosine and cystidine. Each nucleotide consists of a pentose sugar deoxy ribose, phosphoric acid residue and a side chain consisting of bases namely, adenine, thymine, guanine, and cytosine.
The sequence with which the bases are arranged forms the genetic code.
RNA is a single strand structure, which contains ribose sugar instead of deoxyribose and uracil instead of thymine base.
Protein synthesis depends on the genetic code present in DNA. The transfer of genetic information from DNA to the ribosomes occurs through mRNA and is called transcription. The mRNA binds to polyribosomes in the cytoplasm and assemble the amino acids with the help of tRNA. This process is called translation. The rough endoplasmic reticulum contains ribosomes on its surface and protein synthesis takes place on it. The synthesized protein undergoes post-translational modification, with cleavage of bonds and side chain changes. It moves to Golgi apparatus for packaging and forms secretory vesicle.
Source: Textbook of Physiology, 3E (Chandramouli) (2010)