Central dogma in molecular biology

After the discovery of double helical structure of DNA, Francis Crick pondered the problem of how DNA is functionally related to proteins. This led him to propose what he called the central dogma of molecular biology.The central dogma, simply stated, is that DNA codes for the production of RNA, RNA codes for the production of protein, and protein does not code for the production of protein, RNA, or DNA . In Crick’s words, “once ‘information’ has passed into protein it cannot get out again.”
The expression of a gene to form a polypeptide occurs in two major steps:
Transcription copies the information of a DNA sequence (the gene) into corresponding information in an RNA sequence.
Translation converts this RNA sequence into the amino acid sequence of a polypeptide.
Crick proposed two hypotheses.
THE MESSENGER HYPOTHESIS AND TRANSCRIPTION. Crick and his colleagues proposed that an RNA molecule forms as a complementary copy of one DNA strand of a particular gene. The process by which this RNA forms is called transcription . This messenger RNA,then travels from the nucleus to the cytoplasm, where it serves as a template for the synthesis of proteins. Each gene sequence in DNA that codes for a protein is expressed as a sequence in mRNA.
THE ADAPTER HYPOTHESIS AND TRANSLATION;According to  Cricks  adapter hypothesis: there must be an adapter molecule that can bind a specific amino acid with one region and recognize a sequence of nucleotides with another region. In due course, these adapters, called transfer RNA, or tRNA, were identified. Because they recognize the genetic message of mRNA and simultaneously carry specific amino acids, tRNAs can translate the language of DNA into the language of proteins. The tRNA
adapters line up on the mRNA so that the amino acids are in the proper sequence for a growing polypeptide chain—a process called translation .
The human immunodeficiency virus (HIV) and certain rare tumor viruses also have RNA as their genome, but do not replicate it as RNA-to-RNA. Instead, after infecting a host cell,
they make a DNA copy of their genome and use it to make more RNA. This RNA is then used both as genomes for more copies of the virus and as mRNA to produce viral proteins. Synthesis of DNA from RNA is called reverse transcription, and not surprisingly, such viruses are called retroviruses.