7 Central dogma

Prof. Sunil Kumar Khare

  1. Objective
  2. Introduction : What is central dogma of molecular biology
  3. Origen of central dogma
  4. How it transfer the genetic information
  5. Explain the reverse of central dogma

Concept Map

 

  1. Introduction

Central dogma is all about the flow of genetic information in biological systems. It was first stated by Francis Crick in 1958. According to Crick there is a sequential transfer of genetic information from DNA to RNA to protein and it could not be transferred from protein to protein or to nucleic acid. This was the statement of Central dogma of molecular biology. However, it was questioned many times by molecular biologists, but the work of Dr. Howard Temin showing that RNA viruses can use viral RNA as a template for DNA synthesis forced Crick to re-explain the concept of central dogma. In 1970 Crick cleared all the ambiguities in the concept of central dogma through one of his article in Nature publication covered later in section 3.2. In mid-1950s situation in molecular biology was very confusing as molecular mechanism of DNA replication, translation and transcription was not known that time. So, it was very difficult to propose central dogma. Let see how the concept of central dogma originated and what led Crick to propose this.

 

3.1 Origen of central dogma

 

In 1953, Watson and Crick proposed the structure of DNA and stated that it fulfil all the requirements of being genetic material such as self-replication. In 1954 Gamow proposed that DNA could directly determines the sequence of protein. DNA was the component of chromosome which was present in the nucleus and protein synthesis occurs in cytoplasm. Along with this RNA was abundant in cytoplasm. Direct role of DNA in protein formation could not explain the abundance of RNA in the cytoplasm and the rate at which protein synthesis takes place. In addition they also found microsomes in the cytoplasm made up of RNA and proteins where protein synthesis does not occurs. In contest to these observations Crick got one clue for the central dogma that once information reached to protein it will never get back to nucleic acid. Later on for the complementation of central dogma, Crick postulated two more hypothesis first was sequence hypothesis and other was protein folding hypothesis. In sequence hypothesis he assumed that specificity of nucleic acid lies in the arrangement of their bases and that sequence is unique for a particular protein. At this point of time the concept proposed by Crick was totally informational one. He follow the transfer of information with in the macromolecule and just for the time being neglected the conversion of DNA to RNA. However, he got the most important information that sequence of nucleotide and amino acid is important for nucleic acid and protein, respectively. Further elaboration of central dogma was based on the two different experimental observations independently done by Heinz Fraenkel Conrat and Christian Anfinsen. In one experiment Heinz Fraenkel Conrat showed that two different strain of tobacco mosaic virus could be distinguished by difference in their coat protein and RNA is required for the correct production of these coat proteins. Whereas, Christian Anfinsen showed the spontaneous refolding of ribonuclease enzyme denatured in vitro. It was difficult to imagine that, how compactly folded protein can transfer information to another protein or to any nucleic acid. Moreover, there was no machinery available in the cell for this purpose. All these observations allowed Crick to understand the flow of genetic information from DNA to RNA to Protein.

 

3.2 Transfer of genetic material

 

Crick stated that transfer could be divided roughly into three groups:

  1. Group I: For this transfer evidences are available in the nature and they were :

DNA→RNA →Protein

 

There are four kind of transfers under this category

 

DNA→DNA

 

    DNA→ RNA

 

RNA →Protein

 

RNA→ RNA

 

Above three transfers are evident in all the living cells and the last one is found in RNA viruses.

B. Group II: For this group transfer neither any experimental or any strong theoretical evidence were found.

 

DNA→ RNA→ Protein

 

Transfers in this category are:

      RNA → DNA  (evidences provided by Dr. Howard Temin)

 

DNA → Protein (evidences provided by Dr. Gamow)

  1. Group III: This class consist of three transfer

Protein → Protein

 

Protein → RNA

 

This kind of transfer was very unlikely to occur as cell has to evolve totally different machinery for back translation.

 

Central dogma further has four rules:

  1. It does not explains what kind of machinery is used for transfer of genetic material and what kind of errors incorporated during this process.
  2. It does not tell about the rate at which transfer process works.
  3. It was intended to apply only for the present day organism and not to the past events when origin of life took place.
  4. It is not the same as assumed earlier that transfer occur from nucleic acid to protein and back. Finally, Crick postulated that there are three kind of transfer namely
  • General transfer: It occurs in all the cells with few exception such as mammalian reticulocytes. It falls in the Group I stated above.
  • Special transfer: This is not common and occur in only specialised cells such as virus infected cells. It falls in the Group II
  • Unknown transfer: It falls in Group III and no evidence is available for this category. However, a special cell free system containing neomycin in bacterial cell can perform protein to protein transfer of information.

3.3 Reverse of central dogma

 

Central dogma was time to time challenged by many workers few of them are Howard Temin, Satoshi Mizutani and David Baltimore. The discovery of reverse transcriptase, formation of prions, role of chaperones in protein folding, epigenetic modifications of DNA, chromatin that modify gene expression, RNAi, RNA splicing and RNA editing all these new information gave the concept of central dogma reverse. Let see how these discoveries challenged central dogma.

 

In 1970, David Baltimore, discovered reverse transcriptase that had capability to synthesize DNA using RNA as a template. However, it did not contradict central dogma as Crick has explained that these type of transfer may occur in some special cases. But after this discovery these transfers categorised as reverse of central dogma.

 

After the discovery of prions Central dogma got a blow. Prions are protein that become pathogenic after aggregation, which is spontaneous process. There is a transfer of information from pathogenic form to non-pathogenic one. Which is contradictory to Cricks information and against the hypothesis that all information inside the cell originated in the form of liner DNA. Therefore, central dogma does not fit here.

 

Later on in mid-1980s discovery of chaperonins again challenged one of the statement of Crick that “Protein folding is the function of the order of amino acid” and no other machinery is required for protein folding. Chaperonins were known for proper folding of the protein and present in the different compartment of the cell which showed that Crick was totally wrong in predicting the protein folding phenomenon. However, this issue was rapidly vanished after knowing the proper function of chaperonins that was only to prevent improper folding of the protein. It again re – established that folding of protein was simply the function of order of amino acid.