Protein Synthesis Notes for NEET 2019, Download PDF!

By Neetu Pathak|Updated : October 30th, 2018

Check here Protein Synthesis notes for NEET 2019 exams. In NEET and other medical entrance examinations, most of the questions come from Protein Synthesis. Here are some simple steps to understand all the important facts based on the topic.

Protein Synthesis Notes for NEET 2019, Download PDF!

Central Dogma

  • One way flow of genetic information from DNA to the protein through RNA is called central dogma.
  • It is proposed by Crick and it includes 3 major events:-
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  1. Replication – DNA produces its own identical copy during S-phase of the cell cycle.
  2. Transcription – Formation of RNA from DNA template.
  3. Translation – Formation of protein by RNA. Here genetic languages change from nitrogenous base form to amino acid form.
  • Reverse transcription
  • It is proposed by Temin and Baltimore and commonly called teminsm.
  • They discovered it in retroviruses, whose genetic material is single-stranded RNA, but it can transform into double-stranded DNA within the host by reverse transcription.
  • The catalyzing enzyme is reverse transcriptase or RNA dependent DNA polymerase.
  • After the discovery of Temin, the concept of central dogma proposed by Crick is modified or not applicable for retroviruses.

Another important event came into confirming that existence of RNA world.

Semi-conservative mode of DNA replication 

  • In such a process the daughter DNA formed contains one strand of parental and one strand new.
  • So, 50% parental character is conserved in daughter DNA. 


  • Messelson and Stahl in 1954 proved the semi-conservative model of DNA replication in E-coli by density gradient ultracentrifugation.
  • CsCl medium is considered because it is heavier than DNA in which DNA floats .E-coli DNA having a light density (N14N14) is marked in the test tube.
  • E-coli are cultured in heavy nitrogen medium (not radioactive, in NH4Cl medium) (15N15N) and it is marked heavy density in the test tube.
  • After culturing 20min in a normal medium the E-coli having heavy density produced hybrid density and by culturing more time hybrid density and light density appeared but heavy density never appeared.
  • This proves the semi-conservative model of DNA replication.

 The process of DNA Replication 

  • The process of Prokaryote and Eukaryote are nearly the same.
  • The main polymerizing enzyme is DNA polymerase.
  • In prokaryotes, there are 3 types of DNA polymerases.
  1. DNA polymerase I: It is most abundant and discovered by Kornberg. So it is called as Kornberg’s enzyme.

It causes proofreading.

  1. DNA polymerase II: It is discovered after DNA polymerase I

It causes DNA Repair.

  1. DNA polymerase III: It is known finally. It is the main polymerizing enzyme that polymerizes nucleotides.
  • In Eukaryote there are 5 types of DNA polymerases (α,β,σ,θ,ε)

DNA polymerase σ is the main polymerizing enzyme.

α and β are the proofreading enzyme.

ε is the repairing enzyme.

  • The process of replication follows the following steps:-
  1. Origin of replication
  2. Formation of the replication fork
  3. Formation of RNA primer
  4. The polymerisation of the daughter strand
  5. Proofreading

Origin of Replication

  • DNA is a much long molecule. Beginning of replication is definite. In prokaryote, it is a single point origin or Ori, but in Eukaryotes, there are multiple Ori.
  • From the point of Ori replication proceeds in both direction.

Formation of the replication fork

  • Both the strands of parent DNA isolated and upon each strand, a new daughter strand polymerizes.
  • Helicase enzyme cut H bonds between the parent strand from the point of Ori.
  • After proceeding to certain length supercoils are formed in a rear part which may be removed by topoisomerase enzyme.
  • A polymeric protein is called a single strand binding protein (SSBP) binds to one of the unwinding strands to prevent rewinding.
  • In prokaryotes, DNA gyrase performs the role of helicase and topoisomerase.
  • Sum total events form Y shaped replication fork.

          Formation of RNA primer

  • After the formation of the replication fork, both parent strands behave as template i.e. upon which new strand polymerize.
  • Although the key polymerizing enzyme is DNA polymerase III / DNA polymerase σ, they never start polymerization because they can’t distinguish between 5’ and 3’ terminal of nucleotide.
  • Therefore they depend upon a short segment of RNA nucleotide called RNA primer polymerized by primase enzyme. RNA primase behaves as the 5’ terminal of daughter strand, therefore, DNA polymerase select 3’ terminal for polymerization.

The polymerization of the daughter strand 

- Now DNA polymerase III in prokaryotes polymerize nucleotides by adding to the 3’terminal, therefore, the growth of daughter strand occurs 5’->3’

- DNA polymerase III forms phosphodiester bond energetically. Therefore they use the substrate nucleoside tri-phosphate whose terminal high energy phosphate bonds are broken by phosphatase enzyme to provide energy for phosphor diester bond. These events overcome the use of additional ATP and unnecessary ATP load in the cell.

- The polymerization of daughter occurs unidirectional i.e. from 5’->3’. Unwinding and polymerization are a simultaneous process. Therefore two types of daughter strands are produced.

1) Leading Strand

2) Lagging Strand

- Leading strand polymerizes upon the template strand whose polarity is 3’->5’

- Whereas the lagging strand polymerize upon the parent strand whose polarity is 5’->3’

- Leading strand is continuous; lagging strand is discontinuous bearing Okazaki fragments having separate primers (Okazaki belongs to Japan).

- After completion of polymerization, Okazaki fragments are joined by DNA ligase to make it continuous.

 Proof Reading

 - Usually, a mistake may arise after an interval of 1 lakh nucleotide although the action of polymerase enzyme is accurate.

- If this misleading is not to be corrected it may create a gene mutation.

- Thereafter DNA polymerase I and III correct these misleading and replace RNA primers by DNA nucleotide and replace RNA primers by DNA nucleotide.

- Finally, H bond established between parent and daughter strand.


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