Protein Synthesis

Cellular Level of Organization: 

Protein Synthesis 


  • Proteins are responsible for the characteristics of the cell, some form its structure while some form its functional parts.

  • All the information for protein synthesis is stored in the DNA of the organism.

  • DNA has the instructions for making all proteins required  in an organism but…   DNA CANNOT make proteins!  DNA doesn't leave the nucleus, but proteins are made outside the nucleus in the ribosomes which are present in the cytoplasm. 

  • DNA is used to build RNA.  RNA leaves the nucleus and builds proteins in the ribosome.  Proteins create an organism's physical traits and functions.

  • 3 Types of RNA are made from DNA template strand:

    • Ribosomal RNA (rRNA):  Used to build ribosomes.

    • Transfer RNA (tRNA):  Used to carry amino acids from cytosol to ribosomes.

    • Messenger RNA (mRNA):  Carries the instructions for making ONE protein from nucleus to ribosome.


  • RNA has one chain (strand) of RNA nucleotides

  • RNA nucleotides have a ribose sugar (not deoxyribose)

  • RNA is made from 4 different RNA nucleotides:

  • Adenine (A)          Uracil (U)

  •  Guanine (G)         Cytosine (C).

  • Uracil bonds ionically to the base Adenine.

  • RNA nitrogen bases are attracted to DNA nitrogen bases (example:  RNA Uracil base is attracted to DNA Adenine base). 


  1. Transcription (making RNA from DNA in the nucleus).

  2. Translation (using RNA to build proteins in the ribosome).


  • Takes place in the nucleus.

  • During Transcription (in the nucleus) the instructions for building one protein is copied from a DNA template strand gene into messenger RNA (mRNA).


  1.  Enzyme Helicase opens the Helical structure of DNA.

  2. RNA polymerase enzymes sit on DNA strands along a gene code.

  3. The point where the code starts is called the Promoter end while where it ends is called  Terminator end.

  4. Free RNA nucleotides floating in the nucleus bond ionically to exposed bases on the DNA template strand.

  5. The RNA nucleotides that bond to the DNA template are covalently linked together to form a pre-messenger RNA molecule.

  6. Pre Messenger RNA contains two regions…

    1. Exons: The actual protein code.

    2. Introns: The code which is not useful for a particular protein.

  7. The special enzymes called  “Spliceosomesdelete the introns and join all exons together to form a mRNA. The process is called “Splicing.”

  8. The messenger RNA leaves the nucleus and the two DNA strands of DNA come back together.

  9.  When mRNA is made from DNA all of the 3 letter DNA words in a gene are converted to 3 letter RNA words (codons) in the mRNA molecule. 

  10. A typical mRNA molecule will contain more than 100 codons, with each codon specifying the placement of one particular amino acid in a protein chain. 


  • Takes place in cytoplasm.

  • Messenger RNA (mRNA) brings instructions for making ONE protein from the nucleus to the ribosome.

  • The mRNA binds with a small ribosomal unit.

  • This binding initiates binding of a large ribosomal unit with the small ribosome unit-mRNA complex to form an assembly line for protein production.

  • The large Ribosomal unit has three binding sites

    • A: Attachment site.

    • P: Pairing Site.

    • E: Exit site.

  • The completion of small ribosomal unit-mRNA- large ribosome unit complex activates movement of tRNA.

  • Transfer RNA (tRNA) molecules bring amino acids from the cytosol into the ribosome to build the protein.

  • Different types of tRNA bring different types of amino acid to the ribosome.

  • Different types of tRNA also have different 3 base anticodon sequences that bond to mRNA “codons”. 

  • The codon-anticodon binding takes place at Attachment site.

  • The start codon is the first codon of a messenger RNA. It is always AUG which stands for amino acid Methionine.

  • tRNA’s will only bond to mRNA where the mRNA codon complements the tRNA anticodon.

  • Molecules in the ribosome covalently bond adjacent amino acids together to form the protein chain at the P site.

  • The empty tRNA molecule leaves the complex at E site to bind with another amino acid.

  • As the process continues, the length of amino acids grows.

  • When the mRNA stop codon appears in the ribosome, the complete protein chain is released and the small ribosomal unit-mRNA- large ribosome unit complex gets separated.

Commonly Asked Questions.

  1. Write a short note on “Protein Synthesis”.

  2. Write a note on “translation”.

  3. Write a note on “Transcription”.

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