Unit E – PROTEIN SYNTHESIS



PROTEIN SYNTHESIS – STEP 1 - TRANSCRIPTION

 

Introduction :

- *DNA Replication produces an identical DNA strand.

 

- *Protein Synthesis uses DNA to produce proteins.

Do not confuse these 2 processes!!!!!!

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 - DNA is the master copy (or template) containing instructions for the production of a protein's primary structure.

 

- Proteins are built out in the cytoplasm at protein factories called RIBOSOMES, but DNA never leaves the nucleus. An imitated, mobile copy of the DNA must be made. This copy is called messenger RNA (mRNA). Only genes for required proteins are copied into mRNA. The process of making mRNA is called “TRANSCRIPTION”.

-Then mRNA travels into the cytoplasm where it is Translated into proteins.

 

DNA--------------> mRNA----------------> Protein

Transcription Translation

PROTEIN SYNTHESIS : Step 1 - TRANSCRIPTION

 

A. The gene in DNA is selected, isolated and uncoiled by enzymes. Each gene codes for a particular protein that will eventually be produced.

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B.     The specific gene is then unzipped by enzymes.

(H - bonds broken between bases)

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C.  Free – floating RNA nucleotides join complementary to the selected gene ( this portion of DNA strand is called the template) - In diagram below, the strand on the right is acting as the template.

- *Because there is no Thymine in RNA; A different base called Uracil always joins across from Adenine*

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D. The Ribose – Phosphates then join together to complete the backbone. The hydrogen bonds that temporarily formed between the RNA bases and the DNA bases are broken by enzymes and the new mRNA strand is free of the template DNA strand. 

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 - We now have a mRNA strand that is mobile and can leave the nucleus. Steps A, B, C and D = Transcription.

 

- The gene segment of DNA then zips back together. The newly formed mRNA passes through the nuclear pores of the nuclear envelope and out into the cytoplasm. It will then be used to complete the second part of Protein Synthesis known as "TRANSLATION".

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 E.  The segment of DNA will then recoil back into its double helix shape.

Genetic Code:

- The nitrogenous bases in DNA contain the instructions for making proteins. Every 3-base sequence in a DNA strand codes for a specific amino acid.

- Each 3 base set is called a “CODON”.

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- There are 64 possible combinations (Codons) :

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- However, we only have about 20 amino acids, therefore most amino acids have more than 1 codon. Often the codons only differ in the last base.

- Example: CAA and CAG

Both codons code for the amino acid Glutamine.

 

- Duplicate codons may be a way of protecting/reducing the effects of mutations. This property of having duplicate codons for the same AA is called DEGENERACY.

 

- The genetic code is basically “Universal”, the same codons stand for the same A.A. in all living things.

- AUG which is for Methionine is a start codon, whereas UAA, UGA and UAG are stop codons. These stop codons signal that the polypeptide has all its amino acids.

- At the end of each mRNA strand there are long chains of Adenine bases (Polyadenine Tail). This is so the cell can detect that the mRNA is still functional. When enough of a given protein has been created, the polyadenine tail is cut off. This tells the cell that this particular mRNA is no longer needed to produce its protein at this time. The cell will then digest and destroy the no-longer functional mRNA. Enzymes will use water to hydrolyze this strand of mRNA.

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Template Strand

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