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  • Writer's pictureMegan Chou

Biology with Bob: Protein Synthesis


Secondary Contributors: Matthew To, Caden Leung


Maybe you have difficulties following a recipe, but did you know your cells have to constantly follow protein recipes? This process is known as protein synthesis.


Overview:

  • DNA's function

  • Role & importance of mRNA in transcription and translation

  • How amino acids are coded for & produced

  • Basics of codons, anticodons, and tRNA


Up for a challenge? We've included some practice problems at the end and throughout the article!

 

1) The DNA holds all of the information in a cell, like how a cookbook holds all of the recipes!



DNA - double helix structure

DNA is the carrier that contains our genetic information. It's located inside of a cell's nucleus. It's similar to a cookbook for baking since it contains many protein recipes for your cell to function.


Now key point: The DNA does not leave the nucleus! That's where mRNA comes in to copy its code!

Fun Fact - Open me!

Just like how DNA cannot leave the nucleus, the Coca Cola recipe, dating back to 1886, is locked in a vault located in Atlanta!




 

2) The mRNA acts as a recipe!

Now just like you have to open up the cookbook to get access to the many recipes that it holds, the same applies with DNA.


Likewise, before DNA's code is turned into mRNA (messenger RNA), a specific part of DNA is unzipped by an enzyme called RNA polymerase.


Now that we have our specific piece of DNA (recipe to make a certain protein), mRNA will copy the code of the DNA segment in its particular order. In other words, this process transcription, takes a portion of DNA and turns it into mRNA form.

During transcription, complementary RNA base pairs (A, U, C, G) attach to the DNA nucleotides (A, T, C, G) to make the mRNA strand, which is a copy of the DNA strand.


View the rules of pairing for transcription! Also, try out the practice problem for yourself!

 

TRANSCRIPTION PAIRING RULES

Keep in mind that the bases for DNA are:

Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).


The bases for RNA are:

Adenine (A), Uracil (U), Cytosine (C), and Guanine (G).


In the process of transcription when converting DNA to mRNA:

  • A (DNA) is paired with U (RNA)

  • T (DNA) is paired with A (RNA)

  • C (DNA) is paired with G (RNA)

  • G (DNA) is paired with C (RNA)

 


So, if my DNA segment was:


T A C T A T G G T A T T


What would the mRNA strand be if the DNA segment was transcribed? (Hint: Starts with A U G...)

Click for the answer!

A U G A U A C C A U A A


Remember when DNA is transcribed into mRNA, use the base pairing rules to get the sequence of the mRNA strand!


 

3) The Ribosome, or the Chef, begins to make the protein!


Now what happens to the mRNA strand? The mRNA strand will detach from the DNA and travel down to the ribosome. This is where translation occurs, the process of turning the mRNA strand (with all of the DNA's code) into proteins.


Remember that proteins are all made of amino acids and that there are 20 essential amino acids that need to be synthesized.


So how does the mRNA strand become a series of amino acids that make proteins? A codon is a sequence of 3 nucleotides (in a row) that code for a certain amino acid!

Why do codons have to be 3 letters/bases long? Why not 2?

Let's do some math here. Here are two things for a refresher:

  • There are 4 bases in RNA: A, U, C, and G.

  • There are 20 different amino acids that need to be synthesized.

If there are only 2 bases, then you would only get 16 combinations of amino acids:


4 x 4 = 16 | (16 is less than 20 amino acids!)


Now let's try that with 3 bases:

4 x 4 x 4 = 64 | ( 64 is more than 20 amino acids!)


This means that there are 64 codons to represent 20 essential amino acids. Hence, there can be multiple codons that represent the same amino acid!



 

Example:

mRNA strand: A U G A U A C C A U A A


AUG AUA CCA UAA

Codon 1 Codon 2 Codon 3 Codon 4

 

Which amino acids are associated with the 3-letter codons?


The table below shows each amino acid associated with the codon. Here's how you use it:

1) Find the first base on the left column, labeled "1."

  • 2) Find the second base on the horizontal row, labeled "2."

  • 3) Find the third base on the right column, labeled "3."

That would give you the amino acid that is assigned to that certain codon.


Let's practice: Which amino acid does the codon GAC stand for?

ANSWER

Aspartic acid (Asp).

Explanation: The first base is G (look for G on the left column), the second base is A (look for A on the horizontal row), and the third base is C (look for C on the right column). You should end up on the GAC square, which corresponds with the amino acid aspartic acid (Asp).


Here are some special codons:

  • AUG - the ONLY START codon which starts the process of protein synthesis. The amino acid associated with AUG is methionine (Met). EVERY PROTEIN BEGINS WITH THE CODON AUG (amino acid - Met).

  • UAA - STOP Codon

  • UAG - STOP Codon

  • UGA - STOP Codon

 

Anticodons & tRNA


An anticodon is a set of three nucleotides on the tRNA that base-pair

with an mRNA codon at the ribosome (A-U C-G). The anticodon needs to base pair with its complementary codon to ensure that the correct amino acid is added to the protein chain.

If you take a quick look at the image, the tRNA is the floating gold structure and the colored balls represent the amino acids it produces. You can see the anticodons of the tRNA pairing with the codons on the mRNA.

This process will continue all the way until it reaches one of the STOP codons.


The protein will end up being a chain of amino acids!

 

Gene Expression:


A gene has been "expressed" when the protein is made from a segment of the DNA strand.

 

PRACTICE PROBLEM:


DNA Strand: TACAACGTACAGAGCAGT

mRNA Strand (Separate into codons): __________

Amino Acid Protein Chain (Use Chart): ___________


*BONUS: List the anticodons for the mRNA Strand



ANSWER KEY

mRNA Strand: AUG | UUG | CAU | GUC | UCG | UGA

Amino Acid Protein Chain: Met-Leu-His-Val-Ser-stop

Bonus: TAC | AAC | GUA | CAG | AGC | ACU


 

Summary:

  • DNA is the carrier of our genetic information & carries our cell's protein recipes.

  • mRNA copies a segment of the DNA recipe through transcription and travels to the ribosome (kitchen) for translation

  • The anticodons on the tRNA (the chef) base-pair with the mRNA codons to create amino acids.

  • The amino acids are chained together by peptide bonds to make the protein.

 

Next time you bake with a recipe, just remember that your cells are "baking" some tasty proteins all day long!


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