Translation

The core concept of translation is the connecting of a codon to an amino acid, which is accomplished with the Transfer RNAs. What that leaves us with then is the actual mechanism of amino acid polymerization.

Initiation of Translation

Protein function is determined by the sequence of amino acids. This sequence allows the protein to fold into the correct configuration to produce activity. Any variation in the sequence can produce alterations to function, or even result in non-functionality. In order to generate the correct sequence, we must first establish the correct reading frame of codons. We must first find the start codon on mRNA.

The small subunit of the Ribosome (40s in eukaryotes) is built to find the Start Codon (AUG) and will align the full ribosome with the correct reading frame. A number of proteins will help the alignment and in the formation of the full (holoenzyme) Ribosome.
The diagram below shows the overall formation of the initiation complex, complete with a tRNA (the yellow structure with a pink circle attached). Again, the function of this replication complex is to find the start codon and set the reading frame for the Ribosome. Notice that the large ribosomal subunit (60s in eukaryotes) only attaches after AUG has been found. As before, it is not necessary at this academic level to memorize all of the factors involved. What is more critical is that it is a multifactor system designed to find the correct start point, and thus the same reading frame.

Key Feature: Notice that the first tRNA is already linked to the small subunit. Why? Its anticodon is complementary to the codon on mRNA. Specifically, it has the anticodon for the start codon (AUG). So we are using base complementarity to find AUG.

Elongation

The polymerization of amino acids occurs during elongation. This is where the P and A sites become important (NOTE: P and A sites are the active sites of the enzyme). P stands for Peptidyl, while A stands for Aminoacyl. These are chemical terms, which shows the orientation of the amino acid. The exit site, represented by E, is not an active site. Consider it a disposal point for spent tRNAs. [NOTE: you may also find references to a fourth site where the tRNA first comes into the complex. Don't worry about this optional site.]

The P and A sites reveal a single codon on mRNA and can hold a single complimentary tRNA. During elongation, when you have a filled P and A site, the amino acid from the P site will be linked to the amino acid in the A site. This is a process that you will have to visualize, so use the diagram below as reference:

https://karimedalla.files.wordpress.com/2012/11/translation_elongation.jpg

The amino end of the amino acid is free. The carboxyl end is attached to the tRNA. Starting at the top of the above diagram, the growing amino acid chain is attached to a tRNA in the P site. A new tRNA with an amino acid (charged tRNA) is brought into the A site. Using GTP, the Ribosome (large subunit) takes the growing peptide chain and links (carboxyl to amino) it to the individual amino acid in the A site. When this is done, the entire ribosome shifts downstream to the next codon (the new codon appears in the A site).

The spent tRNA that started in the P site is now moved to the E site, where it is removed from the ribosome. NOTE: It takes 2 GTP to create the peptide bond, then another GTP to move the ribosome. So a total of 3 GTP are used in one 'round' of Ribosomal action. REMEMBER THIS! In addition, it took a triphosphate to charge tRNA (so a total of 4 for each amino acid added to the polymer).
When both the P and A sites have charged tRNA, the growing chain from the P site is added to the single amino acid in the A site. The ribosome shifts and the process continues. This elongation process of adding amino acids (amino acid polymerization) will continue until a STOP codon is reached (UAA, UAG, and UGA).

Question: How much ATP will you need to expend to make a protein with 100 amino acids? How about a 150 amino acid protein? GOAL: Recognize and be able to articulate why protein synthesis is an energy consumptive process, and be able to discuss why it is critical for cells to regulate energy consumptive processes.

Termination

To create a functional protein, translation must end with the appropriate amino acid. If translation stops to soon, the protein will be too short and may not bend (configure) correctly. If it is too long, then it may not bend (configure) correctly. Termination is a critical process. Termination begins when a STOP CODON (UAA, UAG, and UGA) is reached. In eukaryotes, a releasing factor is used to separate the ribosomal subunits. KEY CONCEPT: The stop codon signals the end of the coded message.

http://www.proteinsynthesis.org/wp-content/uploads/2013/06/protein-synthesis-steps-termination.jpg

Once completed, proteins can be further modified as fits their function (such as adding sugars). This is known as post-translational modification. The image below shows the posttranslational modifications needed in the production of insulin. Production starts with a ribosome-bound on the Rough Endoplasmic Reticulum (RER). Processing will occur in the RER and in the Golgi body. This is only one example of posttranscriptional modification, and a majority of proteins require such modifications before they are functional. [NOTE: as a general rule, there is a less extensive posttranscriptional modification in prokaryotes, but they have numerous proteins that do require modification].

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Daily Challenge

In your own words, describe the process of translation. Discuss initiation, elongation, and termination. Make sure that you discuss the P and A site, as well as the importance of the start and stop codon. Afterwards, give a BRIEF discussion on how this is an energy consumptive process that needs to be regulated.

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Optional Challenge: Genomics and Proteomics

Read the following articles:
A brief guide to genomics - NIH fact sheet
Transcriptome - NIH fact sheet
Proteomics
The genome can be seen as the genetic potential of an individual (think Genotype), while the proteome shows what is actually produced at a given time, under a given condition (consider this the phenotype). Provide a discussion of the importance of genomic and proteomic studies in modern biological research, and make sure that you provide a description of both the genome and the proteome of an organism.

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Optional Video

This video deals with advanced topics, so I give it to you in case you are curious.

Codons, Anticodons & Amino Acids

Translation is the process of "reading" mRNA and using the code to construct a protein. But what is the code? The nucleotide language of mRNA can be divided into codons. Three sequential nucleotides that represent a genetic (nucleotide) word. So, how do you read this code or nucleotide language?

mRNA showing Codons

In the image to the right, you have sequential nucleotides divided up into codons. Notice that AUG is listed as Codon 1. This is important! AUG is the Universal Start Codon. Nearly every organism (and every gene) that has been studied uses the three ribonucleotide sequence AUG to indicate the "START" of protein synthesis (Start Point of Translation).

As we will see tomorrow, it takes more than a start codon to initiate transcription, but for now, just remember that this is the codon that indicates the START point of the instructions on how to make a protein.
The start codon established the Reading Frame for translation. From the start codon, every three sequential nucleotides will be viewed as a codon. This is critical! Mutations can affect reading frames. For example, if a nucleotide is inserted between codon 2 and 3 (G G), would you have the same reading frame down stream? What if you deleted the first nucleotide of codon 4? What is the effect of changing the reading frame? What would happen to the resulting protein?

Insertions and deletions can change reading frames, but point mutations can also occur. In this case, one nucleotide is change to a different nucleotide. What would happen if the final nucleotide of condon 3 were changed to a C? To an A? How about the second nucleotide in codon 4? Change the U to an A, what happens?

Each codon is a "genetic word," and refers to a specific amino acid (thus changes to these words can result in changes to final proteins). The tRNA is the agent of translation. On one end of the tRNA, you will find an anti-codon. Anti-codons are complimentary to codons. Example: Codon 1 reads AUG. The corresponding tRNA would have an anticodon reading UAC. (Question: Would these be antiparallel?). Codon 2 reads ACG, so the anticodon would read UGC. Oppisite the anticodon, you will find a binding site for a specific amino acid.

Aminoacyl tRNA Synthase - Addition of an Amino Acid to tRNA
http://canmedia.mcgrawhill.ca/istudy3/books/0070741751/images/figures/bro41751_1115L_lg.jpg

An amino acid can be attached to the free 3' end of the tRNA. There is a class of enzymes capable of attaching an amino acid to a tRNA: Aminoacyl tRNA Synthetase. Below is a very basic cartoon of how an amino acid is added to a tRNA.
Note that an ATP is needed to complete the binding. There is an Aminoacyl tRNA Synthetase for each tRNA-Amino Acid combination.
Below is a diagram showing the pairing of codon to anticodon. The diagram also contains a version of the Genetic Code table, showing the relationship between codon and amino acid.

Note that three codons are referred to as STOP codons: UAA, UAG, and UGA. These are used to terminate translation; they indicate the end of the gene's coding region. What would happen if you lost a Stop codon?

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 Challenge

How the change of one amino acid caused the configuration change in the protein.  Amino Acids are coded due to a codon.  In Sickle Cell Anemia, there is a mutation that changes one amino acid: Valine (Val) is found in place of Glutamic Acid (Glu).  If we look at the sequences, we find that at the sixth codon, the wild type reads GAG, but the sickle type reads GUG.  This is a single nucleotide polymorphism.  At the end is a video to explain SNPs (pronunciation: Snips)

Here is the mRNA code for the wild type (non-mutant) and mutant β-hemoglobin molecule.

https://thestrangeandspectacularworldofbiochemistry.files.wordpress.com/2013/03/sickle.jpg

Today, consider the consequence of a SNP.  What would happen if it occurred in a Start or Stop codon.  What would happen if an AAG upstream (before) the start codon had a SNP that changed the second nucleotide from an A to a U?  What would happen if CGC changed to CGG?  How about CAU to GAU?

After considering these, and looking at the video, what are some of the consequences of a SNP?  How could a SNP either stop translation or prolong it?  Are all of the results harmful, or can they be neutral?

SNP Video