Translation
My AP Biology Thoughts
Unit 6 Gene Expression and Regulation
Welcome to My AP Biology Thoughts podcast, my name is Saarim Rizavi and I am your host for episode #108 called Unit 6 Gene Expression and Regulation: Translation. Today we will be discussing everything there is to know about translation. I will first be giving a brief overview of what translation is, it’s overall function, the 3 steps involved in translation, and some of the different components and organelles involved in translation. I’ll then go into greater detail on the individual steps of translation which will involve the organelles and different components mentioned before. Finally, I will relate the process of translation to the broader topic of gene expression and regulation. Before I begin, I would like to give credit to Khan Academy, biologydictionary.com, and nature.com for the information they provided me with in order for this podcast to be possible. So thanks to them. Alright, so here we go:
Segment 1: Introduction to Translation
- Translation is the process of creating proteins from an mRNA template
- A cell reads information from mRNA molecules and uses this information to build a protein – involves decoding an mRNA and using its information to build a polypeptide, and multiple polypeptide chains form a protein
- Three basic steps of translation – initiation, elongation, and termination
- Initiation – the ribosomes get together with the mRNA and the first tRNA so translation can begin
- Elongation – the amino acids are brought to the ribosome by tRNAs and linked together to form a chain of amino acids
- Termination – the finished polypeptide is released to go and do its job in the cell
- In mRNA, the instructions for building a polypeptide come in groups of 3 nucleotides called codons – there are 61 codons for amino acids and each of them is read to specify a certain amino acid out of the 20 possible amino acids
- Stop codons tell the cell when polypeptide is complete and the AUG codon is the start codon which signals the start of protein construction
- In translation, the codons of an mRNA are read in order, from the 5’ end to the 3’ end, by tRNAs.
- tRNA’s = molecular bridges that connect mRNA codons to the amino acid they encode
- One end of the tRNA has a sequence of 3 nucleotides called an anticodon, which binds to a matching mRNA codon through base pairing; the other end of the tRNA carries the amino acid specified by the codons
- tRNAs bind to mRNAs inside the ribosomes – ribosomes are made up of protein and ribosomal RNA
- The ribosomes provide a set of slots where tRNAs can find their matching codons on the mRNA template and deliver their amino acids. As these tRNAs enter slots in the ribosome and bind to codons, their amino acids are linked to the growing polypeptide chain in a chemical reaction.
Segment 2: More About Translation
- Initiation
- Ribosome, an mRNA with instructions for the protein to be built, and an initiator tRNA carrying the first amino acid in the protein – these components come together to form the initiation complex which is the molecular setup needed to make a new protein
- The tRNA carrying the methionine attaches to the small ribosomal subunit – they bind to the 5’ end of the mRNA by recognizing the 5’ GTP cap which was added during processing in the nucleus
- They go along the mRNA in the 3’ direction, stopping when they reach the start codon (eukaryotic cells)
- In bacteria, the small ribosomal subunit attaches directly to certain sequences in the mRNA – these Shine-Dalgarno sequences mark the start of each coding sequence, letting the ribosome find the right start codon for each gene.
- Elongation
- The amino acid chain gets longer and the mRNA is read one codon at a time, and the amino acid matching each codon is added to a growing protein chain
- Detailed:
- The first methionine- carrying tRNA (methionine is an amino acid specified by the start codon, AUG) starts out in the middle slot of the ribosome, called the P site.
- A fresh codon is exposed in another slot, called the A site. The A site will be the landing site of the next tRNA whose anticodon would be complementary to the exposed codon
- Once the matching tRNA has landed in the A site, the formation of the peptide bond that connects one amino acid to another occurs
- A methionine from the first tRNA in transfered onto the amino acid of the second tRNA in the A site
- The mRNA is pulled onward through the ribosome by one codon after peptide bond formed – this shift allows the first, empty tRNA to drift out via the E or exit site. It also exposes a new codon in the A site, so the cycle can repeat itself.
- Summary:
- When a new codon is exposed, a matching, complementary tRNA binds to the codon, the existing polypeptide is linked onto the amino acid of the tRNA, and the mRNA is shifted one codon over in the ribosome which allows a new exposed codon to be read and the cycle continues.
- Termination
- The finished polypeptide chain is released
- A stop codon in the mRNA enters the A site
- Stop codons are recognized by release factors, which fit into the P site
- Release factors mess up the enzyme that normally forms peptide bonds by making the enzyme add a water molecule to the last amino acid of the chain
- This reaction separates the chain from the tRNA, and the newly made protein is released out of the ribosome.
- Processing
- Editing the polypeptides so that they are ready to then perform their correct function in the cell.
- The new polypeptide will fold into a distinct 3D structure, and may join with other polypeptides to make a multi-part protein
- Amino acids may be chemically altered or removed
- Some proteins contain special amino acid sequences that direct them to certain parts of the cell – through protein targeting, the proteins reach their destination.
Segment 3: Connection to Gene Expression & Regulation
- A DNA molecule is divided up into functional units called genes – each gene provides instructions for a functional product, or a molecule needed to perform a job in the cell (polypeptide molecule)
- Genes provide instructions for building polypeptides which fold up and combine to make complex proteins
- Transcription – the process in which the DNA sequence of a gene is copied to make an RNA molecule
- If the gene that is transcribed encodes a protein, the RNA molecule will be read to make a protein in translation (when the sequence of the mRNA is decoded to specify the amino acid sequence of a polypeptide)
- Central dogma – transcription + translation = process of info going from DNA to RNA to protein
Thank you for listening to this episode of My AP Biology Thoughts. For more student-ran podcasts and digital content, make sure that you visit www.hvspn.com. ((Learning new things doesn’t have to be challenging. It’s really easy once you have a goal in mind and a purpose for everything you do)!
Music Credits:
- “Ice Flow” Kevin MacLeod (incompetech.com)
- Licensed under Creative Commons: By Attribution 4.0 License
- http://creativecommons.org/licenses/by/4.0/
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