Where Does Transcription and Translation Occur? Tips to Remember the Difference

If you are starting to learn about protein synthesis in biology class, the terms “transcription” and “translation” probably sound familiar. These are the two major phases by which the code in our DNA is read and translated into specific proteins. If you need a crash course on transcription and translation, where the processes occur, and how to remember the difference, you are in the right place.

Background

Plants, animals, fungi, protists, and most algae have eukaryotic cells. This means that they contain membrane-bound organelles, including a nucleus where DNA resides. DNA never leaves the nucleus, but the production of the proteins that DNA codes for occurs in the cytoplasm of the cell, outside of the nucleus. To make this possible, transcription and translation must occur.

DNA

DNA is a complex molecule. It has an antiparallel, double-helix structure, with complimentary base pairs held together by hydrogen bonds. In other words, it is a double-stranded molecule that looks like a twisty ladder. The rungs of the ladder are the nucleotide bases – adenine, thymine, guanine, and cytosine. To form the rungs, adenine bonds with thymine and guanine bonds with cytosine These nucleotide base pairs exist in groups of threes called codons. Codons correspond to amino acids that build a protein during translation. For example, if one strand of DNA has the sequence AAT GCG, the corresponding bases that would bond to it on the antiparallel strand would have the complementary sequence of TTA CGC.

DNA also has a directionality component because of its complex chemical structure. One end of a single DNA strand is the 5’ end and the other is the 3’ end. On the complementary strand, it is the opposite orientation with the 3’ end next to the first strand’s 5’ end. For a basic understanding of protein synthesis, it’s only necessary to know that DNA reads in the 3’ to 5’ direction during transcription. By logical extension, the complementary, antiparallel pre-mRNA builds from its 5’ to 3’ end during transcription.

Transcription: In the Nucleus

Transcription occurs inside a cell’s nucleus. It is the process of writing mRNA by using the existing DNA as a template. Messenger RNA, or mRNA, is a type of nucleic acid that can travel out of the nucleus into the cytoplasm of the cell. Since DNA cannot exit the nucleus, its code transfers to RNA molecules that can. For the following example, imagine a hypothetical gene, abc, that codes for protein Abc. Here are the steps of transcription for the synthesis of protein Abc from gene abc.

1) Helicase unwinds the DNA

First, the enzyme helicase finds the region of the DNA that corresponds to the abc gene and causes that section to “unwind” by interfering with the hydrogen bonds between bases. Helicase “unzips” the DNA, exposing a template strand and a complimentary strand (the “coding” strand). Only the template strand transcribes into a single-stranded pre-mRNA molecule.

2) RNA Polymerase Synthesizes Pre-mRNA

Next, the enzyme RNA polymerase binds to the template strand of the DNA. RNA polymerase reads the DNA from the 3’ end to the 5’ end and catalyzes the reactions that form bonds with compatible nucleotides. The nucleotides being added are found free-floating in the nucleus. The pre-mRNA, consisting of nucleotides complementary to the template strand, will therefore have the same bases as the coding strand of the original abc gene. The only difference is that the nucleotide uracil replaces thymine when building the pre-mRNA. So, if the template strand from the original DNA was ATCGAC, then the coding strand must have been TAGCTG and the pre-mRNA strand will be UAGCUG.

3) Post-transcription Modifications

After transcribing the pre-mRNA for the abc gene, there are modifications to prepare it to exit the nucleus through pores in the nuclear membrane. There are 3 key modifications to the pre-mRNA before it becomes mature mRNA. First, a “5’ cap” is added to the 5’ end of the pre-mRNA molecule. Second, a “poly(A) tail” is added to the 3’ end of the molecule. Finally, introns (sequences in the gene that are not expressed in the RNA product) are spliced from the pre-mRNA so only exons remain (parts of the gene that will be expressed in the final RNA). For the purposes of this article, details of the chemical compositions and functions of the 5′ cap and poly(A) tail are not necessary

The final components of mature mRNA are the 5’ cap, 5’ UTR (a region prior to the initiation codon), exons, the 3’ UTR, and the 3’ Poly(A) tail. UTR refers to untranslated regions that are important to gene regulation after transcription. These sections of mRNA exist at both ends of the molecule before the initiation codon and after the stop codon. The 5’ and 3’ UTR sequences do not translate in the next phase of protein synthesis, only the sequence between the initiation (start) codon and stop codon.

Translation: In the Cytoplasm

Translation occurs in the cytoplasm of the cell after finishing transcription in the nucleus. It is the process by which ribosomes translate mRNA molecules into polypeptide chains made of amino acids. The region of the mRNA that the ribosome translates consists of codons (sets of three nucleotides) that each code for an amino acid. The mRNA codons correspond to amino acids, the amino acids bind together to form a polypeptide chain, and the polypeptide chain folds to become a functioning protein.

1) Ribosome Synthesizes Polypeptide Chain

In the cytoplasm, the large and small subunits of a ribosome bind to the mRNA strand for gene abc at the start codon. The start codon is always AUG (adenine, uracil, guanine). The tRNA molecules (transfer RNA) in the cytoplasm assist the ribosomes in creating the polypeptide chain that will form protein Abc. The ribosome reads the mRNA from the 5’ to 3’ end. Meanwhile, tRNA molecules bring free-floating amino acids to the corresponding codons. Each tRNA molecule is specific to each codon and delivers a specific amino acid. Covalent bonds form between the adjacent amino acids to create a chain.

2) Ribosome Releases Polypeptide Chain

As the ribosome is reading the mRNA, it will eventually reach a stop codon. The possible stop codons which do not correspond to an amino acid are always UAA, UAG, or UGA. No tRNA molecule recognizes these codons and a release factor protein induces the release of a complete polypeptide chain from the ribosome. This polypeptide chain will then fold to become the functioning Abc protein.

Protein Folding: In the Cytoplasm

After translation, polypeptide chains undergo multiple structural transformations by folding. The specific structure of the final protein dictates its function. This folding is due to electrostatic forces between ions and the formation of chemical bonds. The primary structure of a protein is the polypeptide chain. Folding it forms the secondary structure of the protein. After more folding, it forms the tertiary structure, and sometimes, additional folding forms a quaternary structure. When the Abc protein is in its final form and functioning, its effects are the manifestation of the abc gene.

Remembering the Difference Between Transcription and Translation

A good way to remember the order and basic processes of transcription versus translation is to think about writing a story in a different language. First, you sit down and write the story as someone tells it to you; you transcribe the story. Then, since it is in a different language you must translate it to form your final product. First you write the story, then you translate the story. In protein synthesis, mRNA is first written and then it is translated into a polypeptide chain. Like in the example, the target gene abc was transcribed, then translated, and finally folded into the protein Abc.

A good way to remember where transcription or translation occurs is by remembering if the molecule being acted upon is DNA or mRNA. DNA NEVER LEAVES THE NUCLEUS. Therefore, when DNA is being acted upon during transcription, you know it must be in the nucleus. When mRNA is being acted upon during translation, it is in the cytoplasm.

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