What are the steps of transcription?

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trophyhunter1 eNotes educator| Certified Educator

The double helix of DNA contains our hereditary information in a sequence of nucleotides that make up various genes. Genes direct the production of proteins that produce the traits seen in an organism's phenotype, along with the production of RNA molecules which help to synthesize these proteins. 

In order for gene expression to occur, there is a flow of information whereby DNA directs protein synthesis in two stages called transcription and translation. 

Transcription in eukaryotic cells occurs inside the nucleus. In prokaryotes, their DNA is in the cell not bound inside a nucleus. One of two DNA strands will serve as a template to be copied into messenger RNA (mRNA) using base-pairing rules. This mRNA will faithfully copy the DNA template and after processing, it will leave the nucleus via pores and travel to the cytoplasm. Here, it will attach to a ribosome where its genetic code can be translated and a polypeptide can be constructed. This polypeptide will be further processed and folded and become a functional protein which can become incorporated into a cell organelle or be sent to the Golgi apparatus to be packaged and exported outside of the cell for use somewhere else in the body.

There are differences between DNA and RNA including: DNA is a double stranded molecule while RNA is a single stranded molecule. Their subunits are called nucleotides. Each nucleotide contains a sugar, a phosphate and one of four bases.  The sugar in DNA is called deoxyribose and in RNA it is called ribose. There are four bases in DNA including adenine (A), thymine (T), cytosine (C) and guanine (G) and they pair according to rules--A pairs with T and C pairs with G in the DNA double helix. RNA has the bases adenine, guanine and cytosine, however it lacks thymine and has the base uracil instead. So, where a thymine would pair with adenine in DNA, if RNA were to pair with adenine, the base uracil is substituted for thymine. 

When transcription takes place, the mRNA copies the DNA template using base-pairing rules. For example, if the template strand of DNA reads: TAC TTT GAG CCT, the complementary mRNA would read:                               AUG AAA CUC GGA

The transcript is the copy of a protein-coding gene. Once transcription is completed, the mRNA travels to the cytoplasm and the small and large ribosomal subunits attach to it and translation begins. 

In translation, the triplets on the mRNA, called codons specify which amino acids will be added to the growing polypeptide and in which order they are added, as originally dictated by the DNA genetic code. Once assembled and later folded, they will form a functional protein. For example, the codon AUG is a message that means start-- a signal to begin protein synthesis, and it also codes for methionine--an amino acid. Each triplet in the mRNA that follows represents another amino acid to be added to the growing polypeptide. Special transfer RNA molecules (tRNA) will carry the appropriate amino acid to the ribosome to help assemble the polypeptide chain that will eventually become a protein. Eventually, a stop codon is reached and translation ends. 

An analogy for transcription and translation would be a chef's original recipe for a cake which is like a blueprint (similar to DNA)  that she has someone write down (transcription) and then once the recipe is made with specific ingredients in a specific order (translation) the finished product--the cake is produced, just like a protein is manufactured at the end of translation.


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