MOL - Transcription [LESSON]

Transcription

It has been stated several times over that DNA controls the functioning of a cell and ultimately the organism through the storing and transmission of genetic information. Gene expression is the process by which DNA controls the synthesis of proteins that regulate the function of an organism. This expression occurs in two stages, transcription, and translation. Each gene provides instructions for a product, and in most cases, the product is a protein.

Mendel's flower color gene provides instructions for a protein that helps make colored molecules in flower petals.

Transcription is the process of forming an mRNA strand from a DNA strand. This process begins in the nucleus when a portion of a DNA strand is used as a template to create an RNA strand. The synthesis of RNA is directed by an enzyme, RNA polymerase, which binds to the promoter sequence after general transcription factors bind.  RNA polymerase opens the DNA molecule and reads in the 3' to 5' direction, but adds new RNA nucleotides to create the new strand in the 5’ to 3’ direction. Using the same base pair rules as DNA replication, RNA polymerase directs the addition of nucleotides based on the DNA template strand. However, uracil is used in place of thymine. Note here that the promoter is accessible, as we will talk about the regulation of transcription heavily later on in this module.

Which way is the new strand of mRNA synthesized?

The new strand is ALWAYS synthesized in the 5’ arrow pointing right. 3’ direction

Watch the Transcription video below to learn more about transcription and eukaryotic post-transcriptional modification.

What are the three major modifications to the RNA transcript that occur after transcription in eukaryotes?

Addition of the 5’ cap, addition of the poly-A tail, and intron removal/exon splicing

In eukaryotic cells, the mRNA strands are modified before translation begins, but in prokaryotic cells, transcription and translation typically occur simultaneously.

The picture shows differences in location and timing of transcription in prokaryotic and eukaryotic cells.

The location of transcription is also different between prokaryotes and eukaryotes. Eukaryotic transcription takes place in the nucleus, where the DNA is stored, while protein synthesis takes place in the cytosol. Because of this, a eukaryotic mRNA must be exported from the nucleus before it can be translated into a polypeptide. Prokaryotic cells, on the other hand, don't have a nucleus, so they carry out both transcription and translation in the cytosol.

Experiment with the model below. When you click the correct letter button, a nucleotide will fly up and add itself to the RNA molecule, but if you click the wrong button nothing will happen. When you think you know what's going on, answer the Questions to Know below. (You can always come back to the model if you're unsure.) Practice your coding to correctly transcribe the entire gene below.

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