Course: SAT 170 Introductory Biotechnology

Unit 4 Learning Module

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- Unit 4
- Table of Contents (Selected)
- Unit 4 Glossary (Selected)

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Prokaryotes vs Eukaryotes

Prokarytoic DNA is located within a region of the cytoplasm known as the nucleoid. Since the ribosomes are also located in the cytoplasm, translation can occur simultaneously with transcription

For eukaryotes transcription occurs in the nucleus where the DNA is located. Once transcription is complete, the mRNA must be modified and sent to the cytoplasm before translation can occur. These mRNA modifications that occur within a eukaryote include the addition of a 5’ cap and a 3’ poly A tail as well as RNA splicing.

See Figure 7: The process of transcription and the mRNA modifications that follow occur within the nucleus of eukaryotes (notice the mRNA initially contains both aqua green and golden yellow regions; these golden regions are the introns that are eventually removed via RNA splicing). Once the modifications are complete, the mRNA is sent out of the nucleus through the nuclear pores of the nuclear envelope (shown in yellow) for the process of translation on a ribosome.

Figure 7

Figure 8

- The 5’ cap is a modified guanine that is located at the 5’ end of the mRNA molecule. The function of this structure is to protect the mRNA from degradation. There are enzymes inside the cytoplasm that are known as nucleases. These enzymes help to protect us from foreign nucleic acids, such as those that are introduced into our cells by viruses. Although these enzymes help to protect us against foreign nucleic acids, they will also attack our own nucleic acids that are floating around in the cytoplasm including these mRNA molecules. This is why it is so important to protect the mRNA using this 5’ cap. The 5’ cap also plays an important role in the process of translation as it directs the ribosomes where to bind to the mRNA in order to place the start codon into the P site of the ribosome.
- At the 3’ end of the mRNA a chain of adenines is added to also serve as protection against the nucleases which would otherwise destroy the mRNA. This chain of adenines is known as the 3’ poly A tail.

- Finally, the mRNA is modified by removing segments that are known as introns. These introns are referred to as non-coding regions. Once these non-coding introns are removed what remains in the mRNA are the coding regions which are also known as exons. This process of mRNA splicing is carried out by the spliceosome. This is a straight forward explanation of the process for the purpose of clarification. The role of these introns is much more complex than this, however. Through a process known as alternative RNA splicing a single primary transcript can be spliced into different mRNAs by the inclusion of different sets of exons. This process explains how 25,000 genes of the human genome can encode the more than 80,000 different mRNAs. Alternative RNA splicing is an important component of eukaryotic gene expression, it is estimated that 35 to 59% of human genes exhibit some form of alternative splicing. It has also been identified that 15% of known human genetic disorders are due to altered splicing.

See Figure 8: Before the process of translation can begin in eukaryotes, the mRNA must be modified through the addition of a 5’ cap and 3’ poly A tail. Additionally, note the removal of introns that takes place bringing together the spliced exons.

Eukaryotes can also traffic proteins to certain locations within the cell since they have ribosomes that are both free floating in the cytoplasm as well as attached to the rough endoplasmic reticulum (ER). For proteins that will carry out their function in the cytoplasm, they will be produced on free floating ribosomes in the cytoplasm. If a particular protein is destined to be in a specific organelle or location in the cell it will be synthesized on a ribosome in the rough ER. The rough ER can then send that protein to the golgi complex where it will be packaged and shipped to its final destination (recall the function of the golgi complex from our unit 2 material).

See Figure 9: The removal of the non-coding introns (blue) must occur to complete the mRNA modifications. The result is the coding exons are spliced together to create a transcript that is ready for translation.

Figure 9