Problem 13 - Biology Today and Tomorrow with Physiology

Question

Put the following processes in order of their occurrence during expression of a eukaryotic gene: a. mRNA processing b. translation c. transcription d. RNA leaves nucleus

Step-by-Step Solution

Answer

The correct order is: c, a, d, b.

Step 1: Identify the First Process

The first step in the expression of a eukaryotic gene is transcription. During transcription, the DNA sequence of a gene is copied into mRNA. This occurs in the nucleus.

Step 2: Continue to mRNA Processing

After transcription, the primary mRNA transcript undergoes processing to become mature mRNA. This involves capping, polyadenylation, and splicing and also occurs in the nucleus.

Step 3: mRNA Leaves the Nucleus

Once processed, the mature mRNA must leave the nucleus and enter the cytoplasm where it can be translated into a protein.

Step 4: Translation

In the cytoplasm, the mRNA is used as a template for protein synthesis in a process called translation. Ribosomes read the mRNA sequence and assemble the corresponding amino acids into a protein.

Key Concepts

Transcription

Transcription is the first step in the process of gene expression. In eukaryotic cells, this process occurs within the nucleus. During transcription, a specific segment of DNA is transcribed, or copied, into messenger RNA (mRNA). The process is initiated when RNA polymerase, an enzyme, binds to the DNA at a region known as the promoter. RNA polymerase then unwinds the DNA and synthesizes a complementary strand of RNA based on the DNA template. This newly formed RNA strand undergoes a process called elongation, where the RNA polymerase adds nucleotides in a sequence that is complementary to the DNA. Finally, the process concludes when RNA polymerase encounters a termination signal on the DNA, signaling the end of transcription. At this point, the RNA polymerase releases the newly formed mRNA strand, which is not yet ready for translation into protein.

mRNA Processing

After transcription, the primary mRNA transcript requires further processing to become mature mRNA. This complex phase ensures that the mRNA is protected and properly configured before it can undergo translation. The processing includes several key modifications:

• Capping: A special structure called a 5' cap is added to the beginning of the mRNA strand. This cap protects the mRNA from degradation and assists in ribosome binding during translation.
• Polyadenylation: At the end of the mRNA molecule, a tail of adenine nucleotides, known as the poly-A tail, is added. This tail also helps protect the mRNA and aids in its export from the nucleus.
• Splicing: The mRNA contains non-coding regions called introns, which need to be removed. The exons, or coding regions, are then spliced together to form a continuous sequence ready for translation.

Once these steps are completed, the mRNA is considered mature and ready to leave the nucleus.

Translation

Translation is the final step in gene expression, where the genetic code carried by mRNA is used to build proteins. This process occurs in the cytoplasm of the cell and involves the ribosome, a complex molecular machine made up of RNA and proteins. The mRNA, carrying the genetic information from the DNA, serves as a template for protein synthesis. The ribosome reads the sequence of mRNA three nucleotides at a time, with each triplet known as a codon representing a specific amino acid. Transfer RNA (tRNA) molecules bring the corresponding amino acids to the ribosome, where they are linked together in a growing polypeptide chain. Every tRNA molecule has an anticodon that pairs with the complementary codon on the mRNA, ensuring that the correct amino acids are added sequentially. The process continues until a stop codon on the mRNA is reached. At this point, the polypeptide chain is released, and it eventually folds into a functional protein, completing the translation process.

Eukaryotic Cells

Eukaryotic cells are complex cells with a defining characteristic: a nucleus that houses their genetic material. Unlike prokaryotic cells, eukaryotic cells possess membrane-bound organelles, each having specific functions that contribute to the cell's overall operation. In the context of gene expression, eukaryotic cells compartmentalize the processes of transcription and translation. Transcription occurs inside the nucleus, where the DNA resides safely. This separation allows for intricate regulation and modification of the genetic material before it exits the nucleus. Once mRNA is processed and leaves the nucleus, it enters the cytoplasm where the process of translation takes place. The organization of eukaryotic cells allows them to conduct complex and regulated processes that are vital for the proper function of multicellular organisms. This compartmentalization plays a crucial role in the accurate and efficient expression of genes.