Transcription and mRNA processing | Biomolecules | MCAT | Khan Academy

Recent questions

• What is transcription in biology?

  Transcription is a fundamental biological process where the information encoded in DNA is converted into messenger RNA (mRNA). This process is crucial for gene expression, particularly for genes that code for proteins. In eukaryotic cells, transcription begins in the nucleus, where the DNA is located. The initial step involves the transformation of DNA into pre-mRNA, which is then processed to form mature mRNA. This mature mRNA is essential for the next stage of protein synthesis, as it carries the genetic information from the DNA to the ribosomes, where proteins are synthesized.

• How does RNA polymerase function?

  RNA polymerase is a vital enzyme that plays a key role in the transcription process. It binds to a specific region of the DNA known as the promoter, which signals the start of a gene. Once attached, RNA polymerase unwinds the DNA strands and begins synthesizing mRNA in the 5' to 3' direction, using one of the DNA strands as a template. This synthesis involves pairing complementary RNA nucleotides with the DNA template, where adenine (A) pairs with uracil (U) instead of thymine (T). The activity of RNA polymerase continues until it encounters a terminator sequence, which signals the end of transcription.

• What happens after transcription in eukaryotes?

  After transcription in eukaryotic cells, the pre-mRNA undergoes several processing steps to become mature mRNA. This processing includes the addition of a 5' cap, which is a modified guanine nucleotide that protects the mRNA from degradation and assists in ribosome binding during translation. Additionally, a poly-A tail, consisting of a sequence of adenine nucleotides, is added to the 3' end of the mRNA to enhance its stability. Importantly, non-coding sequences known as introns are removed through a process called splicing, leaving only the coding sequences, or exons. The final product, mature mRNA, is then transported out of the nucleus to the ribosomes for translation into proteins.

• What is the role of exons and introns?

  Exons and introns are two types of sequences found within a gene in eukaryotic DNA. Exons are the coding sequences that contain the information necessary for producing proteins, while introns are non-coding sequences that do not contribute to the final protein product. During the processing of pre-mRNA, introns are removed through a process called splicing, which ensures that only exons are included in the mature mRNA. This distinction is crucial because the presence of introns allows for alternative splicing, a mechanism that can generate multiple protein variants from a single gene, thereby increasing the diversity of proteins that can be produced by an organism.

• Why is mRNA important for protein synthesis?

  Messenger RNA (mRNA) is essential for protein synthesis as it serves as the intermediary between the genetic information encoded in DNA and the actual production of proteins. After transcription, mature mRNA carries the specific sequence of nucleotides that corresponds to the amino acid sequence of a protein. Once transported out of the nucleus to the ribosomes, mRNA is translated into a polypeptide chain, which then folds into a functional protein. The accuracy and efficiency of this process are critical for proper cellular function and overall organismal health, as proteins play a myriad of roles in biological processes, including catalyzing reactions, providing structural support, and regulating cellular activities.