Transcription is the first step in gene expression: the process of copying a DNA sequence into RNA. This step determines which genes are active in any given cell, making it central to how cells differentiate and respond to their environment. Understanding transcription, the types of RNA it produces, and how eukaryotic cells process mRNA before translation will give you a strong foundation for the rest of this unit.
Transcription Initiation
RNA Polymerase and Promoter Regions
RNA polymerase is the enzyme that builds an RNA strand using one strand of DNA as a template. It reads the template strand 3' to 5' and synthesizes the new RNA strand 5' to 3'.
Before transcription can begin, RNA polymerase needs to know where to start. That's the job of promoter regions, which are specific DNA sequences located upstream (before) the gene. One well-known promoter element in eukaryotes is the TATA box, a short sequence rich in adenine and thymine that helps position RNA polymerase correctly.
Here's how initiation works:
- Transcription factors recognize and bind to the promoter region (e.g., the TATA box).
- These transcription factors recruit RNA polymerase to the promoter, forming a transcription initiation complex.
- RNA polymerase unwinds a small stretch of DNA and begins synthesizing RNA from the template strand.
Transcription Factors and Gene Regulation
Transcription factors are proteins that bind to specific DNA sequences to regulate whether a gene gets transcribed, and how much RNA is made. They come in two main types:
- Activators bind to DNA sequences called enhancers and increase the rate of transcription by helping recruit RNA polymerase.
- Repressors bind to sequences called silencers and decrease or block transcription.
The specific combination of transcription factors present in a cell is what determines which genes are turned on or off. This is a big part of why a muscle cell and a neuron can have identical DNA yet behave so differently.
RNA Types
Messenger RNA (mRNA)
mRNA carries the genetic instructions from DNA in the nucleus to the ribosomes in the cytoplasm, where proteins are built. The nucleotide sequence of the mRNA directly determines the amino acid sequence of the resulting protein through the genetic code.
A few key characteristics of mRNA:
- It's synthesized during transcription and serves as the template for translation.
- mRNA molecules are typically short-lived. After being translated (sometimes multiple times), they're degraded by cellular enzymes. This gives the cell tight control over how much of a protein gets made.
Transfer RNA (tRNA) and Ribosomal RNA (rRNA)
tRNA acts as the translator during protein synthesis. Each tRNA molecule carries a specific amino acid on one end and has a three-nucleotide anticodon on the other end that pairs with the complementary codon on the mRNA. Enzymes called aminoacyl-tRNA synthetases are responsible for loading ("charging") each tRNA with the correct amino acid.
rRNA is a structural and functional component of ribosomes. It doesn't just hold the ribosome together; rRNA actually catalyzes peptide bond formation between amino acids during translation. This makes rRNA a ribozyme (an RNA molecule with enzymatic activity).
mRNA Processing
In eukaryotes, the initial RNA transcript (called pre-mRNA) must be processed before it can leave the nucleus and be translated. Prokaryotes skip this step because they lack a nucleus and can translate mRNA while it's still being transcribed.
Introns, Exons, and Splicing
Eukaryotic genes contain a mix of exons (coding sequences that will be expressed) and introns (non-coding sequences that interrupt the exons). The pre-mRNA includes both, so the introns need to be cut out.
- The spliceosome, a large complex made of proteins and small nuclear RNAs (snRNAs), recognizes the boundaries between introns and exons.
- The spliceosome cuts out each intron and joins the remaining exons together into a continuous coding sequence.
One particularly important consequence of this system is alternative splicing: by including or excluding certain exons, a single gene can produce multiple different mRNA variants. Each variant can code for a slightly different protein. This is a major source of protein diversity in eukaryotes. For example, the human genome has roughly 20,000 genes but can produce well over 100,000 different proteins, largely thanks to alternative splicing.
5' Cap and Poly-A Tail
Two protective modifications are added to the pre-mRNA, and both happen in the nucleus (co-transcriptionally, meaning while transcription is still occurring):
- 5' cap: A modified guanine nucleotide (7-methylguanosine) is added to the 5' end of the mRNA. It protects the mRNA from being degraded by enzymes, helps the mRNA get exported from the nucleus, and is recognized by the ribosome to initiate translation.
- Poly-A tail: A string of 100–200 adenine nucleotides is added to the 3' end. It increases mRNA stability, aids in nuclear export, and influences how efficiently the mRNA is translated.
Together, the 5' cap and poly-A tail act like bookends that protect the mRNA and help it function properly once it reaches the cytoplasm. Without these modifications, the mRNA would be rapidly degraded and never translated.