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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Transcription Process
Transcription is the first step in protein synthesis, where the genetic information stored in DNA is copied into RNA. This process is essential for turning the genetic code into functional proteins that carry out most of the work in our cells.
Key Definitions:
DNA is made up of two strands that twist around each other to form a double helix. Each strand consists of nucleotides containing:
The bases pair in a specific way: A with T and G with C. This base pairing is crucial for transcription.
RNA differs from DNA in several important ways:
These differences allow RNA to perform its specific functions in protein synthesis.
Transcription occurs in the nucleus of eukaryotic cells and can be divided into three main stages: initiation, elongation and termination.
Transcription begins when an enzyme called RNA polymerase binds to a specific region of DNA called the promoter. The promoter is a sequence of DNA that signals where transcription should start.
A specific DNA sequence that marks where transcription should begin. It's like a "start" signal for RNA polymerase.
The enzyme that reads the DNA template and builds the RNA strand by adding complementary nucleotides.
The DNA double helix unwinds and separates at the promoter region, exposing the bases that will be transcribed.
Once RNA polymerase is attached to the promoter, it begins to move along the DNA strand, reading the template strand and adding complementary RNA nucleotides to create a growing RNA chain.
During transcription, RNA nucleotides pair with DNA nucleotides following these rules:
This complementary base pairing ensures that the RNA copy accurately reflects the DNA sequence.
As RNA polymerase moves along the DNA, it creates a "transcription bubble" where:
This process happens at a rate of about 40 nucleotides per second!
Transcription continues until RNA polymerase reaches a termination signal in the DNA. At this point:
The result is a single-stranded RNA molecule that is a complementary copy of one strand of the DNA.
Transcription produces several types of RNA, each with different functions:
Carries the genetic code from DNA to the ribosomes, where it directs protein synthesis. mRNA is the most common product of transcription.
Small RNA molecules that transport amino acids to the ribosomes during protein synthesis. Each tRNA binds to a specific amino acid.
Forms part of the structure of ribosomes, the cellular factories where proteins are made. rRNA is the most abundant type of RNA.
In eukaryotic cells, the RNA produced during transcription (called pre-mRNA) must undergo several modifications before it can leave the nucleus and be used for protein synthesis:
Eukaryotic genes contain non-coding regions called introns that are removed from the pre-mRNA. The remaining coding regions, called exons, are joined together to form the mature mRNA. This process is called splicing and is carried out by complexes of proteins and RNA called spliceosomes.
The mature mRNA also receives:
These modifications protect the mRNA from degradation and help with its export from the nucleus and translation at the ribosomes.
Errors in transcription can lead to the production of abnormal proteins, which can cause diseases. For example, cystic fibrosis is caused by mutations in the CFTR gene. If these mutations are transcribed, they can result in a faulty CFTR protein that doesn't function properly. This protein normally helps move salt and water in and out of cells and when it's faulty, thick, sticky mucus builds up in the lungs and digestive system, causing the symptoms of cystic fibrosis.
This highlights the importance of accurate transcription in maintaining our health. Fortunately, cells have proofreading mechanisms that help minimize errors during transcription, though they're not as extensive as those used during DNA replication.
Not all genes are transcribed all the time. Cells regulate which genes are expressed (turned on) or repressed (turned off) depending on their needs.
Proteins called transcription factors can bind to specific DNA sequences near genes to either enhance or inhibit transcription. They work by:
External factors can affect which genes are transcribed:
Transcription is a crucial step in the central dogma of molecular biology: DNA → RNA → Protein. It ensures that the genetic information stored in DNA can be used to make proteins, which carry out most of the functions in our cells.
Remember these key points:
In the next session, we'll explore the second major step of protein synthesis: translation, where the information in mRNA is used to build proteins.
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