Summary
Highlights
The human genome contains over 3 billion base pairs of DNA, distributed across 23 pairs of chromosomes, and includes about 30,000 genes that code for proteins. A significant portion (95%) of DNA consists of non-coding regions called introns, some of which control gene expression, while the purpose of much of it remains unknown. Gene expression, the process by which coded information in DNA leads to functional products like proteins, involves two main steps: transcription (DNA to RNA) and translation (RNA to protein). This video focuses on transcription.
Transcription occurs in two broad steps: first, the enzyme RNA polymerase forms a pre-messenger RNA (pre-mRNA) molecule in the nucleus, using the DNA template strand. Second, the pre-mRNA undergoes editing by other enzymes to produce a mature messenger RNA (mRNA), which is the final product of transcription. The formation of pre-mRNA involves three key stages: initiation, elongation, and termination.
The main enzyme in transcription is RNA polymerase, which copies DNA information into a new RNA molecule. It synthesizes the new strand from the 5' to 3' direction. Initiation begins when RNA polymerase binds to the promoter region of a gene, which signals where to start. The enzyme then separates the DNA strands, exposing a single-stranded template, specifically the 3' to 5' DNA strand for transcription.
Elongation is the process of copying bases from the DNA template strand to form a new complementary RNA strand. RNA polymerase adds nucleotides according to base-pairing rules, with one crucial difference from DNA replication: uracil (U) is added in RNA instead of thymine (T) when adenine (A) is present on the template strand. This process accurately builds the RNA copy.
Just as promoter regions signal the start, terminator regions of the gene signal RNA polymerase where to stop. Once the enzyme reads and transcribes these terminator sequences, the newly formed RNA transcript is released from the transcription complex. This marks the completion of the first section of transcription, producing a pre-mRNA molecule.
The pre-mRNA is not yet ready for translation and needs to be processed into a mature mRNA molecule. This processing involves two main steps: first, the addition of a protective cap and tail (short amino acid sequences) to the ends of the pre-mRNA, which shields it from enzymatic degradation. Second, a process called RNA splicing occurs, where non-coding sequences called introns are removed by a protein-RNA complex called a spliceosome. The remaining coding regions (exons) are then joined together to create the mature, functional mRNA molecule.