Summary
Highlights
Transcription converts DNA into messenger RNA (mRNA), while translation uses mRNA to build proteins. Ribosomes are the organelles responsible for protein manufacturing.
Transcription starts with initiation, where RNA polymerase binds to the promoter region (TATA box) of DNA and separates the strands. During elongation, RNA polymerase adds nucleotides to synthesize mRNA from the 5' to 3' end, reading the DNA template strand in the 3' to 5' direction. The non-template strand, or coding strand, matches the mRNA sequence with uracil replacing thymine. Termination involves the separation of RNA polymerase and mRNA from the DNA. The mRNA is then capped at both 5' and 3' ends (poly-A tail) to protect it from degradation. This initial mRNA (pre-mRNA) contains introns (non-coding sequences) and exons (coding sequences); introns are removed via RNA splicing, leaving only exons for protein synthesis.
A practice problem demonstrates how to convert a DNA nucleotide sequence (3' to 5' direction) into an mRNA sequence (5' to 3' direction), explaining the base pairing rules: G with C, C with G, A with U (uracil in RNA instead of thymine), and T with A.
Translation begins when the synthesized mRNA leaves the nucleus and binds to a ribosome in the cytosol. Ribosomes facilitate the interaction between mRNA and transfer RNA (tRNA) molecules. Each set of three nucleotides on mRNA is a codon, which pairs with a specific anticodon on tRNA, bringing a corresponding amino acid. This process builds a protein based on the information stored in the mRNA.
Translation occurs in three main steps. Initiation starts with the start codon AUG, which corresponds to the UAC anticodon on a tRNA carrying methionine, entering the ribosome's P-site. During elongation, subsequent tRNA molecules enter the A-site, forming covalent bonds between amino acids, with tRNA molecules exiting from the E-site as the polypeptide chain grows. Termination occurs when a stop codon (UAA, UAG, UGA) is encountered, which signals a release factor to enter the A-site, causing the ribosome to disassemble and the protein to be released. The newly formed protein then moves to the Golgi body for further processing, folding, and modification to achieve its specific function.