transcription

• an mRNA copy of a gene is made from DNA
• in eukaryotic cells, transcription takes place in the nucleus
• 1. starts when RNA polymerase attaches to the DNA double helix at the beginning of a gene
• 2. the hydrogen bonds between DNA strands in the gene are broken down by DNA helicase attached to the RNA polymerase. this separates the DNA strands, exposing some of the bases
• 3. one of the strands is then used as a template to make an mRNA copy
• 4. RNA polymerase lines up free RNA nucleotides alongside the exposed bases on the template strand. free bases are attracted to the exposed bases. complementary base pairing means mRNA strand ends up being a complementary copy of the DNA template strand (except T being U)
• 5. once the RNA nucleotides have paired up with their specific bases on the DNA strand, they're joined together by RNA polymerase, forming an mRNA molecule
• 6. the RNA polymerase moves along the DNA, separating the strands and assembling the mRNA strand
• 7. the hydrogen bonds between the uncoiled strands of DNA re-form and the strands coil back into a double helix
• 8. when RNA polymerase reaches a particular sequence of DNA called a stop signal, it stops making mRNA and detaches from the DNA
• 9. in eukaryotes, mRNA moves out of the nucleus via a nuclear pore and attaches to a ribosome in the cytoplasm