transcription

• transcription 


  an mRNA copy of a gene (a section of DNA) is made in the nucleus.
• transcription is the first stage of protein synthesis
• 1. RNA polymerase attaches to the DNA

  • Transcription starts when RNA polymerase attaches to the DNA double-helix at the beginning of a gene.
  • The hydrogen bonds between the two DNA strands in the gene break, separating the strands, and the DNA molecule uncoils at that point.
  • One of the strands is then used as a template to make an mRNA copy
• 2. Complementary mRNA is formed

  • The RNA polymerase lines up free RNA nucleotides alongside the template strand.
  • Complementary base pairing means that the mRNA strand ends up being a complementary copy of the DNA template strand (except the base T is replaced by U in RNA).
  • 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 strand
• 3. RNA polymerase moves down the DNA strand

  • The RNA polymerase moves along the DNA, assembling the mRNA strand.
  • The hydrogen bonds between the uncoiled strands of DNA reform once the RNA polymerase has passed by and the strands coil back into a double-helix
• first stage of transcription

  A) RNA polymerase

  B) template strand of DNA

  
• stage 2 of transcription

  A) RNA nucleotides joined together

  B) free RNA nucleotides

  
• stage 3 of transcription

  A) direction RNA polymerase is moving

  
• 4. mRNA leaves the nucleus

  • When RNA polymerase reaches a stop codon, it stops making mRNA and detaches from the DNA.
  • The mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm, where the next stage of protein synthesis takes place