Chapter 13 - DNA to Protein

DNA -----transcription---> RNA -----translation----> protein

Comparison of DNA and RNA

• nucleotides have a different sugar [Figure 13.2]
• nitrogen-containing base is Uracil, not Thymine; C=G and A=U
• has only 1 strand
• are 3 kinds of RNA
  1. messenger RNA (mRNA) - contains the protein-building instructions
  2. ribosomal RNA (rRNA) - major component of ribosomes
  3. transfer RNA (tRNA) - carries amino acids to ribosomes

Transcription

• DNA to RNA
• in the nucleus
• only one DNA strand is transcribed
• only a small portion of the DNA is transcribed (a gene)
• uses RNA polymerase
• all 3 types of RNA are made this way
• process [see Figure13-4]
  1. RNA polymerase unwinds DNA.
  2. Free RNA nucleotides pair to the exposed template strand.
  3. Enzyme adds the new nucleotides one by one to the growing RNA molecule.
  4. Enzyme keeps adding and unwinding as it goes (DNA strands re-wind behind the enzyme).

Translation

• mRNA to protein (rRNA and tRNA will also be used)
• in the cytoplasm
• the mRNA genetic code is read in sets of 3 nucleotides (codon) [Figure 13-6]
• each codon specifies only one amino acid, but there may be several codons specifying the same amino acid [Figure 13-7]
• structure of tRNA [Figure 13-8] - anticodon
• structure of ribosome [Figure 13-9]
• process of translation [see Figure 13-10]
  1. A ribosome binds to mRNA.
  2. The ribosome has room for the first 2 of the mRNA codons in it.  Two tRNAs (with amino acids attached), with matching anticodons, bind to the codons in the ribosome.  The ribosome links the 2 amino acids together.
  3. The ribosome moves to the next codon.  This leaves 1 tRNA still in the ribosome holding the newly made protein and the other tRNA is ejected from the ribosome after its amino acid is broken off.  That tRNA's amino acid is part of the protein now.
  4. A tRNA with matching anticodon for the next codon enters the ribosome.
  5. The ribosome adds this tRNA's amino acid to the protein.
  6. The ribosome moves to the next codon.
  7. The used tRNA is ejected from the ribosome.
  8. This continues until the end of the mRNA is reached.  When the ribosome reaches the end, it falls off of the mRNA and the new protein is free.

© Carol L. Wymer, 2003