DNA & Heredity Transcription & Translation

Protein Synthesis Involves Transcription & Translation

• The 2 steps of protein synthesis:
  

  Process	Where	Major Components	Notes
  Transcription	In the nucleus	DNA gene RNA polymerase	Gene is activated. A copy of the code is made from RNA (m-RNA) m-RNA leaves the nucleus, goes to cytoplasm.
  Translation	In the cytoplasm, on the ribosomes	m-RNA Ribosomes Peptidyl transferase enzyme Transfer RNAs	Ribosome decodes the m-RNA and makes the correct protein.

• Keeping the main copy of the genes in the nucleus protects it from damage
• There are many DNA repair mechanisms in the nucleus
• When a protein is to be made the DNA code is transferred into a working copy made of messenger RNA (transcription)
  • DNA must unwind to do transcription
  • Genes can be turned on and off and are transcribed one at a time
• The m-RNA leaves the nucleus and translation takes place in the cytoplasm, on the ribosomes

Three Types of RNA are Involved in Protein Synthesis

• RNA properties:
  • RNA = nucleotide bases + 5 carbon sugar (ribose instead of deoxyribose) + phosphate
  • RNA has 4 types of nucleotide bases: A, C, G, U (U replaces T)
  • RNA is usually a single strand, not a helix
• The 3 types of RNA:
  

  Messenger RNA	m-RNA	An RNA copy of a gene
  Ribosomal RNA	r-RNA	Ribosomal structure; also includes the enzyme peptidyl transferase (makes peptide bonds)
  Transfer RNA	t-RNA	Transfer amino acids to ribosome; have anticodons which match the m-RNA codons. At least 20 types required- one for each amino acid.

  • The transcribed code is carried from the nucleus to the ribosomes by messenger RNA (m-RNA)
  • The ribosomes contain ribosomal RNA (r-RNA). This type is structural and also acts as an enzyme when the protein is lengthened
  • Transfer RNA (t-RNA) carries amino acids to the ribosomes: there must be at least 20 types of t-RNA

Proteins are Made on the Ribosomes

• Decoding (translation) occurs on the ribosome
• Ribosomes are constructed of a special RNA (r-RNA) and protein
• They have 2 sub-units which come together to form a decoding machine
• Some ribosomes attached to membranes (rough endoplasmic reticulum), others not attached (free ribosomes)

Messenger RNA Leaves the Nucleus and Looks for a Ribosome

• After the code is transcribed the m-RNA is further processed: special head and tail regions are added and some parts are spliced out
• RNA leaves the nucleus and carries the code into the cytoplasm, then attaches to a ribosome

The RNA Code is Translated into Protein

• Ribosome finds start codon (AUG), then decodes the message, 3 bases at a time
• When the ribosome reaches the stop codon the protein is released and the decoding can start over to make another proteins.

In Translation Amino Acids are Carried to the Ribosomes by Transfer RNA

• There are at least 20 kinds of t-RNA because there are 20 different amino acids
• Each transfer RNA has a site that attaches to a specific amino acid, and a site with 3 nucleotide bases (anticodon) that must match up with the RNA code letter for that amino acid
• Amino acids attach to the new protein only if the t-RNA anticodon matches (is complementary to) the m-RNA codon
• 
• Drawing from the copyright-free collection, The Sourcebook of Medical Illustration. Park Ridge, NJ: Parthenon, 1989, edited by Peter Cull.

Proteins are Finished Off in the Endoplasmic Reticulum & Golgi Apparatus

• Most new proteins are not immediately functional- they must be finished off
• In the endoplasmic reticulum and Golgi apparatus they are folded into the proper shape and sometimes chemical groups are added or clipped off
• Some are routed to the cytoplasm, others to the mitochondria, etc.

Review of the Coding: DNA -> RNA -> Protein

• DNA makes RNA makes Protein
• The amino acids are determined by the m-RNA codons

Many Hormones Act by Turning on Genes

• Hydrophobic hormones (sex hormones, cortisone, aldosterone, thyroxine) pass through the cell membrane and enter the nucleus
• In the nucleus they bind to receptors which turn on genes

More Information:

Gwen Childs of the University of Texas at Galveston has a detailed discussion of protein synthesis, including the roles of the ribosomes, endoplasmic reticulum and Golgi apparatus.

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