Protein Synthesis Involves Transcription & Translation
- The 2 steps of protein synthesis:
Process Where Major Components Notes Transcription In the nucleus DNA gene
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
Peptidyl transferase enzyme
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
- 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
- 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)
- 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
- 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.
- 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.
- 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.
|Start||1st Codon||2nd Codon||3rd Codon|
- DNA makes RNA makes Protein
- The amino acids are determined by the m-RNA codons
- 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
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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