Codon Recognition tRNA anticodon matched to mRNA codon in the A site.

Peptide Bond Formation A peptide bond is formed between the new AA and the polypeptide chain in the P-site. Bond formation is by rRNA acting as a ribozyme

After bond formation The polypeptide is now transferred from the tRNA in the P-site to the tRNA in the A-site.

Translocation tRNA in P-site is released. Ribosome advances 1 codon, 5’ 3’. tRNA in A-site is now in the P-site. Process repeats with the next codon.

Comment Elongation takes 60 milliseconds for each AA added.

Termination Triggered by stop codons. Release factor binds in the A-site instead of a tRNA. H2O is added instead of AA, freeing the polypeptide. Ribosome separates.

Polyribosomes Cluster of ribosomes all reading the same mRNA. Another way to make multiple copies of a protein.

Prokaryotes

Comment Polypeptide usually needs to be modified before it becomes functional.

Examples Sugars, lipids, phosphate groups added. Some AAs removed. Protein may be cleaved. Join polypeptides together (Quaternary Structure).

Signal Hypothesis “Clue” on the growing polypeptide that causes ribosome to attach to ER. All ribosomes are “free” ribosomes unless clued by the polypeptide to attach to the ER.

Result Protein is made directly into the ER . Protein targeted to desired location (e.g. secreted protein). “Clue” (the first 20 AAs are removed by processing).

Mutations Changes in the genetic makeup of a cell. May be at chromosome (review chapter 15) or DNA level

DNA or Point Mutations Changes in one or a few nucleotides in the genetic code. Effects - none to fatal.

Types of Point Mutations 1. Base-Pair Substitutions 2. Insertions 3. Deletions

Base-Pair Substitution The replacement of 1 pair of nucleotides by another pair.

Sickle Cell Anemia

Types of Substitutions 1. Missense - altered codons, still code for AAs but not the right ones 2. Nonsense - changed codon becomes a stop codon.

Question? What will the "Wobble" Effect have on Missense? If the 3rd base is changed, the AA may still be the same and the mutation is “silent”.

Comment Silent mutations may still have an effect by slowing down the “speed” of making the protein. Reason – harder to find some tRNAs than others.

Missense Effect Can be none to fatal depending on where the AA was in the protein. Ex: if in an active site - major effect. If in another part of the enzyme - no effect.

Nonsense Effect Stops protein synthesis. Leads to nonfunctional proteins unless the mutation was near the very end of the polypeptide.

Sense Mutations The changing of a stop codon to a reading codon. Result - longer polypeptides which may not be functional. Ex. “heavy” hemoglobin

Insertions & Deletions The addition or loss of a base in the DNA. Cause frame shifts and extensive missense, nonsense or sense mutations.

Question? Loss of 3 nucleotides is often not a problem. Why? Because the loss of a 3 bases or one codon restores the reading frame and the protein may still be able to function.

Mutagenesis Process of causing mutations or changes in the DNA.

Mutagens Materials that cause DNA changes. 1. Radiation 2. Chemicals ex: UV light, X-rays 2. Chemicals ex: 5-bromouracil

Spontaneous Mutations Random errors during DNA replication.

Comment Any material that can chemically bond to DNA, or is chemically similar to the nitrogen bases, will often be a very strong mutagen.

What is a gene? A gene is a region of DNA that can be expressed to produce a final functional product. The product can be a protein or a RNA molecule

Protein vs RNA Protein – usually structure or enzyme for phenotype RNA – often a regulatory molecule which will be discussed in future chapters.

Summary Know Beadle and Tatum. Know the central dogma. Be able to “read” the genetic code. Be able to describe the events of transcription and translation.

Summary Be able to discuss RNA and protein processing. Be able to describe and discuss mutations. Be able to discuss “what is a gene”.