1. Translation and Point Mutations
Chapter 17.4 and 17.5

2. TRANSFER RNA
tRNA brings the correct amino acid from the cytoplasm to the ribosome.
The cell keeps a “pool” of all 20 amino acids by synthesizing them or taking them from the food we eat.
Each tRNA translates a particular mRNA codon into a specific amino acid
The tRNA has the amino acid on one end and the anticodon (complimentary base pairs to the codon) on the other
Codon by codon, the genetic message of DNA is translated and the ribosome joins the amino acids into a polypeptide chain.

3. Structure and function of Transfer RNA
Like mRNA, tRNA is made in the nucleus and must travel to the cytoplasm where it can be used repeatedly
There are complimentary strands along the length of tRNA that can hydrogen bond to each other allowing the tRNA to fold and form a 3- dimensional structure.
20 different enzymes called aminoacyl-tRNA synthetases make sure the tRNA carries the correct amino acid to the ribosome.
WOBBLE - flexible base-pairing between the 3rd base of a codon and the corresponding base of the anticodon. This is why several codons can code for the same amino acid.

4. Ribosomes
2 subunits to a ribosome, called large and small, and are made of rRNA.
Large and small join to form a functioning ribosome when they attach to a mRNA molecule.
Bacterial and eukaryotic ribosomes differ slightly and are medically significant: antibiotic drugs can inactivate bacterial ribosomes without inhibiting the eukaryote’s ability to make proteins. (ex. tetracycline used to fight bacterial infections).
mRNA binding sites – A, P, and E
A – holds the tRNA carrying the next amino acid
P – holds the tRNA attached to the growing polypeptide
E – discharged tRNA leaves the ribosome

5. Building a polypeptide
The 3 stages of translation:
Initiation stage: brings together mRNA, a tRNA bearing the first amino acid of the polypeptide, and the two subunits of the ribosome. An specific initiator tRNA carries the start codon, methionine.
Elongation stage: amino acids are added to the growing polypeptide one by one. Each addition involves proteins, called elongation factors, and includes 3 stages.
1. Codon Recognition – the anticodon pairs with the complimentary codon in the A site.
2. Peptide Bond Formation – an rRNA molecule catalyzes the formation of a peptide bond between the new a.a. and the carboxyl end of the growing polypeptide in the P site.

6. Building a polypeptide
3: Translocation – the empty tRNA in the P site is moved to the E site where it is released. The mRNA brings its next codon to be translated to the A site.
Termination of translation (stage three) : elongation continues until a stop codon reaches the A site. A release factor protein binds directly to the stop codon at the A site and causes the addition of a water molecule to the end of the polypeptide chain. This hydrolyzes (breaks) the bond between the polypeptide and the tRNA in the P site.

7. Polyribosomes Multiple ribosomes translate mRNA at the same time.
Many polypeptides can be made simultaneously from the same strand of mRNA.
Multiple ribosomes can attach to the strand of mRNA. These strands of ribosomes are called polyribosomes (or polysomes).
Polysomes enable a cell to make copies of a polypeptide very quickly.

8. Completing and targeting the functional protein
The polypeptide begins to coil and fold forming a protein with a specific shape, a 3-dimensional molecule with secondary and tertiary structure.
2 or more polypeptides, that are synthesized separately, may come together becoming a protein that has quaternary structure (ex) hemoglobin

9. Free vs. bound ribosomes
Free Ribosomes – suspended in the cytosol and synthesize proteins that will stay there.
Bound Ribosomes – attached to the ER or the nuclear envelope. These ribosomes make proteins of the endomembrane system (ex. nuclear envelope and lysosomes) as well as proteins secreted from the cytosol (ex. insulin).
The ribosomes are identical and can switch their status from free to bound.

10. Point mutations
Changes to the genetic information of a cell and responsible for the huge diversity among organisms.
Mutations are the source of new genes and lead to new phenotypes.
Point mutations – change in a single base pair of a gene
In a gamete, a point mutation may be transmitted to offspring and future generations.
Ex. sickle cell disease: mutation of a single base pair in a gene that codes for the beta-globin polypeptide in hemoglobin. Individuals who are homozygous for the mutant gene will have multiple symptoms associated with sickle cell.

11. Types of point mutations: substitutions
Base pair substitution – the replacement of one nucleotide and its “partner” with another pair of nucleotides.
Missense mutations – substitution of a single amino acid for another amino acid.
Nonsense mutation – point mutation that changes a codon for an amino acid into a STOP codon. Translation ends prematurely and leads to a nonfunctional protein.

12. Mutations : insertions and deletions
Additions or losses of nucleotide pairs in a gene, leading to disastrous effects on the resulting protein because the “reading frame” has been altered.
Frameshift mutations – alters the triplet groupings of bases in the mRNA. All nucleotides “downstream” of the insertion or deletion will be improperly grouped into codons.
THE FAT CAT ATE THE RAT- deletion mutation of first F
THE ATC ATA TET HER AT – makes no sense and the protein will be nonfunctional

13. mutagens
Physical and chemical agents, called mutagens, may interact with DNA to cause mutations.
X-rays, UV light - cause genetic changes
Cancer causing chemicals – interfere with correct DNA replication and/or cause chemical changes in bases that change their pairing properties

14. summary Transcription – mRNA is transcribed from a DNA template.
RNA Processing – pre-mRNA is spliced and modified to produce mRNA which moves from the nucleus into the cytoplasm.
The mRNA leaves the nucleus and attaches to a ribosome.
Each amino acid attaches to its proper tRNA
Translation – a succession of tRNAs add their amino acids to a polypeptide chain as the mRNA is moved through the ribosome one codon at a time.
When completed, the polypeptide is released from the ribosome.
The protein expresses the gene that the DNA coded for.