Protein Synthesis

A. Transcription-the process by which DNA is copied to RNA The enzyme responsible for transcription is RNA polymerase RNA polymerase must bind with DNA at a promoter Promoter series of repeating nucleotides up stream of the gene to be transcribed The most common promotor is a TATA box

3. RNA polymerase slides along the DNA molecule 3’ 5’ until it hits a start codon The DNA start codon is TAC This creates the first RNA codon AUG and the nucleotides are added 5‘ 3' 4. RNA polymerase will copy the DNA codons until it encounters a stop codon

5. Prokayotic and Eukaryotic transcription differs because of the type of RNA molecule produced The end product of prokaryotic transcription is m-RNA (messenger RNA) Eukaryotic transcription produces hn-RNA (homologous nuclear RNA or pre m-RNA) hn-RNA contains both intron (sequences of nucleotides that do not code for protein) and exon (sequences of nucleotides that code for protein). m-RNA contains only exon.

How do RNA and DNA differ?

6. DNA and RNA differ in the following ways: DNA has two strands, RNA has one DNA contains the sugar deoxyribose, RNA has the sugar ribose DNA has the base thymine which is replaced in RNA by the base uracil

B. RNA Processing- the conversion of hn-RNA to m-RNA in Eukaryotes The 5' end of the RNA molecule gets a GTP cap and the 3' end gets a poly A tail. The 5' cap prevents hydrolysis of the RNA and aids in binding the molecule to a ribosome. The poly A tail facilitates the export of the molecule out of the nucleus.

RNA Processing continued 2. Introns are spliced out of the RNA by units called “snurps” (small nuclear ribonuclear proteins) that form a large assembly called a spliceosome

C. Following processing, the m-RNA molecule moves into the cytoplasm to a ribosome where the polypeptide is manufactured 1. About 60% of the ribosome is r-RNA and 40% is protein 2. Ribosomes are composed of 2 subunits. A large and small subunit.

3. The large subunit has three binding sites for t-RNA The P site (peptidyl-binding site) that holds the t-RNA carrying the growing polypeptide chain The A site (aminoacyl-tRNA site) holds the t-RNA molecule which carries the next amino acid to be added to the chain The E site (exit site) where discharged (t-RNA without amino acids) leave the ribosome

D. Translation- the conversion of m-RNA to a polypeptide chain The m-RNA leave the nucleus and binds with the small ribosomal subunit The initiator t-RNA brings the amino acid methionine and binds to the m-RNA at the start code forming an initiation complex The large ribosomal subunit joins the initiation complex with the initiator t-RNA in the P site The new m-RNA codon is read and another t-RNA brings the proper amino acid into the A site

Transcription Initiation

5. The t-RNA molecule is 80 nucleotides long in the shape of a cloverleaf.

t-RNA Structure the 3' end at the top of the molecule contain the amino acid attachment site The other end t-RNA molecule has a 3 base sequence called the anticodon where the t-RNA binds with the m-RNA codon The enzyme aminoactyl-tRNA synthetase catalyzed the union of t-RNA molecules to specific amino acids

Only 45 tRNAs Exist for 61 Codons Some tRNAs have anticodons that recognize two or more different codons Because the anticodon is curved on the tRNA, the base pairing rule for third base is flexible. This is called wobble The RNA base inosine can bond with U, C or A

Elongation of Polypeptide Chain

6. When the new amino acid is in place in the A site, the enzyme peptidyl transferase joins the amino acid to the growing peptide chain in a condensation reaction. This forms a peptide between the amino acids 7. When growing peptide chain is connected to a t-RNA in the A site, it is translocated to the P site and the discharged t-RNA in the P site moves to the E site 8. The discharge t-RNA is now removed from the E site and the next m-RNA codon is read in the A site

9. The process is repeated until the ribosome encounters a stop codon in the A site 10. A protein called a Release Factor binds directly to the stop codon. This frees the completed polypeptide and dissociates the ribosomal subunits and m-RNA.

E. Mutation- change in the genetic code 1. Gene Mutation or Point Mutation- a nucleotide base is added, subtracted or changed to produce a change in the amino acid sequence of a protein

Proline Glutamic Acid Glutamic Acid A change in a single base in the DNA strand will result in a change in the m-RNA strand and the resulting protein Normal Hemoglobin Amino Acids DNA RNA 5 6 7 GGA CTC CTC CCU GAG GAG Proline Glutamic Acid Glutamic Acid Sickle Cell Hemoglobin Proline Valine Glutamic Acid GGA CAC CTC CCU GUG GAG

2. Chromosome Mutation- involves a change in many genes a) Deletion- part of a chromosome is lost b) Inversion- part of a chromosome is flipped around c) Translocation- part of a chromosome is added to another chromosome

3. Somatic & Germ Mutations a) Somatic mutations: change that occurs in body cells. Affects only the individual. Ie. cancer b) Germ Mutations- changed in the genetic code of gametes that will affect the individuals offspring 4. Mutagens- substances capable of causing damage to DNA 5. Most mutations are harmful