Chapter 11.4-11.6 From DNA to Proteins College Prep Biology Mr. Martino
Introduction Protein synthesis is the process of making proteins 2 stages: 1. Transcription – process of transferring the DNA gene onto an mRNA “transcript” 2. Translation – process of tRNA and rRNA assembling the amino acids in the proper sequence according to mRNA Involves RNA
11.4 Transcription 3 main classes of RNA: mRNA: messenger RNA carries the instructions from DNA tRNA: transfer RNA delivers amino acids to ribosome to form protein rRNA: ribosomal RNA a major component of ribosomes
More Transcription RNA is similar yet different from DNA Single stranded 4 types of nucleotides (uracil substitutes for thymine – pairing with adenine) Ribose substitutes for deoxyribose Different types of RNA Found in nucleus (like DNA) but also in cytoplasm
Uracil is only found in RNA
Transcription again Transcription is similar to replication – but differs in 3 important ways: 1. Only a small section of DNA (gene) serves as template Promoter: a base sequence on DNA that signals the beginning of a gene 2. RNA polymerase attaches nucleotides instead of DNA polymerase 3. Results in a single strand of RNA – not double helix
3 Phases of transcription: 1. Initiation: when RNA polymerase attaches to the promotor DNA 2. Elongation: growing of the RNA strand As it continues, RNA strand pulls away from DNA & DNA rejoins 3. Termination: RNA reaches a specific sequence that codes for stop Polymerase detaches from RNA and gene
Not all of the new mRNA is used Introns: base sequences that are removed before translation Exons: parts of the mRNA that gets translated into a protein
11.5 Genetic Code Gobind Khorana and Marshall Nirenberg realized ribsomes read nucleotides 3 at a time Codon: set of 3 nucleotides = amino acid 64 codons code for 20 amino acids AUG – codes for methionine /“start” 3 codons for “stop” Several codons code for some amino acids Codons are universal for all living organisms
Structure and Function of tRNA & rRNA tRNA: (transfer RNA) carry amino acids One end has anticodon while other carries a.a. Anticodon: nucleotide triplet that base pairs with codon Specific for 1 amino acid rRNA: component of ribosomes
11.5 Translation Translation has 3 stages – initiation, elongation, and termination: 1. Initiation: Step 1: mRNA binds to small subunit initiator tRNA locates & binds to start codon (AUG) Step 2: large subunit binds to small one
2. Elongation: After initiation, A.A.’s are added one by one Step 1: Codon recognition tRNA anticodon matches mRNA codon Step 2: Peptide Bond Formation A.A. separates from tRNA and attaches to polypeptide (P to A) Step 3: Translocation A site tRNA moves to P site
Termination: stop codon on mRNA is reached Triggers a release factor to bind to ribosome Enzyme detaches mRNA and polypeptide Many polypeptides then float freely in cytoplasm – others enter rough ER before being sent to their ultimate destinations
11.6 Mutations Gene mutations: change in nucleotide sequence of DNA genes Substitution: one amino acid is substituted for another Ex. Sickle-cell anemia Frame shift mutations: change “down-stream” codons Insertion: an extra base is put into a gene Deletion: loss of a base Transposable Elements: (translocation) DNA segments that move spontaneously
Causes of Mutations Many mutations occur spontaneously during replication Small number of mistakes do sneak by proofreaders Mutagen: mutation causing agents in environment UV light, gamma rays, X-rays Carcinogen: cancer causing agents – disrupt base pairing Only spontaneous mutations of germ cells or gametes are passed on to offspring