From DNA to Proteins

From DNA to Proteins Chapter 14

Marvelous Mussel Adhesive • Mussel binds itself to rocks with threads coated with the protein bysuss • Gene for bysuss has been put into yeast • Yeast synthesize the protein based on the instructions in the mussel DNA

Steps from DNA to Proteins Same two steps produce all proteins: 1) DNA is transcribed to form RNA • Occurs in the nucleus • RNA moves into cytoplasm 2) RNA is translated to form polypeptide chains, which fold to form proteins

Three Classes of RNAs • Messenger RNA • Carries protein-building instruction • Ribosomal RNA • Major component of ribosomes • Transfer RNA • Delivers amino acids to ribosomes

A Nucleotide Subunit of RNA uracil (base) phosphate group sugar (ribose) Figure 14.2Page 228

Base Pairing during Transcription DNA G C A U G C A T RNA C G T A C G T A DNA DNA base pairing in DNA replication base pairing in transcription

Transcription & DNA Replication • Like DNA replication • Nucleotides added in 5’ to 3’ direction • Unlike DNA replication • Only small stretch is template • RNA polymerase catalyzes nucleotide addition • Product is a single strand of RNA

Promoter • A base sequence in the DNA that signals the start of a gene • For transcription to occur, RNA polymerase must first bind to a promoter

Gene Transcription DNA to be transcribed unwinds transcribed DNA winds up again mRNA transcript RNA polymerase Figure 14.4cPage 229

Adding Nucleotides 5’ 3’ growing RNA transcript 5’ 3’ direction of transcription Figure 14.4dPage 229

Genetic Code • Set of 64 base triplets • Codons • 61 specify amino acids • 3 stop translation Figure 14.7Page 230

tRNA Structure codon in mRNA anticodon amino-acid attachment site amino acid OH Figure 14.8Page 231

Ribosomes tunnel small ribosomal subunit large ribosomal subunit intact ribosome Figure 14.9b,cPage 231

Three Stages of Translation Initiation Elongation Termination

Initiation • Initiator tRNA binds to small ribosomal subunit • Small subunit/tRNA complex attaches to mRNA and moves along it to an AUG “start” codon • Large ribosomal subunit joins complex Fig. 14.10a-cPage 232

Binding Sites binding site for mRNA A (second binding site for tRNA) P (first binding site for tRNA) Figure 14.10dPage 232

Elongation • mRNA passes through ribosomal subunits • tRNAs deliver amino acids to the ribosomal binding site in the order specified by the mRNA • Peptide bonds form between the amino acids and the polypeptide chain grows

Elongation Fig. 14.10e-gPage 233

Termination • Stop codon into place • No tRNA with anticodon • Release factors bind to the ribosome • mRNA and polypeptide are released mRNA new polypeptide chain Fig. 14.10j-kPage 233

What Happens to the New Polypeptides? • Some just enter the cytoplasm • Many enter the endoplasmic reticulum and move through the cytomembrane system where they are modified

Transcription Overview rRNA tRNA mRNA Mature mRNA transcripts ribosomal subunits mature tRNA Translation

Gene Mutations Base-Pair Substitutions Insertions Deletions

Base-Pair Substitution a base substitution within the triplet (red) original base triplet in a DNA strand During replication, proofreading enzymes make a substitution possible outcomes: or original, unmutated sequence a gene mutation Figure 14.11Page 234

Frameshift Mutations • Insertion • Extra base added into gene region • Deletion • Base removed from gene region • Both shift the reading frame • Result in many wrong amino acids

Frameshift Mutation mRNA parental DNA arginine glycine tyrosine tryptophan asparagine amino acids altered mRNA DNA with base insertion altered amino- acid sequence arginine glycine leucine leucine glutamate Figure 14.12Page 234

Mutation Rates • Each gene has a characteristic mutation rate • Average mutation rate is approximately 1 in every 100,000 genes • Only mutations that arise in germ cells can be passed on to next generation

Mutagens • Ionizing radiation (X rays) • Nonionizing radiation (UV) • Natural and synthetic chemicals

From DNA to Proteins