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From DNA to Proteins 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 – occurs at the ribosome Three Main Classes of RNAs • Messenger RNA – Carries protein-building instruction • Ribosomal RNA – Major component of ribosomes • Transfer RNA – Delivers amino acids to ribosomes Differences Between DNA and RNA 1. Sugars – DNA = deoxyribose, RNA = ribose 2. DNA = double strand, RNA = single strand 3. DNA has Thymine as one base, RNA has Uracil instead A Nucleotide Subunit of RNA uracil (base) phosphate group sugar (ribose) Figure 14.2 Page 228 Base Pairing during Transcription DNA G C A T RNA G C A U DNA C G T A DNA C G T A 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 • A base sequence in the DNA that signals the start of a gene (TATA) • For transcription to occur, RNA polymerase must first bind to a promoter (in eukaryotes this requires many transcription factors) • Once RNA polymerase is bound it will unwind the DNA and nucleotides can be added Gene Transcription transcribed DNA winds up again DNA to be transcribed unwinds mRNA transcript RNA polymerase Figure 14.4c Page 229 Adding Nucleotides 5’ growing RNA transcript 3’ 5’ 3’ direction of transcription Figure 14.4d Page 229 Once the termination site is reached, the RNA molecule will be released. In eukaryotic cells, the RNA polymerase will actually pass the termination point before the RNA molecule is released. Transcript Modification unit of transcription in a DNA strand 3’ exon intron exon transcription intron 5’ exon into pre-mRNA poly-A tail 3’ cap 5’ snipped out snipped out 5’ 3’ mature mRNA transcript Figure 14.5 Page 229 RNA processing1. 5' cap with a modified guanine nucleotide is added. 2. At the 3' end 30-200 adenine nucleotides are added (poly-Atail). -These modifications prevent the mRNA from being degraded and signal the ribosome where to attach. 3. There are noncoding regions (introns) that are removed in eukaryotic cells. The remaining regions (exons) are joined together. A particle called a spliceosome removes the introns. Spliceosomes are composed of smaller particles called snRNP (made of proteins and snRNA). - The average immature RNA is 8000 nucleotides long but the mature mRNA is 1200 nucleotides long. Genetic Code • Set of 64 base triplets • Codon = 3 bases • 61 specify amino acids. There is "redundancy" in the code or more than one codon codes for the same amino acid. One that codes for methionine also codes for “start”. • 3 stop codons Figure 14.7 Page 230 tRNA Structure codon in mRNA anticodon amino-acid attachment site amino acid OH Figure 14.8 Page 231 Ribosomes tunnel small ribosomal subunit large ribosomal subunit intact ribosome Figure 14.9b,c Page 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-c Page 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-g Page 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-k Page 233 What Happens to the New Polypeptides? • Some just enter the cytoplasm • Many enter the endoplasmic reticulum lumen and move through the cytomembrane system where they are modified Transcription Overview mRNA Mature mRNA transcripts Translation rRNA ribosomal subunits tRNA mature tRNA http://www.fed.cuhk.edu.hk/~johnson/teaching/ genetics/animations/transcription.htm http://www.stolaf.edu/people/giannini/flashanim at/molgenetics/translation.swf http://www.youtube.com/watch?v=WsofH466lqk Gene Mutations Base-Pair Substitutions (point mutation) Insertions (frameshift) Deletions (frameshift) Base-Pair Substitution •Results in 1 wrong amino acid •Protein may still function 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 14.11 Page 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 arginine glycine leucine leucine glutamate DNA with base insertion altered aminoacid sequence Figure 14.12 Page 234 Transposons • DNA segments that move spontaneously about the genome • When they insert into a gene region, they usually inactivate that gene Mutation Rates • Each gene has a characteristic mutation rate • Average rate for eukaryotes is between 10-4 and 10-6 per gene per generation • Only mutations that arise in germ cells can be passed on to next generation Mutagens – things that can lead to mutations • Ionizing radiation (X rays) • Nonionizing radiation (UV) • Natural and synthetic chemicals