Chapter 12 DNA, RNA, Gene function, Gene regulation, and Biotechnology

DNA Double helix “Rungs” are base pairs joined by hydrogen bonds Adenine (A) pairs with thymine (T) Cytosine (C) pairs with guanine (G) Complementary strands Strands oriented in opposite directions –5’ to 3’ or 3’ to 5’

–Relationship between nucleic acids and proteins is a flow of information –Part 1: Transcription – cell templates DNA to RNA –Part 2: Translation – RNA information used to manufacture proteins –Developed by Watson & Crick in 1950’s The “central dogma” So let’s compare DNA with RNA…

DNA vs. RNA

3 types of RNA –Messenger RNA (mRNA) – carries info. specific to a protein, 3 RNA bases form a codon specifying an amino acid –Ribosomal RNA (rRNA) – combines with proteins to form a ribosome –Transfer RNA (tRNA) – carries specific amino acid to ribosome RNA

Transcription 1.Initiation –Enzymes unwind DNA exposing template strand –RNA polymerase binds to promoter 2.Elongation –RNA polymerase moves 3’ to 5’ 3.Termination –RNA polymerase reaches terminator sequence at end of gene –RNA separates – may be mRNA, tRNA or rRNA (for translation to occur, it must be mRNA) –DNA reforms helix

Translation Genetic code –mRNA codon with 3 bases specifies amino acid –Also contains start and stop codons

Translation requires these players: –mRNA – genetic information specifying amino acid order in codons –tRNA – brings specific amino acid to ribosome by pairing anticodon to mRNA codon –Ribosome – rRNA and proteins

tRNA vs. rRNA

3 steps in translation 1.Initiation –mRNA start codon binds to small ribosomal subunit –1 st tRNA binds to mRNA codon 2.Elongation –Large ribosomal subunit attaches –tRNA corresponding to 2 nd codon attaches –Covalent bond forms between amino acids –Ribosome release empty 1 st tRNA –Ribosome shift down one codon allowing 3 rd tRNA to bind –Polypeptide grows one amino acid at a time 3.Termination –Stop codon reached –New polypeptide released

Protein folding –Must achieve final functional shape – some regions attract or repel, enzymes catalyze bonding, “chaperone” proteins stabilize –Errors in folding can lead to illness –Some proteins must be altered Insulin has amino acids removed Hemoglobin has 4 separate polypeptides After Translation

Regulation Protein synthesis is fast and efficient Tremendous ATP requirement Cells save energy by not producing unneeded proteins

Mutations Change in cell’s DNA sequence Can be good, bad, or silent Point mutations –Substitute one DNA base for another –“Silent” is same amino acid specified (no change caused by mutation) –May cause disease – sickle cell anemia

Base insertions and deletions –Frameshift mutation caused by addition or deletion by any number other than a multiple of 3 –Expanding repeat – number of copies of 3 or 4 nucleotide sequence increases over several generations

Causes of mutations 1.Spontaneous – DNA replication error 2.Meiotic error – duplication or deletion 3.Chromosome inversion and translocations 4.Transposons – moveable DNA sequences 5.Mutagen – external agent – radiation, chemicals

Somatic mutations occur in nonsex cells All cells derived from that cell carry mutation Not passed to offspring Heritable mutations –Germline mutation Heritable – passed in every gamete Mutations are important –Create new gene variants (alleles) –Random mutations results in antibiotic resistant bacteria Types of Mutations

Human Genome Project 3.2 billion base pairs 25,000 genes produce 400,000 different proteins –Removing different combinations of introns makes different proteins Only about 1.5% of genome encodes protein –98.5% encodes regulatory sequences, pseudogenes, and transposons

Transgenic organism receives recombinant DNA Recombinant DNA – genetic material spiced together from multiple organisms –Transgenic bacteria make drugs –Transgenic crops resist disease –Transgenic human disease models Transgenic Organisms

Biotechnology Gene therapy – replacing faulty genes Block gene expression to silence harmful gene or study gene function –Antisense RNA, gene knockouts DNA microarrays or DNA chips – use collection of known DNA sequences Proteomics – genome changes little but proteins different in different cells and different times