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Chapter 2 An Introduction to Genes and Genomes Introduction to Molecular Biology Prokaryotic Cell Structure Prokaryotic Cell Eukaryotic Cell Eukaryotic Cell Structure Eukaryotic Cell Structure Animal Cell Plant Cell Let’s in on a cell! DNA Zoom Interactive DNA Discovery (visit DNAi.org) Miescher – identified a nuclear substance he called nuclein Griffith – performed the first transformation Avery, McCarty, and Macleod – identified Griffith’s transforming factor as DNA Chargaff – proved that the percentage of the DNA bases adenine always equaled thymine and guanine always equaled cytosine Wilkins, Franklin, Watson & Crick – demonstrated the structure of DNA Structure of DNA • Deoxyribose Sugar • Phosphate • Nitrogen Base Structure of DNA Purines – double ring Pyrimidines – single ring Structure of DNA Nucleic Acid Overview Structure of DNA DNA Replication When DNA makes an exact copy of itself DNA Replication DNA Replication The first step in DNA replication is for the enzyme, helicase, to unzip the double stranded DNA molucule. DNA Replication Proteins hold the two strands apart. An RNA primer lays down on each strand of DNA. DNA Replication DNA polymerase extends the primer by adding complementary nucleotides. DNA polymerase can only extend in the 5’ → 3’ direction DNA Replication Leading strand follows helicase. Lagging strand must wait for replication fork to open and therefore forms discontinous Okazaki fragments. Ligase seals the nicks in the DNA backbone between the Okazaki fragments. helicase Let’s put it all together Click on the animation below. Select the button for the “whole picture”. DNA Replication Animation Transcription Making an RNA copy from a DNA template RNA polymerase RNA Structure Uracil instead of thymine Ribose sugar instead of deoxyribose sugar Single stranded Can leave the nucleus RNA Structure mRNA – RNA copy of DNA that carries genetic information from the nucleus to the ribosomes rRNA – makes up the ribosomes tRNA – carries amino acids to ribosomes for protein synthesis Transcription RNA polymerase binds to a promoter region on double stranded DNA and unzips the double helix. Transcription Free RNA nucleotides pair with the complementary DNA of the template strand Transcription RNA is processed Introns are spliced out 7 methyl guanosine cap Poly-A tail Transcription mRNA leaves the nucleus and travels to the ribosomes in the cytoplasm ribosome nucleus Let’s put it all together Transcription Animation Practice Central Dogma of Molecular Biology Animation Click to see Video Translation Making protein from mRNA Translation Important Definitions A codon is composed of 3 RNA nucleotides Each codon codes for one amino acid Protein does the work in a cell Translation Translation First Base U C A G Third Base Second Base U C A G U C A G phenylalanine serine tyrosine cysteine phenylalanine serine tyrosine cysteine Leucine serine (stop) (stop) Leucine serine (stop) tryptophan U C A G leucine proline histidine arginine leucine proline histidine arginine leucine proline glutamine arginine leucine proline glutamine arginine U C A G isoleucine threonine asparagine serine isoleucine threonine asparagine serine isoleucine threonine lysine arginine met (start) threonine lysine arginine U C A G valine alanine apartic acid glycine valine alanine apartic acid glycine valine alanine glutamic acid glycine valine alanine glutamic acid glycine Translation Translation Translation Translation • Asparagine, Serine, Methionine • Tryptophan, Glycine, Lysine • Proline, Leucine, Serine • Aspartic acid, Histidine, Threonine Translation Always begins at a start codon and ends at a stop codon. The region between the start and stop codons is called the open reading frame (ORF) Practice Click on the animation to transcribe and translate a gene. Click to see animation Translation Initiation mRNA attaches to the small subunit of a ribosome tRNA anticodon pairs with mRNA start codon Large ribosomal subunit binds and translation is initiated amino acid tRNA anticodon Translation Elongation Anticodon of tRNA carrying next amino acid binds to codon on mRNA A peptide bond joins the amino acids and the first tRNA is released. Translation Termination Amino acid chain continues until a stop codon is read. The amino acid chain is released and all of the translation machinery is recycled to translate another protein. Let’s put it all together Click on the animation below Translation Animation Translation Video Let’s put it all together 5’-GATCTGAATCGCTATGGC-3’ Coding: Template: 3’-CTAGACTTAGCGATACCG-5’ mRNA: mRNA 5’-GAUCUGAAUCGCUAUGGC-3’ tRNA: CUAGACUUAGCGAUACCG amino acid: Asp, Leu, Asn, Arg, Tyr, Gly Control of Gene Expression Control of Gene Expression Control of Gene Expression Prokaryotes cluster genes into operons that are transcribed together to give a single mRNA molecule. Control of Gene Expression Lac Operon Promoter region allows RNA polymerase to attach and begin transcription. Operator region is in the middle of the promoter. Control of Gene Expression If a repressor protein is bound to the operator, RNA polymerase cannot pass to transcribe the genes. Control of Gene Expression When the inducer (lactose) binds to the repressor protein, it changes shape and falls off of the operator region. Now RNA polymerase can pass and transcribe the genes into mRNA. Let’s put it all together Click on the animation below. Animation of lac operon Video of lac operon Mutations Mutations are changes in the DNA sequence. Mutations can be inherited or acquired. Mutations Mutagens are agents that interact with DNA to cause mutations. Examples are chemicals and radiation. Mutations Point mutation changes a single base Point mutations can be silent, meaning they code for the same amino acid. Mutations Point mutations can also code for a structurally similar amino acid. Mutations Point mutations are not always harmless. If the mutation occurs on a critical amino acid in the active site of the protein, it can be detrimental, as in the case of sickle cell anemia. Mutations Frameshift mutations cause a shift in the reading frame by adding or deleting nucleotides. Mutations An example of a deletion causing a premature stop codon.