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5/29/2012 Chapter 4 Genetics and Cellular Function • • • • DNA Structure • DNA – threadlike molecule with uniform diameter, but varied length DNA and RNA – the nucleic acids DNA replication Genes and their action DNA to Proteins – How many in most human cells? (a) A T G C A T A • Double helix T G C A T C G • Composed of Nucleotides T A G C (b) T C G 1. phosphate group 2. deoxyribose sugar 3. nitrogenous base T A G Sugar–phosphate backbone 4-1 DNA Molecular Structure 4-2 (c) Nitrogenous Bases of DNA Purines • DNA and other nucleic acids are polymers (chains) of nucleotides Adenine NH2 N • Each nucleotide consists of – one sugar - deoxyribose – one phosphate group – one nitrogenous base C Hydrogen bond Sugar–phosphate backbone HC N H C C C N P O O CH2 OH H H H H OH Phosphate NH2 H Deoxyribose N C C N – – – – Adenine (A) Guanine (G) Cytosine (C) Thymine (T) NH N Adenine (A) Guanine (G) Pyrimidines H C NH2 CH3 C HC C N N H C NH N H O Cytosine (C) • DNA bases - ATCG O C HC C O Thymine (T) O O C N H CH CH Uracil (U) (b) Mitosis? • Nitrogenous bases united by hydrogen bonds Mitosis: division of cells that results in daughter cells with the same the genetic information that the original cell had. G C T • DNA base pairing A–T C–G C G • Law of Complementary Base Pairing T – one strand determines base sequence of other A G • Depending on which chromosome – 50 to 250 million base pairs CH NH C NH2 C – – C C N HN Complementary Base Pairing N C CH HN C C H CH N O HO O • There are only four nitrogenous bases 46 C 46 Hydrogen bond Sugar–phosphate backbone 46 Sugar–phosphate backbone Tiny piece of DNA Diploid 2n Diploid 2 n 27-6 1 5/29/2012 DNA Replication DNA Replication and Cell Cycle • before cell divides, it must must duplicate its DNA (replication) - so each new cell gets exact copy of DNA 1. Helicase breaks H bonds at replication fork 2. DNA Polymerase adds complimentary nucleotides Don’t forget Law of Complementary Base Pairing – When does this occur during the cell cycle? 3. DNA ligase joins segments together 4-7 2 DNA each with ½ of original DNA = Semi-conservative DNA Replication Key: Continuous Incoming nucleotides replication 3-52 = Adenine = Thymine = Cytosine = Guanine (e) Replication fork Parental DNA (b) DNA polymerase DNA polymerase Discontinuous Replication fork Old (template) strand (a) Newly made strand DNA helicase Key A C Leading strand T (c) Gap in replication (d) New strand forming Lagging strand DNA ligase G Old (template) strand DNA of one chromatid DNA polymerase Human Anatomy and Physiology, 7e by Elaine Marieb & Katja Hoehn Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings. Mutations Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. • DNA polymerase does make mistakes – But: – checks new base pairs and tends to fix mistakes – result is only 1 error per 1 billion bases replicated • mutations - changes in DNA structure due to replication errors or environmental factors (radiation, viruses, chemicals) – some mutations = no problem/some kill the cell, turn it cancerous or cause genetic defects in future generations 4-12 2 5/29/2012 DNA Function DNA Function • Gene – segment of DNA that codes for a specific protein • Make Proteins!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! • Gene – segment of DNA that codes for a specific protein – humans have ~ 35,000 • 2% of total DNA • other 98% is non-coding DNA – plays role in chromosome structure – regulation of gene activity (on-off sites) – no function at all – “junk” DNA – humans have ~ 35,000 • 2% of total DNA • other 98% is non-coding DNA – plays role in chromosome structure – regulation of gene activity – no function at all – “junk” DNA 4-13 4-14 Genetic Code RNA: Structure and Function • Millions of different proteins, all made from 20 amino acids, – encoded by genes made of 4 nucleotides (A,T,C,G) • • Genetic code – Arrangement of nucleotides that code for amino acid sequence of proteins RNA – Much smaller than DNA - a single nucleotide chain - Ribose sugar (not deoxyribose) - No thymine nitrogenous base (replaced by Uracil) i.e., nucleotides arrangement determines amino acid arrangement • General: a DNA triplet – a sequence of 3 DNA nucleotides will code for 1 amino acid 3 types of RNA 1. messenger RNA (mRNA) over 10,000 bases 2. ribosomal RNA (rRNA) 3. 3. transfer RNA (tRNA) 70 - 90 bases – codon - the 3 base sequence in mRNA • Function – interprets code in DNA – uses those instructions for protein synthesis 4-15 4-16 From DNA to Protein Overview of Protein Synthesis • all body cells contain identical genes (except sex cells and some immune cells) Nuclear envelope • different genes are activated in different cells Transcription • Once activated a gene: DNA 1. Makes messenger RNA (mRNA) – a mirror-image copy of the gene is made • migrates from the nucleus to cytoplasm • its code is read by the ribosomes mRNA 2. mRNA attaches to ribosomes (ribosomal RNA (rRNA) and enzymes) Ribosome Translation 3. transfer RNA (tRNA) – delivers amino acids to the ribosome 4. ribosomes assemble amino acids in the order directed by codons of mRNA Polypeptide 4-17 Figure 3.33 3 5/29/2012 Transcription: RNA Polymerase • • • • The enzyme that oversees mRNA synthesis Starts at a promoter site Breaks H bonds & unwinds DNA Adds complementary nucleotides on DNA template strand – following Law of Complimentary Base Pairing – Joins RNA nucleotides together to match DNA coding strand • Reads a termination signal to stop transcription Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. Coding strand Termination signal Promoter Template strand Transcription unit IRNA polymerase binds to promoter and unwinds 16–18 base pairs of the DNA template strand RNA polymerase Unwound DNA RNA polymerase bound to promoter RNA nucleotides mRNA RNA nucleotides RNA polymerase mRNA synthesis begins RNA polymerase moves down DNA; mRNA elongates Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. mRNA synthesis is terminated DNA (a) mRNA transcript Coding strand RNA polymerase Unwinding of DNA Rewinding of DNA Template strand RNA nucleotides mRNA RNA-DNA hybrid region (b) 22 Figure 3.34 Nucleus From DNA to Protein Nuclear membrane RNA polymerase Nuclear pore Nuclear envelope mRNA Template strand of DNA Transcription Released mRNA DNA Pre-mRNA RNA Processing mRNA Ribosome Translation Polypeptide Figure 3.33 Figure 3.36 4 5/29/2012 Nucleus Nuclear membrane Nucleus Nuclear membrane RNA polymerase Nuclear pore RNA polymerase Nuclear pore mRNA mRNA Template strand of DNA Polysome: mRNA binding to ribosome Template strand of DNA Released mRNA 1 1 After mRNA processing, mRNA leaves nucleus and attaches to ribosome, and translation begins. After mRNA processing, mRNA leaves nucleus and attaches to ribosome, and translation begins. Small ribosomal subunit Codon 15 Codon 16 Codon 17 Amino acids Released mRNA tRNA Aminoacyl-tRNA synthetase Small ribosomal subunit Codon 15 Direction of ribosome advance Portion of mRNA already translated Codon 16 Codon 17 Direction of ribosome advance Portion of mRNA already translated Large ribosomal subunit Large ribosomal subunit Energized by ATP, the correct amino acid is attached to each species of tRNA by aminoacyl-tRNA synthetase enzyme. Figure 3.36 Figure 3.36 Nucleus Nuclear membrane Nucleus Nuclear membrane RNA polymerase Nuclear pore RNA polymerase Nuclear pore mRNA mRNA Template strand of DNA Template strand of DNA Amino acids Released mRNA 1 After mRNA processing, mRNA leaves nucleus and attaches to ribosome, and translation begins. Codon 16 Codon 17 1 tRNA After mRNA processing, mRNA leaves nucleus and attaches to ribosome, and translation begins. Aminoacyl-tRNA synthetase Small ribosomal subunit Codon 15 Amino acids Released mRNA tRNA Aminoacyl-tRNA synthetase Small ribosomal subunit Codon 15 Direction of ribosome advance Portion of mRNA already translated Codon 16 Codon 17 Direction of ribosome advance Portion of mRNA already translated tRNA “head” bearing anticodon tRNA “head” bearing anticodon Large ribosomal subunit 2 Incoming aminoacyltRNA hydrogen bonds via its anticodon to complementary mRNA sequence (codon) at the A site on the ribosome. Large ribosomal subunit Energized by ATP, the correct amino acid is attached to each species of tRNA by aminoacyl-tRNA synthetase enzyme. Energized by ATP, the correct amino acid is attached to each species of tRNA by aminoacyl-tRNA synthetase enzyme. Incoming aminoacyltRNA hydrogen bonds via its anticodon to complementary mRNA sequence (codon) at the A site on the ribosome. 2 As the ribosome moves along the mRNA, a new amino acid is added to the growing protein chain and the tRNA in the A site is translocated to the P site. 3 Figure 3.36 Figure 3.36 Translation of mRNA Nucleus Nuclear membrane RNA polymerase Nuclear pore Cytosol 8 mRNA Nucleus DNA Template strand of DNA Ribosomal subunits rejoin to repeat the process with the same or another mRNA. Amino acids 7 5 Released mRNA mRNA 1 After mRNA processing, mRNA leaves nucleus and attaches to ribosome, and translation begins. Codon 15 Codon 16 Codon 17 4 tRNA 1 Aminoacyl-tRNA synthetase Small ribosomal subunit mRNA leaves the nucleus. 2 Ribosome binds mRNA. Direction of ribosome advance The preceding tRNA hands off the growing protein to the new tRNA, and the ribosome links the new amino acid to the protein. tRNA anticodon binds to complementary mRNA codon. GU Protein A CGU C A U GC 3 Portion of mRNA already translated tRNA “head” bearing anticodon Large ribosomal subunit Once its amino acid is released, tRNA is ratcheted to the E site and then released to reenter the cytoplasmic pool, ready to be recharged with a new amino acid. 3 As the ribosome moves along the mRNA, a new amino acid is added to the growing protein chain and the tRNA in the A site is translocated to the P site. ADP tRNA 2 4 Incoming aminoacyltRNA hydrogen bonds via its anticodon to complementary mRNA sequence (codon) at the A site on the ribosome. Energized by ATP, the correct amino acid is attached to each species of tRNA by aminoacyl-tRNA synthetase enzyme. A tRNA binds an amino acid; binding consumes 1 ATP. ATP 6 + After translating the entire mRNA, the ribosome dissociates into its two subunits. Pi Free tRNA tRNA is released from the ribosome and is available to pick up a new amino acid and repeat the process. Free amino acids 4-30 Figure 3.36 5 5/29/2012 Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. 31 Protein Synthesis Information Transfer from DNA to RNA Template strand Figure 3.38 Review of Peptide Formation 3-43 Genetic Code: • RNA codons code for amino acids according to a genetic code • Remember there are 20 amino acids 1 DNA double helix 2 triplets on the template strand of DNA 3 Corresponding codons of mRNA transcribed from the DNA triplets 4 The anticodons of tRNA that bind to the mRNA codons 5 The amino acids carried by those six tRNA molecules 6 The amino acids linked into a peptide chain 4-35 Figure 3.35 6 5/29/2012 Protein Packaging and Secretion Mechanism of Gene Activation Prolactin Prolactin receptors 1 1 Exocytosis Protein formed by ribosomes on rough ER. ATP 2 Protein packaged into transport vesicle, which buds from ER. Nucleus Casein 7 3 Transport vesicles fuse into clusters that unload protein into Golgi complex. 2 4 Golgi complex modifies protein structure. Secretory vesicles AD P + Pi 6 Golgi complex 5 Golgi vesicle containing finished protein is formed. Regulatory protein (transcription activator) 6 Secretory vesicles release protein by exocytosis. Ribosomes Clathrin-coated transport vesicle Golgi complex Rough Endoplasmic reticulum 5 3 Rough ER 4 Lysosome Casein gene Figure 4.11 4-37 mRNA for casein RNA polymerase 4-38 7