Genetics For Dummies 4th Edition PDF

Genetics Genetics 4th Edition by René Fester Kratz, PhD Lisa J. Spock, PhD, CGC Genetics For Dummies®, 4th Edition Published by: John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030-5774, www.wiley.com Copyright © 2024 by John Wiley & Sons, Inc., Hoboken, New Jersey Media...

Genetics Genetics 4th Edition by René Fester Kratz, PhD Lisa J. Spock, PhD, CGC Genetics For Dummies®, 4th Edition Published by: John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030-5774, www.wiley.com Copyright © 2024 by John Wiley & Sons, Inc., Hoboken, New Jersey Media and software compilation copyright © 2024 by John Wiley & Sons, Inc. All rights reserved. Published simultaneously in Canada No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except as permitted under Sections 107 or 108 of the 1976 United States Copyright Act, without the prior written permission of the Publisher. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http://www.wiley.com/go/permissions. 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Library of Congress Control Number: 2023920017 ISBN: 978-1-394-21019-0 (pbk); ISBN 978-1-394-21020-6 (EPUB); ISBN 978-1-394-21025-1 (EPDF) Contents at a Glance Introduction......................................................... 1 Part 1: The Lowdown on Genetics: Just the Basics....................................................... 5 CHAPTER 1: Welcome to Genetics: Just About Everyone Loves a Good Pair of Genes....................................... 7 CHAPTER 2: Basic Cell Biology............................................... 13 CHAPTER 3: Visualize Peas: Discovering the Laws of Inheritance................. 35 CHAPTER 4: Law Enforcement: Mendel’s Laws Applied to Complex Traits......... 55 Part 2: DNA: The Genetic Material.............................. 73 CHAPTER 5: DNA: The Pattern for Life......................................... 75 CHAPTER 6: Chromosomes: The Big Picture................................... 91 CHAPTER 7: Replication: A Copy Machine for DNA............................ 105 CHAPTER 8: Transcription: Getting Instructions from DNA..................... 125 CHAPTER 9: Translating the Genetic Code................................... 141 CHAPTER 10: Gene Expression: Finding the Right Tool for the Job............... 157 Part 3: Genetics and Your Health............................. 173 CHAPTER 11: What Could Go Wrong: Changes in DNA Sequence................ 175 CHAPTER 12: Chromosome Disorders: It’s All a Numbers Game................. 193 CHAPTER 13: Taking a Closer Look at the Genetics of Cancer...................... 211 CHAPTER 14: Genetic Counseling, Risk Assessment, and Genetic Testing......... 229 CHAPTER 15: Treating Genetic Disorders and Using Genetics to Tailor Treatment............................................ 249 Part 4: Genetics and Your World.............................. 263 CHAPTER 16: Tracing Human History and the Future of Populations............. 265 CHAPTER 17: Solving Mysteries Using DNA................................... 283 CHAPTER 18: DNA Sequencing: Decoding the Genome......................... 305 CHAPTER 19: Genetic Makeovers: Using Genetic Engineering to Change the Genome........................................ 319 CHAPTER 20: Giving Ethical Considerations Their Due.......................... 339 Part 5: The Part of Tens......................................... 353 CHAPTER 21: Ten Hot Issues in Genetics..................................... 355 CHAPTER 22: Ten Pathways to a Career in Genetics............................ 365 Index............................................................... 371 Table of Contents INTRODUCTION.................................................... 1 About This Book............................................... 1 Foolish Assumptions........................................... 3 Icons Used in This Book........................................ 3 Beyond This Book............................................. 4 Where to Go from Here........................................ 4 PART 1: THE LOWDOWN ON GENETICS: JUST THE BASICS.................................................... 5 CHAPTER 1: Welcome to Genetics: Just About Everyone Loves a Good Pair of Genes.............................. 7 Unwrapping Life’s Secrets...................................... 8 Classical Genetics: Studying How Traits Are Transmitted within Families...................................................... 9 Molecular Genetics: Diving into DNA Science..................... 10 Population Genetics: Studying the Genetics of Groups............. 11 Quantitative Genetics: Exploring the Heredity of Complex Traits.... 11 CHAPTER 2: Basic Cell Biology......................................... 13 Entering the World of the Cell.................................. 14 Cells without a nucleus..................................... 14 Cells with a nucleus........................................ 16 Exploring Your DNA, Chromosomes, and Genes.................. 18 Examining the basics of chromosomes....................... 18 Finding your genes........................................ 21 Mitosis: Splitting Up.......................................... 22 Step 1: Time to grow....................................... 23 Step 2: Dividing up the chromosomes........................ 25 Step 3: The big divide...................................... 27 Meiosis: Making Cells for Sexual Reproduction................... 27 Meiosis I................................................. 28 Meiosis II................................................. 31 Meiosis in the human body................................. 32 CHAPTER 3: Visualize Peas: Discovering the Laws of Inheritance...................................... 35 Gardening with Gregor Mendel................................. 36 Speaking the Language of Inheritance........................... 38 Simplifying Inheritance........................................ 40 Establishing dominance.................................... 40 Segregating alleles......................................... 43 Table of Contents vii  Declaring independence.................................... 45 Predicting with Punnetts................................... 45 Finding Unknown Alleles...................................... 46 Applying Basic Probability to the Likelihood of Inheritance......... 47 Solving Genetics Problems..................................... 49 Deciphering a monohybrid cross............................ 49 Tackling a dihybrid cross................................... 50 CHAPTER 4: Law Enforcement: Mendel’s Laws Applied to Complex Traits.............................. 55 Dominant Alleles Rule... Sometimes........................... 56 Blending in with incomplete dominance...................... 56 Keeping it fair with codominance............................ 57 Hanging back with incomplete penetrance.................... 58 Alleles Causing Complications.................................. 59 More than two alleles...................................... 59 Lethal alleles.............................................. 61 Making Life More Complicated................................. 61 When genes interact....................................... 62 Genes in hiding........................................... 63 One gene with many phenotypes............................ 64 Genes linked together......................................65 Uncovering More Exceptions to Mendel’s Laws................... 68 Epigenetics............................................... 68 Genomic imprinting....................................... 70 Anticipation.............................................. 71 Environmental effects...................................... 71 PART 2: DNA: THE GENETIC MATERIAL........................ 73 CHAPTER 5: DNA: The Pattern for Life............................... 75 Chemical Ingredients of DNA................................... 76 Covering the bases........................................ 77 Adding a spoonful of sugar and a little phosphate............. 79 Assembling the Double Helix: The Structure of DNA............... 80 Starting with one: Forming the backbone..................... 80 Putting the double in double helix........................... 82 Examining Different Varieties of DNA........................... 85 Nuclear DNA.............................................. 85 Mitochondrial DNA........................................ 86 Chloroplast DNA.......................................... 87 Digging into the History of DNA................................ 88 Discovering DNA.......................................... 88 Obeying Chargaff’s rules................................... 89 Hard feelings and the helix: Franklin, Wilkins, Watson, and Crick................................................. 89 viii Genetics For Dummies CHAPTER 6: Chromosomes: The Big Picture......................... 91 Packaging the Double Helix.................................... 92 Anatomy of a Chromosome.................................... 93 Two Chromosomes Are Better than One (or Three)............... 95 Sex Chromosomes: Is It a Boy or Girl?........................... 97 Sex determination in humans............................... 97 Sex determination in other organisms....................... 102 CHAPTER 7: Replication: A Copy Machine for DNA............... 105 Unzipped: Creating the Pattern for More DNA................... 106 How DNA Copies Itself....................................... 109 Meeting the replication crew............................... 110 Splitting the helix......................................... 113 Priming the process...................................... 115 Leading and lagging...................................... 116 Joining all the pieces...................................... 117 Proofreading replication................................... 118 Replication in Eukaryotes..................................... 119 Pulling up short: Telomeres................................ 120 Finishing the job......................................... 121 How Circular DNAs Replicate.................................. 122 Theta................................................... 122 Rolling circle............................................. 122 D-loop.................................................. 123 CHAPTER 8: Transcription: Getting Instructions from DNA.... 125 RNA: DNA’s Close Cousin..................................... 126 Using a slightly different sugar............................. 126 Meeting a new base: Uracil................................ 127 Stranded!............................................... 128 Transcription: Copying DNA’s Message into RNA’s Language.......130 Getting ready to transcribe................................ 130 Initiation................................................ 134 Elongation............................................... 136 Termination............................................. 136 Post-transcription Processing................................. 136 Adding cap and tail....................................... 137 Editing the message...................................... 138 CHAPTER 9: Translating the Genetic Code......................... 141 Discovering the Good in a Degenerate......................... 142 Considering the combinations............................. 143 Framed! Reading the code................................. 144 Almost universal......................................... 145 Meeting the Translating Team................................. 145 Table of Contents ix  Taking the Translation Trip................................... 146 Initiation................................................ 146 Elongation............................................... 150 Termination............................................. 150 Proteins Are Powerful........................................ 152 Recognizing radical groups................................ 153 Giving the protein its shape................................ 154 CHAPTER 10: Gene Expression: Finding the Right Tool for the Job.......................................... 157 Getting Your Genes Under Control............................. 158 To Be Expressed or Not To Be Expressed?...................... 158 Regulating Eukaryotic Gene Expression: A Time and Place for Everything.................................................. 160 Controlling Transcription Before It Starts....................... 161 Regulation of Gene Transcription: Flipping the Switch............ 162 Proteins controlling transcription........................... 162 Sequences controlling genes............................... 163 Hormones controlling transcription......................... 164 Post-transcriptional Control................................... 166 Nip and tuck: RNA splicing................................. 166 Shut up! RNA silencing.................................... 167 mRNA expiration dates.................................... 168 Gene Control Lost in Translation.............................. 169 Modifying where translation occurs......................... 169 Modifying when translation occurs......................... 169 Modifying the protein shape............................... 170 Prokaryotic Gene Expression.................................. 170 Bacterial gene organization................................ 171 Bacterial gene expression................................. 171 PART 3: GENETICS AND YOUR HEALTH..................... 173 CHAPTER 11: What Could Go Wrong: Changes in DNA Sequence....................................... 175 Heritable or Not Heritable?................................... 176 Facing the Consequences of Sequence Variants................. 177 Sorting Out Terminology..................................... 177 The phenotypic effect of a DNA sequence change............. 178 The type of DNA sequence change.......................... 178 What Causes Sequence Variants?.............................. 181 Spontaneous mutation.................................... 182 Induced mutations....................................... 186 Evaluating Options for DNA Repair............................. 190 x Genetics For Dummies CHAPTER 12: Chromosome Disorders: It’s All a Numbers Game......................................... 193 Chromosome Numbers: No More and No Less.................. 194 Aneuploidy of the Autosomal Chromosomes.................... 196 When chromosomes go missing............................ 196 When too many chromosomes are left in.................... 197 Aneuploidy of the Sex Chromosomes.......................... 199 Extra Xs................................................. 199 Extra Ys................................................. 200 Monosomy X............................................ 200 Exploring Variations in Chromosome Structure.................. 201 Duplications............................................. 201 Deletions................................................ 202 Inversions............................................... 203 Translocations........................................... 203 Other things that go awry with chromosomes................ 205 How Chromosomes Are Studied............................... 206 Big enough to see........................................ 207 Too small for the naked eye................................208 Non-Invasive Prenatal Testing for Aneuploidy................... 210 CHAPTER 13: Taking a Closer Look at the Genetics of Cancer.... 211 Defining Cancer............................................. 212 Benign growths: Not always so harmless.................... 212 Malignancies: Seriously scary results........................ 213 Metastasis: Cancer on the move............................ 215 Recognizing Cancer as a DNA Disease.......................... 215 Exploring the cell cycle and cancer.......................... 216 Demystifying chromosome abnormalities................... 223 Breaking Down the Types of Cancers........................... 224 Hereditary breast cancer.................................. 226 Hereditary colorectal cancer............................... 227 CHAPTER 14: Genetic Counseling, Risk Assessment, and Genetic Testing.................................... 229 Getting to Know Genetic Counselors........................... 230 Building and Analyzing a Family Tree........................... 231 Autosomal Inheritance: No Differences Among the Sexes......... 234 Autosomal dominant traits and disorders.................... 234 Autosomal recessive traits and disorders.................... 236 Found on Sex Chromosomes: Sex-linked Inheritance............. 239 X-linked recessive traits................................... 239 X-linked dominant traits................................... 242 Y-linked traits............................................ 243 Table of Contents xi  Sex-limited traits......................................... 244 Sex-influenced traits...................................... 244 Testing for Genetic Disorders................................. 244 Diagnostic testing........................................ 245 Prenatal diagnosis........................................ 246 Carrier testing........................................... 247 Predictive and susceptibility testing......................... 247 Preimplantation genetic diagnosis.......................... 248 Pharmacogenetic testing.................................. 248 CHAPTER 15: Treating Genetic Disorders and Using Genetics to Tailor Treatment......................... 249 Alleviating Genetic Disease through Gene Therapy............... 250 Inserting Healthy Genes into the Picture........................ 250 Finding Vehicles to Get Genes to Work......................... 253 Viruses that join right in................................... 254 Viruses that are a little standoffish.......................... 255 Progress on the Gene Therapy Front........................... 255 Utilizing Genetic Information for Precision Medicine............. 257 Pharmacogenetics (and pharmacogenomics)................. 257 Cytochrome P450 and drug metabolism..................... 258 Decreasing the risk of side effects of treatment............... 260 Increasing the effectiveness of treatment.................... 260 PART 4: GENETICS AND YOUR WORLD...................... 263 CHAPTER 16: Tracing Human History and the Future of Populations.................................. 265 Genetic Variation Is Everywhere............................... 266 Allele frequencies........................................ 267 Genotype frequencies.....................................268 Breaking Down the Hardy-Weinberg Law of Population Genetics... 269 Relating alleles to genotypes............................... 270 Populations in balance.................................... 271 Violating the law......................................... 273 Mapping the Gene Pool...................................... 275 One big happy family..................................... 276 Ancestry testing.......................................... 277 Uncovering the secret social lives of animals................. 278 Changing Forms over Time: The Genetics of Evolution............ 279 Genetic variation is key.................................... 279 Where new species come from............................. 280 Growing the evolutionary tree.............................. 281 xii Genetics For Dummies CHAPTER 17: Solving Mysteries Using DNA......................... 283 Rooting through Your DNA to Find Your Identity................. 284 Investigating the Scene: Where’s the DNA?...................... 287 Collecting biological evidence.............................. 287 Moving to the lab......................................... 289 Employing DNA to Catch Criminals (And Free the Innocent)....... 295 Matching the evidence to the bad guy....................... 295 Taking a second look at guilty verdicts....................... 297 It’s All Relative: Finding Family................................. 298 Paternity testing.......................................... 298 Relatedness testing....................................... 301 CHAPTER 18: DNA Sequencing: Decoding the Genome........... 305 Sequencing: Reading the Language of DNA..................... 306 Identifying the players in DNA sequencing................... 306 Finding the message in sequencing results................... 309 Newer, cheaper, faster.................................... 309 Sequencing Your Way to the Human Genome................... 311 Trying on a Few Genomes.................................... 315 The yeast genome........................................ 317 The roundworm genome.................................. 317 The chicken genome...................................... 318 CHAPTER 19: Genetic Makeovers: Using Genetic Engineering to Change the Genome................. 319 Genetically Modified Organisms Are Everywhere................ 320 Old Genes in New Places..................................... 322 Making a transgenic (recombinant) organism................ 323 Making a transgene using recombinant DNA technology....... 323 Modifying the gene to reside in its new home................ 325 Looking at the GMO Menagerie............................... 325 Transgenic animals....................................... 326 Transgenic insects........................................ 328 Transgenic bacteria....................................... 328 Transgenic Plants........................................... 330 Getting new genes into the plant........................... 330 Exploring commercial applications.......................... 331 Weighing points of contention............................. 332 Changing the Blueprint: Gene Editing.......................... 334 CRISPR-Cas9 gene editing..................................335 Germline versus somatic gene editing....................... 337 Discussing the ethics of gene editing........................ 337 Table of Contents xiii  CHAPTER 20: Giving Ethical Considerations Their Due........... 339 Profiling Genetic Discrimination............................... 340 Ordering Up Designer Babies................................. 341 Ethical Issues Surrounding Genetic Testing..................... 342 Informed consent........................................ 343 Patient autonomy........................................ 346 Privacy and confidentiality................................. 346 Incidental findings........................................ 347 Direct-to-consumer testing................................ 349 Practicing Safe Genetic Treatments............................ 350 Genetic Property Rights...................................... 350 PART 5: THE PART OF TENS.................................... 353 CHAPTER 21: Ten Hot Issues in Genetics............................ 355 Direct-to-Consumer Genetic Testing........................... 355 Whole Exome Sequencing.................................... 357 Whole Genome Sequencing................................... 358 Stem Cell Research.......................................... 359 The ENCODE Project......................................... 360 Proteomics................................................. 361 Gene Chips................................................. 362 Evolution of Antibiotic Resistance.............................. 363 Circumventing Mother Nature................................ 363 Genetics from Afar.......................................... 364 CHAPTER 22: Ten Pathways to a Career in Genetics.............. 365 Laboratory Technicians...................................... 365 Graduate Student........................................... 366 Post-doctoral Fellows........................................ 366 Research Scientist........................................... 367 College or University Professor................................ 367 Clinical Laboratory Director................................... 368 Clinical Geneticist........................................... 368 Genetic Counselor........................................... 369 Genetic Counseling Assistant................................. 370 Forensic Geneticist.......................................... 370 INDEX.............................................................. 371 xiv Genetics For Dummies Introduction G enetics seeks to understand how the traits of all living things are deter- mined. Although sometimes complicated and always diverse, all genetics comes down to basic principles of heredity — how traits are passed from one generation to the next — and how DNA is put together. As a science, genetics is a fast-growing field because of its untapped potential — for good and for bad. Despite its complexity, genetics can be surprisingly accessible. Genetics is a bit like peeking behind a movie’s special effects to find a deceptively simple and ele- gant system running the whole show. About This Book Genetics For Dummies, 4th Edition, is an overview of the entire field of genetics. Our goal is to explain every topic so that anyone, even someone without any genetics background at all, can follow the subject and understand how it works. As in the first three editions, we include many examples from the frontiers of research. We also make sure that the book has detailed coverage of some of the hottest topics that you hear about in the news, including gene therapy, pharmacogenetics, and gene editing. And we address the practical side of genetics: how it affects your health and the world around you. In short, this book is designed to be a solid introduction to genetics basics and to provide some details on the subject. Genetics is a fast-paced field; new discoveries are coming out all the time. You can use this book to help you get through your genetics course or for self-guided study. Genetics For Dummies, 4th Edition, provides enough information for you to get a handle on the latest press coverage, understand the genetics jargon that mystery writers like to toss around, and translate information imparted to you by medical professionals. The book is filled with stories of key discoveries and “wow” developments. Although we try to keep things light and inject some humor when possible, we also make every effort to be sensitive to whatever your circumstances may be. This book is a great guide if you know nothing at all about genetics. If you already have some background, then you’re set to dive into the details of the subject and expand your horizons. Introduction 1  It would be very easy for us to use specialized language that you’d need a transla- tor to understand, but what fun would that be? Throughout this book, we try to avoid jargon as much as possible, but at the same time, we use and carefully define terms that scientists actually use. After all, it may be important for you to understand some of these terms in the course of your studies or your or a loved one’s medical treatment. To help you navigate through this book, we use the following typographical conventions: »» We use italic for emphasis and to highlight new words or terms that we define in the text. The names of genes are also italicized because that is standard practice in the field of genetics. »» We use boldface to indicate keywords in bulleted lists or the action parts of numbered steps. »» We use monofont for websites and email addresses. We designed this book to cover background material in the first two parts and then all the applications in the rest of the book. We think you’ll find it quite accessible. »» Part 1, “The Lowdown on Genetics: Just the Basics”: Part 1 explains how traits are inherited. The first chapter introduces you to the field of genetics. The second chapter gives you a handle on how genetic information gets divided up during cell division; these events provide the foundation for just about every- thing else that has to do with genetics. From there, we explain simple inheri- tance of one gene and then move on to more complex forms of inheritance. »» Part 2, “DNA: The Genetic Material”: Part 2 covers what’s sometimes called molecular genetics. Don’t let the word “molecular” scare you off. We give you details, but we break them down so that you can easily follow along. We track the progress of how your genes work from start to finish: how your DNA is put together, how it gets copied, and how the building plans for your body are encoded in the double helix. »» Part 3, “Genetics and Your Health”: Part 3 is intended to help you see how genetics affects your health and well-being. We cover the subjects of genetic counseling; inherited diseases; genetics and cancer; and chromosome disorders such as Down syndrome. We also include a chapter on gene therapy, a practice that may hold the key to cures or treatments for many of the disorders we describe in this part of the book. »» Part 4, “Genetics and Your World”: Part 4 explains the broader impact of genetics and covers some hot topics that are often in the news. We explain 2 Genetics For Dummies  how various technologies work and highlight both the possibilities and the perils of each. We delve into population genetics (of both humans, past and present, and endangered animal species), evolution, DNA and forensics, genetically modified plants and animals, and the issue of ethics, which is raised on a daily basis as scientists push the boundaries of the possible with cutting-edge technology. To help you understand how scientists explore the secrets stored in your DNA, we also cover how DNA is sequenced. In the process, we relate the fascinating story behind the Human Genome Project. »» Part 5, “The Part of Tens”: In Part 5, you get our lists of ten of the next big things in the field and ten careers in genetics. Foolish Assumptions It’s a privilege to be your guide into the amazing world of genetics. Given this responsibility, you were in our thoughts often while we were writing this book. Here’s how we imagine you, our reader: »» You’re a student in a genetics or biology class. »» You’re curious to understand more about the science you hear reported in the news. »» You’re an expectant or new parent or a family member who’s struggling to come to terms with what doctors have told you. »» You’re affected by cancer or some hereditary disease, wondering what it means for you and your family. If any of these descriptions fit, you’ve come to the right place. Icons Used in This Book All For Dummies books use icons to help readers keep track of what’s what. Here’s a rundown of the icons we use in this book and what they all mean. This icon points out stories about the people behind the science and accounts of how discoveries came about. Introduction 3  This icon flags information that’s critical to your understanding or that’s partic- ularly important to keep in mind. These details are useful but not necessary to know. If you’re a student, though, these sections may be especially important to you. Points in the text where we provide added insight on how to get a better handle on a concept are found here. We draw on our personal experience for these tips and alert you to other sources of information you can check out. This icon alerts you to concepts that are typically confusing to people new to the field of genetics. We help you avoid these common misunderstandings. Beyond This Book In addition to the abundance of information and guidance related to genetics that we provide in this book, you get access to the following help and information online at Dummies.com: »» Cheat Sheet: To access this book’s online Cheat Sheet, go to www.dummies. com and search for “Genetics For Dummies Cheat Sheet.” »» Bonus chapters: To access this book’s three bonus chapters, go to www. dummies.com/go/geneticsfd4e. You’ll find chapters about cloning and defining events in the genetics, as well as a handy online glossary. Where to Go from Here With Genetics For Dummies, 4th Edition, you can start anywhere, in any chapter, and get a handle on what you’re interested in right away. We make generous use of cross-references throughout the book to help you get background details that you may have skipped earlier. The table of contents and index can point you to specific topics in a hurry, or you can just start at the beginning and work your way straight through. If you read the book from front to back, you’ll get a short course in genetics in the style and order that it’s often taught in colleges and universities — Mendel first and DNA second. 4 Genetics For Dummies 1 The Lowdown on Genetics: Just the Basics IN THIS PART... Discover the basics of genetics. Explore how cells divide and separate their chromosomes. Learn about Mendelian genetics and the fundamentals of how genes and traits are inherited. Go beyond the fundamentals of inheritance to see how multiple genes can interact to determine your traits.  IN THIS CHAPTER »» Defining the subject of genetics and its various subdivisions »» Studying the genetics of individuals and families »» Exploring DNA science through molecular genetics »» Studying genetic diversity and its evolution within populations of a species »» Examining how a trait can vary from one individual to another Chapter 1 Welcome to Genetics: Just About Everyone Loves a Good Pair of Genes W elcome to the complex and fascinating world of genetics. Genetics is all about physical traits and the DNA code that supplies the building plans for any organism. This chapter defines the field of genetics and explains what geneticists do. You get an introduction to the big picture and a glimpse at some of the details found in other chapters of this book. CHAPTER 1 Welcome to Genetics: Just About Everyone Loves a Good Pair of Genes 7 Unwrapping Life’s Secrets Take a moment to think about the vast diversity of living things in the world around you. The instructions to build all of those diverse forms — from a long, limbless snake to a tentacled octopus to a tall redwood tree — are encoded in the chemical pattern of DNA. Genetics is the field of science that examines how DNA determines traits and how these traits are passed from one generation to the next. Simply put, genetics affects everything about every living thing on earth. An organism’s genes are segments of DNA (deoxyribonucleic acid) that are the fundamental units of heredity. Genes play an essential role in how the organism looks, behaves, and reproduces. Because all biology depends on genes, genetics is a critical foundation for all other life sciences, including agriculture and medicine. From a historical point of view, genetics is still a young science. The principles that govern inheritance of traits by one generation from another were described (and promptly lost) less than 150 years ago. Around the turn of the 20th century, the laws of inheritance were rediscovered, an event that transformed biology for- ever. Even so, it wasn’t until the 1950s that the importance of DNA was really understood. Now technology is helping geneticists push the envelope of knowl- edge every day. Genetics is generally divided into four major subdivisions. We discuss each of these more in the sections that follow: »» Classical, or Mendelian, genetics: A discipline that describes how physical characteristics (traits) are passed along from one generation to another. »» Molecular genetics: The study of the chemical and physical structures of DNA, its close cousin RNA (ribonucleic acid), and proteins. Molecular genetics also covers how genes do their jobs. »» Population genetics: A division of genetics that looks at the genetic makeup of larger groups. »» Quantitative genetics: A highly mathematical field that examines the statistical relationships between genes and the traits with which they are associated. In the academic world, many genetics courses begin with classical genetics and proceed through molecular genetics, with a nod to population and quantitative genetics. In general, this book follows the same path, because each division of knowledge builds on the one before it. That said, it’s perfectly okay, and very easy, to jump around among disciplines. No matter how you take on reading this book, it provides lots of cross-references to help you stay on track. 8 PART 1 The Lowdown on Genetics: Just the Basics Classical Genetics: Studying How Traits Are Transmitted within Families At its heart, classical genetics is the genetics of individuals and their families. It focuses mostly on studying physical traits, or phenotypes, as a way to understand the genes behind those traits. Gregor Mendel, a monk and part-time scientist, founded the discipline of genet- ics. Mendel was a gardener with an insatiable curiosity to go along with his green thumb. His observations may have been simple, but his conclusions were jaw- droppingly elegant. This man had no access to technology, computers, or a pocket calculator, yet he determined, with keen accuracy, fundamental laws of inheri- tance that have stood the test of time. Classical genetics is sometimes referred to as: »» Mendelian genetics: You start a new scientific discipline, and it gets named after you. Seems fair. »» Transmission genetics: This term refers to the fact that classical genetics describes how traits are passed on, or transmitted, from parents to their offspring. No matter what you call it, classical genetics includes the study of cells and chromosomes, which we cover in Chapters 2 and 6. Cell division is the machine that drives inheritance, but you don’t have to understand combustion engines to drive a car, right? Likewise, you can dive straight into simple inheritance in Chapter 3 and work up to more complicated forms of inheritance in Chapter 4 without knowing anything whatsoever about cell division. (Mendel didn’t know anything about chromosomes and cells when he figured this whole thing out, by the way.) The genetics of sex and reproduction are also part of classical genetics. Various combinations of genes and chromosomes (strands of DNA) determine biological sex. But the subject of sex gets even more complicated and interesting: The envi- ronment plays a role in determining the sex of some organisms (like crocodiles and turtles), and other organisms can even change sex with a change of address. If this has piqued your interest, you can find out all the astonishing details in Chapter 6. (Of note, we use the term sex throughout this book instead of the term gender. Sex is what defines males and females from a biological perspective. A person’s gender, on the other hand, may also be influenced by social and cultural factors, and may differ from one’s biological sex.) CHAPTER 1 Welcome to Genetics: Just About Everyone Loves a Good Pair of Genes 9  Classical genetics provides the framework for many subdisciplines. The study of chromosome disorders such as Down syndrome, which we cover in Chapter 12, relies on cell biology and an understanding of what happens during cell division. Genetic counseling, which we cover in Chapter 14, also relies on understanding patterns of inheritance to interpret people’s medical histories from a genetics perspective. In addition, forensics, covered in Chapter 17, uses Mendelian genetics to determine paternity and to work out who’s who with DNA fingerprinting. Molecular Genetics: Diving into DNA Science Classical genetics concentrates on studying outward appearances, while the study of actual genes falls under the heady title of molecular genetics. The area of opera- tions for molecular genetics includes all the machinery that runs cells and manu- factures the structures called for by the plans found in genes. The focus of molecular genetics includes the physical and chemical structures of the double helix, DNA, which is broken down in all its glory in Chapter 5. The messages hid- den in your DNA (your genes) constitute the building instructions for your appear- ance and everything else about you — from how your muscles function and how your eyes blink to your blood type, your susceptibility to particular diseases, and everything in between. How that DNA (and the immense amount of information it contains) is packaged in your cells is covered in Chapter 6, which reviews the structure and function of chromosomes. Your genes are expressed through a complex system of interactions that begins with transcription — copying DNA’s messages into a somewhat temporary form called RNA, which is short for ribonucleic acid and is covered in Chapter 8. RNA car- ries the DNA message through the process of translation, covered in Chapter 9, which in essence is like taking a blueprint to a factory to guide the manufacturing process. Where your genes are concerned, the factory makes the proteins (from the RNA blueprint) that get folded in complex ways to make the various compo- nents of the cells and tissues in the human body. The study of gene expression (how genes get turned on and off, which we review in Chapter 10) and how the genetic code works at the levels of DNA and RNA are considered parts of molecular genetics. Research on the causes of cancer and the hunt for better treatments, which we address in Chapter 13, focuses on the molecular side of things because tumors result from changes in the DNA, called mutations. Chapter 11 covers mutations in detail. Gene therapy, covered in Chapter 15, and genetic engineering, covered in Chapter 19, are subdisciplines of molecular genetics. 10 PART 1 The Lowdown on Genetics: Just the Basics Population Genetics: Studying the Genetics of Groups The science of genetics has a strong foundation in math. (In fact, Gregor Mendel studied to be a math teacher.) One area in which calculations are used to describe what goes on genetically is population genetics. If you use Mendelian genetics and examine the inheritance patterns of many ­different individuals who have something in common, like geographic location, you can study population genetics. Population genetics is the study of the genetic diversity of a subset of a particular species (for details, you can flip ahead to ­Chapter 16). Basically, it’s a search for patterns that help describe the genetic ­signature of a particular group, such as the consequences of migration, isolation from other populations, and mating choices. Population genetics helps scientists understand how the collective genetic diversity of a population influences the health of individuals within the population. For example, cheetahs are lanky cats; they’re the speed demons of Africa. Population genetics has revealed that all cheetahs are extremely genetically similar; in fact, they’re so similar that a skin graft from one cheetah would be accepted by any other cheetah. Because the genetic diversity of cheetahs is so low, conservation biologists fear that a disease could sweep through the population and kill off all the individu- als of the species. It’s possible that no animals would be resistant to the disease, and therefore, none would survive, leading to the extinction of this amazing predator. Evolutionary genetics is a type of population genetics that involves studying how traits change over time. We review evolutionary genetics in Chapter 16. Describing the genetics of populations from a mathematical standpoint is also critical to forensics, as explained in Chapter 17. To pinpoint the uniqueness of one DNA fin- gerprint, geneticists need to sample the genetic fingerprints of many individuals and decide how common or rare a particular pattern may be. Likewise, medicine uses population genetics to determine how common particular DNA changes are and to develop new medicines to treat disease (discussed in Chapter 15). Quantitative Genetics: Exploring the Heredity of Complex Traits Quantitative genetics examines traits that vary in subtle ways and relates those traits to the underlying genetics of an organism. A combination of whole suites of genes and environmental factors controls characteristics like retrieving ability in CHAPTER 1 Welcome to Genetics: Just About Everyone Loves a Good Pair of Genes 11  dogs, egg size or number in birds, and running speed in humans. Mathematical in nature, quantitative genetics takes a rather complex statistical approach to esti- mate how much variation in a particular trait is due to the environment and how much is actually genetic. One application of quantitative genetics is determining how heritable a particular trait is. This measure allows scientists to make predictions about how offspring will turn out based on characteristics of the parent organisms. Heritability gives some indication of how much a characteristic (like seed production) can change when selective breeding (or, in evolutionary time, natural selection) is applied. GREAT GENETICS WEBSITES TO EXPLORE The Internet is an unparalleled source of information about genetics. With just a few mouse clicks, you can find the latest discoveries and attend the best courses ever offered on the subject. Here’s a quick sample. To see a great video that explains genetics and gives it a human face, check out “Cracking the Code of Life”: https://www.pbs.org/wgbh/nova/genome/ program.html. New discoveries are unveiled every day. To stay current, log on to www. sciencedaily.com/news/plants_animals/genetics and https:// www.sciencenews.org/topic/genetics. For students, http://learn.genetics.utah.edu can’t be beat. From the basics of heredity to virtual labs to cloning, it’s all there in easy-to-grasp animations and language. Want to get all the details about genes and diseases? Medline Plus provides straightforward explanations on numerous topics: https://ghr.nlm.nih.gov. You could also start at https://www.ncbi.nlm.nih.gov/books/NBK22183 for a review of the basics. More advanced (and greatly detailed) information is available at Online Mendelian Inheritance in Man (OMIM): www.ncbi.nlm.nih.gov/omim. If you’re interested in a career in genetics, the American Society for Human Genetics is ready to help: https://www.ashg.org/careers-learning/career- flowchart. (You can also check out some career descriptions in Chapter 22.) 12 PART 1 The Lowdown on Genetics: Just the Basics  IN THIS CHAPTER »» Getting to know the cell »» Understanding the basics of chromosomes, DNA, and genes »» Exploring simple cell division »» Appreciating the complexities of meiosis Chapter 2 Basic Cell Biology G enetics is closely related to the study of how cells work. The process of passing genetic material from one generation to the next depends com- pletely on how cells grow and divide. Organisms such as bacteria or yeast that reproduce asexually simply copy their DNA (through a process called replica- tion, which is covered later in Chapter 7) and split in two. But organisms that reproduce sexually go through a complicated dance that includes mixing and matching strands of DNA (a process called recombination) and then halving the amount of DNA for special sex cells that join to create completely new genetic combinations for their offspring. These amazing processes are part of what makes you unique from every other human on the planet. In this chapter, we provide a brief introduction to cell structure, DNA, and chro- mosomes. In addition, you need to be familiar with the processes of mitosis (cell division) and meiosis (the production of sex cells) to appreciate how genetics works. So come inside your cell and let us introduce you to the basics. Later in this book, we will spend more time on the details of DNA and chromosomes, since these topics lay the groundwork for all things in genetics. CHAPTER 2 Basic Cell Biology 13 Entering the World of the Cell There are two basic kinds of cell types, distinguished by whether or not they have a nucleus (a compartment filled with DNA surrounded by a membrane): »» Prokaryotes: Organisms whose cells lack a nucleus and therefore have DNA floating loosely in the liquid center of the cell. »» Eukaryotes: Organisms that have a well-defined nucleus to house and protect the DNA. The basic qualities of prokaryotes and eukaryotes are similar but not identical. Because all living things fall into one of these two groups, understanding the dif- ferences and similarities between cell types is important. In this section, you will learn how to distinguish the two kinds of cells from each other, and you will get a quick tour of the insides of cells — both with and without nuclei (plural of nucleus). Figure 2-1 shows you the structure of each type of cell. Cells without a nucleus Scientists classify organisms composed of cells without nuclei as prokaryotes, which means “before nucleus.” The prokaryotes you’ve probably heard of are bacteria, which are mostly single-celled organisms. Their cells lack nuclei and have relatively small amounts of DNA (you can flip to Chapter 8 for more on the amounts of DNA different organisms possess). Prokaryotes are the most common forms of life on earth. They exist in every type of environment, including within and on other living things. In fact, at this very moment, trillions of bacterial cells live in and on your body. Most of the bacteria in your body are completely harmless, and others are beneficial to your health; for example, the digestion going on in your intestines is partially powered by bacteria that break down the food you eat. The bacteria that make news headlines, how- ever, are the small percentage that can be deadly, causing rapidly transmitted diseases such as cholera, a severe intestinal disease found most often in popula- tions without a clean supply of water. The vast majority of prokaryotic cells are beneficial to human life and the ecosystems we live in, performing vital functions such as photosynthesis and recycling the dead into usable nutrients for other forms of life. In fact, it’s not too much to say that no other forms of life would survive for long on the planet without the prokaryotes. 14 PART 1 The Lowdown on Genetics: Just the Basics  FIGURE 2-1: A prokaryotic cell (a) is very simple compared to a eukaryotic cell (b). Note that these cells are not drawn to scale: Prokaryotic cells are normally 10–100 times smaller than eukaryotic cells. The basic structure of a prokaryotic cell is shown in Figure 2-1. Scientists call the boundary that separates all cells from their environment the plasma membrane. Membranes are thin, flexible sheets of molecules that act as a barrier. The plasma membrane regulates the exchange of nutrients, water, and gases between cells and their environment. In addition to the plasma membrane, many prokaryotic cells are surrounded by a cell wall that primarily serves as protection from CHAPTER 2 Basic Cell Biology 15  pressure changes caused by movement of water into and out of the cell. The liquid interior of the cell is called the cytoplasm. The cytoplasm provides a cushiony, watery home for the DNA and other cell machinery that carry out the business of living. Prokaryotes typically have a single hoop-shaped piece of DNA that floats in the cytoplasm. Scientists call the pieces of DNA inside cells chromosomes (see the section “Examining the basics of chromosomes” later in the chapter). Cells with a nucleus Scientists classify organisms that have cells with nuclei as eukaryotes, which means “true nucleus.” Eukaryotes range in complexity from simple, single-celled organisms to complex, multicellular organisms like you. Eukaryotic cells are fairly complicated and have numerous parts to keep track of (refer to Figure 2-1). Like prokaryotes, eukaryotic cells have a selective border called a plasma membrane, and sometimes a cell wall surrounds the membrane (plants, for example, have cell walls). But that’s where the similarities end. The distinctive feature of the eukaryotic cell is the nucleus — the membrane- surrounded compartment that houses the DNA. The nucleus protects the DNA from damage during day-to-day living. The DNA of eukaryotes is divided into separate pieces called chromosomes. Eukaryotic chromosomes are usually long, string-like segments of DNA instead of the hoop-shaped ones found in prokary- otes. Another hallmark of eukaryotes is the way the DNA is packaged: Eukaryotes usually have much larger amounts of DNA than prokaryotes, and to fit all that DNA into the tiny cell nucleus, it must be tightly wound around special proteins. (For all the details about DNA packaging in eukaryotes, flip to Chapter 6.) Unlike prokaryotes, eukaryotes have all sorts of cell parts, called organelles, that perform specific functions for the cell. Membranes form the boundaries of the organelles. Two of the most important organelles found in the cytoplasm are: »» Mitochondria: The powerhouses of the eukaryotic cell, mitochondria pump out energy by transferring energy from food to ATP (adenosine triphosphate). ATP acts like a battery of sorts, storing energy until it’s needed for day-to-day living. Almost all eukaryotes have mitochondria. »» Chloroplasts: Found in plants and algae, chloroplasts capture energy from the sun and store it in sugars they make from carbon dioxide and water. These sugars can be used as a source of energy for plants (or for organisms that eat the plants). In most multicellular eukaryotes, cells come in two basic varieties: body cells (called somatic cells) or sex cells (called gametes). The two cell types have different functions and are produced in different ways. 16 PART 1 The Lowdown on Genetics: Just the Basics  »» Somatic cells: Somatic cells are produced by a type of cell division called mitosis (see the section “Mitosis: Splitting Up” for details). Somatic cells of multicellular organisms like humans are differentiated into special cell types. Skin cells and muscle cells are both somatic cells, for instance, but if you were to examine your skin cells under a microscope and compare them with your muscle cells, you’d see that their structures are very different. The various cells that make up your body all have the same basic components (mem- brane, organelles, and so on), but the arrangements of the elements change from one cell type to the next so that they can carry out various jobs such as digestion (intestinal cells), energy storage (fat cells), or oxygen transport to your tissues (blood cells). Changes that occur only in the DNA of somatic cells (that is, changes that occur over the course of a person’s lifetime) cannot be passed from parent to child. »» Gametes: Gametes are specialized cells produced by a type of cell division called meiosis for sexual reproduction, as shown in the human life cycle in Figure 2-2. Sexual reproduction combines genetic material from two organisms and requires special preparation in the form of a reduction in the amount of genetic material allocated to gametes (see “Meiosis: Making Cells for Sexual Reproduction” later in the chapter for an explanation). Only eukaryotic organisms engage in sexual reproduction, which we cover in detail at the end of this chapter in the section “Meiosis in the human body.” In humans, the two types of gametes are eggs and sperm. Unlike those in somatic cells, changes in the DNA of gametes can be passed from parent to child. FIGURE 2-2: The Human Life Cycle. CHAPTER 2 Basic Cell Biology 17 Exploring Your DNA, Chromosomes, and Genes The nucleus of a eukaryotic organism is home to most of its DNA. (The mitochon- dria and chloroplasts also have DNA, which we discuss more in Chapter 5.) In humans, nearly all the body’s cells have a nucleus filled with DNA (mature red blood cells do not). Each of your cells contains two copies of your genome – that is, all the DNA you inherited from your mother (minus the mitochondrial DNA) and all the DNA you inherited from your father. Your genome contains all the information necessary for the human body to grow and develop and for the vari- ous parts of the body to function every day. The amount of information carried in your DNA is staggering. It is basically the “how-to guide” for your body. DNA is generally measured in base pairs (which we discuss in detail in Chapter 5). Your genome has approximately 3 billion base pairs. The two copies of your genome add up to 6 billion base pairs per cell, which means the DNA from a single cell would measure just over 6 feet (or 1.8 m) if stretched out and laid end-to-end! In order to organize all that information and to fit it into each of your cells, your DNA is packaged into chromosomes. If you think of your genome as a multivolume how-to guide, then each chromosome is a single book that belongs to the set. Your genome is contained in 23 chromosomes, and each cell contains two sets of those 23 chromosomes (for a total of 46 chromosomes in every cell). Examining the basics of chromosomes Chromosomes are threadlike strands composed of DNA. To pass genetic traits from one generation to the next, the chromosomes must be copied (see Chapter 7), and then the copies must be divided up into the cells that will form the next genera- tion. Most prokaryotes have only one circular chromosome that, when copied, is passed on to the daughter cells (new cells created by cell division). Eukaryotes have more complex problems to solve (like sorting the chromosomes properly to make gametes for sexual reproduction), and their chromosomes behave differently dur- ing mitosis and meiosis. This section gets into the intricacies of chromosomes in eukaryotic cells because they’re so complex. Counting out chromosomes Each eukaryotic organism has a specific number of chromosomes per cell — but it is different for each organism. For example, the human genome is contained in 23 chromosomes, while the fruit fly genome is contained in only 4 chromosomes. 18 PART 1 The Lowdown on Genetics: Just the Basics (Remember that you have two copies of every chromosome for a total of 46 per cell; adult fruit flies would have a total of 8.) Chromosomes come in two varieties: »» Sex chromosomes: These chromosomes determine sex — male or female. Human cells contain two sex chromosomes. Females should have two X chromosomes, while males should have an X chromosome and a Y chromo- some. (To find out more about how sex is determined by the X and Y chromo- somes, flip to Chapter 6.) »» Autosomal chromosomes: Autosomal simply refers to nonsex chromosomes. Sticking with the human example, if you do the math, you can see that humans have 44 autosomal chromosomes (46 total chromosomes minus two sex chromosomes equals 44 autosomal chromosomes). Human chromosomes can be divided into pairs and numbered to create a karyo- type like the one pictured in Figure 2-3. You have 22 pairs of uniquely shaped autosomal chromosomes plus 1 pair of sex chromosomes for a total of 23 chromo- some pairs. Your autosomal chromosomes are identified by numbers — 1 through 22 — and are generally arranged based on size, with chromosome 1 being the largest and chromosome 22 being the smallest. So you have two chromosome 1s, two 2s, and so on. Scientists and doctors can learn about certain health conditions by looking at a person’s karyotype; you can discover more about karyotyping in Chapter 12. The individual chromosomes in each pair of your chromosomes are homologous, meaning that the paired chromosomes are identical to one another with respect to which genes they carry. In addition, your homologous chromosomes are identical in shape and size. These pairs of chromosomes are sometimes referred to as homologs for short. Chromosome numbers can be a bit confusing. Humans are diploid, meaning we have two copies of each chromosome. Some organisms (like bees and wasps) have only one set of chromosomes (cells with one set of chromosomes are called ­haploid); other organisms have three, four, or as many as sixteen copies of each chromosome! The number of chromosome sets held by a particular organism is called the ploidy. For more on chromosome numbers, see Chapter 12. The total number of chromosomes doesn’t tell you what the ploidy of an organism is. For that reason, the number of chromosomes an organism has is often listed as some multiple of n. A single set of chromosomes referred to by the n is the haploid number. Humans are 2n = 46, indicating that humans are diploid and their total number of chromosomes is 46. Human sex cells such as eggs or sperm are haploid (refer to Figure 2-2). CHAPTER 2 Basic Cell Biology 19  FIGURE 2-3: The 46 human chromosomes are divided into 23 pairs. Geneticists believe that the homologous pairs of chromosomes in humans started as one set (that is, haploid), and the entire set was duplicated at some point in some distant ancestor, many millions of years ago. Examining chromosome anatomy Chromosomes are often depicted in stick-like forms, like those you see in Figure 2-4. Chromosomes don’t look like sticks, though. In fact, most of the time they’re loose and string-like. Chromosomes only take on this distinctive shape and form when cell division is about to take place (during metaphase of meiosis or mitosis). They’re often drawn this way so that the special characteristics of eukaryotic chromosomes are easier to see. Figure 2-4 points out the important features of eukaryotic chromosomes. The centromere is the part of the chromo- some that appears pinched, while the telomeres are the ends of the chromosomes. Telomeres are made of densely packed DNA and serve to protect the chromosome from being broken down. 20 PART 1 The Lowdown on Genetics: Just the Basics  FIGURE 2-4: Basic structure of eukaryotic chromosomes. Finding your genes The differences in shapes and sizes of chromosomes are easy to see, but the most important differences between chromosomes are hidden deep inside the DNA. Chromosomes carry genes — sections of DNA that provide the building plans for specific molecules (such as proteins) and that are associated with various traits. The genes tell the body how, when, and where to make all the structures that are necessary for the processes of living. In continuing the “how-to guide” analogy, each gene could be a chapter with a very specific set of instructions. Your genome contains approximately 22,000 genes, with each chromosome containing a varying number of genes, from fewer than 300 genes on the smallest chromo- some (the Y chromosome) to more than 3,000 genes on the largest (chromosome 1). The smallest human genes are only a few hundred base pairs in length. The largest is the gene that encodes the muscle protein dystrophin, which has 2.4 million base pairs. You have two copies of each gene, with the exception of genes on the sex chromo- somes in males (who have only one copy of most genes on the X and Y chromo- somes). You inherit one copy of each gene from each parent. Each pair of homologous chromosomes carries the same — but not necessarily identical — genes. For example, both chromosomes of a particular homologous pair may con- tain a gene that controls hair color, but one can be a “brown hair” version of the gene and the other can be a “blond hair” version of the gene — alternative ver- sions of genes are called alleles (refer to Figure 2-4). One of these alleles would be from your mother and the other from your father. CHAPTER 2 Basic Cell Biology 21  Any given gene can have one or more alleles. In Figure 2-4, one chromosome car- ries the allele A while its homolog carries the allele a (the relative size of a gene is very small relative to the whole chromosome; the alleles for a given gene are large here so you can see them). The alleles are associated with different physical traits (phenotypes) you see in organisms, like hair color or flower shape. You can find out more about how alleles affect phenotype in Chapter 3. Each point along the chromosome is called a locus (Latin for “place”). The plural of locus is loci (pronounced low-sigh). Most of the phenotypes that you see are produced by multiple genes (that is, genes occurring at different loci and often on different chromosomes) acting together. For instance, human eye color is deter- mined by at least three genes that reside on two different chromosomes. You can find out more about how genes are arranged along chromosomes in Chapter 11. Mitosis: Splitting Up Most cells have simple lifestyles: They grow, divide, and eventually die. Figure 2-5 illustrates the basic life cycle of a typical somatic, or body, cell. FIGURE 2-5: The cell cycle: mitosis, cell division, and all points in between. The cell cycle, the stages a cell goes through from one division to another, is tightly regulated; some cells divide all the time, and others never divide at all. Cells that aren’t dividing are in interphase, the phase during which cells grow and perform their function for the body. When cells get a signal to divide, they enter mitosis, the phase of cell division. Your body uses mitosis to provide new cells when you grow and to replace cells that wear out or become damaged from injury. Talk about multitasking — you’re going through mitosis right now, while you read this book! Some cells divide only part of the time, when new cells are needed to handle 22 PART 1 The Lowdown on Genetics: Just the Basics certain jobs like fighting infection. Cancer cells, on the other hand, get carried away and divide too often. In Chapter 14, you can find out how the cell cycle is regulated and what happens when it goes wrong. Cell division includes mitosis (the process of dividing the cell nucleus) and ­cytokinesis (the process of separating the cytoplasm into two cells). Together, mitosis and cytokinesis create two identical new cells from one original cell. To ensure that both new cells get everything they need, cells grow, duplicate their organelles, and make a copy of their DNA during interphase. During mitosis, they send a complete collection of all the chromosomes (in humans, 23 pairs) to each of the two resulting cells. Some eukaryotic organisms use mitosis to reproduce themselves asexually, producing offspring that are genetically identical to the original parent (you’ve seen this action if you’ve ever taken a cutting from a plant to make a new one, or found an old potato that’s begun to sprout). You should remember two important points about mitosis: »» Mitosis produces two identical cells. The new daughter cells are identical to each other and to the cell that divided to create them (the mother cell). »» Cells created by mitosis have exactly the same number of chromosomes as the original cell did. If the original cell had 46 chromosomes, the new cells each have 46 chromosomes. In order to make sure that each daughter cell receives the right number of chro- mosomes, cells carefully control the process of mitosis. In the following sections, you will be guided through the phases of the cell cycle and learn exactly what hap- pens during each one. Step 1: Time to grow Interphase is the part of the cell cycle during which the cell grows, copies its DNA, and prepares to divide. Interphase occurs in three stages: the G1 phase, the S phase, and the G2 phase. G1 phase When a cell begins life, such as the moment an egg is fertilized, the first thing it does is begin to grow. Scientists call this period of growth the G1 phase of inter- phase. Lots of things happen during G1, including physical growth of the cell and the creation of more organelles. Cells that aren’t dividing exit the cell cycle and remain in G0. Cells in G0 are like an idling car, the engine is running, but they aren’t going anywhere. The cells are CHAPTER 2 Basic Cell Biology 23  alive, functioning, and doing their job for the body, but they aren’t growing and reproducing. If an injury occurs and the cells get a signal that they must divide, they can return to the cell cycle and progress towards mitosis. Some types of cells opt out of the cell cycle permanently and remain in G0. Your brain cells, for exam- ple, have retired from the cell cycle. Mature red blood cells and muscle cells don’t divide, either. In fact, human red blood cells have no nuclei and thus possess no DNA of their own. (Your body constantly produces new red blood cells from the division of stem cells in your bone marrow.) Cells that receive signals to divide progress from G1 to the next phase of the cell cycle. Actively dividing cells go through the whole cell cycle every 24 hours or so. After a predetermined period of growth that lasts from a few minutes to several hours, the cell arrives at the first checkpoint (refer to Figure 2-5). When the cell passes the first checkpoint, there’s no turning back. Various proteins control when the cell moves from one phase of the cycle to the next. At the first checkpoint, proteins called cyclins and enzymes called kinases con- trol the border between G1 and the next phase. Cyclins and kinases interact to cue up the various stages of the merry-go-round of cell division. Two proteins, G1 cyclin and cyclin dependent kinase (CDK), hook up to escort the cell over the bor- der from G1 to S — the next phase. S phase Cells copy their DNA during S phase (S refers to synthesis, or copying, of the DNA). When the cell enters the S phase, activity around the chromosomes really steps up. Cells copy all their chromosomes, making exact replicas to pass on to the newly formed daughter cells produced by cell division. DNA replication is a very complex process that gets full coverage in Chapter 7. When cells copy their chromosomes during S phase, the two identical copies of each chromosome stay together as a unit that’s joined at the centromere. Scien- tists call the two identical copies sister chromatids. You can see what chromosomes look like before and after S phase in Figure 2-4; notice that we often draw repli- cated chromosomes (with their two identical sister chromatids) to look like X’s. Sister chromatids are alike in every way: they carry the exact same copies of the exact same genes (notice the letters that represent genes in Figure 2-4 are alike for each sister chromatid). During mitosis, cells separate the sister chromatids from each chromosome, sending one to each of the new daughter cells. G2 phase The G2 phase is the last phase before actual mitosis gets underway. During this phase, the chromosomes, now copied and hooked together as sister chromatids, stay together inside the cell nucleus. (The DNA is still “relaxed” at this point and 24 PART 1 The Lowdown on Genet