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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
TWO FROM ONE Condensed and easy step-in resource to the vast universe of cell cycle control and cell division Two from One: A Short Introduction to Cell Division Mechanisms is an easy and solid step-in for students and all individuals starting to learn about cell and molecular biology, as well as professionals looking for an avenue into the subject, emphasizing general concepts and universal aspects of eukaryotic cell division without getting lost in the vast amount of detail across the overall field. The text enables readers to learn about general concepts and discoveries from various systems and approaches to elucidate the process of cell division, with descriptions of scientific processes included throughout in order to aid in reader comprehension. The content and material have been taught, revised, and simplified based on student feedback, to be as accessible as possible to a broader audience. It can be read in a few hours by anyone with an interest in the topic and an undergraduate background. In Two from One, readers can expect to find coverage on a myriad of essential topics, such as: * Cell theory, mitosis, chromosome theory of heredity, DNA, and why/how cell cycles come in many flavors * Cell growth and division, covering balanced growth and cell proliferation, measures of cell growth, and the relationship between cell growth and division * Assaying cell cycle progression, covering measuring cell cycle phases, single-cell imaging, labeled mitoses, and frequency distributions * Duplicating the genome, covering DNA replication, origin firing, chromatin, checkpoints, and the DNA damage checkpoint Undergraduates, graduate students, and early career professionals in cell biology, biomedicine, and biology, along with post docs changing subject area or needing further information on cell division, will find Two from One to be an immensely useful, accessible, and reader-friendly resource in a traditionally highly complex field.
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Veröffentlichungsjahr: 2022
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Table of Contents
Cover
Title Page
Copyright Page
Dedication Page
Foreword
Preface
Symbols and Abbreviations
1 History and Context
1.1 From Cells to Their Nuclei
2 Cell Growth and Division
2.1 Balanced Growth and Cell Proliferation
2.2 Measures of Cell Growth
2.3 The Relationship Between Cell Growth and Division
2.4 Patterns of Growth in the Cell Cycle
2.5 Sizers vs. Adders
3 Assaying Cell Cycle Progression
3.1 Measuring Cell Cycle Phases
3.2 Growth Limitations and Variations in the Duration of Cell Cycle Phases
3.3 Synchronous Cultures
4 The Master Switch
4.1 Genetic Analyses Leading the Way
4.2 All Roads Lead to the Same Control System
4.3 Making Sense of it All
5 Controlling the Master Switch
5.1 Cyclins in Cdk Complexes
5.2 Cdk as a Target of Phosphorylations
5.3 Other Proteins in Cyclin/Cdk Complexes
5.4 What Are Its Targets and How Cdk Phosphorylates Them
5.5 Ordering Cdk Phosphorylation in the Cell Cycle
6 A Full Circle of the Switch
6.1 Modeling a Cell Cycle Oscillator
6.2 The M‐Cdk Switch
6.3 The G1/S Cdk Switch
6.4 Transcriptional Waves Until the End of the Cell Cycle
6.5 Comments on Overall Gene Expression in the Cell Cycle
7 Duplicating the Genome
7.1 DNA Replication
7.2 Checkpoints
8 Segregating the Chromosomes
8.1 Blind Men's Riddle
8.2 The Mitotic Spindle
8.3 The MT Organizing Centers (MTOCs)
8.4 The Kinetochore
8.5 The Spindle Assembly Checkpoint (SAC)
9 Segregating Organelles and the Cytoplasm
9.1 The Golgi
9.2 Mitochondria
9.3 Lysosomes and Vacuoles
9.4 Mitotic Fragmentation of the Nuclear Envelope
9.5 Cytokinesis: Two from One
References
Index
End User License Agreement
List of Tables
Chapter 4
Table 4.1 Mutants, proteins, and associated information, as mentioned in th...
Chapter 6
Table 6.1 The Rb pathway in human cancer.
Chapter 9
Table 9.1 List of proteins or complexes and their function.
List of Illustrations
Chapter 1
Figure 1.1 Illustration of mitotic cells.
Figure 1.2 The duration of the interphase is much longer than the mitotic st...
Figure 1.3 Embryonic cell cycles are usually devoid of gap phases.
Figure 1.4 Polytene chromosomes from the salivary gland of a fruit fly larva...
Chapter 2
Figure 2.1 Measuring the rate of cell proliferation. In exponentially prolif...
Figure 2.2 Possible outcomes of the relation between cell growth and divisio...
Figure 2.3 The Coulter principle measures changes in impedance (
Z
), which ar...
Figure 2.4 Different aspects of cell growth are captured by different techni...
Figure 2.5 Increasing ploidy also proportionately increases cell size.
Figure 2.6 Plots of exponential and linear growth over one doubling are very...
Figure 2.7 The ways (a) fission and (b) budding yeast cells grow in the cell...
Figure 2.8 A constant added mass per generation could account for size homeo...
Figure 2.9 Sizers and adders. (a) Plotting added size (the difference betwee...
Figure 2.10 Daughter budding yeast cells are born smaller and stay longer in...
Chapter 3
Figure 3.1 In the Fucci4 marker set, a combination of reporter proteins has ...
Figure 3.2 Overview of the labeled mitoses technique.
Figure 3.3 Expected graph of a labeled mitoses experiment. The length of eac...
Figure 3.4 A hypothetical DNA content histogram, from haploid yeast cells. T...
Figure 3.5 The relative cell numbers (
y
‐axis) along the cell cycle (
x
‐axis) ...
Figure 3.6 When cell growth is limited, the G1 phase of the cell cycle lengt...
Figure 3.7 Releasing the cells from a double thymidine block is a popular ap...
Figure 3.8 Centrifugal elutriation separates cells in a continuous flow cent...
Figure 3.9 In budding yeast daughter cells, which increase in size exponenti...
Chapter 4
Figure 4.1 The
cdc
screen.
Figure 4.2 Budding yeast cells commit to a new round of cell division in lat...
Figure 4.3 The unexpected recovery of the
wee1
mutant.
Figure 4.4 Functional complementation and conservation of the cell cycle eng...
Figure 4.5 Oocyte maturation.
Figure 4.6 Cyclin abundance in the early embryonic cell cycles. After egg fe...
Figure 4.7 Mitosis is dominant over all other phases of the cell cycle. Inje...
Figure 4.8 MPF oscillations. On the
x
‐axis is the time after fertilization w...
Figure 4.9 Cyclin is necessary and sufficient to trigger mitosis in frog ooc...
Figure 4.10 Cyclin synthesis turns MPF on, while cyclin degradation turns it...
Figure 4.11 The G1 cyclin Cln3 alters the critical size at Start. The domina...
Figure 4.12 Diagram of the G2/M network in fission yeast, based on the phosp...
Chapter 5
Figure 5.1 The predicted structure of the budding yeast Cdk (Cdc28) monomer ...
Figure 5.2 Cyclin/Cdk activity toward a peptide containing a full consensus ...
Figure 5.3 Cdk activation by cyclin binding and phosphorylation of Cdk at a ...
Figure 5.4 Phosphorylation‐mediated positive feedback of M‐Cdk activation.
Figure 5.5 The only essential function of G1 and G1/S cyclins in yeast is to...
Figure 5.6 Engineered Cdk (Cdk
as
) that can use bulky ATP analogs and label t...
Figure 5.7 The Loog model of how interactions between substrate proteins and...
Chapter 6
Figure 6.1 Three simple signal‐response behaviors.
Figure 6.2 Cyclin re‐localization in the cell cycle. Immunofluorescence pat...
Figure 6.3 M‐Cdk activates the APC, which then targets numerous proteins for...
Figure 6.4 Schematic of some major M‐Cdk positive and negative feedbacks (sh...
Figure 6.5 The rise of M‐Cdk eventually leads to the activation of the Cdc14...
Figure 6.6 Schematic of the ideal behavior of a M‐Cdk oscillator. Notice the...
Figure 6.7 Active, preformed G1‐Cdk is sufficient to drive cells out of quie...
Figure 6.8 Schematic of competing models about how growth inputs control Sta...
Figure 6.9 The G1/S switch and some of its feedbacks.
Figure 6.10 Structures of three FDA‐approved inhibitors of Cdk4/6.
Figure 6.11 Transcriptional networks late in the budding yeast cell cycle.
Chapter 7
Figure 7.1 Model for MCM loading.
Figure 7.2 Regulation of Cdc6 and Cdt1 (the latter via geminin in human cell...
Figure 7.3 The unexpected roles of Cdc6 at the M/G1 transition.
Figure 7.4 Model for MCM helicase activation. Most of the factors that are n...
Figure 7.5 Cohesin complexes previously associated with un‐replicated DNA co...
Figure 7.6 Checkpoint mutants fail to arrest the cell cycle upon DNA damage ...
Figure 7.7 Diagram summarizing one example of how the DNA damage checkpoint,...
Figure 7.8 Diagram summarizing one example of how the DNA damage checkpoint,...
Chapter 8
Figure 8.1 The structure of microtubules. (a) Electron cryo‐micrograph of un...
Figure 8.2 Schematic of the mitotic spindle of animal somatic cells. Spindle...
Figure 8.3 Microtubule behavior in metaphase.
Figure 8.4 Electron tomogram of centrioles in wing disc fly cells. The cartw...
Figure 8.5 Microtubules are nucleated at the centrosome and grow at their pl...
Figure 8.6 Licensing SPB duplication by Cdk is analogous to the licensing of...
Figure 8.7 The deterministic “mooring” mechanism of chromosome biorientation...
Figure 8.8 Model about how Aurora‐B kinase may destabilize weak MT‐kinetocho...
Figure 8.9 Schematic of SAC activation and inhibition of anaphase, through i...
Chapter 9
Figure 9.1 In animals, M‐Cdk triggers fragmentation of the Golgi apparatus (...
Figure 9.2 In budding yeast, the cell cycle machinery upregulates mitochondr...
Figure 9.3 Visualization of the cleavage furrow in sea urchin (
L. pictus
) em...
Figure 9.4 Spindle position determines cleavage site. (a) Typically, once an...
Figure 9.5 Diagram of how the centralspindlin complex from MTs activates Rho...
Figure 9.6 The contractile ring during cytokinesis. Actin filaments become a...
Guide
Cover Page
Title Page
Copyright Page
Dedication Page
Foreword
Preface
Symbols and Abbreviations
Table of Contents
Begin Reading
References
Index
Wiley End User License Agreement
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Two from One
A Short Introduction to Cell Division Mechanisms
Michael Polymenis
Department of Biochemistry and Biophysics
Texas A&M University
College Station, Texas
Illustrations by Athená Polymenis
This edition first published 2023© 2023 John Wiley & Sons Ltd
All rights reserved. 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 or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.
The right of Michael Polymenis to be identified as the author of this work has been asserted in accordance with law.
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Limit of Liability/Disclaimer of WarrantyIn view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.
Library of Congress Cataloging‐in‐Publication DataNames: Polymenis, Michael, author.Title: Two from one : a short introduction to cell division mechanisms / Michael Polymenis, Professor, Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas.Description: First edition. | Hoboken, NJ, USA : John Wiley & Sons, Inc., 2023. | Includes bibliographical references and index.Identifiers: LCCN 2022038433 (print) | LCCN 2022038434 (ebook) | ISBN 9781119930143 (Paperback) | ISBN 9781119930150 (Adobe pdf) | ISBN 9781119930167 (Epub)Subjects: LCSH: Cell division. | Cell cycle.Classification: LCC QH605 .P57 2023(print) | LCC QH605(ebook) | DDC 571.8/44–dc23/eng/20220920LC record available at https://lccn.loc.gov/2022038433LC ebook record available at https://lccn.loc.gov/2022038434
Cover Design: WileyCover Image: Reproduced under CC BY license, Figure 3 © Siegel D, Kepa JK, Ross D (2012)
For Helene, my best friend, lover, wife, and mother of our children, who is dealing with the catastrophic consequences of too much cell division.
Foreword
This book is about cell division, the basis of all life on this planet. It is based on the material covered in a short, five‐week course developed by the author. It was designed for first‐year graduate students in the life sciences or undergraduate juniors and seniors, who have some general biology and biochemistry background, but not much beyond that. The book is meant to be a solid step in learning about the subject, not the last for anyone interested in cell division. Learning about cell division is an excellent way to illustrate the unity and power of life sciences. The cell division field is a spectacular “melting pot,” where general concepts and discoveries are synthesized from various systems and approaches.
I do not consider myself an expert in all things cell cycle. Although my research is in the general area, it is focused on a tiny slice of that area in the coupling of protein synthesis with cell division in yeast. Hence, I have probably overlooked some areas or overemphasized others. Some of these errors, hopefully not too many, may result from my poor knowledge of specific processes. The rest were mainly intentional, looking for ways to simplify the story without missing key pieces and telling it in a way that would make it more digestible to my students and other readers.
The text has been taught twice, revised, and simplified, based on student feedback, to be as accessible as possible. Emphasis is on general concepts. The “curse” of modern descriptions of cell division mechanisms is that they quickly morph into an “alphabet soup” of gene and protein names. There are fewer than a hundred such names on these pages. Hence, I must apologize for the many proteins and their cell cycle roles discovered by numerous scientists that are not covered here. I also wish to thank colleagues who pointed out errors of fact for the proteins and processes I do cover and apologize for any remaining mistakes. The book is not about providing the most comprehensive assembly of the current knowledge on cell division mechanisms. It can be read in a few hours by anyone with some interest in the topic and minimal undergraduate background. The book aims to outline the beauty and logic of the most vital biological process, making two cells from one.
My daughter, Athená Polymenis, “touched‐up” most of the illustrations in the figures. Her talent for drawing is astonishing. I am glad she did not inherit my drawing skills.
I am very grateful to all the Department of Biochemistry and Biophysics students who commented on early versions of the text. They faced the material at the “frontlines,” in the classroom. Their suggestions made a difference as the book evolved, especially toward simplifying it as much as possible. Likewise, the comments from anonymous reviewers were much appreciated. I thank all those at Wiley who transformed a preliminary draft into what you are reading. I am especially grateful to Julia Squarr, Senior Commissioning Editor, who saw potential in the concept and supported it. She kept me in line with her regular emails regarding decisions, deadlines, etc. I also thank Rosie Hayden, Joss Everett, and Naveen Kumaran Shanmugam for their help at various editorial steps. Finally, I thank my friends, lab members, and family who encouraged me to do this, even though it meant that, on occasion, they had to deal with an even more overworked and neurotic version of me.
—Michael PolymenisTexas A&M University,College Station, TX, USA
Preface
The cell cycle is what needs to happen from the time one cell is generated at the end of a cell division for that cell to divide into two. To make two cells from one, all the parts of one cell are duplicated and segregated into two cells. In the last 50 years, there have been several excellent monographs on cell division. The overwhelming emphasis in most of them is on the replication and segregation of the cell's chromosomes. Endowing the next generation of cells with the correct genetic material is the primary task a dividing cell must accomplish. But focusing only on chromosomes is like watching a movie with a single actor. After all, chromosomes make up only a small fraction of the cells' parts. As it turns out, when and how cells make and segregate their other components, even large molecules, such as proteins and RNAs, or smaller ones, such as lipids in membranes, is crucial in itself. It also impacts when and how cells duplicate and segregate their chromosomes.
But how does one go about describing all that information without getting lost in a blizzard of gene names and regulatory pathways? The book tries the following:
A brief history of the cell cycle and its prominent landmarks, looking at cellular parts that are easily seen to duplicate and segregate.
Discuss in some detail “bulk” cellular components that also need to be duplicated and segregated. But this process of cell “growth” is amorphous, with no apparent beginning or end. And yet, cell “growth” is coupled with cell division in ways that profoundly affect how fast cells multiply.
Take a short detour to the current methods of monitoring cell division and their shortcomings. For example, some of these methods may perturb cellular physiology and the normal coordination between cell growth and division.
Learn about the master control system of the cell cycle.
Examine how cell cycle switches are put together and how they are turned on and off.
Looking at gene expression patterns in the cell cycle, or at what goes up and down and when.
After all the above, then we can better describe how the genome is duplicated and segregated.
More than an afterthought, we will see how some organelles are duplicated and segregated.
Finally, the last act, cytokinesis, yields two cells from one.
But other related topics will not be covered (e.g., meiosis, prokaryotic cell cycles, cell cycle controls during animal development, some unusual cell cycles). Not because these topics are uninteresting or unimportant. To keep the book short and more accessible, it can only focus on the common, universal aspects of eukaryotic cell division. Although a serious effort was made to minimize their number, some protein names are necessary. The final tally came to slightly less than a hundred. A table with all the protein names mentioned in the book and their function is given at the end.
Michael Polymenis, Texas A&M University,College Station, TX, USA
January 2023
Symbols and Abbreviations
[γ‐
32
P]‐ATP
adenosine triphosphate, labeled on the gamma phosphate group with
32
P
˧
inhibition
°C
degrees Celsius
time delay, hysteresis
1N
haploid genome content
2N
diploid genome content
3H
tritium, a hydrogen atom that has two neutrons in the nucleus and one proton
APC
anaphase promoting complex
ATM
ataxia telangiectasia mutated
ATP
adenosine triphosphate
ATPγS
adenosine 5
′
‐
O
‐(3‐thio)triphosphate
ATR
ataxia telangiectasia and Rad3 related
cdc
cell division cycle
Cdk
cyclin‐dependent protein kinase
Cdk‐as
analog‐sensitive Cdk
CKI
Cdk inhibitor
dCTP
deoxycytidine triphosphate
DDK
Cdc7‐Dbf4 kinase
DNA
deoxyribonucleic acid
dNTP
deoxyribonucleotide triphosphate
FACS
fluorescence‐activated cell sorting
FDA
(United States) Food and Drug Administration
FUCCI
fluorescent ubiquitinated cell cycle indicator
G0
quiescent state, outside of the replicative cell cycle
G1 phase
the phase of the cell cycle lasting from the birth of the cell until initiation of DNA replication
G2 phase
the phase of the cell cycle when cells have fully replicated their DNA but have not started mitosis
GDP
guanosine diphosphate
GO
gene ontology
GTP
guanosine triphosphate
h
hours
I
input
k
specific proliferation (or volume growth) rate constant
K
M
Michaelis constant
M phase
mitosis
MPF
maturation promoting factor
mRNA
messenger RNA
MT
microtubule
MTOC
microtubule organizing center
N:C
nuclear:cytoplasmic ratio
nm
nanometer
nN
nanonewton
NPC
nuclear pore complex
O
output
PCNA
proliferating cell nuclear antigen
R
2
coefficient of determination
RNA
ribonucleic acid
RNase
ribonuclease
RO‐3306
(5
Z
)‐5‐(quinolin‐6‐ylmethylidene)‐2‐(thiophen‐2‐ylmethylimino)‐1,3‐thiazolidin‐4‐one; a small molecule Cdk inhibitor
rRNA
ribosomal RNA
S phase
the phase of the cell cycle when cells replicate their DNA
SAC
spindle assembly checkpoint
SCF
Skp, Cullin, F‐box containing
SPB
spindle pole body
T
total cell cycle time
T
G1
length of the G1 phase
TOR
target of rapamycin
tRNA
transfer RNA
Ub
ubiquitin
1History and Context
1.1 From Cells to Their Nuclei
Until the last decades of the previous century, the history of the cell cycle is nearly the same as the history of the chromosome theory of heredity. And before chromosomes, we have to go back to when cells were first seen and then recognized as the unit of life. By the early seventeenth century, advances in lens grinding in the Netherlands allowed several individuals in Europe to experiment with and construct microscopes. Robert Hooke first used the word “cell” to describe what he saw when looking with his microscope at plant tissue. School children today have seen Hooke's famous drawings of cork sections. The drawings resemble a honeycomb, and they first appeared in Hooke's Micrographia in 1665. Hooke did not recognize that what he saw were the skeletal remains of all life's basic units. That realization did not happen until much later. In the decades after Hooke, numerous microscopists made a series of observations of cells. In 1719, looking at red cells from fish (which do not lose their nuclei as ours do), Antonie van Leeuwenhoek probably made the first drawings of the nucleus (1). The term “nucleus” was introduced in 1831 by Robert Brown (also of “Brownian” motion fame) when he used it to describe cells from orchids (1).
The discovery of the nucleus was highly significant. Not only because the nucleus contains the chromosomes (which had not been discovered at the time), but also because it is a visible cellular landmark. A marker that scientists can look for and monitor. Cellular landmarks, morphological or molecular, have driven and continue to drive cell cycle research. But just because a landmark is there does not necessarily mean that it may also make certain things happen. An example is the nucleolus and its role in the generation of new cells. Rudolf Wagner clearly described the nucleolus within the nucleus in 1835, which he assumed to be the “germinative spot” for cell formation. The nucleolus has important roles, but not in “seeding” new cells.
1.1.1 The Cell Theory
Theodor Schwann and Matthias Schleiden are usually credited with the formulation of key tenets of the cell theory. That the cell is the fundamental unit of life, and that all plants and animals are made of cells, each cell having a nucleus and a nucleolus (2). But it was not clear to them how new cells were made. They favored the idea that intracellular or extracellular matter crystallizes somehow into new cells (1, 2). Schleiden thought that new nuclei form without any relationship to preexisting ones. In essence, the views of Schwann and Schleiden on how new cells are made fell squarely into the realm of the spontaneous generation of life from nonliving matter, which was a popular theory at the time. Until Louis Pasteur put an end to spontaneous generation with his unambiguous col de cygnet (swan neck flask) experiment in 1859.
Explicit descriptions of cell division and the concept of new cells arising from preexisting ones came from Robert Remak (1, 2). Remak reached these conclusions from observations in multiple contexts, from red blood cells of chicken embryos to frog eggs immediately after fertilization (1). The splitting of eggs after fertilization had been observed by others before. But Remak's histological manipulations allowed him to visualize the membrane of the egg, and follow the origin of the embryonic cells from the fertilized egg. The continuity of all the cells in the embryo from one fertilized ancestor was now a settled issue. This placed cell division and the cell cycle as the basis of how multicellular organisms develop. Remak's data fit nicely with Pasteur's that there is no spontaneous generation of life. The notion that new cells arise only from cell division was opposed by many, including Rudolf Virchow. But Virchow changed his tune later. Virchow's famous aphorism omnis cellula e cellula (all cells arise only from preexisting cells) encapsulated succinctly and popularized a key part of the cell theory, a pillar of modern biology. Together with the other pillar of modern biology, Darwin's theory of evolution, we arrive at a stunning and profound conclusion: Starting with a single cell, all life that ever was, is, and will be on this planet results from cell division. Spend a moment to reflect on this. If you had any doubt that the topics we will discuss in this book are important, now is the time to put those doubts to rest.
1.1.2 Mitosis
By the mid‐nineteenth century, the cell theory was established, and microscopy was getting better. Several individuals (including Remak) had documented distinct stages of nuclear division, including nuclear elongation in some cases, and nuclear dissolution in others. Wilhelm Hofmeister noticed that the nuclei of plant cells dissolved, but some nuclear material remained, in what he thought were coagulates, which then segregated into the nuclei of daughter cells. Although Hofmeister could not realize the biological role of what he was seeing, his descriptions were remarkably close to the stages of mitosis we recognize today (1). Scientists kept looking and looking under the microscope. They also processed and stained the cells with various techniques and dyes ‐all searching for crisp landmarks. Walther Flemming experimented with basic dyes and fixatives. To this day, pathologists use similar methods to look at cells in tissues. Flemming found that a nuclear substance, which was presumably acidic and negatively charged, stained very strongly with basic, positively charged dyes. He used the term chromatin (from the Greek “colored”) to describe that substance. Flemming used salamander cells, because they were big and had big nuclei. As always, choosing the right experimental system for a research objective can reap enormous rewards.
Imagine you have a simple microscope, and you have figured out how to stain cells, with some nuclear substance being intensely stained. Assuming that it takes several hours to days for typical proliferating animal cells to divide, what would you expect to see? For the most part, not much. Each cell would be very similar to others and to itself, from its birth until it divides. Shortly before division, Flemming noticed that the colored nuclear substance was organized into threadlike structures (the Greek word for a thread is μίτος/mitos), which were then distributed into the daughter cells, in a process he called mitosis (Figure 1.1).
The nuclear threads/filaments were named chromosomes a few years later by Waldeyer. Unlike others that had also seen thread‐like structures forming and segregating, Flemming was the first to discover their splitting during mitosis lengthways (1). Flemming imaged and documented the complete series of events during mitosis, a fundamental cellular process, in all its glory: first, chromosomes appear, becoming denser and more compact over time, while at the same time the nuclear membrane disappears in most animal and plant cells (prophase); second, the compact chromosomes reach an equilibrium position at an “equator” position (metaphase); third, the chromosomes split lengthways and move toward the opposite poles of the cell (anaphase); fourth, the chromosomes arrive to the poles and the daughter nuclei appear, with reconstituted nuclear membrane around them (telophase).
Figure 1.1 Illustration of mitotic cells.
Source:(3) Walther Flemming (1882), F.C.W. Vogel.
Figure 1.2 The duration of the interphase is much longer than the mitotic stages in the cell cycle. In most cells, interphase is devoid of any morphological landmarks.
Now we can divide the cell cycle into two phases. The relatively short phase of mitosis (“M” phase) starts when threads appear in the nucleus and ends when two nuclei appear (Figure 1.2). Mitosis is typically followed very quickly with cytokinesis when the cell's cytoplasm segregates to two daughter cells, each with a nucleus. The rest of the cell cycle, which in most cells lasts a lot longer than the M phase, is called the interphase (Figure 1.2). During interphase nothing much appears to be happening. The dramatic visual landmarks of mitosis are hard to miss and continue to guide research about when and how the nucleus divides. But it would be many decades later, in the middle of the twentieth century, when interphase landmarks were discovered. Until then, scientists followed what they had, the mysterious chromosomes.
1.1.3 The Chromosome Theory of Heredity
When the chromosomes were first seen, their role was unknown. That the chromosomes carry the genetic information would not be clearly formulated until 1903 from Walter Sutton, and at about the same time from Theodor Boveri. The chromosome theory of heredity would be proven beyond doubt in the first decades of the twentieth century. Mendel's laws, which by 1900 were rediscovered, were abstract and could apply to any substance that carried the genetic information. As we will discuss later, the chromosomes do behave according to Mendel's laws. But what was the evidence that led to the idea that chromosomes carry the genetic information?
It was becoming more apparent that the nucleus, and not the cytoplasm, carried the genetic information. The unambiguous pieces of evidence for the role of the nucleus in carrying the genetic information came from fertilization experiments. Several individuals had already shown that the male and female parents make equal genetic contributions to the zygote, even though the egg is usually much bigger than the sperm. But these observations do not necessarily exclude cytoplasmic contributions to heredity. Until Eduard Strasburger showed in 1884 that during orchid fertilization, it is only the nucleus and not the cytoplasm (he coined the term) that is forced out at the end of the pollen tube into the embryo sac (1). Suppose both parents make equal genetic contributions to the zygote, but only the male parent's nucleus makes it into the embryo. In that case, the nucleus must carry the genetic information.
What nuclear component could be the carrier of genetic information? There was nothing better to look at than the chromosomes. If chromosomes carry the genetic information, then their continuity from generation to generation must be preserved. Again, the choice of the experimental system was critical. If you want to follow chromosomes, pick a system where you can track them easily. Edouard van Beneden chose Ascaris (a genus of roundworms) with only four (and sometimes just two), large, and easily distinguishable chromosomes, where he could follow the male and female‐derived chromosomes in the zygote after fertilization. In 1883 he stated that each parent's chromosomes never mix and maintain their identity in the zygote and possibly in all the nuclei derived from it as the embryo develops (1). Boveri used the same system, Ascaris univalens, and extended van Beneden's observations to show that the number, morphology, and identity of each chromosome is maintained, division after division. Although in interphase the chromosomes are not visible, they are not destroyed. William Sutton made similar observations by looking at grasshopper cells, noting that each of their 11 pairs of chromosomes was morphologically distinct (4). In diploid organisms, two sets of chromosomes are brought together at fertilization, and they are inherited by all the cells of the new individual. It is only in the germinal cells through meiosis that the haploid state is generated again from the diploid state (1). Both Sutton and Boveri noted that their data about the constancy of chromosomes fit very nicely with Mendel's rules of inheritance: Genetic traits are stable from generation to generation, come in pairs (in diploid organisms), with each pair separating in gametes (during meiosis), but each pair segregating in gametes independently of how the other pairs segregate (because different chromosome pairs orient at random during division). All the evidence pointed to the direction that chromosomes and their behavior constitute the physical basis of Mendel's laws. Still, however, the evidence was mostly indirect. What was needed to unambiguously test the Sutton–Boveri chromosome theory was evidence that specific genetic traits localize to specific chromosomes.
Looking at a random gathering of people, say in a classroom, there are many traits one could identify. However, most show a broad distribution among individuals (e.g., height, weight, coloration of hair or skin, etc.), which could complicate their study. But one trait is unmistakably distributed in a precise pattern: a one‐to‐one male‐to‐female ratio. Could gender be linked to a particular chromosome? Nettie Stevens answered precisely that, looking at cells from Tenebrio beetles, which have a small Y chromosome that is easily distinguished from the larger X. Stevens saw that in Tenebrio (and as we know now in people too) the female was XX and the male XY (5). Somehow, Stevens' observation did not receive the attention it deserved. Thomas Hunt Morgan and his students developed Drosophila as a genetic system. They showed that specific mutant phenotypes are always associated with the transmission of particular chromosomes (6). Those triumphant and elegant Drosophila experiments have a special place in the history of genetics. They “sealed the deal” on the chromosome theory of heredity, and some are still standard laboratory exercises in undergraduate genetics courses.
