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 Chromosomes, the Cell Cycle, and Cell Division

Chapter 4 provided a definition of a cell as a self-replicating structure. Chapter 9 moves from biochemistry and cell structure to the processes and controls of cell division. The chapter focuses on eukaryotic cell division and starts with the cell cycle and an example of some of the molecules that control cell division. This leads into a detailed description of mitosis, then meiosis.
chapter outline

Read the chapter, not just the summary.
Things are presented in an order different from that used in lecture.
READINGS: Chap. 9: 193-212
Genetic information (genes) lies on long
DNA polymers called chromosomes
Prokaryotic chromosomes are
Almost universally circular (Fig. 9.2)
Single molecules
Largely free of attached proteins
Localized to a nucleoid
Following replication of the chromosome,
prokaryotes undergoe fission (Fig. 9.3)
Mitochondria and plastids behave like
their prokaryotic ancestors
Eukaryotic DNA is
Linear (Fig. 9.6)
Very long (10 Mb ó 10 Gb)
In multiple chromosomes
Complexed with many proteins
Located in the nucleus
The primary structure is provided by the
DNA helix itself (~2 nm)
The DNA is complexed with proteins that
give it higher order structure (Fig. 9.8)
The most obvious are nucleosomes
(~10 nm) containing histones
H2-H5 form a core, H1 is a linker
Nucleosomes are packed in a hexagonal array
termed a solenoid (~30 nm)
Solenoids can form chromatin fiber
(~200 nm)
As a general rule, eukaryotic cells possess
chromosomes in pairs, which are
called homologs
Homologs are similar, but not identical
Organisms can have other numbers of each
chromosome set:
Haploid = one of each
Diploid = two of each
Polyploid = more than two. e.g.,
Triploid = three of each
Tetraploid = four of each
In general, every cell in an organism has a
complete set of chromosomes
Therefore, before a cell divides, it must
replicate (duplicate) its chromosomes
The set of events that defines the life of a cell
is called the cell cycle (Fig. 9.4)
The cell cycle consists of four main stages:
S when new DNA is synthesized
G1 the period before S (Gap 1)
G2 the period after S (Gap 2)
M mitosis and cytokinesis
Mitosis is the distribution of replicated
chromosomes to two new nuclei
Cytokinesis is the physical separation of one
binucleate cell into two cells
Cells usually spend most of their time in G1,
although cells that are not dividing at all
may enter G0
The cell cycle is regulated by the interaction
between cyclin and cyclin-dependent kinases
(Fig. 9.5)
In the presence of cyclin, a mitosis-
promoting kinase
activates proteins promoting mitosis
inactivates those inhibiting mitosis
All eukaryotes seem to have the same
system, suggesting a common origin
During S phase each chromosome in the cell
is replicated
Replicated chromosomes are joined at
a point called the centromere
The two copies of a chromosome are
called sister chromatids
At mitosis, each of the two new nuclei
receives one of each sister chromatid
Mitosis is a continuous process that can be
characterized by four phases (9.9):
Prophase chromosome preparation
Metaphase chromo. arrangement
Anaphase chromosome separation
Telophase return to interphase
Interphase is the time between mitoses
During prophase:
Chromosomes condense
The nuclear envelope breaks down
Microtubules develop from microtubule-
organizing centers (MTOC) at the cellís poles
(Fig. 9.10)
In animals, the MTOC surrounds the
centrioles, and asters are formed
Some microtubules attach to regions of the
centromeres that are called kinetochores
The kinetochore of one sister
chromatid attaches to microtubules
leading to one pole, while the other
kinetochore links to the other pole
The entire microtubule array is called the
mitotic spindle
During metaphase:
All of the chromosomes align at the
metaphase plate
The chromosomes are maximally
During anaphase:
The centromeres separate, and sister
chromatids become chromosomes
The chromosomes disjoin and migrate
to the cellís poles
During telophase:
The chromosomes decondense
The microtubules disassemble
The nuclear envelope reforms
The cell then physically divides (Fig. 9.11)
In animals, the cell is pinched in two
by a contractile ring
In plants, the cell is cut in two by the
formation of a cell plate
The result is two genetically identical cells
Mitochondria and chloroplasts replicate their
DNA and divide during interphase
In multicellular organisms the population
of organelles segregates randomly
during mitosis and cytokinesis
When organisms (or cells) reproduce
asexually, they simply produce copies
These copies are called clones
Bacteria, some fungi, algae and plants
All eukaryotes at some point reproduce
sexually, yielding non-clonal progeny
Special cells from each parent, called
gametes, fuse to form a zygote
In plants and animals, the gametes
are egg and sperm
Gametes have half of the chromosome
complement of the parent organism
The normal chromosome complement is
termed "2n", and thus sex cells are "n"
The way chromosome number is halved is
through meiosis
Meiosis is similar to mitosis except that:
There is only one replication, but two
divisions, yielding a haploid cell
Which homolog of a pair goes to
which cell is random
The two nuclear and cellular divisions are
referred to as meiosis I and meiosis II
In contrast to mitotic progeny cells, each
meiotic progeny cell:
Is unique
Contains only one chromosome of
each and every pair
Prophase I is the most different in meiosis
(Figs. 9.15, 9.18)
Chromosome condensation begins
(= mitosis)
Maternal and paternal homologs
synapse to form tetrads
The homologs are fastened together
by the synaptonemal complex
Non-sister chromatids break and join
each other, termed crossing over
This results in chromosomal
recombination (Figs. 9.16; 9.17)
Homologous chromosomes separate
slightly, held together by chiasmata
The nuclear envelope breaks down
(= mitosis)
During metaphase I:
The kinetochores of sister chromatids
become attached to microtubules
from the same pole
Tetrads line up at the metaphase plate
During anaphase I:
Homologous chromosomes go to
opposite poles
Each chromosome is still two sister
During and after telophase I:
Chromosomes may decondense
Nuclear envelope may reform
Cytokinesis usually occurs
Interkinesis may occur
Meiosis II is very similar to mitosis except
The cells that are dividing are n
The sister chromatids are not identical
due to crossing over
Therefore, meiosis
yields four genetically different haploid
instead of two genetically identical
diploid cells
Fertilization and meiosis alternate
(Fig. 9.13)
Meiosis does not necessarily yield gametes
In animals, the only n cells are gametes
In other eukaryotes, meiosis may be
followed by mitotic divisions
Plants and fungi have a wide variety of
life cycles
Some have very short n periods
Some spend most of their time as n
Many are in between
Meiosis doesnít always work right
First studied and understood were
chromosomal aberrations, e.g.
Incorrect number of individual
chromosomes (aneuploidy;
polyploidy is lethal)
Usually occurs by nondisjunction
(Fig. 9.19)
In hymans almost all aneuploidy is lethal at
the embryo or fetal stage, except:
Trisomy 21 leads to Down syndrome
Mental and physical retardation
Abnormalities of the face, eyelids,
tongue, hands, stature, etc.
Susceptibility to Alzheimer's
More common as the age of the
mother increases
Key terms

Chromosome A contiguous piece of DNA and its associated proteins in the cell.
Mitosis Exact duplication of a eukaryotic cell.
Interphase A stage in the life of a cell. Most cells are in interphase for an extended period of time. It is important to point out that all of the processes discussed to date are occurring in interphase cells.
Cell cycle The sum of the events occurring in the life of a cell. Defined by several phases of varying length. Three of these phases, S, G1, and G2, collectively form interphase, and the final phase, M, is the cell division phase.
Cyclin-dependent kinase A group of proteins that control the timing of each stage of the cell cycle. These kinases are only active when present in a complex with other proteins called cyclins. This is an example of an allosteric type of activation, with the cyclin playing the role of the regulatory subunit.
Growth factors
Chromatids The subunit structure of a chromosome during cell division. Formed following the duplication of DNA during S phase. Students continually confuse chromosomes and chromatids.
Centromere A specialized structure that joins the two chromatids prior to division.
Histone proteins
Centrosomes Commonly confused with centromeres. Regions of the cell where the polymerization of the microtubules necessary for the spindle is initiated. Also referred to as mitotic centers. May or may not contain centrioles.
Prophase The first stage of mitosis. Characterized by the appearance of chromosomes and the mitotic spindle. Late in prophase, chromosomes attach to the spindle.
Mitotic center
Polar microtubules Microtubules that run from one centrosome and interdigitate with the microtubules emanating from the other centrosome.
Spindle The microtubule-based mitotic structure involved in positioning and locomotion of chromosomes.
Kinetochores Specialized regions of the centrosome where kinetochore microtubules attach. One is found on each chromatid.
Prometaphase A stage of mitosis characterized by the dissolution of the nuclear envelope and chromosome movement to the center of the cell.
Kinetochore microtubules Microtubules that emanate from the centrosome and attach to one of the two kinetochores. The two kinetochores on a pair of chromatids are each attached to separate kinetochore microtubules emanating from opposite centrosomes.
Equatorial plate, metaphase plate Position in the cell where the chromosomes are located during metaphase. One half of the distance between the centrosomes.
Anaphase A stage of mitosis characterized by chromatid movement toward each of the poles. During anaphase, (1) the kinetochore microtubules shorten while the chromosomes move along them, while (2) the polar microtubules slide across each other to increase the distance between the poles, and hence the overall separation of the chromatids
Telophase The final phase of mitosis. Characterized by disintegration of the spindle, decondensing of the DNA, and formation of two separate nuclei.
Cytokinesis Division of the cytoplasm of the cell into two cells, each containing one nucleus.
Asexual reproduction
Clone A group of identical cells all derived from one ancestral cell.
Sexual reproduction
Gamete Sexual cell that contains a single set of chromosomes or the haploid (1/2) amount of DNA in an organism.
Zygote The product of fertilization, the fusion of two complementary gametes.
Meiosis Process that produces sexual cells in eukaryotes.
Spores A quiescent resting stage in plants and fungi, usually haploid.
Homologous pair Two identical chromosomes found in a diploid organism. One is derived from each parent.
Meiosis I The first of two nuclear divisions during meiosis.
Synapsis Pairing of homologous chromosomes during meiosis I.
Meiosis II The second nuclear division during meiosis. The events are similar to mitosis, but the amount of DNA is decreased by one half.
Prophase I A stage in meiosis distinctly different from mitosis. During this phase, homologous chromosomes synapse. When the chromosomes synapse, genetic material is exchanged between the chromatids of homologous chromosomes. These events do not happen during mitosis.
Prometaphase I This stage of meiosis I is characterized by the formation of the spindle and by nuclear dissolution. Also, only one kinetochore forms per chromosome. This is a key point for your students.
Metaphase I Homologous chromosomes line up at the equatorial plate.
Anaphase I A stage in meiosis characterized by the movement of entire chromosomes to the poles, and not chromatids as in mitosis. It is this division that changes the daughter cells from diploid to haploid.
Telophase I If present, a reformation of the nuclei and cell division. Followed by interkinesis, a phase analogous to interphase in mitosis.
Nondisjunction The result of uneven partitioning of the chromosomes into daughter nuclei. When a chromosome is absent, the condition is aneuploidy, and when an extra chromosome is present the condition is referred to as trisomy.
Apoptosis A cell-signaling-dependent series of events that lead to death of that cell.
chapter cross-references

A number of other chapters are referred to in Chapter 9. Below are the chapters referred to and the corresponding page number(s) where the reference is found in Chapter 9.
Chapter Page(s)
4 The Organization of Cells 201, 202, 203
10 Transmission Genetics: Mendel and Beyond 205, 211
11 DNA and Its Role in Heredity 203
12 From DNA to Protein: Genotype to Phenotype 198
14 The Eukaryotic Genome and Its Expression 198, 200
15 Development: Differential Gene Expression 213
17 Molecular Biology and Medicine 197
26 Protists and the Dawn of Eukarya 204
Thought Questions
1. Why does a triploid plant make seedless fruit?
2. Did meiosis evolve from an error in mitosis?