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1. Cell Division
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Cell division is the basis for all forms of organismal reproduction.
Single-celled organisms divide to reproduce. Cell division in
multicellular organisms produces specialized reproductive cells,
such as egg and sperm.
In order for a cell to divide, the genome must also divide, so, in all
types of cell division in all organisms, DNA replication precedes
cell division.
Cell division can be grouped into asexual and sexual cell division.
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
2. Types of cell division
In prokaryotes there is only one simple type
of cell division, which produces two
identical daughter cells from one progenitor
cell (asexual cell division).
Eukaryotes also show asexual cell division;
this converts a single fertilized egg cell, a
zygote, into a multicellular organism, or a
single unicellular organism into a population
or a colony.
The asexual cell division in eukaryotes is
called mitosis (M). Both haploid (n) and
diploid (2n) cells can divide asexually.
The sexual cell division in eukaryotes is
called meiosis (Mei) and occur in specialized
cells, the meiocytes, which divides twice,
resulting in four haploid cells called a tetrad.
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
3. Life cycles of humans, plants, and fungi
In humans and many
plants, three cells of the
meiotic tetrad abort.
The abbreviation n
indicates a haploid cell,
2n a diploid cell; gp
stands for
gametophyte, the small
structure of haploid
cells that will produce
gametes.
In flowering plants the gametophyte stage is radically reduced, but in others (such as
mosses) the gametophyte is the main vegetative stage
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
4. G1, S, G2, M
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Cells spend most of their life in interphase, the period between nuclear divisions, and
comparatively little time in mitosis. Interphase is divided into three stages, G1, S, and G2.
In G1cells are growing and synthesizing the materials necessary for their proper functioning.
The cells are "doing their thing", so if they are nose cells they are producing mucus, if they
are muscle cells they are contracting and relaxing, etc. Some cells, such as our nerve cells
(neurons) and red blood cells, never leave this stage and it is then called G0. A cell which is
in the G0 stage will not divide. It will not grow, either, but will continue to function until it
dies.
Cells which will divide pass through a specific phase in
G1 which acts as a gateway into the S stage. Once cells
pass this "point of no return" they will proceed through
S, G2 and mitosis. S is the stage when DNA synthesis
(chromosome replication) occurs. The chromosomes
consist of two identical strands once replication is
completed. Each of these strands is called a chromatid.
During mitosis the chromatids will separate and each
chromatid will become a separate chromosome.
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
5. Mitosis
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Mitosis (M) is usually the shortest segment of the cell cycle, lasting for
approximately 5 to 10 percent of the cycle. DNA synthesis takes place during the S
period. G1 and G2 are gaps between S and M. Together, G1, S, and G2 constitute
interphase, the time between mitoses. (Interphase used to be called "resting period";
however, cells are active in many ways during interphase, not the least of which, of
course, is DNA replication.) The chromosomes cannot be seen during interphase,
mainly because they are in an extended state and are intertwined with one another
(chromatine).
For the sake of study, biologists divide mitosis into four stages called prophase,
metaphase, anaphase, and telophase. It must be stressed, however, that any nuclear
division is a dynamic process on which we impose such arbitrary stages only for our
own convenience.
In each of the resultant daughter cells, the chromo-some complement is identical
with that of the original cell. Of course, what were referred to as chromatids now
take on the role of full-fledged chromosomes in their own right.
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
6. The four stages of mitosis
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Prophase
Metaphase
Anaphase
Telophase
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
7. Prophase
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The onset of mitosis is characterized by the chromosomes becoming distinct. They
get progressively shorter through a process of contraction, or condensation, into a
series of spirals or coils; the coiling produces structures that are more easily moved
around.
As the chromosomes become visible, they appear double-stranded, each
chromosome being composed of two longitudinal halves, the sister chromatids. The
two chromatids formed by one chromosome each contain one of the replicated DNA
molecules.
Because of semiconservative replication these replicate DNA molecules are each
"half old and half new"; that is, in each double helix one of the nucleotide strands is
newly polymerized. These sister chromatids are joined at a region called the
centromere. At this stage the centromere has already divided into a pair of sister
centromeres.
The nuclear membrane begins to break down, and the nucleoplasm and cytoplasm
become one.
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
8. Metaphase
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At this stage, the nuclear spindle becomes
prominent. The spindle is a birdcage-like
structure that forms in the nuclear area; it
consists of a series of parallel proteinaceous
fibers that point to each of two cell poles. These
spindle fibers are polymers of a protein called
tubulin. The chromosomes move to the
equatorial plane of the cell, where one sister
centromere becomes attached to a spindle fiber
from one pole; the other sister centromere, to
the other pole.
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
9. Anaphase and telophase
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Anaphase begins when the pairs of sister chromatids separate, one of a
pair moving to each pole. The centromeres, which now clearly appear
to have divided, separate first. As each chromatid moves, its two arms
appear to trail its centromere; a set of V-shaped structures results, with
the points of the V's directed at the poles.
At telophase, a nuclear membrane re-forms around each daughter
nucleus, the chromosomes uncoil, and the nucleoli reappear,
effectively re-forming the interphase nuclei. By the end of telophase,
the spindle has dispersed, and the cytoplasm has been divided into two
by a new cell membrane.
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
10. Cells in active proliferation
Longitudinal section of
an onion root tip (apical
meristem) x40.
The apical meristem x400. Most of the cells even in this area of
active cell division are at interphase. Those with visible
chromosomes are at some stage of mitosis.
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
11. Meiosis: Prophase 1
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Meiosis consists of two nuclear divisions, distinguished as meiosis I and meiosis II. The
events of meiosis I are quite different from those of meiosis II, and the events of both differ
from those of mitosis. Each meiotic division is formally divided into prophase, metaphase,
anaphase, and telophase.
PROPHASE I. The chromosomes become visible as long, thin single threads (which are
composed of pairs of replicated DNA molecules), and continue contracting during the entire
stage. Homologous chromosomes form pairs (this does not happen in Mitosis); each
chromosome has a pairing partner, and the two become progressively paired, or synapsed,
along their lengths. Thus, the number of homologous pairs of chromosomes in the nucleus is
equal to the haploid number n. The beadlike chromomeres align precisely in the paired
homologs, producing a distinctive pattern for each pair. Since each member of a homologous
pair produces two sister chromatids, the synapsed structure now consists of a bundle of four
homologous chromatids, the tetrad. At this stage, cross-shaped structures called chiasmata
(singular, chiasma) appear between nonsister chromatids. Each homologous group of four
generally has one or more chiasmata. Chiasmata are the visible manifestations of events
called crossovers. A crossover is a precise breakage, swapping, and reunion between two
nonsister chromatids.
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
12. Meiosis: Metaphase I, Anaphase I, Telophase I
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METAPHASE I. The nuclear membrane and nucleoli disappear, and each pair of
homologs takes up a position in the equatorial plane. The sister centromeres do not
appear to have divided, so they act as one. This apparent lack of division
represents a major difference from mitosis. The two nonsister centromeres attach
to spindle fibers from opposite poles.
ANAPHASE I. Homologous chromosomes move directionally to opposite poles.
This is the stage at which haploid nuclei are formed.
TELOPHASE I. In many organisms, this stage do not exist, no nuclear membrane
re-forms, and the cells proceed directly to meiosis II. In other organisms, telophase I
and the interkinesis are brief in duration; the chromosomes elongate and become
diffuse, and the nuclear membrane re-forms. In any case, there is never DNA
synthesis at this time, and the genetic state of the chromosomes does not change.
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
13. Meiosis I Stages of Trillium erectum
Late prophase I
Metaphase I
Anaphase I
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
14. Meiosis II Stages of Trillium erectum
Metaphase II
Anaphase II
The centromers have
separated, half chromosomes
are drawn to the poles
Early telophase II.
The nuclei are haploid, the
chromosomes single-stranded
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
15. Comparing mitosis with meiosis
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
16. Inheritance Patterns of Individual Genes (1)
1) MITOSIS
We will use the alleles A and a as "typical" alleles of a gene. We can represent gene duplication and
segregation as follows:
Haploid cells can be of
Genotype A
genotype A or a, and the
diploids can be
homozygous, A/A and
Genotype a
a/a, or heterozygous, A/a.
Because each of the
chromosomes is
Genotype A/A replicated faithfully, the
genotypes of the daughter
cells must be identical
Genotype A/a with the progenitor.
Genotipe A/a
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini
17. Inheritance Patterns of Individual Genes (2)
2) MEIOSIS
Meiosis in homozygous meiocytes can produce only one genotype in the four haploid
products of meiosis (the tetrad), whereas starting with a meiocyte of genotype A/a,
meiosis produces four haploid cells, of which half are A and half are a, as follows:
The reason for this is that
in the A/a meiocyte, the A
chromosome produces a
pair of sister chromatids
Genotype A/A A/A, and the homologous
chromosome produces a
pair a/a. These four
copies of the gene end up
in the four meiotic
Genotype A/a product cells. This result,
was first observed by
Mendel.
Genotipe A/a
Genetica per Scienze Naturali
a.a. 06-07 prof S. Presciuttini