2. In prokaryotic cells, reproduction is simple, because
prokaryotic cells possess a single molecule.
In eukaryotic cells, multiple chromosomes must be
copied and distributed to each of the new cells, and
so cell reproduction is more complex.
Cell division in eukaryotes takes place through
mitosis and meiosis
For any cell to reproduce successfully, three
fundamental events must take place:
(1) its genetic information must be copied,
(2) the copies of genetic information must be
separated from each other, and
(3) the cell must divide.
Cell division
3. Eukaryotic chromosomes: Eukaryotic chromosomes
are separated from the cytoplasm by the nuclear
envelope.
The nucleus has a highly organized internal
scaffolding called thenuclear matrix.
This matrix consists of a network of protein fibers
that maintains precise spatial relations among the
nuclear components and takes part in DNA
replication, the expression of genes, and the
modification of gene products before they leave the
nucleus.
Each eukaryotic species has a characteristic number
of chromosomes per cell: potatoes have 48
chromosomes, fruit flies have 8, and humans have
46.
two sets is a consequence of sexual reproduction:
4.
5. Each chromosome in one set has a corresponding
chromosome in the other set, together constituting
a homologous pair
Human cells, for example, have 46 chromosomes,
constituting 23 homologous pairs.
The two chromosomes of a homologous pair are
usually alike in structure and size, and each carries
genetic information for the same set of hereditary
characteristics (traits).
For example, if a gene on a particular chromosome
encodes a characteristic such as hair color, another
copy of the gene (each copy is called an allele) at
the same position on that chromosome’s homolog
also encodes hair color.
the DNA molecules in eukaryotic chromosomes are
highly folded and condensed
6.
7. functional chromosome has three essential elements:
a centromere, a pair of telomeres, and origins of
replication.
The centromere is the attachment point for spindle
microtubules, which are the filaments responsible for
moving chromosomes during cell division
The centromere appears as a constricted region.
Before cell division, a protein complex called the
kinetochore assembles on the centromere; later spindle
microtubules attach to the kinetochore.
On the basis of the location of the centromere,
chromosomes are classified into four types:
metacentric, submetacentric, acrocentric, and
telocentric
One of the two arms of a chromosome (the short arm
of a submetacentric or acrocentric chromosome) is
8.
9.
10. Telomeres are physical ends, the tips, of a linear
chromosome; they serve to stabilize the
chromosome ends.
Telomeres contain repeated nucleotide sequences
that enable the ends of chromosomes to be
efficiently replicated.
Telomeres also perform another function: the
repeated telomere DNA sequences, together with
the regions adjoining them, form structures that
protect the end of the chromosome from being
mistaken by the cell for a broken DNA molecule in
need of repair
Telomeres act as protective caps to chromosome
ends, preventing end-to-end fusion of chromosomes
and DNA degradation resulting after chromosome
breakage.
Telomeres provide chromosome stability.
11. Nonhistone proteins make complexes with telomeric
DNA to protect the ends of chromosomes from
nucleases located within the cell.
The telomeric region also plays a role in synapsis during
meiosis.
Chromosome pairing appears to be initiated in the
subtelomeric regions
Origins of replication are the sites at which the DNA
replication machinery assembles to initiate replication.
They are found some 30-40 kb apart throughout the
length of each eukaryotic chromososme.
prokaryotic chromosomes typically have one origin of
replication
Eukaryotic chromosomes contain many origins of
replication to ensure that the entire chromosome can be
replicated rapidly
12. The cell cycle involves the stages through which it
passes from one division to the next
This process is critical to genetics because, through
the cell cycle, the genetic instructions for all
characteristics are passed from parent to daughter
cells.
A new cycle begins after a cell has divided and
produced two new cells.
Each new cell metabolizes, grows, and develops. At
the end of its cycle, the cell divides to produce two
cells, which can then undergo additional cell cycles.
Progression through the cell cycle is regulated at key
transition points called checkpoints.
Cell cycle and mitosis
13.
14.
15. The cell cycle consists of two major phases.
The first is interphase, the period between cell divisions, in
which the cell grows, develops, and prepares for cell
division.
The second is the M phase (mitotic phase), the period of
active cell division.
The M phase includes mitosis, the process of nuclear
division, and cytokinesis, or cytoplasmic division.
Interphase: Interphase is the extended period of growth
and development between cell divisions.
Interphase includes several checkpoints, which regulate
the cell cycle by allowing or prohibiting the cell’s division.
These checkpoints, like the checkpoints in the M phase,
ensure that all cellular components are present and in
good working order before the cell proceeds to the next
stage.
Checkpoints are necessary to prevent cells with damaged
16. Interphase is divided into three phases:
G1, S, and G2
Interphase begins withG1 (gap 1).
In G1, the cell grows, and proteins necessary for cell
division are synthesized; this phase typically lasts several
hours.
There is a critical point termed theG1/S checkpoint near
the end of G1. The G1/S checkpoint holds the cell in G1
until the cell has all of the enzymes necessary for the
replication of DNA.
After this checkpoint has been passed, the cell is
committed to divide.
Before reaching the G1/S checkpoint, cells may exit from
the active cell cycle in response to regulatory signals and
pass into a nondividing phase called G0, which is a stable
state during which cells usually maintain a constant size.
17. They can remain in G0 for an extended period of time,
even indefinitely, or they can reenter G1 and the active
cell cycle.
Many cells never enter G0; rather, they cycle
continuously
After G1, the cell enters the S phase (for DNA
synthesis), in which each chromosome duplicates.
Although the cell is committed to divide after the G1/
S checkpoint has been passed, DNA synthesis must
take place before the cell can proceed to mitosis.
If DNA synthesis is blocked (by drugs or by a mutation)
, the cell will not be able to undergo mitosis.
Before the S phase, each chromosome is composed
of one chromatid; after the S phase, each
chromosome is composed of two chromatids
18. After the S phase, the cell entersG2 (gap 2). In this
phase, several additional biochemical events
necessary for cell division take place.
The importantG2/M checkpoint is reached near the
end of G2.
This checkpoint is passed only if the cell’s DNA is
undamaged.
Damaged DNA can inhibit the activation of some
proteins that are necessary for mitosis to take place.
After the G2/M checkpoint has been passed, the
cell is ready to divide and enters the M phase.
Although the length of interphase varies from cell
type to cell type, a typical dividing mammalian cell
spends about 10 hours in G1
19. M phase (mitosis) is a brief interlude in the cell
cycle, usually <1 h in duration.
Throughout interphase, the chromosomes are in a
relaxed state, and individual chromosomes cannot
be seen with the use of a microscope.
20. M phase: The M phase is the part of the cell cycle
in which the copies of the cell’s chromosomes
(sister chromatids) separate and the cell
undergoes division.
The separation of sister chromatids in the M
phase is a critical process that results in a
complete set of genetic information for each of
the resulting cells.
M phase is divided into six stages: the five stages
of mitosis (prophase, prometaphase, metaphase,
anaphase, and telophase), and cytokinesis.
21.
22.
23.
24.
25. During interphase, the chromosomes are relaxed
and are visible only as diffuse chromatin, but they
condense during prophase, becoming visible under a
light microscope.
Each chromosome possesses two chromatids
because the chromosome was duplicated in the
preceding S phase, resulting from DNA replication
plus the histones and other chromosomal proteins
associated with them
The mitotic spindle, an organized array of
microtubules that move the chromosomes in
mitosis, forms.
In animal cells, the spindle grows out from a pair of
centrosomes that migrate to opposite sides of the
cell.
Within each centrosome is a special organelle, the
26. The identical daughter DNA molecules and associated
chromosomal proteins that form one chromosome
are referred to as sister chromatids.
Sister chromatids are attached to each other by
protein cross-links along their lengths.
In vertebrates, these become confined to a single
region of association called the centromere as
chromosome condensation progresses.
During interphase, the portion of the cell cycle
between the end of one M phase and the beginning of
the next, the outer nuclear membrane is continuous
with the endoplasmic reticulum.
With the onset of mitosis in prophase, the nuclear
envelope retracts into the endoplasmic reticulum in
most cells from higher eukaryotes, and Golgi
membranes break down into vesicles.
27. Disintegration of the nuclear membrane marks the start
of prometaphase.
Spindle microtubules, which until now have been
outside the nucleus, enter the nuclear region.
The ends of certain microtubules make contact with
the chromosomes.
a multiprotein complex, the kinetochore, assembles at
each centromere.
The kinetochores of sister chromatids then associate
with microtubules coming from opposite spindle poles
For each chromosome, a microtubule from one of the
centrosomes anchors to the kinetochore of one of the
sister chromatids;
a microtubule from the opposite centrosome then
attaches to the other sister chromatid, and so the
chromosome is anchored to both of the centrosomes.
28. During metaphase, the chromosomes become
arranged in a single plane, the metaphase plate,
between the two centrosomes.
A spindle assembly checkpoint ensures that each
chromosome is aligned on the metaphase plate and
attached to spindle fibers from opposite poles.
During the anaphase period of mitosis, sister
chromatids separate.
29.
30. After the chromatids have separated, each is considered a
separate chromosome.
Telophase is marked by the arrival of the chromosomes at
the spindle poles.
Once chromosome separation is complete, the mitotic
spindle disassembles and chromosomes decondense
during telophase.
The chromosomes relax and lengthen, once again
disappearing from view.
The nuclear membrane reforms around each segregated
set of chromosomes as they decondense, producing two
separate nuclei within the cell.
31. In many cells, division of the cytoplasm (cytokinesis) is
simultaneous with telophase.
The physical division of the cytoplasm, called cytokinesis,
then yields two daughter cells as the Golgi complex re-
forms in each daughter cell.
Following mitosis, cycling cells enter the G1 phase,
embarking on another turn of the cycle.
In multicellular organisms, most differentiated cells “exit”
the cell cycle and survive for days, weeks, or in some
cases (e.g., nerve cells and cells of the eye lens) even the
lifetime of the organism without dividing again.
Suchpostmitotic cells generally exit the cell cycle in G1,
entering a quiescent phase called G0.
Some G0 cells can return to the cell cycle and resume
replicating; this reentry is regulated, thereby providing
control of cell proliferation.
32. Genetic Consequences of the Cell Cycle
From a single cell, the cell cycle produces two cells that
contain the same genetic instructions. These two cells are
genetically identical with each other and with the cell that gave
rise to them.
They are genetically identical because DNA synthesis in the S
phase creates an exact copy of each DNA molecule, giving rise
to two genetically identical sister chromatids.
Mitosis then ensures that one chromatid from each replicated
chromosome passes into each new cell.
Second is each of the cells produced contains a full
complement of chromosomes—there is no net reduction or
increase in chromosome number.
Each cell also contains approximately half the cytoplasm and
organelle content of the original parental cell, but no precise
mechanism ensures that organelles are evenly divided.
Consequently, not all cells resulting from the cell cycle are
identical in their cytoplasmic content.
33. Meiosis
•
•
•
•
•
The realization that gametes are haploid came from an
observation that also suggested that chromosomes carry
genetic information.
In 1883, it was discovered in a study of roundworms that the
nucleus of an unfertilized egg and that of a sperm each contain
two chromosomes, whereas the fertilized egg (zygote)
contains four.
This led to the chromosome theory of heredity, which
explained the long-standing paradox that the maternal and
paternal contributions to the character of the progeny seem to
be equal, despite the enormous difference in size between the
egg and sperm
haploid germ cells arise from a special kind of cell division in
which the number of chromosomes is precisely halved.
This type of division, called meiosis—the Greek word for
34.
35. Differences in mitosis and meiosis:
Mitosis consists of a single nuclear division
accompanied by a single cell division.
Meiosis consists of two divisions.
After mitosis, chromosome number in newly
formed cells is the same as that in the original
cell
In meiosis chromosome number in the newly
formed cells is reduced by half.
Finally, mitosis produces genetically identical
cells, whereas meiosis produces genetically
variable cells.
36. meiosis is preceded by an interphase stage that
includes G1, S, and G2 phases
Meiosis consists of two distinct processes:
meiosis I and meiosis II, each of which includes a
cell division.
The first division, meiosis I, is termed the
reduction division because the number of
chromosomes per cell is reduced by half
The second division, meiosis II, is sometimes
termed the equational division.
The events of meiosis II are similar to those of
mitosis
37.
38. In meiosis, (and mitosis), the chromosomes have
replicated their DNA (in meiotic S phase), and the two
copies are tightly bound together bycohesin
complexes along their entire length and are called
sister chromatids
meiosis has to produce gametes with half as many
chromosomes as their diploid precursor cells.
a single round of DNA replication is followed by two
successive rounds of chromosome segregation in
meiosis
In division I of meiosis (meiosis I), by contrast to
mitosis, the duplicated paternal and maternal
homologs (including the two replicated sex
chromosomes) pair up along side each other and
exchange genetic information through the process of
genetic recombination.
39.
40. Homologous chromosomes line up at the equator of the
meiotic spindle, after which the duplicated homologs
rather than the sister chromatids are pulled apart and
segregated into the two daughter cells.
in division II of meiosis (meiosis II), which occurs without
further DNA replication, the sister chromatids are pulled
apart and segregated to produce haploid daughter cells.
In this way, each diploid cell that enters meiosis produces
four haploid cells, each of which inherits either the
maternal or paternal copy of each chromosome, but not
both
During mitosis in most organisms, homologous
chromosomes behave independently of each other.
During meiosis I, however, it is crucial that homologs
recognize each other and associate physically in order for
the maternal and paternal homologs to undergo genetic
recombination and to segregate to different daughter cells
41. Prophase I is a lengthy stage in which the
chromosomes form homologous pairs and crossing
over takes place.
First, the chromosomes condense, pair up, and
begin synapsis, a very close pairing association.
Each homologous pair of synapsed chromosomes
consists of four chromatids called a bivalent or
tetrad.
The chromosomes become shorter and thicker, and
a synaptonemal complex develops between
homologous chromosomes.
Crossing over takes place, in which homologous
chromosomes exchange genetic information.
The progressive juxtaposition of homologs occurs
during prophase 1 which can take hours in yeasts,
days in mice, and weeks in higher plants.
42. It is during early prophase I that the homologs begin
to associate along their length in a process called
pairing, which, occurs initially through interactions
between complementary DNA sequences (called
pairing sites) in the two homologs
As prophase I progresses, the homologs become
more closely juxtaposed, forming a four-chromatid
structure called a bivalent.
43. The replicated chromosomes undergo major
rearrangements within the nucleus during prophase I.
The ends of the chromosomes (thetelomeres) are
tightly bound to the inner surface of the nuclear
envelope.
They are initially distributed diffusely there, but they
then cluster transiently at one spot on the envelope
and, later still, disperse again.
although they are thought to make prophase I faster
and more efficient. One possibility is that they help
prevent chromosome entanglements during
prophase I.
44. The cohesin complexes assemble on the DNA during S
phase and bind the sister chromatids together during
meiosis are major components of the axial core of each
homolog
Some of the cohesin subunits that operate in meiosis are
the same as those that function in mitosis, whereas
others are specific for meiosis.
Both the crossovers and the cohesin complexes play
crucial parts in segregating the homologs during meiotic
division I
45. ◦
◦
◦
◦
◦
◦
Prophase I: it is subdivided into five phases based on
chromosomal behaviour, i.e., Leptotene, Zygotene,
Pachytene, Diplotene and Diakinesis.
leptotene stage: prophase starts with leptotene.
homologs condense and pair, and genetic recombination
begins.
the chromosomes become gradually visible under the light
microscope.
The compaction of chromosomes continues throughout
leptotene.
Zygotene Stage:
chromosomes start pairing together and this process of
association is called synapsis.
Such paired chromosomes are called homologous
chromosomes.
Electron micrographs of this stage indicate that
chromosome synapsis is accompanied by the formation of
complex structure called synaptonemal complex.
46. ◦
◦
◦
◦
◦
◦
the synaptonemal complex begins to assemble in local
regions along the homologs;
The complex formed by a pair of synapsed homologous
chromosomes is called a bivalent or a tetrad.
However, these are more clearly visible at the next stage.
The first two stages of prophase I are relatively short-
lived compared to the next stage
Pachytene Stage:
the assembly process is complete, and the homologs
are synapsed along their entire lengths.
bivalent chromosomes now clearly appears as tetrads.
47.
48. ◦
◦
◦
◦
◦
◦
◦
This stage is characterised by the appearance of
recombination nodules, the sites at which crossing over
occurs between non-sister chromatids of the homologous
chromosomes.
Crossing over is the exchange of genetic material between
two homologous chromosomes.
Crossing over leads to recombination of genetic material on
the two chromosomes.
Recombination between homologous chromosomes is
completed by the end of pachytene, leaving the
chromosomes linked at the sites of crossing over
The pachytene stage can persist for days or longer, until
desynapsis begins
Diplotene Stage:
The beginning of diplotene is recognised by the dissolution/
disassembly of the synaptonemal complex
the tendency of the recombined homologous chromosomes
of the bivalents to separate from each other except at the
sites of crossovers.
49. ◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
These X-shaped structures, are called chiasmata.
chiasmata (singular chiasma), plays a crucial part in holding
the compact homologs together.
In oocytes of some vertebrates, diplotene can last for months
or years.
There is concomitant condensation and shortening of the
chromosomes.
The homologs are now ready to begin the process of
segregation.
Diakinesis:
The final stage of meiotic prophase I is diakinesis.
This is marked by terminalisation of chiasmata.
During this phase the chromosomes are fully condensed and
the meiotic spindle is assembled to prepare the homologous
chromosomes for separation.
By the end of diakinesis, the nucleolus disappears and the
nuclear envelope also breaks down.
Diakinesis represents transition to metaphase.
50. ◦
◦
◦
◦
Metaphase I:
The bivalent chromosomes align on the equatorial. The
microtubules from the opposite poles of the spindle attach
to the pair of homologous chromosomes.
A microtubule from one pole attaches to one chromosome
of a homologous pair, and a microtubule from the other pole
attaches to the other member of the pair.
Anaphase I:
The two chromosomes of a homologous pair are pulled
toward opposite poles.
The homologous chromosomes separate, while sister
chromatids remain associated at their centromeres
51. ◦
◦
◦
◦
◦
◦
Telophase I:
the chromosomes arrive at the spindle poles and the
cytoplasm divides
cytokinesis follows and this is called as diad of cells.
Interkinesis:
The period between meiosis I and meiosis II is
interkinesis, and is generally short lived.
the nuclear membrane re-forms around the
chromosomes clustered at each pole, the spindle
breaks down, and the chromosomes relax.
Although in many cases the chromosomes do undergo
some dispersion, they do not reach the extremely
extended state of the interphase nucleus.
Interkinesis is followed by prophase II, a much simpler
prophase than prophase I.
52. One fundamental difference between meiosis I and
mitosis (and meiosis II) is that in meiosis I homologs
rather than sister chromatids separate and then
segregate into the two daughter cells.
This difference depends on three features of meiosis I
that distinguish it from mitosis
thekinetochores (protein complexes associated with
the centromeres) on the two sister chromatids of a
homolog attach to microtubules emanating from the
same pole of the meiosis I spindle and thus segregate
together into the same daughter cell at anaphase I;
this contrasts with mitosis (and meiosis II), in which
the kinetochores on the two sister chromatids of a
chromosome attach to opposite poles of the spindle
and therefore segregate into different daughter cells
at anaphase.
53. ◦
◦
◦
◦
◦
◦
Meiosis II: In contrast to meiosis I, meiosis II resembles a
normal mitosis.
Prophase II:
Meiosis II is initiated immediately after cytokinesis,
usually before the chromosomes have fully elongated
and the events of interkinesis are reversed.
the chromosomes recondense and again become
compact, the spindle reforms, and the nuclear
envelope once again breaks down.
The nuclear membrane disappears by the end of
prophase II.
In interkinesis in some types of cells, the
chromosomes remain condensed, and the spindle
does not break down.
These cells move directly from cytokinesis into
metaphase II
Metaphase II:
At this stage the chromosomes align at the equator
and the microtubules from opposite poles of the
spindle get attached to the kinetochores of sister
chromatids.
54. ◦
◦
◦
◦
◦
Anaphase II:
It begins with the simultaneous splitting of the
centromere of each chromosome (which was holding
the sister chromatids together)
the kinetochores of the sister chromatids separate and
the chromatids are pulled to move toward opposite
poles of the cell.
Each chromatid is now a distinct chromosome
telophase II:
the chromosomes arrive at the spindle poles, a nuclear
envelope re-forms around the chromosomes, and the
cytoplasm divides.
The chromosomes relax and are no longer visible.
55.
56.
57. Meiosis comprises two divisions; so each original cell
produces four cells
Chromosome number is reduced by half; so cells
produced by meiosis are haploid.
Cells produced by meiosis are genetically different
from one another and from the parental cell.
Genetic differences among cells result from two
processes that are unique to meiosis.
The first is crossing over, which takes place in
prophase I.
Crossing over refers to the exchange of genes
between nonsister chromatids (chromatids from
different homologous chromosomes).
Consequences of meiosis
58. After crossing over has taken place, the sister
chromatids may no longer be identical.
Crossing over is the basis for intrachromosomal
recombination, creating new combinations of alleles
on a chromatid.
The second process of meiosis that contributes to
genetic variation is the random distribution of
chromosomes in anaphase I of meiosis after their
random alignment in metaphase I.
