Concept 12.2: The mitotic phase alternates with interphase in the cell cycle Flashcards
In 1882, a German anatomist named ____________________ developed dyes that allowed him to observe, for the first time, the behavior of chromosomes during mitosis and cytokinesis.
Walther Flemming
In fact, Flemming coined the terms
mitosis and chromatin
includes both mitosis and cytokinesis, is usually the shortest part of the cell cycle
mitotic (M) phase
The mitotic phase alternates with a much longer stage called
interphase
interphase often accounts for about
90% of the cycle
Interphase can be divided into three phases:
the G1 phase (“first gap”), the S phase (“synthesis”), and the G2 phase (“second gap”)
the first gap, or growth phase, of the cell cycle, consisting of the portion of interphase before DNA synthesis
G1 phase
the synthesis phase of the cell cycle; the portion of interphase during which DNA is replicated
S phase
the second gap, or growth phase, of the cell cycle, consisting of the portion of interphase after DNA synthesis occurs
G2 phase
The G phases were misnamed as “gaps” when they were first observed because the cells appeared inactive, but we now know that intense metabolic activity and growth occur throughout
interphase
During all three phases of interphase, in fact, a cell grows by producing proteins and cytoplasmic organelles such as
mitochondria and endoplasmic reticulum
Duplication of the chromosomes, crucial for eventual division of the cell, occurs entirely during the
S phase
a cell grows (G1), continues to grow as it copies its chromosomes (S), grows more as it completes preparations for
cell division (G2), and divides (M).
The daughter cells may then
repeat the cycle.
watch animation the cell cycle
A particular human cell might undergo one division in
24 hours
Of this time, the M phase would occupy less than 1 hour, while the S phase might occupy
10–12 hours, or about half the cycle.
The rest of the time would be apportioned between the
G1 and G2 phases.
The G2 phase usually takes 4–6hours; in our example, G1 would occupy about 5–6hours.
5-6 hours
G1 is the most variable in length in different types of
cells.
Mitosis is conventionally broken down into five stages:
prophase, prometaphase, metaphase, anaphase, and telophase
Overlapping with the latter stages of mitosis, cytokinesis completes the
mitotic phase.
study figure 12.7 exploring mitosis in animal cell, watch bioflix animation mitosis and video animal mitosis
Many of the events of mitosis depend on the ________________, which begins to form in the cytoplasm during prophase.
mitotic spindle
This structure consists of fibers made of microtubules and associated
proteins.
While the mitotic spindle assembles, the other microtubules of the cytoskeleton partially disassemble, providing the material used to construct the
spindle.
The spindle microtubules elongate (polymerize) by incorporating more subunits of the protein tubulin (see Table 6.1) and shorten (depolymerize) by
losing subunits.
In animal cells, the assembly of spindle microtubules starts at the ____________, a subcellular region containing material that functions throughout the cell cycle to organize the cell’s microtubules.
centrosome
A pair of centrioles is located at the center of the centrosome, but they are not essential for cell division: If the centrioles are destroyed with a laser microbeam, a spindle nevertheless forms during
mitosis.
centrioles are not even present in plant cells, which do form
mitotic spindles.
During interphase in animal cells, the single centrosome duplicates, forming two centrosomes, which remain near the
nucleus.
The two centrosomes move apart during prophase and _____________ of mitosis as spindle microtubules grow out from them.
prometaphase
By the end of prometaphase, the two centrosomes, one at each pole of the spindle, are at opposite ends of the
cell.
An _________, a radial array of short microtubules, extends from each centrosome.
aster
The spindle includes the
centrosomes, the spindle microtubules, and the asters.
a structure made up of proteins that have assembled on specific sections of DNA at each centromere
kinetochore
The chromosome’s two kinetochores face in
opposite directions.
During prometaphase, some of the spindle microtubules attach to the kinetochores; these are called
kinetochore microtubules (The number of microtubules attached to a kinetochore varies among species, from one microtubule in yeast cells to 40 or so in some mammalian cells.)
When one of a chromosome’s kinetochores is _____________ by microtubules, the chromosome begins to move toward the pole from which those microtubules extend.
“captured”
However, this movement comes to a halt as soon as microtubules from the opposite pole attach to the
kinetochore on the other chromatid.
At metaphase, the centromeres of all the duplicated chromosomes are on a
plane midway between the spindle’s two poles.
This plane is called the ____________________, which is an imaginary plate rather than an actual cellular structure
metaphase plate,
Meanwhile, microtubules that do not attachto kinetochores have been elongating, and by metaphase they overlap and interact with other
nonkinetochore microtubules from the opposite pole of the spindle
By metaphase, the microtubules of the asters have also grown and are in contact with the
plasma membrane.
watch video spindle formation during mitosis
The structure of the spindle correlates well with its function during .
anaphase
Anaphase begins suddenly whenthe cohesins holding together the sister chromatids of each chromosome are cleaved by an enzyme called
separase.
Once separated, the chromatids become individualchromosomes that move toward opposite ends of the
cell.
How do the kinetochore microtubules function in this poleward movement of chromosomes? Apparently, two mechanisms are in play, both involving
motor proteins.
Results of a cleverly designed experiment suggested that motor proteins on the kinetochores “walk” the chromosomes along the microtubules, which depolymerize at their kinetochore ends after the motor proteins have passed (Figure 12.9). (This is referred to as the
“Pac-man” mechanism because of its resemblance to the arcade game character that moves by eating all the dots in its path.)
However, other researchers, working with different cell types or cells from other species, have shown that chromosomes are “reeled in” by
motor proteins at the spindle poles and that the microtubules depolymerize after they pass by these motor proteins at the poles.
The general consensus now is that both mechanisms are used and that their relative contributions vary among
cell types.
study figure 12.9 and watch animation microtubule depolymerization
In a dividing animal cell, the nonkinetochore microtubules are responsible for elongating the whole cell during
anaphase.
Nonkinetochore microtubules from opposite poles overlap each other extensively during
metaphase
During anaphase, the region of overlap is reduced as motor proteins attached to the microtubules walk them away from one another, using
energy from ATP.
As the microtubules push apart from each other, their spindle poles are pushed apart, elongating the
cell.
At the same time, the microtubules lengthen somewhat by the addition of
tubulin subunits to their overlapping ends
As a result, the microtubules continue to
overlap.
At the end of anaphase, duplicate groups of chromosomes have arrived at opposite ends of the elongated
parent cell.
Nuclei re-form during
telophase.
Cytokinesis generally begins during anaphase or telophase, and the spindle eventually disassembles by
depolymerization of microtubules.
watch animation mitosis
In animal cells, cytokinesis occurs by a process known as
cleavage.
The first sign of cleavage is the appearance of a __________________, a shallow groove in the cell surface near the old metaphase plate
cleavage furrow
On the cytoplasmic side of the furrow is a contractile ring of
actin microfilaments associated with molecules of the protein myosin.
The actin microfilaments interact with the
myosin molecules, causing the ring to contract
The contraction of the dividing cell’s ring of microfilaments is like the pulling of a drawstring. The cleavage furrow deepens until the parent cell is pinched in two, producing
two completely separated cells, each with its own nucleus and its own share of cytosol, organelles, and other subcellular structures.
figure 12.10 cytokinesis in animal and plant cells, and watch video cytokinesis in an animal cell
Cytokinesis in plant cells, which have cell walls, is markedly different. There is no
cleavage furrow.
Instead, during telophase, vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce, producing a
cell plate.
Cell wall materials carried in the vesicles collect inside the
cell plate as it grows
The cell plate enlarges until its surrounding membrane fuses with the
plasma membrane along the perimeter of the cell.
Two daughter cells result, each with its own plasma membrane. Meanwhile, a new cell wall arising from the contents of the cell plate forms between the
daughter cells.
watch animation cytokinesis, figure 12.11
Prokaryotes (bacteria and archaea) can undergo a type of reproduction in which the cell grows to roughly double its size and then
divides to form two cells.
The term __________________, meaning “division in half,” refers to this process and to the asexual reproduction of single-celled eukaryotes, such as the amoeba in Figure 12.2a.
binary fission
However, the process in eukaryotes involves mitosis, while that in prokaryotes
does not.
In bacteria, most genes are carried on a single bacterial chromosome that consists of a
circular DNA molecule and associated proteins.
In some bacteria, the process of cell division is initiated when the DNA of the bacterial chromosome begins to replicate at a specific place on the chromosome called
the origin of replication, producing two origins.
As the chromosome continues to replicate, one origin moves rapidly toward the opposite end of the
cell
While the chromosome is replicating, the cell
elongates.
hen replication is complete and the bacterium has reached about twice its initial size, proteins cause its plasma membrane to pinch inward, dividing the parent bacterial cell into two daughter cells. In this way, each cell inherits a complete
genome.
figure 12.12 bacterial cell division by binary fission, animation cell division in bacteria
In most bacterial species studied, the two origins of replication end up at opposite ends of the cell or in some other very specific location, possibly anchored there by
one or more proteins.
Polymerization of one protein resembling eukaryotic actin apparently functions in bacterial chromosome movement during cell division, and another protein that is related to tubulin helps pinch the plasma membrane inward, separating the
two bacterial daughter cells.
Given that prokaryotes preceded eukaryotes on Earth by more than a billion years, we might hypothesize that mitosis evolved from simpler
prokaryotic mechanisms of cell reproduction
As eukaryotes with nuclear envelopes and larger genomes evolved, the ancestral process of binary fission, seen today in bacteria, somehow gave rise to
mitosis.
Variations on cell division exist in different groups of organisms. These variant processes may be similar to mechanisms used by ancestral species and thus may resemble steps in the evolution of mitosis from a
binary fission-like process presumably carried out by very early bacteria
Possible intermediate stages are suggested by two unusual types of nuclear division found today in certain
unicellular eukaryotes—dinoflagellates, diatoms, and some yeasts
These two modes of nuclear division are thought to be cases where ancestral mechanisms have remained relatively unchanged over evolutionary time. In both types, the nuclear envelope remains intact, in contrast to what happens in most
eukaryotic cells.
figure 12.13 mechanisms of cell division in several groups of organisms, video nuclear envelope breakdown
