CELL DIVISION, CELL DIVERSITY AND CELLULAR ORGANISATION
(a) The cell cycle
● To include the processes taking place during interphase (G1, S and G2),
mitosis and cytokinesis, leading to genetically identical cells.
During the cell cycle, there are two main phases, the mitotic division) phase, and
interphase. Cells do not divide continuously, there is a long period of growth and
normal working separate divisions. Indeed, nuclear and cytoplasmic division, called
the M phase, occupy only a small part of of the cell cycle; therefore, cells spend the
majority of time in the interphase, between each M phase. It is a very active phase,
and it is sub-divided into growth and synthesis stages. The cell is carrying out all its
major functions, such as producing enzymes or hormones, while also actively
preparing for cell division.
Indeed, it may enter a G0 phase, in which some cells leave the cell cycle, either
temporarily or permanently. The reasons for this include differentiation, apoptosis
and senescence. A cell that become specialised to carry out a specific function is no
longer able to divide. Many cells will then remain in this stage for a very long time,
possibly indefinitely. Some types of cell, though, can be stimulated back into the cell
cycle, e.g. lymphocytes in an immune response. The cell may be damaged, in which
case it may have to undergo programmed cell death, or it may become senescent.
This may be programmed, such as in erythrocytes, or due to the natural dividing-age
of the cell. Indeed, numbers of senescent cells are linked with age-related diseases
such as arthritis and cancer.
There are then three main phases of interphase, the G1 phase, in which protein
from which organelles are synthesized and produced and organelles replicate and the
cell increases in size; S (synthesis), in which DNA is replicated in the nucleus; G2, the
second growth phase, in which the cell continues to increase in size, energy stores
are increased and the duplicated DNA is checked for errors. During these stages, DNA
is replicated and checked for errors in the nucleus, protein synthesis occurs in the
cytoplasm, mitochondria grow and divide, increasing the number in the cytoplasm, as
do chloroplasts in plant and algal cells and the normal metabolic processes of the cell
occur.
The mitotic phase is the period of cell division. It can be divided into two
stages. First, in mitosis, the cell nucleus divides, then, second, once mitosis, is
complete, the cell undergoes cytokinesis. The cell splits in two so each new cell
contains a nucleus. In animal cells, the plasma membrane folds inwards and ‘nips in’
the cytoplasm. Whereas, in plants, an end plate forms where the equator of the
spindle was, and a new plasma membrane and cellulose cell-wall material are laid
down on with side of this end-plate.
(b) How the cell cycle is regulated
● To include an outline of the use of checkpoints to control the cycle.
There are two main checkpoints used to regulate the cell cycle, the G1/S checkpoint,
also called the restriction point, and the G2/M checkpoint. There are other
checkpoints, for example there is one halfway through mitosis and one in early G1.
The purpose of the checkpoint is to prevent uncontrolled division that would lead to
(a) The cell cycle
● To include the processes taking place during interphase (G1, S and G2),
mitosis and cytokinesis, leading to genetically identical cells.
During the cell cycle, there are two main phases, the mitotic division) phase, and
interphase. Cells do not divide continuously, there is a long period of growth and
normal working separate divisions. Indeed, nuclear and cytoplasmic division, called
the M phase, occupy only a small part of of the cell cycle; therefore, cells spend the
majority of time in the interphase, between each M phase. It is a very active phase,
and it is sub-divided into growth and synthesis stages. The cell is carrying out all its
major functions, such as producing enzymes or hormones, while also actively
preparing for cell division.
Indeed, it may enter a G0 phase, in which some cells leave the cell cycle, either
temporarily or permanently. The reasons for this include differentiation, apoptosis
and senescence. A cell that become specialised to carry out a specific function is no
longer able to divide. Many cells will then remain in this stage for a very long time,
possibly indefinitely. Some types of cell, though, can be stimulated back into the cell
cycle, e.g. lymphocytes in an immune response. The cell may be damaged, in which
case it may have to undergo programmed cell death, or it may become senescent.
This may be programmed, such as in erythrocytes, or due to the natural dividing-age
of the cell. Indeed, numbers of senescent cells are linked with age-related diseases
such as arthritis and cancer.
There are then three main phases of interphase, the G1 phase, in which protein
from which organelles are synthesized and produced and organelles replicate and the
cell increases in size; S (synthesis), in which DNA is replicated in the nucleus; G2, the
second growth phase, in which the cell continues to increase in size, energy stores
are increased and the duplicated DNA is checked for errors. During these stages, DNA
is replicated and checked for errors in the nucleus, protein synthesis occurs in the
cytoplasm, mitochondria grow and divide, increasing the number in the cytoplasm, as
do chloroplasts in plant and algal cells and the normal metabolic processes of the cell
occur.
The mitotic phase is the period of cell division. It can be divided into two
stages. First, in mitosis, the cell nucleus divides, then, second, once mitosis, is
complete, the cell undergoes cytokinesis. The cell splits in two so each new cell
contains a nucleus. In animal cells, the plasma membrane folds inwards and ‘nips in’
the cytoplasm. Whereas, in plants, an end plate forms where the equator of the
spindle was, and a new plasma membrane and cellulose cell-wall material are laid
down on with side of this end-plate.
(b) How the cell cycle is regulated
● To include an outline of the use of checkpoints to control the cycle.
There are two main checkpoints used to regulate the cell cycle, the G1/S checkpoint,
also called the restriction point, and the G2/M checkpoint. There are other
checkpoints, for example there is one halfway through mitosis and one in early G1.
The purpose of the checkpoint is to prevent uncontrolled division that would lead to
