Download Lecture 9 Summary – Cell Division and Differentiation

Lecture 9 Summary – Cell Division and
Differentiation
1. Discuss the stages, events and significance of somatic cell division.
Interphase:
The interphase is a long process. In an actively dividing cell, the cell is still performing normal
functions during the interphase but is preparing for division/duplication. The DNA of the cell as well as
all its membranes and organelles need to be duplicated. It is during the interphase that DNA is
replicated. Can be further broken down into:
 G0 phase; groups cells that remain in the G1 phase for a long time and will most likely never
divide again eg: some neurons, red blood cells.
 G1 phase; the active cell duplicates most of its organelles and cytosolic components. The
replication of chromosomes begins.
 S phase; DNA is replicated by semi-conservative replication.
 G2 phase; further cell growth and final preparations for mitosis. Centrioles are replicated and
position themselves at opposite ends of the cell creating "poles". Proteins required for mitosis
are being produced. Cell is now focussed on replication rather than normal cell function.
Mitotic Phase;
It is during the mitotic phase that nuclear division occurs. It is a generally fast process between 1-3
hours in duration. During this time, the cell is metabolic inactive and is focussing on the division. Can
be further broken down into:
 Early prophase; chromosomes have been duplicated and become highly coiled and condensed
at the centromere of the cell.
 Late prophase; the nuclear envelope has disappeared, centrioles are at opposite poles of the
cell and the microtubules involved in separation are present. The chromatids attach to the
microtubules via the kinetochore (close to the centromere).
 Metaphase; chromatids align on the metaphase plate (the middle of the dividing cell).
 Anaphase; chromatids move apart to either pole and their centromere's split. One chromatid
moves to each pole of the dividing cell. The daughter chromosomes are pulled to the poles
from the interaction between the kinetochore and the microtubules.
 Telophase; the nuclear envelope begins to reform and the separation of the cytoplasm of the
dividing cell begins. This is called a cleavage furrow.
 Cytokinesis; the cytoplasm of the daughter cell divides completely to form two daughter cells.
This divide occurs as a result of the constriction of actin filaments that are distributed around
the cleavage furrow. It is tightened and tightened until completely constricted and a complete
split of the cytoplasm occurs, forming the two identical daughter cells.
What is the significance of somatic cell division? The body is able to control homeostasis by regulating
the number of cells in the body and their rate of division. If a cell is damaged in some way and can no
longer function correctly, the body responds by sending
signals to begin apoptosis (cell condenses and is broken
down into different fragments) and cell division of a normal
functioning cells begins to replace the damaged cell and its functions. In short, it helps to keep the
body functioning correctly and healthily.
2. Understand the difference between mitosis and meiosis.
Meiosis is a very specialised form of cell division designed for sex cell (gamete) production. Meiosis
involves two separate division processes to form 4 daughter cells that each contain only half the
number of chromosomes and therefore, half the amount of genes/hereditary information in order to
become complete during fertilisation. Therefore, there is always two sets of division in meiosis
whereas there is only one in mitosis because this ensures that each of the newly replicated daughter
cells has a complete set of DNA to ensure correct protein synthesis and also to provide correct
information involved in other important processes in the body.
3. Describe the signals that induce somatic cell division.
Various signals tell a cell if it needs to exist as it is, divide or die. Within a cell there are enzymes called
cyclin dependant protein kinases (Cdk’s) that are involved in the attachment or detachment of a
phosphate group to/from a protein. This movement (activation/deactivation respectively) is very
important in the regulation of DNA replication, mitosis and cytokinesis. Switching the Cdk’s on and off
is controlled by proteins called cyclins and the joining of a cyclin to a Cdk molecule is what triggers
various events that control cell division.

The activation of specific cyclin-Cdk complexes is responsible for the progression of a
cell through the steps of the interphase stage of mitosis. If any of these steps is
delayed, the entire process is delayed in order to maintain the normal sequence of
steps.
 The levels of cyclin in the cell determine the timing of cell division processes. A high
level of cyclin will help the cell to pass through to the end of the interphase stage and
then onto the other stages of mitosis but levels of cyclin will drop off toward the end
to ensure mitosis ends, rather than continuing leading to uncontrolled cell divisions.
4. Describe how cells differ in size and shape.
The largest cell is the oocyte which is usually around 140µm in length whereas red blood cells are
usually around 8µm in length/diameter. Cells may be round, oval, flat, cube-shaped, column-shaped,
elongated, star-shaped, cylindrical or disc shaped depending on its function in the body. For example;
a sperm cell has a long tail (flagellum) which helps it move and travel (swim) toward the oocyte for
fertilisation. Disc shapes of the red blood cells give them a larger surface area which aids in its passing
of oxygen to other cells around the body. The long, spindle shape of a muscle cell helps it to contract
and lengthen depending on the body’s needs. Nerve cells also have long extensions called axons
which help them to communicate and conduct nerve impulses over great distances throughout the
body.