
Growth, asexual reproduction, tissue repair and maintenance are
processes that require the creation of new cells

In eukaryotic cells, division of the nucleus to form 2 genetically
identical nuclei is called MITOSIS

The life of the cell can be considered an ordered sequence of
events called the cell cycle
M
-
Mitosis and cytokinesis
G1 -
Growth phase (cell grows and organelle number increase)
S
-
Synthesis phase (chromosomes are replicated in
preparation for the next cell division)
G2 -
Another growth phase (leading to M phase again)
Newly produced cells may:
 grow and reproduce again
 differentiate to become specialised cells
 perform a particular function, age then die
R – Point –
The R-Point is known as the RESTRICTION POINT. It
is the most important of the control points of the cell
cycle. It is here that the cell determines whether it will
undergo mitosis or not. If it has a mutation or is
cancerous it may be terminated or remain dormant.

Apoptosis is the programmed death of a cell

Some cells live for a lifetime (brain cells) others different times Eg. Skin cells (few weeks), stomach lining (few days) and RBCs (few
months)

Cancer cells avoid apoptosis and continue growing unchecked

During Apoptosis, the programmed cell:
 signals that it is going to die
 shrinks
 digestive enzymes are released breaking the cell up
 some parts are engulfed by macrophages whilst other organelles
can be reused by neighbouring cells

Cell death is important as:
 it protects from ill health by destroying infected cells
 help divide cells in an ordered manner
 it helps shape organs and tissues

Necrosis is a form of cell death that results from irrepairable injury to the cell

4 Phases of Mitosis
1. PROPHASE
2. METAPHASE
3. ANAPHASE
4. TELOPHASE

Mitosis is the process used by cells to make new,
genetically identical, cells. This is predominantly used in
growth and repair

INTERPHASE is a phase in a non-dividing cell where all the
chromosomes exist in a single chromatid form. Cells spend
the majority of their lives in this form
 P.M.A.T.I OR
I.P.M.A.T

PROPHASE
 chromosomes become shorter and fatter by coiling.
Repeated coiling is called supercoiling
 microtubules grow from the poles of the cell and the
centrioles
 these form a spindle called the MITOTIC SPINDLE

METAPHASE
 spindle microtubules attach to the chromosome
centromeres
 chromosomes are moved to the equator. Each
chromatid moves to the opposite pole

ANAPHASE
 the pairs of sister chromatids separate, centromeres
split and the microtubules pull them to opposite poles

TELOPHASE
 chromatids at each pole gather together and become
enclosed in a nuclear membrane
 the whole cell splits at the equator and forms 2 new
cells
 each cell has the SAME number of chromosomes as
the parent cell and contains the same genetic
information
 cell enters INTERPHASE again
UNDERSTAND THE PROCESS DON’T MEMORISE THE STEPS
 VIDEO: CRASHCOURSE MITOSIS
 VIDEO: THE EUKARYOTIC CELL CYCLE

Meiosis is a type of cell division that produces
gametes (sex cells)

Cells produced are haploid (contain HALF the original
chromosome number)

Process accounts for great genetic variation due to
 RANDOM ASSORTMENT
 CROSSING OVER

2 cell divisions:
1st:
PROPHASE I
METAPHASE I
ANAPHASE I
2nd: PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE I
TELOPHASE II
 PROPHASE I
 cell has 2n (diploid) number of chromosomes
 chromosomes become visible
 homologous pairs line-up
 crossing over occurs and exchange of genetic material
(see right)
 METAPHASE I
 spindle microtubules move homologous pairs to the
equator of the cell
 orientation of paternal and maternal chromosomes at either
side of the equator is random (RANDOM ASSORTMENT)
 ANAPHASE I
 Homologous pairs are separated. One chromosome of
each pair moves to each pole
 TELOPHASE I
 Nuclear membranes form around chromosomes
 cell membranes form around each nucleus
 reduction of chromosome number from diploid to haploid
is completed (2 haploid cells now formed)
 cytokinesis occurs
 PROPHASE II
 nuclear envelope begins to disintegrate
 spindle microtubules form
 chromosomes (with 2 chromatids) condense and
become visible
 METAPHASE II
 chromosomes line up along the equator (in a
straight line)
 ANAPHASE II
 centromeres separate and chromatids are moved
to opposite poles
 TELOPHASE II
 chromatids reach opposite poles
 nuclear envelope forms
 cell membrane re-forms
 cytokinesis occurs

At this stage there are 4 GENETICALLY DIFFERENT
daughter cells

They contain HALF the original chromosome number
(haploid)

Phase II is very similar to mitosis
Random Assortment and Crossing Over play an important role in
increasing genetic variation within a species.
They do this by increasing the number of different combinations
that can be formed in the gametes.
CROSSING OVER cont…..
During crossing over the 4 chromatids are
lined up perfectly next to one another so that
the corresponding genes are side by side
This process is known as SYNAPSIS and is
a bit like doing up a zipper
The chromatids are broken at corresponding
places and rejoined to form new chromatids.
When the chromatids are crossed over the
arrangement is called CHIASMATA

The inheritance of TOO MANY of TOO FEW chromosomes

Not whole chromosome sets, rather INDIVIDUAL CHROMOSOMES

The most common aneuploidies are TRISOMIES
Eg. Trisomy 21 (Down’s Syndrome)
3 copies of chromosome 21 rather than 2

Aneuploidy  can occur in autosomes/sex chromosomes
 different aneuploidy result in particular chromosome
abnormality
 the older the mother the more likely the occurrence


Egs: Trisomy 21 – 3 @ 21 (Down’s Syndrome)
Trisomy 13 – 3 @ 13 (Patau Syndrome)
45
- XO
(Turner’s Syndrome)
47
- XXX
(Klinefelter’s Sydrome)
Can result from NON-DISJUNCTION of homologous chromosomes during
meiosis

The inheritance of additional FULL SETS
OF CHROMOSOMES

Common in plants – often larger with bigger
fruits, flowers and seeds
 During Anaphase I, if the
homologous pair fails to separate
then one cell will receive BOTH
chromosomes
 In the 2nd division the paired
chromatids separate but the
gametes will receive 2 copies of
the chromosome
 Can also result from FAILURE
OF PAIRED CHROMOSOMES
TO SEPARATE at anaphase II.
One gamete receives both
chromosomes and the other
none
 VIDEO: CRASHCOURSE MEIOSIS
 VIDEO: NON-DISJUNCTION
 VIDEO: CROSSING OVER AND GENETIC VARIABILITY