2.3 CELL CONTINUITY
2.3.1 Cell Continuity and Chromosome
Explanation of the term “Cell Continuity”
This is the way that all cells come from pre-existing cells. It refers to the way in which cells
divide and give rise to new cells.
Cell continuity is vital for all species to allow for growth and for the continuity of life.
Three things have to happen to a cell before is able to form a new cell
 It has to grow to a suitable size
 It has to make all the materials needed to divide
 It has to reproduce and form new cells
Basically cell continuity involves growth, synthesis and reproduction. It can be
summarised in a cycle called the "cell cycle". The cell cycle describes a cell’s state of nondivision (interphase) and division (mitosis)
A cell spends most of its time making the materials it needs to live and grow. It will spend
only a short time doing cell division.
Explanation of the term “chromosome”.
Chromosomes are structures found in the nucleus of a cell.
Each chromosome is made of 60% protein and 40% DNA.
The DNA (deoxyribonucleic acid) is wrapped around the
protein.
When the cell is not dividing the chromosomes are in the
form of long thin invisible threads called chromatin.
Genes are located along the length of each chromosome.
Each gene is a section of DNA and contains the code for
making a particular protein. Each gene or group of genes
controls the development of a particular characteristic or
trait of an individual.
Each species have their own definite number of chromosomes in the nucleus of each of their
cells e.g. humans have 46 chromosomes in each cell.
2.3.2 Haploid, Diploid
Definition of "haploid" number.
A haploid cell has one of each type of chromosome i.e. it has a single set of chromosomes
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.3 CELL CONTINUITY
The letter n is used to represent haploid
Human egg and sperm cells are haploid and each contains 23 chromosomes.
Haploid cell - No
matching pairs
just single
chromosomes-
n=2
n=3
Haploid
number-count
the single
chromosomes
n=4
Definition of "diploid" number.
A diploid cell has two of each type of chromosome i.e. it has a two sets of chromosomes.
In a diploid cell the chromosomes are in pairs. They are called homologous pairs because they
have similar genes. One of the chromosomes came from the father and the other one come
from the mother.
The symbol 2n is used to represent diploid.
Normal human cells have a diploid number of 46 i.e. 2n = 46.
The chromosomes
are all in pairs so
this cell is a
diploid cell
Diploid number count the total
chromosomes they
happen to be in pairs
2n = 4
2n = 6
2.3.3 The Cell Cycle
The cell cycle describes the cell’s activities in the state of non-division (interphase) and
division (mitosis).
The cell cycle describes the life cycle of the cell.
It has 3 main stages
 Interphase – non-dividing longest phase. A cell
spends most of its time making the materials it
needs to live and grow.
 Mitosis – nuclear division forming two nuclei with
identical sets of chromosomes.
 Cytokinesis –division of cytoplasm forming two
genetically identical cells
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.3 CELL CONTINUITY
Cancer
Cancer is a group of disorders in which certain cells lose normal regulation over both mitotic
rate and the number of divisions they undergo. This results in the uncontrolled
multiplication of abnormal cells.
The uncontrolled growth of cells forms a malignant tumour.
Two possible causes of cancer
1. Cancer is caused when normal genes are changed or mutated to form cancer-causing
genes called oncogenes.
2. Cancer is caused by chemical, biological or physical agents called carcinogens
Common carcinogens are:
 Carcinogenic chemicals like cigarette smoke, asbestos fibres and benzene
 Certain cancer causing viruses
 Radiation like uv light, x rays and nuclear radiation.
2.3.4 Mitosis
Mitosis is a form of cell division where one cell divides to form two new cells, each identical
to the other and to the parent cell
Basically mitosis is a form of cell replication (cell copying) in which the chromosome number
remains constant (or the same) in each of two identical cells generated from one.
Just before the cell divides, chromosomes become visible in the nucleus (short, thick and
duplicated). The nuclear membrane disappears, and fibres are formed to which the
chromosomes attach. The chromosomes are pulled apart to opposite ends of the cell.
A nuclear membrane forms around each set of chromosomes and the cell divides in two.
Each new daughter cell now contains the same number of chromosomes as the parent cell.
So, during mitosis the genetic material divides and the cytoplasm, organelles and
biomolecules are partitioned into two cells.
2.3.5 Function of Mitosis (the role of mitosis)
Primary function in single-celled and multicellular organisms.

In single-celled organisms, mitosis allows the organisms to multiply i.e. it is for asexual
reproduction in unicellular organisms e.g. asexual reproduction in the amoeba, binary
fission in bacteria and budding in yeast cells

In multicellular organisms, mitosis is primarily for growth (mitosis increases the number
of cells in an organism) and repair.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.3 CELL CONTINUITY
2.3.6 Meiosis
Meiosis is a type of nuclear division where four daughter nuclei are produced each containing
half the number of chromosomes as the parent cell
Meiosis is called reduction division. In meiosis a diploid (2n) cell produces 4 haploid (n)
cells.
2.3.7 Functions of Meiosis in multicellular organisms (the role of meiosis)
Meiosis is important in multicellular organisms for :
(a) keeping the parental chromosome number the same by
forming haploid cells (gametes) in sexual reproduction
Meiosis halves the chromosome number when gametes are
formed. This means that the normal chromosome number is
restored again at fertilisation.
(b) introducing variation in the species by exchange of genetic
material between homologous chromosomes
The daughter cells produced in meiosis are not identical due to
crossing over or the exchange of genetic material that takes place when the haploid cells or
gametes are formed. This results in variations or differences in organisms made as a result of
sexual reproduction
H.2.3.8 Stages of Mitosis (Extended Study)
Mitosis occurs in somatic cells i.e. cells not involved in reproduction. In animals mitosis
occurs throughout the body whereas in plants mitosis occurs in specific growing areas
(meristematic regions) e.g. tips of roots and shoots
CELL CYCLE
1. INTERPHASE ( Non Division Stage )
Cell membrane
Centriole
Nuclear membrane
membrane
Chromatin
nucleolus
Ms. B. Fennessy
1. These changes cannot be seen
2. A store of energy and enzymes is made
3. Cell organelles are duplicated
4. Chromosomes (DNA) are replicated
5. Chromosomes are in the form of
Chromatin
Loreto Secondary School
Fermoy
Co.Cork
2.3 CELL CONTINUITY
2. MITOSIS
PROPHASE
Centromere
1. Chromatin threads shorten and thicken
forming chromosomes
2. Chromosomes are double stranded
joined at the centromere
3. Centrioles move to opposite sides of
the cell
4. The nucleolus disappears and the
nuclear membrane breaks down
Spindle
METAPHASE
1. The chromosomes move and line up
along the equator
2. Spindle fibres attach to the
Centromeres
Chromosome
ANAPHASE
1. The spindle fibres contract and the
centromeres split
2. The sister chromosomes are pulled to
opposite sides of the cell
TELOPHASE
1. The nuclear membrane reforms around
each group of chromosomes
2. The chromosomes unwind to become
chromatin
3. The nucleoli reform and the spindles
break down
Chromosome
unwinds
3. CYTOKINESIS (DIVISION OF THE CYTOPLASM)
In animal cells the cell membrane pinches in forming 2 separate cells i.e. a cleavage furrow
forms.
In plant cells the cell wall is too rigid to pinch in so plant cells form a cell plate of pectin and
later a cell wall of cellulose.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork