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Chapter 13: The Cell Cycle
Leaving Certificate Biology
Higher Level
Cell Continuity
• Cells that arise from cells of the same type
– Living organisms maintain cell continuity by mitosis and cell
division
Chromatin and Chromosomes
• Chromatin is elongated DNA
• Chromosomes are composed of DNA and
protein
• Chromosomes are arranged into homologous pairs – pairs of
chromosomes that contain genes that control the same
characteristics
Haploid and diploid
• Haploid means one set of chromosomes
• Diploid means two sets of chromosomes
– In humans the haploid number is 23
– The diploid number is 46 – arranged into 23 pairs
Cell cycle
– The cell cycle occurs under three distinct stages:
1. Interphase
2. Mitosis
3. Cell division
Interphase
• Interphase is the stage where a cell is not
dividing and a cell spends the vast
majority of its time in this stage
• Chromosomes are elongated and the cell
goes about its daily functions
• Towards the end of interphase the cell
organelles and DNA are replicated
End of interphase
The DNA is replicated and condenses into
duplicated chromosomes which are held together
by centromeres
Centromere
DNA
replication
4n
2n
Nucleus
Mitosis
• Mitosis is nuclear division that leads to the
formation of two identical daughter cells
• Stages:
– Prophase
– Metaphase
– Anaphase
– Telophase
Prophase
2. The nuclear membrane begins to disappear
and spindle fibres begin to appear:
Nucleus
Metaphase
• The replicated chromosomes line up along the
equator of the cell held in place by spindle fibres:
Nuclear membrane
has completely
disappeared
Spindle
fibre
Anaphase
• The spindle fibres begin to contract pulling one
chromosome from each pair to each pole:
Centromeres split
at anaphase
Single chromosome
pulled to pole
Telophase
• The chromosomes at each pole begin to unravel
and lengthen; spindle fibres disappear; and the
nuclear membranes reform at each pole:
Chromatin and
nuclear
membranes
reform
Cell Division
• Cell division occurs immediately after mitosis and involves
formation of cleavage furrow (in animals cells) or cell
plate (in plant cells) prior to cell splitting
Cleavage furrow (animals);
cell plate (plants)
Cell Division – Mitosis v Meiosis
2n
2n
Mitosis
Meiosis
2n
2n
n
n
n
n
Cancer
• Cancer is a disease whereby a cell loses its
ability to control both the rate of mitosis and cell
division
• Mitosis is normally carefully controlled but when
it is out of control it can result in cancer
• Cancer can be either benign or malignant
– Benign cancer involves cells that divide out of control
for a limited period of time and do not spread
– Malignant cancers involve cells that divide rapidly
uncontrollably and spread and invade other tissues –
destroying that tissue in the process
Cancer
• Anything that has the potential to cause
cancer is called a carcinogen – which
damage DNA
• Two carcinogens:
– UV light – damages DNA of skin cells
– Cigarette smoke – damages the DNA of cells
lining the mouth, airways and lungs