Download CELL CYCLE and MITOSIS 2001

Cell Cycle and Mitosis
Background
• All cells come from pre-existing cells
• Humans begin life with a single cell from
which 600 billion trillion cells are made
(6x1023)
• Each second, millions of cells die. They
must be replaced to ensure the proper
functioning of our bodies
DNA Basics
• DNA or Deoxyribonucleic Acid is a
large molecule of the sugar
deoxyribose and phosphate linked to
four different bases (A,T,C,G)
• DNA contains the genetic information
that specify the proteins that allow
life. It is the “blueprint” of the cell
DNA (con’t)
• DNA is organized into specific informationcontaining sections called genes
• The genes specify which proteins are made
and the proteins then determine
observable traits (eg. Eye colour)
• Mitosis is designed to ensure exact copies
of the DNA in chromosomes are passed on
to daughter cells
Cell Cycle
• The cell cycle is an ordered set of events,
culminating in cell growth and division into
two daughter cells
• Non dividing cells are not considered to be
in cell cycle
• The cell cycle is divided into two distinct
phases – Interphase and Mitosis
Cell Cycle
• Interphase consist of
three phases:
• - gap 1 (G1) – cell
growth
- S (Synthesis) –
replication of DNA
- gap 2 (G2) – cell
growth
• Mitosis (M) consists
of four phases which
will be studied in
detail
Interphase
• The cell is engaged in metabolic
activity and performing its
preparations for mitosis
• Chromosomes are not clearly
discerned
Chromosomes
• Carry cell’s DNA along with some proteins
• Found in pairs that are similar but not
identical. Known as homologues or is said
to be homologous
• For most of the cell cycle, chromosomal
material organized into long, thin strands
of thread-like material known as chromatin
Chromosomes (con’t)
• During mitosis, chromatin coils and
thickens to form the short, thick
chromosomes
Chromosome Structure
• Before a cell divides,
each chromosome
duplicates itself
• A duplicated
chromosome is made
up of two identical
chromatids (sister
chromatids) held
together by a
centromere
Mitosis
•
Mitosis is divided into four distinct
stages
1.
2.
3.
4.
Prophase
Metaphase
Anaphase
Telophase
Prophase
1.
2.
3.
4.
5.
Chromatin begins to
shorten and thicken
Nucleolus and nuclear
membrane break down
Spindle fibres begin to
form near the poles.
These fibres are
microtubules and they
form a microskeleton
Centrosomes are made up
of 2 centrioles and they
migrate to opposite poles
Asters begin to form
Metaphase
1. Chromatids are
shortened and
thickened into
chromosomes
2. Sister chromatids
align along the
middle or
equatorial plate
Anaphase
1. Centromeres divide
and chromatids
separate
2. Spindle fibres pull
separated
chromosomes to
opposite centrosome
3. Nuclear membranes
assemble around
chromosome clusters
4. Cytokinesis usually
begins
Telophase
1. Daughter
chromosomes arrive
at opposite poles
2. Chromosomes begin to
uncoil into chromatin
3. Nuclear membrane
begins to form around
the chromatin
4. Nucleolus reappears
and the spindle fibres
dissappear
Cytokinesis
• Once the two nuclei have been formed, a
division of the cytoplasm, called
cytokinesis occurs
• Begins in anaphase and continues
throughout telophase
• In plants – cell plate forms followed by a
the formation of the cell wall
Cytokinesis (con’t)
• In animals – a cleavage furrow forms
around the middle of the parent cell
pinching the cell into two equal halves
• Each daughter cell now begins a new
cell cycle starting with the first
growth stage
Cancer
• Carcinogenic agents may disrupt the
normal mitotic process
• Cells reproduce at a rate far in
excess of normal cell division and lose
control of growth, differentiation and
replication