The Cell Cycle
Omnis cellula e cellula
The cell cycle is an ordered set of events,
culminating in cell growth and division into
two daughter cells
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Structure
1. The role of cell division – what is a
need to divide?
2. The mitotic cell cycle – how to divide?
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Chromosome
duplication and
distribution during
mitosis
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1. The role of cell division
• In reproduction –
main difference between living
organism and nonliving matter
• In growth –
for multicellular organism (including cell
renewal and cell repair)
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Reproduction and growth
Unicellular organism
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Multicellular organism
The important definitions
• Cell division is a finely controlled process that results in the
distribution of identical hereditary material – DNA – to two
daughter cells.
• A dividing cell:
Precisely replicates its DNA
Allocates the two copies of DNA to opposite ends of
the cell
Separates into two daughter cells containing identical
hereditary information
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The important definitions
• The DNA is distributed among 1 to more then 50 long
linear stretches called chromosomes.
• The number of chromosomes is a characteristic of every
eukaryote.
For example our somatic cells – all cells except
reproductive – contain 46 chromosomes (2n). Our
gametes (reproductive cells) have only 23 chromosomes
(n).
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The important definitions
• The total DNA in the chromosomes of an organism is
referred as its genome.
• Chromosome contains thousands of genes – units of DNA
that contains the information to specify synthesis of single
polypeptide chain.
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The important definitions
Characteristics of chromosomes:
They are supercoils of a DNA-proteins complex called
chromatin. Each chromosome consists of :
Genes
Proteins
They exist in characteristic number (somatic - 2x, gametes –
x)
They exists in different states:
Loosely folded during interphase
Highly folded during mitotic phase
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• The DNA structure is maintained via association with
different protein – formation of the DNA-protein complex –
chromatin.
•Before the cell can divide all DNA must be copied and then
separated to daughter cells.
•After copying each duplicated chromosome consists of
two sister chromatids –identical copies of
chromosome’s DNA molecule.
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centromere
2. The mitotic cell cycle
• The cell cycle is composed from interphase (I)
and mitotic (M) phase
• Interphase consists of several subphases
• Mitotic phase consists of both mitosis and
cytokinesis. The former one is divided to five
subphases with special characteristic for each
one
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The
eukaryotic
cell cycle
M
G1
G2
G0
S
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Short summary of interphase
• Comprised by three (four) subphases: G1, (G0),
S and G2 .
• Common feature of all (except G0) subphases:
growth of the cell by producing proteins and
cytoplasmic organelles.
• G1 – “first gap”
• S phase – chromosome duplication
• G2 – “second gap”, DNA repair
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Overview of
mitosis
Parental cell
(2n) in G1
Chromatid
Parental cell
(4n) in G2
DNA replication
Mitotic
apparatus
Metaphase cell
Anaphase cell
Daughter
cells (2n)
Cytokinesis
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Interphase (G2)
Duplicated chromatin
Centrosomes
with centriole pairs
•
•
•
•
•
The nucleus is well defined
It contains one or more nucleoli
Two centrosomes are formed
Pair of centrioles, aster around
Duplicated chromosomes are
still not visible
Nucleolus
Nuclear envelope
Plasma membrane
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Prophase
Early mitotic
spindle
Chromosome, consisting
of two sister chromatids
• Chromatin condenses into
discrete chromosome
• The nucleoli disappear
• Mitotic spindle begins to
form
•Centrosomes move apart
Centromere
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Prometaphase
• Fragmentation of
nuclear envelope
• Formation of
kinetochore
•Extension of spindle
fibers
• Invasion of
microtubules into the
nucleus and attachment
to kinetochores
Fragments of
nuclear envelope
Spindle pole
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Kinetochore
Nonkinetochore
microtubules
Kinetochore
microtubules
Metaphase
• Centrosomes are at
opposite poles of the cell
•Chromosomes move to
the metaphase plate
• Formation of imaginary
metaphase plate:
alignment of all
chromosome at the
centre
• Spindle formation
Spindle
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Metaphase plate
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The mitotic spindle at metaphase
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Anaphase
•Centromeres separate and
sister chromatids move
toward opposite poles of the
cell as their kinetochore
microtubules shorten
•Nonkinetochore
microtubules elongate
moving the poles further
apart
Daughter chromosomes
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Telophase
Cleavage furrow
•Forming of nuclear envelope
•Chromatin is less condensed
•Mitosis is complete
Nucleolus
forming
•Cytokinesis: formation of
cleavage furrow and
separation of two daughter
cells
Nuclear envelope
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Cytokinesis in animal and plant cells
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Cleavage of an animal cell
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Cell plate formation in a plant cell
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Mitosis in a plant cell
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Mitosis in an onion root
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Bacterial cell division (binary fission).
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Evolution of
mitosis
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Evolution of
mitosis
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Summary
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Summary
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Summary
1. The role of cell division – reproduction and growth (cell
renewal and repair).
2. The mitotic cell cycle – consists of:
- interphase (I=G1+S+G2) – important steps for DNA
duplication and initiation of mitosis;
- mitosis (P+PM+M+A+T) – separation of two daughter
cells with equal amount of genetic material
(chromosomes).
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Reading
1. Ch. 12 pp. 228-237
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Evolution of mitosis (hypothesis)
A. Bacteria
Chromosome
Microtubules
D. Animal cell
Chromosomes
Fragments of
nuclear envelope
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B,C. Unicellular algae
Kinetochore microtubules
Cytokinesis in animal and plant cells
contractile ring
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formation of phragmoplast
formation of a new cell wall