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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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Unicellular organism Multicellular organismReproduction and growth

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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 DNAAllocates the two copies of DNA to opposite

ends of the cellSeparates 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 definitionsCharacteristics of chromosomes:

They are supercoils of a DNA-proteins complex called chromatin. Each chromosome consists of :

GenesProteins

They exist in characteristic number (somatic - 2x, gametes – x)

They exists in different states:Loosely folded during interphaseHighly 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.

centromere

•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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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

G0

S

G1

M

G2

Interphase

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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

Daughter cells (2n)

Anaphase cell

Chromatid

Mitotic apparatus

Cytokinesis

Parental cell (4n) in G2

Metaphase cell

DNA replication

Parental cell (2n) in G1

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Interphase (G2) Duplicated chromatinCentrosomes

with centriole pairs

Nucleolus

Nuclear envelopePlasma membrane

• 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

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ProphaseEarly mitotic

spindle

Centromere

Chromosome, consisting of two sister chromatidsEarly mitotic

spindle

Centromere

Chromosome, consisting of two sister chromatids

• Chromatin condenses into discrete chromosome• The nucleoli disappear• Mitotic spindle begins to form•Centrosomes move apart

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Prometaphase

Fragm ents of nuclear envelope

N onkinetochore m icrotubules

K inetochore

Spindle poleK inetochorem icrotubules

• Fragmentation of nuclear envelope • Formation of kinetochore•Extension of spindle fibers• Invasion of microtubules into the nucleus and attachment to kinetochores

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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

M e t a p h a s e p l a t e

S p i n d l e

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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

N u c l e a r e n v e l o p e

C l e a v a g e f u r r o w

N u c l e o l u s f o r m i n g

•Forming of nuclear envelope•Chromatin is less condensed•Mitosis is complete

•Cytokinesis: formation of cleavage furrow and separation of two daughter cells

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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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Summary1. 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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Reading1. Ch. 12 pp. 228-237

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Evolution of mitosis (hypothesis)Chromosome

Chromosomes

Kinetochore microtubulesCentrosome

Fragments of

nuclear envelope

Microtubules

A. Bacteria

B,C. Unicellular algae

D. Animal cell

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Cytokinesis in animal and plant cells contractile ring

formation of a new cell wall

formation of phragmoplast