By SHRUTI GUPTANCAAH

Eukaryotic cell cycle

& its Regulation

o The cell cycle, or cell-division cycle, is the series of events that take place in

a cell leading to its division and duplication.

o In cells without a nucleus (prokaryotic), the cell cycle occurs by a process termed as binary fission.

o In cells with a nucleus (eukaryotes),

the cell cycle can be divided in two

periods:

o The cell cycle consists of

Interphase– normal cell activity, growth,

The mitotic phase – cell divsion

The cell cycle

• Interphase: during which the cell grows, accumulating

nutrients needed for mitosis and duplicating its DNA.

• Mitosis (M) phase during which the cell splits itself into

two distinct cells, often called "daughter cells" .

• Cytokinesis, final phase, where the new cell is completely

divided.

Phases of the Cell Cycle

Interphase G1 - primary growth S - genome replicated G2 - secondary

growth M - mitosis C - cytokinesis

Interphase

G1 - Cells undergo majority of growth

S - Each chromosome replicates (Synthesizes) to produce sister chromatids.– Attached at centromere.– Contains attachment site (kinetochore).

G2 - Chromosomes condense - Assemble machinery for division such as centrioles.

Cyclin-dependent protein kinases drive progression through the cell cycle

• Cyclin-dependent kinases (Cdks) are inactive unless bound to cyclins.

• Active complex phosphorylates downstream targets

• Cyclin helps to direct Cdks to the target proteins

Checkpoints ensure the cell cycle proceeds without errors

Eukaryotic cells divided by two ways:

Mitosis- generally all somatic cells divide by this way. Produces 2 identical cells.Meiosis- germ cells follow this type of division pattern, produces 4 identical cells.

MITOSIS

Also named as equational division.

Some haploid & diploid cells divide by mitosis.

The term Mitosis was introduced by Walther Flemming in 1882.

Mitosis is the process by which a eukaryotic cell separates the

chromosomes in its cell nucleus into two identical sets, in two separate

nuclei

“Open" mitosis, where the nuclear envelope breaks down before the

chromosomes separate, most animals

“Closed" mitosis, where chromosomes divide within an intact cell

nucleus, fungi

Prokaryotic cells, which lack a nucleus, divide by a process called

binary fission

Mitotic division

• G2 OF INTERPHASE

• PROPHASE • PROMETAPHASE

• Centrosomes(with centriole pairs)• Chromatin

(duplicated)

• Early mitoticspindle

• Aster• Centromere

• Fragmentsof nuclearenvelope

• Kinetochore

• Nucleolus• Nuclearenvelope

• Plasmamembrane

• Chromosome, consistingof two sister chromatids

• Kinetochore microtubule

• Nonkinetochoremicrotubules

• METAPHASE • ANAPHASE • TELOPHASE AND CYTOKINESIS

• Spindle

• Metaphaseplate • Nucleolus

forming

• Cleavagefurrow

• Nuclear envelopeforming• Centrosome at

one spindle pole• Daughter

chromosomes

• A nuclear envelope bounds the nucleus.• The nucleus contains one or more nucleoli (singular,

nucleolus).• Two centrosomes have formed by replication of a single

centrosome.• In animal cells, each centrosome features two centrioles.• Chromosomes, duplicated during S phase, cannot be

seen individually because they have not yet condensed.G2 OF INTERPHASE

Centrosomes(with centriole pairs) Chromatin

(duplicated)

Nucleolus Nuclearenvelope

Plasmamembrane

G2 of Interphase

PROPHASEEarly mitoticspindle

AsterCentromere

Chromosome, consistingof two sister chromatids

Prophase• The chromatin fibers become more tightly coiled,

condensing into discrete chromosomes observable with a light microscope.

• The nucleoli disappear.• Each duplicated chromosome appears as two identical

sister chromatids joined together.

• The mitotic spindle begins to form. It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters (“stars”).

• The centrosomes move away from each other, apparently propelled by the lengthening microtubules between them.

Metaphase • It is the longest stage of mitosis, lasting about 20 minutes.• The centrosomes are now at opposite ends of the cell. • The chromosomes convene on the metaphase plate, an

imaginary plane that is equidistant between the spindle’s two poles. The chromosomes’ centromeres lie on the metaphase plate.

• For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles.

• The entire apparatus of microtubules is called the spindle because of its shape.

Daughter chromosomes

Anaphase

• Anaphase is the shortest stage of mitosis, lasting only a few minutes.

• Anaphase begins when the two sister chromatids of each pair suddenly part. Each chromatid thus becomes a full-fledged chromosome.

• The two liberated chromosomes begin moving toward opposite ends of the cell, as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the chromosomes move centromere first (at about 1 µm/min).

• The cell elongates as the non kinetochore microtubules lengthen.

• By the end of anaphase, the two ends of the cell have equivalent—and complete—collections of chromosomes

Telophase • Two daughter nuclei begin to form in the cell.• Nuclear envelopes arise from the fragments of the parent

cell’s nuclear envelope and other portions of the endomembrane system.

• The chromosomes become less condensed.• Mitosis, the division of one nucleus into two genetically

identical nuclei, is now complete.

Nucleolusforming

Cleavagefurrow

Cytokinesis

• Cytokinesis is a separate process that begins

at the same time as telophase.

• Cytokinesis is technically not even a phase

of mitosis, but rather a separate process,

necessary for completing cell division.

• In animal cells, a cleavage furrow (pinch)

containing a contractile ring develops

where the metaphase plate used to be,

pinching off the separated nuclei.

• Each daughter cell has a complete copy

of the genome of its parent cell.

• The end of cytokinesis marks the end of

the M-phase.

Meiosis

• Meiosis is a special type of cell division necessary for sexual

reproduction in eukaryotes. The cells produced by meiosis are

gametes or spores.

• Meiosis differs from mitosis in two important respects:

The chromosomes in meiosis undergo a recombination which

shuffles the genes producing a different genetic combination in

each gamete.

The outcome of meiosis is four (genetically unique) haploid

cells, compared with the two (genetically identical) diploid cells

produced from mitosis.

• Meiosis begins with one diploid cell containing two copies

of each chromosome one from the organism's mother and

one from its father and produces four haploid cells

containing one copy of each chromosome.

• Each of the resulting chromosomes in the gamete cells is a

unique mixture of maternal and paternal DNA, resulting in

offspring that are genetically distinct from either parent.

• This gives rise to genetic diversity in sexually reproducing

populations.

• During meiosis, specific genes are more highly transcribed,

and these are called the meiome (mRNA).

Process of meiosis

Prophase 1

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