The Cell Cycle and Mitosis

Unit 3 Organization and development

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Phases of the Cell Cycle

The cell cycle consists of

• Interphase – normal cell activity• Cell Division – mitosis and cytokinesis

INTERPHASE

Growth

G 1(DNA synthesis)

Growth

G2

Cel

l Div

sion

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DNA

• Genetic information – makes up an organism’s genome

• Packaged into chromosomes

50 µm

Figure 12.3

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Mitosis and Cell Division

• Mitosis makes new cells for repair; to replace old, damaged, or dead cells.

• Mitosis makes new cells for growth.

• Somatic (non sex cells) undergo mitosis.

• In every mitotic division, 2 cells are made.

• These cells are genetically identical to the parent cell.

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Chromosomes

Chromosomes in Humans: 23 pairs, 46 total

•Autosomes: non-sex chromosomes, chromosomes 1 – 22

•Sex chromosomes: determine sex, X and Y, chromosome 23

Sets of chromosomes

•Diploid: two sets of chromosomes, 2n (46 in humans)

•Haploid: one set of chromosomes, n (23 in humans)

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Structure of Chromosomes

• DNA is an extremely long double-stranded molecule.

• One strand of DNA makes up a chromosome.

• Before mitosis, the DNA exists in long, uncondensed strands called - chromatin

• Each unduplicated chromosome contains one DNA molecule.

• Duplicated chromosomes are called sister chromatids.

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Structure of Chromosomes• DNA wraps around a group of histone proteins to form a

nucleosome.

• Higher order coiling and supercoiling also help condense and package the chromatin inside the nucleus:

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• Because of duplication, each condensed chromosome consists of 2 identical sister chromatids joined by a centromere.

• Each duplicated chromosome contains 2 identical DNA molecules (unless a mutation occurred), one in each chromatid:

Chromosome Duplication

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Two unduplicatedchromosomes

Centromere

Sisterchromatids

Sisterchromatids

Duplication

Non-sisterchromatids

Two duplicated chromosomes

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Structure of Chromosomes• The centromere is where sister chromatid are connected.

• The kinetochore serve as points of attachment for microtubules that move the chromosomes during cell division.

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Metaphase chromosome

Kinetochore

Kinetochoremicrotubules

Centromereregion ofchromosome

Sister Chromatids

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Phases of the Cell Cycle Interphase

• G1 - primary growth

• S – DNA synthesis (replication)

• G2 - secondary growth

M - mitosis

C - cytokinesis

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Interphase

G1 – Growth 1 - Cells undergo majority of growth

S – Synthesis - Each chromosome replicates (Synthesizes) to produce sister chromatids

• Attached at centromere• Contains attachment site (kinetochore)

G2 – Growth 2 - Chromosomes condense - Assemble machinery for division such as centrioles

G0 – cells exit the cell cycle from G1 (they are not preparing for division.

** During interphase, the cells are also carrying out their function(s).

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G2 of Interphase

• A nuclear envelope bounds the nucleus.

• Centrosome duplicated (one for each end of the cell).

• Chromatin is not fully are condensed, but DNA has been replicated during S phase.

• Chromosomes are not yet visible.

G2 OF INTERPHASE

Centrosomes(with centriole pairs) Chromatin

(duplicated)

Nucleolus Nuclearenvelope

Plasmamembrane

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Prophase

• Chromatin has condensed -chromosome are visible.

• Each duplicated chromosome appears as two identical sister chromatids joined together at the centromere.

• The mitotic spindle begins to form. It is composed of the centrosomes and the microtubules that extend from them.

• The centrosomes move to opposite ends of the cell and microtubules become longer.

PROPHASE

Early mitoticspindle

Aster

Centromere

Chromosome, consistingof two sister chromatids

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Metaphase

•The centrosomes are now at opposite ends of the cell.

•The chromosomes line up on the metaphase plate, an imaginary plane that is equidistant between the spindle’s two poles.

•The centromeres lie on the metaphase plate.

• Kinetochores are attached to microtubules coming from opposite poles. • The entire apparatus of microtubules is called the spindle because of its shape.

METAPHASE

Spindle

Metaphaseplate

Centrosome at one spindle pole

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Anaphase

• Sister separate. Each chromatid thus becomes a chromosome.

• Chromosomes begin moving toward opposite ends of the cell (their kinetochore microtubules shorten).

• The chromosomes move centromere first.

• The cell elongates as the other microtubules lengthen.

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

ANAPHASE

Daughter chromosomes

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Telophase

• Nuclei begin to form in the “new” cells.

• Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope.

• The chromosomes become less condensed – chromatin.

• Mitosis is completed.

TELOPHASE AND CYTOKINESIS

Nucleolusforming

Cleavagefurrow

Nuclear envelopeforming

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Cytokinesis• Cleavage of cell into two

halves– Animal cells

Constriction of actin filaments

Cleavage furrow

– Plant cells Cell plate

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Mitosis IN an Animal Cell

G2 OF INTERPHASE PROPHASE PROMETAPHASE

Centrosomes(with centriole pairs) Chromatin

(duplicated)

Early mitoticspindle

Aster

CentromereFragmentsof nuclearenvelope

Kinetochore

Nucleolus Nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Kinetochore microtubule

Nonkinetochoremicrotubules

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METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate Nucleolus

forming

Cleavagefurrow

Nuclear envelopeformingCentrosome at

one spindle poleDaughter chromosomes

Mitosis IN an Animal Cell

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Mitosis in a plant cell

1 Prophase. The chromatinis condensing. The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is staring to from.

Prometaphase.We now see discretechromosomes; each consists of two identical sister chromatids. Laterin prometaphase, the nuclear envelop will fragment.

Metaphase. The spindle is complete,and the chromosomes,attached to microtubulesat their kinetochores, are all at the metaphase plate.

Anaphase. Thechromatids of each chromosome have separated, and the daughter chromosomesare moving to the ends of cell as their kinetochoremicrotubles shorten.

Telophase. Daughternuclei are forming. Meanwhile, cytokinesishas started: The cellplate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell.

2 3 4 5

NucleusNucleolus

ChromosomeChromatincondensing

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Cytokinesis In Animal And Plant Cells

Daughter cells

Cleavage furrow

Contractile ring of microfilaments

Daughter cells

100 µm

1 µmVesiclesforming cell plate

Wall of patent cell Cell plate

New cell wall

(a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM)

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Cell Division in Prokaryotes

• Prokaryotes lack nuclei and membrane-bound organelle

• DNA is a circular chromosome

• Binary Fission:

• DNA is copied two circular chromosomes• New membrane begins to develop• Cells grows to double its size• Pinches off into two identical cells