2 - Cell divisions

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Chapter 12:The Cell Cycle


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The Cell Cycle

1. G 1 Phase 1st

growth phase2. S Phase DNA duplicated3. G 2 Phase Final growth phase4. Mitosis5. Cytokinesis

Purpose of the first three phases (Interphase) to duplicatecell contents; 90% of the cells growth cycle

Purpose of Mitosis to divide the genetic material into exact two halves

Purpose of Cytokinesis to divide all other contents (except nucleus) into two cells


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Fig. 19.1


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Important Notes

DNA wraps some special proteins to form more stablestructure called chromosomes

Chromosome are found inside nucleus

Human - 46 chromosomes, 23 pairs (1 set of 23 from egg, 1set of 23 from sperm)

Gene is a segment of DNA that is responsible for controlling

a trait Each chromosome contains thousands of genes (these makeup our traits)


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Hum an fe m ale chro m oso m es


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Hum an fe m ale karyotype


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Hum an m ale chro m oso m es


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Hum an m ale karyotype


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Types of cell division

Mitosis: Growth, development & repair

Asexual reproduction (yields identical cells) Occurs in somatic (body) cells

Meiosis: Sexual reproduction (yields different cells)

Occurs in specific reproductive cells


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Process of mitosis

Aim: Two identical daughter cells exact copy of mother cellformed

Requirements the whole DNA must be copied exactly


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Phases of Mitosis

1. Prophase

2. Prometaphase

3. Metaphase

4. Anaphase

5. Telophase


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1. Prophase

- Chromatin condenses, this causes the chromosomes tobegin to become visible

- Centrosomes separate, moving to opposite ends of the

nucleus

- The centrosomes start to form a framework used toseparate the two sister chromatids called the mitoticspindle, that is made of microtubules

- Nu cleol u s disappears


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2. Prometaphase

- Nuclear envelope fragments

- Chromosomes become more condensed

- A kinetochore is formed at the centromere, the point wherethe sister chromatids are attached

- Microtubules attach at the kinetochores


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3. Metaphase

- Chromosomes align on an axis called the metaphase plate

- Note: the spindle consists of microtubules, one attached toeach chromosome


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4. Anaphase

- Each centromere splits making two chromatids free

- Each chromatid moves toward a pole

- Cell begins to elongate, caused by microtubules not associated with the kinetochore


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5. Telophase

Formation of nuclear membrane and nucleolus

S hort and thick chromosomes begin to elongate to formlong and thin chromatin

Formation of the cleavage furrow - a shallow groove inthe cell near the old metaphase plate

Cytokinesis = division of the cytoplasm


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Mitosis in an onion root


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T he h um an life cycle


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T hree sex u al life cycles differing in the ti m ing of m eiosis and fertilization


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P reparation of a h um an karyotype


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Meiosis

Type of cell division that halves number of chromosomes ( e.g. , 2N to 1N)

Process of 2 successive divisions Product is gamete, essential for sexualreproduction


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Ov er v iew of m eiosis: how m eiosis red u ces chro m oso m e n umb er


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T he res u lts of crossing o v er d u ring m eiosis


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A co m parison of m itosis and m eiosis


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A co m parison of m itosis and m eiosis: s umm ary


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E volutionary advantage

asexual reproduction (mitosis) easy, rapid, effective way to reproduce

useful in stable environment lack of genetic diversity among offspring

sexual reproduction (meiosis) promotes genetic variability

useful in dynamic environment

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2 - Cell divisions