biology chapter 12 - wcjc

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Biology – Kevin Dees

Chapter 12The Cell Cycle


Omnis cellula e cellula

• 1855- Rudolf Virchow– German scientist – all cells arise from a

previous cell– Every cell from a cell

– In order for this to be true, cells must have the ability to divide

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Cell division• Cell division is an

important process:– Reproduction -

• many unicellular organisms are able to reproduce simply by dividing their cells

• Other higher life forms form sexual cells (gametes) or other asexual reproductive cells (spores) by cell divisions


Cell division• Cell division is an

important process:– Growth and

Development• Adding cells to

overall body mass • Physical changes

that occur in cells

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Cell division• Cell division is

an important process:– Maintenance

• Tissue renewal• Replace

damaged cells


Cell Cycle

• The cell cycle is the life of a cell from the time it is produced by the division of another cell until its own division into two cells

• Not all cells complete the cell cycleNot all cells are capable of cell division

• Regulation of the cell cycle is very important – uncontrolled cell division forms the basis of cancers

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Genetic organization

• In order for a cell to divide and produce a viable copy, the genetic material must be passed from one cell to the next

• The entire complement of genetic material in a cell is called the genome– A typical human cell has >2 m of DNA in side

each nucleus– Before the cell can divide, the entire genome

must be copied and separated


• This duplication and separation is manageable because the DNA molecules are organized into either chromatin or chromosomes

• In humans, recall that there are 23 pairs of chromosomes – 46 total in a somatic cell – diploid (2n)

• Gametes – sexual cells are haploid (n)

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• Chromatin –– DNA molecules

and proteins – Very long– Allow for each

gene to be expressed

• Chromosome– Condensed DNA

(chromatin) and proteins


When cells divide, the genome must be copied

• DNA replication is the process in which the genome is copied

• The duplicated DNA is organized into chromosomes

• Each duplicated chromosome consists of a pair of sister chromatids joined by a centromere

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For cell division to occur:• Two processes must occur

– Division of the nucleus – karyokinesis– Division of the cytoplasm – cytokinesis

– Two basic types of karyokinesis• Mitosis – used in somatic cells to produce two identical cells• Meiosis – used to produce gametes

– The cell cycle alternates between cell divisions and periods of normal activity between divisions

• interphase

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Cell cycle of a somatic cell• Interphase

– Gap 1 – growth and normal function

– Synthesis – DNA replication

– Gap 2 – prepares chemically for division

• Mitotic phase– Mitosis –

karyokinesis– cytokinesis


Mitotic phase –karyokinesis by mitosis

• Prophase (your book refers to late prophase as prometaphase)

• Metaphase• Anaphase• Telophase• compare and contrast animal and plant cell mitosis

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Animal cell-Interphase

• Nuclear envelope complete

• Nucleolus present• Chromatin

– Duplicated during S

• Centrosome present


Prophase

• Chromatin condenses into chromosomes

• Nuclear envelope erodes

• Nucleolus disappear• Centrosome divides

and mitotic spindle formation begins

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Metaphase

• Chromosomes attached to mitotic spindle by centromeres

• Chromosomes line up along equator of cell


Anaphase

• Sister chromatids are pulled apart

• Sister chromatids become a chromosome– Recall that each sister

chromatid was a copy of the chromosome made during DNA replication

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Telophase

• Daughter nuclei begin to form

• Chromosomes unwinds into chromatin

• At completion – G1 of interphase

• Cytokinesis occurs– Cleavage furrow


• Cytokinesis in animal cells

• cleavage furrow formed by constriction of proteins

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Mitosis in plant cells• No centrosomes!!!


Cytokinesis in plant cells

• Formation of cell plate by vesicles from Golgi

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Regulation of the cell cycle

• The timing and rate of cell division in different parts of a plant or animal are crucial to normal growth, development and maintenance.– Human epidermal cells divide constantly– Human liver cells rarely divide but can if

needed– Mature muscle and nerve cells do not have

the ability to divide• What causes this? Same genome???

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Current research points to chemical checkpoints

• Primarily using:– Kinase enzymes– Cyclins

– Much of this research is still in its early stages but is crucial to our understanding of cancer

– It is interesting that understanding the basic cycle of cell life seems so simple yet reminds us of how much we really have to learn!


Cancer

• Cancer cells do not respond normally to these control mechanisms

• Cancer cell divide excessively and can invade other tissues

• normal cells divisions are controlled by cell division mechanisms– Contact inhibition – density dependent

inhibition– Cancer cells do not exhibit this regulation

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• Cancer begins when a single cell in a tissue undergoes transformation– Process that converts a normal cell into a

cancerous one– Normally, the immune system recognizes a

transformed cell and destroys it– If the transformed cell evades the immune

system it may form a tumor• Mass of abnormal cells

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Tumors• Benign tumor

– abnormal cells remain at the original site and simply form a ‘lump’

– Most are not harmful and are easily removed

• Malignant tumor– Invasive to other tissues– Can impair function of tissue or organs– May cause serous damage– May also be capable of metastasis

• Able to spread to distant parts of the body through lymph or blood vessels


biology chapter 12 - wcjc

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