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BIO 156

Chapter 16

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Chromosomes, Cell Division, and the Cell Cycle

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The Cell Cycle

The cell cycleis divided intotwo parts: Interphase Cell

division

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Interphase is divided intothree parts :

G1 , S , and G2 During G1, the cell

produces RNA, proteins,and other molecules.

During the S phase, theDNA replicates

During G2, mitochondriadivide and precursors of the spindle fibers form

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Nuclear and cytoplasmic division occur separately. Cell division follows interphase. It requires two separate but related

processes: mitosis , or nuclear division, andcytokinesis , or cytoplasmic division.

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Human body cells contain 46 chromosomes that

contain all of the genetic information required tocontrol cellular activity. Each organism has a set number of

chromosomes All body cells, except the germ cells, are called

somatic cells. Body cells contain a full complement of

chromosomes and are described as diploid. Germ cells or gametes contain half the

number of chromosomes of somatic cells and

are referred to as haploid cells.

The Chromosome

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Chromosomescondense after replication, whichfacilitates mitosis. Condensed

chromosomes aremetabolicallyinactive.

The number of chromatids inchromosomesvaries, dependingon the stage of thecell cycle.

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Amniocentesis

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Normal Karyotype

Downs Syndrome Karyotype

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Cell Division

Mitosis is divided into four stages: Prophase

Metaphase Anaphase Telophase

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Prophase Chromosomes condense Nuclear envelope disappears Centrioles divide and migrate to opposite poles

of the dividing cell Spindle fibers form and attach to chromosomes

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Prophase

ProphaseInterphase

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Metaphase Chromosomes line up on equatorial plate of

the dividing cell. Anaphase

Chromosomes begin to separate.

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Telophase Chromosomes

migrate or arepulled to oppositepoles.

New nuclear envelope forms.

Chromosomesuncoil.

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Cytokinesis, the division of the cytoplasm, ismade possible by contraction of microfilaments beneath the cell membrane. Cytokinesis begins in late anaphase or

early telophase. In human cells, cytokinesis results from the

contraction of microfilaments lying beneaththe plasma membrane.

Mitosis

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Control of the Cell Cycle Many factors appear to control the cell

cycle. Research suggests the cell cycle is

controlled in part by chemical messagesproduced in the cytoplasm.

External controls such as hormones,

growth regulators, and cell contact areimposed on the cell.

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