The Cell Cycle Chapter 12 Biology – Campbell Reece

The Role of Cell Division Reproduction – unicellular organisms (Amoeba) can use cell division to create new offspring Growth – a fertilized egg divides continuously to produce a multicellular organism Repair – replace cells that die from normal wear and tear or accidents

Cell Division Terminology Genome – the genetic information of a cell Chromosome – one very long, linear DNA molecule Chromatin – DNA and the proteins that maintain the structure of the chromosome Sister chromatids – duplicate copies of the chromosome held together at the centromere

Sister Chromatids

More Terminology Somatic cells – all body cells except the reproductive cells –46 chromosomes in human somatic cells Gametes – reproductive cells (sperm and eggs) –½ the number of chromosomes (23 in humans) Mitosis – division of the nucleus Cytokinesis – division of the cytoplasm

The Mitotic Cell Cycle Interphase (~90% of the cycle) – the cell grows and copies the chromosomes –G 1 phase – “first gap”, the cell grows –S phase – duplication of DNA –G 2 phase “second gap”, the cell grows & prepares for mitosis Mitotic (M) phase (~10%) –Mitosis – Prophase, prometaphase, metaphase, anaphase, telophase –Cytokinesis

The Mitotic Cell Cycle

The Mitotic Spindle Begins to form in the cytoplasm during prophase Consists of microtubules and associated proteins Centrosome – a region containing material that functions to organize the microtubules –The spindle microtubules grow out from them Aster – a radial array of short microtubules extending from each centrosome The spindle includes the centrosomes, the spindle microtubules, and the asters

The Mitotic Spindle

Interphase Nucleus is well defined One or more nucleoli Two centrosomes present (each has a pair of centrioles in animal cells) Microtubules extend from asters Chromosomes not visible

Prophase Chromatin becomes tightly coiled Chromosomes are visible (sister chromatids) Nucleoli disappear Mitotic spindle appears Centrosomes move away from each other

Prometaphase Nuclear envelope disappears Microtubules extend from each pole Microtubules attach to the kinetochore (a structure at the centromere of each chromatid)

Metaphase Centrosomes are now at opposite poles Chromosomes line up at the middle of the cell (on the metaphase plate) The spindle is fully formed Longest stage of mitosis

Anaphase The paired centromeres of each chromosome separate Each chromatid moves to opposite poles of the cell By the end, each pole has an equal set of chromosomes

Telophase and Cytokinesis Nuclear envelope reforms around the chromosomes Chromatin becomes less condensed (chromosomes less visible) Cytokinesis begins A cleavage furrow forms & pinches the cell in two

Cytokinesis: Animals vs. Plants Animals: –Occurs by a process called cleavage –A cleavage furrow (a shallow groove on the surface of the cell near the metaphase plate) forms –The cleavage furrow deepens until the parent cell is pinched in two Plants: –No cleavage furrow –A cell plate forms near the middle of the cell –The cell wall continues to grow until it divides the cell in two

Cytokinesis: Animals vs. Plants

Binary Fission “division in half” A type of cell division used by prokaryotes to reproduce It is believed that binary fission gave rise to mitosis

Regulation of the Cell Cycle Some cells divide often (skin cells), some only when needed (liver cells), and some not at all in a mature human (muscle & nerve cells) Cell cycle control system – a cyclically operating set of molecules that triggers & coordinates events in the cell cycle

Cell Cycle Checkpoints Checkpoint – a critical control point where stop and go-ahead signals can regulate the cycle (found in the G 1, G 2 & M phases)

Cell Cycle Clock Cell cycle clock – the fluctuation in the abundance and activity of cell cycle control molecules set the pace for the cycle –Protein kinases, cyclins, and MPF (“maturation-promoting factor”)

Internal & External Cues Internal signals (from kinetochores) – anaphase does not begin until all of the chromosomes are attached to the spindle External signals –Growth factors – proteins that stimulate cell division –Density-dependent inhibition – cells continue dividing until they run out of room –Anchorage dependence – cells must be attached to something to divide

Cancer Cancer cells do not respond normally to the body’s control mechanisms Cell divide excessively & invade other tissues Tumors –Benign – remain at the original site (usually safe) –Malignant – become invasive enough to impair the functions of one or more organs (cancer) Metastasis – spread of cancer cells away from the original site