The Cell Cycle

Ch. 12

Cell Cycle – life of a cell from its origin in the division of a parent cell until its own division into two.

Cell division allows sexually reproducing cells to develop from a single cell (zygote)

Cell division functions in growth, repair, and reproduction

There are two types of cell division: Mitosis – produces two genetically identical

“daughter cells” Meiosis – occurs with sexually reproducing

organisms and results in cells with half the number of chromosomes of the parent cell

Chromosomes

Coiled and condensed strand of DNA and proteins

The fiber made of DNA and proteins is called chromatin

A replicated chromosome has two “sister” chromatids One is an exact copy of the other

The centromere holds the two chromatids together

The kinetochore is a disc-shaped protein on the centromere that attaches the chromatid to the mitotic spindle during cell division

Somatic cells – all body cells except reproductive cells Gametes – reproductive cells (sperm and egg)

Human somatic cells have 46 chromosomes Human gamete cells have 23 chromosomes

Mitotic Cell Cycle

Cycle gone through when a somatic cell is dividing

Two things limit cell size and promote cell division Ratio of volume of cell to surface area Capacity of Nucleus

Interphase Cell grows and copies its chromosomes

3 subphases G1 Phase (first gap)S Phase (DNA synthesis)G2 Phase (second gap)

More than 90% of the life of a cell is spent in interphase When a cell is in interphase and not dividing, the chromatin

is not condensed, it is threadlike Centrosomes – consisting of two centrioles are in the

cytoplasm of an animal cell MTOC’s (microtubules organization centers) are found in

plant cells instead of centrosomes

Mitosis Dividing of the nucleus

Four parts: prophase, metaphase, anaphase, telophase

Cytokinesis Division of the cytoplasm

Steps of Mitosis

Prophase Nuclear membrane begins to disintegrate Chromosomes condense Nucleolus disappears Mitotic spindle begins to form

Metaphase Chromosomes line up at the equator Centrosomes are at opposite poles of the cell

Anaphase Centromeres of each chromosome separate Spindle fibers pull apart sister chromatids

Telophase Chromosomes cluster at opposite sides of cell Nuclear membrane reforms Chromosomes begin to unravel

Cytokinesis Cytoplasm divides In animal cells a cleavage furrow forms down the

middle of the cell as actin and myosin microfilaments pinch in the cytoplasm

Plant cells have a cell plate that forms during telophase from golgi vesicles

Daughter plant cells do not separate from each other, a sticky middle lamella cements the cells together

If cells lose their dependence and inhibition, then they begin to divide uncontrollably

These cells are considered to be “cancerous”