Cell Division

Cell division consists of two phases:● nuclear division● cytokinesis = division of the cytoplasm

There are two kinds of nuclear division:

●MITOSIS = creates two identical daughter cells●MEIOSIS = a reduction division that produces daughter cells containing half the genetic info. of the parent cell

● During periods of non-cell division the genetic material exists in an uncoiled form called Chromatin

● Before cell division can take place the chromatin must begin to condense into tightly coiled bodies called Chromosomes

● Each chromosome is made of two identical halves called Sister Chromatids that are joined by a Centromere

● Diploid Cells (2n): two copies of every chromosome; form a Homologous Pair

● Homologous Pair: one homologue is from the mother, one from the father

● HUMANS = 46 chromosomes, 23 homologous pairs, 92 chromatids

●When a cell is not dividing = chromatin is enclosed in the nuclear envelope●Microtubule Organizing Centers (MTOC) = Centrosomes contain a pair of centrioles used in cell division; located in cytoplasm●The non-dividing period of the cell cycle = INTERPHASE

MITOSIS: there are four phases in mitosis = prophase, metaphase, anaphase, and telophase

PROPHASE:● Nucleoli disappear● Chromatin condenses into chromosomes● Nuclear envelope breaks down● Mitotic Spindleis assembled as Centrosomes

(MTOC) move toward the poles of the cell● Microtubules from the centrosomes connect to a

region of the centromere called the Kinetochore

Metaphase

●Chromosomes line up along the Metaphase Plate●Plate exists between the poles of the cell●Identical chromatids are attached to kinetochore fibers radiating from opposite ends of the cell

Anaphase

●Paired centromeres of each chromosome move apart●Move toward opposite ends of cell●Cell elongates●Result = 2 poles have identical collections of chromosomes

Telophase

●Concludes nuclear division●Nuclear envelope develops around each pole, forming 2 nuclei●Chromosomes uncoil into chromatin●Cytokinesis occurs – divides the cytoplasm into 2 cells

●Once Mitosis is completed, Interphase begins.●Interphase is characterized as a period of growth.

Growth is divided into 3 periods: G1, S, G2 (G = Gap)

●G1 = growth and synthesis of non-DNA compounds like lipids, proteins, and carbs. Begins as soon as mitosis ends.

●S = DNA is synthesized as chromosomes are duplicated

●G2 = second growth phase, metabolic activities, preparation for mitosis

Fast Facts:

●90% of the cell cycle is spent in Interphase●A cell that does not enter the S phase will not divide●Mitosis is unique to Eukaryotes●Mitosis is reliable, only one error per 100,000 cell divisions

MEIOSIS

●Cell division of the sex cells (gametes), sperm cell and egg cell●Consists of two groups of divisions (Meiosis I & Meiosis II)

Interphase I :● Chromosomes replicate as in mitosis● Duplicated chromosomes consist of two

identical sister chromatids

MEIOSIS I Prophase I:

● Nucleolus disappears● Chromatin condenses into chromosomes● Nuclear envelope breaks down● Spindle apparatus develops● Homologous chromosomes pair, a process

called synapsis● Pairs of homologues = Tetrads

Synapsis

●During synapsis, corresponding regions along non-sister chromatids form close associations called Chiasmata = sites where genetic material is exchanged●Crossing Over= the exchange of genetic material between non-sister chromatids

Metaphase I:● Homologous pairs are lined up at the

metaphase plate● Microtubules extend from one pole and attach

to centromere (kinetochore) of one member from each homologous pair

Anaphase I:● Homologues within tetrads uncouple● Move towards opposite poles

Telophase I:● Chromosomes have reached poles● Nuclear envelope redevelops● Each pole has half the number of chromosomes● Each chromosome contains 2 chromatids● Cytokinesis follows● Interphase II may follow (no duplication)

Prophase II:● Nuclear envelope disappears● Spindle fibers form● NO CROSSING OVER!

Metaphase II:● Chromosomes align singly at the Metaphase

plate

Anaphase II:● Each chromosome is pulled apart into 2

chromatids● Chromatids migrate to poles

Telophase II:● Nuclear envelope reappears● Cytokinesis occurs● END RESULT = 4 Haploid daughter cells

Genetic Variation

●Occurs as a result of meiosis = Sexual Reproduction●Reassortment = Genetic Recombination; 3 events are responsible:●Crossing over: Occurs during Prophase I; non-sister chromatids of homologous chromosomes exchange pieces of genetic material●Independent Assortment of Homologues: random separation during Metaphase I●Random Joining of Gametes: which sperm cell fertilizes the egg is random

Why Do Cells Divide?

●Maintain a low surface area to volume ratio●Many microscopic cells are more efficient than a few large ones●Maintain a low genome to volume ratio (genome controls the cell)●Density Dependent Inhibition= cell division stops when cell density reaches a certain maximum