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