Cell Division

*Bacteria and Archaea 1. Chromosomes begin replication. One copy moves to the other end by actin-like protein 2. Chromosome replication continues. One copy is on each side. Cell elongates 3. Replication finishes. Plasma membrane is pinched inwards by a tubulin-like protein. New cell wall is deposited 4. Two daughter cells results

• Replication and Distribution of DNA is possible because DNA is packed into chromosomes: long, linear molecule associated with proteins (histone) • Chromatin: DNA + proteins, long and stringy DNA • Chromosome: Condensed chromatin, higher order of DNA organization • Chromatid: Either of the two strands of replicated chromosomes

• Species have characteristic # of chromosomes • In humans: somatic cells (non - reproductive) have two sets of chromosomes (2 x 23 = 46) = 2n diploid • Gametes (reproductive cells: sperm and egg) have 1 set of chromosomes = (n) haploid *Karyotype: visual representation of the complete set of chromosomes in a cell

• Mitotic (M) phase = mitosis and cytokinesis • Interphase (90%) = cell growth and copying of chromosomes in prep for cell division G1 = gap 1 S = synthesis G2 = gap 2

• Nuclear envelope encloses nucleus • Nucleus contains one or more NUCLEOI (large nucleus structure, ribosome biogenesis) • Two centrosomes form (organize microtubules) • Chromosomes, duplicated from S phase, cannot yet be seen (not condensed)

• Chromatin fibers condense • Nucleoli disappear • Duplicated chromosomes appear as sister chromatids joined at centromeres • Mitotic spindles bein to form • Centromeres move away from each other

• Nuclear envelope USUALLY fragments (microtubules invade nuclear area) • Chromosomes condense further • Each chromatid now has a KINETOCHORE (structure of proteins attached to the centromere) that links each sister chromatid to the mitotic spindle • Microtubules attach to kinetochores • Non-kinetochore microtubules interact with those from opposite pole of spindle

• Centromeres at opposite poles • Chromosomes have arrived at the metaphase plate • Kinetochores of sister chromatids, are attached to kinetochore microtubules coming from opposite poles

• Two sister chromatids separate (cohesion proteins are cleared) = each chromatid becomes a separate chromosome • Daughter chromosomes move toward opposite ends of the cell Non-kinetochore microtubules length = cell elongates

• Two daughter nuclei form in the cell • Nucleoli reappear • Chromosomes become less condensed • Spindle microtubules are depolymerized • Mitosis is complete

• Cleavage furrow is created using a ring of contractile proteins, actin and myosin • When the cleavage furrow reaches the center the cell, it is pinched and the cell becomes two

• A new cell wall is formed by vesicles that merge at the center to become tubular structures that merge to form cell membranes • Pectin and other substances form the middle lamella and cellulose is brought to the lamella by exocytosis which creates the cell wall connecting the two separate cells

• There are multiple checkpoints (3) in the cell cycle • sometimes the cell become unregulated and can lead to cancer