Asexual Cell Division: Asexual cell division makes identical cells where as sexual cell division makes non-identical cells. (Diploid and haploid, 2n and 1n.) Diploid cells have homologous pairs. Haploid cells do not. We will concentrate on eukaryotic cell division, but prokaryotes also divide (a simper process).

• Chromosomes: Chromosomes are composed primarily of DNA and protein (more detail later). The number of chromosomes varies greatly from organisms to organism.
• DNA: The DNA molecule of a eukaryotic chromosome is one long linear molecule.
• Protein: Eukaryotic chromosomes actually have more protein than DNA (by molecular weight). The most abundant protein bound to eukaryotic chromosomes is the class of basic proteins called histones (rich in basic amino acids=plus charge; DNA has negative charge).

• The Cell Cycle: Dividing cell are undergoing the cell cycle: G1 (and G0), S, G2, M (G1=gap 1, S=synthesis, G2=gap 2, M=mitosis). During mitosis both karyokinesis and cytokinesis occur. The pair of centrioles replicates before mitosis in G2 creating 2 pairs of centrioles.
• Cell Cycle Checkpoints: There are at least three checkpoints beyond which the cycle will not proceed if certain events occur (or don't occur).
• The G1/S checkpoint: The cell checks to make sure the cell if of sufficient size and there is no DNA damage
• The G2/M checkpoint: DNA replication must be complete and the DNA must be undamaged in order for the cell to proceed beyond this checkpoint.
• The M checkpoint: The cell checks to insure that the spindle apparatus has properly formed.
• The Stages of the Cell Cycle:

• G1: This is the stage for DNA replication preparation.
• S: This is the stage during which DNA replication (chromosome replication) occurs.
• G2: This is the stage for mitosis preparation.

• M phase (Mitosis): Mitosis is the stage during which the visible events of cell division occur and the replicated chromosomes segregate to separate daughter cells. Mitosis is the type of cell division that makes a multicellular organism from a fertilized egg, replaces old cells, regenerates organs, and is a means of asexual reproduction for many organisms. Mitosis is divided into four stages.



• Prophase: During prophase, chromosomes condense, nuclear membrane disappears, nucleolus disappears, and the spindle apparatus forms. Spindle fibers attach to the kinetochore of the centromere. As chromosomes become visible, it is obvious that chromosome replication has occurred (each chromosome is made up of two sister chromatids held together at the centromere).



• Metaphase: During metaphase, chromosomes line up along equatorial plane and chromosomes are most easily observed. Karyotyping is done using metaphase (or prometaphase) chromosomes. The protein cohesin holds the sister chromatids together. Chromosomes are classified according to their size and the position of their centromeres. This is expressed as the chromatid arm ratio. The short arm of the chromatid is represented by p and the long arm by q, so chromosomes are classified by the q/p ratio.

• Metacentric: Metacentric chromosomes have their centromeres more or less in the middle (q/p = 1.0 - 1.7)
• Submetacentric: The centromere is located nearer to one end (q/p = 1.7 - 3.0)
• Acrocentric: The centromere is located very near one end (q/p = 3.0 - 7.0)
• Telocentric: The centromere located almost at the end (q/p ≥ 7.0). Humans have no true telocentric chromosomes.

• Anaphase: Sister chromatids (now chromosomes) disjoin and move to opposite poles. The movement occurs due to molecular motors and the microtubules of the spindle.

• Telophase: The events of prophase are reversed and cytokinesis occurs. In plants, a cell plate (middle lamella)  forms between the two poles. In animals, a cleavage furrow pinches the cell in two.

Sexual Cell Division: Meiosis: Meiosis is a process that produces 4 haploid cells beginning with one diploid cell through two divisions: meiosis I and meiosis II. Meiosis I is sometimes call a reductional division (?) and meiosis II an equational division (?). In higher organisms, meiosis occurs in the sex organs and produces haploid egg and sperm. Meiosis produces cells that are genetically distinct from each other and from the mother cell (i.e., it is a sexual process).

• Meiosis I: Meiosis I is preceded by an S phase (chromosomes replicate) and is subdivided into 4 stages, just as mitosis is.

• Prophase I: Prophase I is a very busy stage. Besides all the events that occur in prophase of mitosis, two additional events occur: synapsis and crossing over. Prophase I is further subdivided into these stages:
• Leptonema (-tene): This is the stage when chromosome condensation begins. (It continues throughout prophase I.) Chromomeres may be present.
• Zygonema (-tene): Synapsis (pairing) occurs during zygonema with the appearance of the proteinaceous synaptonemal complex, made up of two lateral elements and one central element. Synapsis may start in the end of the chromosomes, the telomeres, which may be bound to the nuclear membrane. It proceeds as the chromosomes "zip up" toward the centromere. Condensation continues. 
• Pachynema (-tene): Tighter pairing and crossing over occurs at this stage. (Details later in the course under Molecular Recombination.) Recombination nodules may appear on the synaptonemal complex during pachynema and are thought to be the sites of crossing over. (There is a correlation between these nodules and crossovers.)
• Crossing Over: Crossing over usually occurs between a pair of chromosomes with most showing double, triple, quadruple (or more) crossovers. However, where along the chromosome the crossover occurs varies with each meiosis. Double crossover may be two-strand, three-strand, or four-strand double crossovers.
• Diplonema (-tene): The synaptonemal complex disappears and pairing is relaxed. However, chromosomes are still held together at the site of the crossovers (chiasmata).
• Diakinesis: The homologous pairs continue to pull apart and terminalization of the chiasmata is seen (the chiasmata slip toward the telomeres).

• Metaphase I: Homologous pairs (still synapsed) line up on the metaphase plate.
• Anaphase I: Homologous pairs (not sister chromatids) disjoin (separate). (If they don't: nondisjuction.)
• Telophase I: Cytokinesis divides the cell into two cells.

• Meiosis II: The events of meiosis II are identical the events of mitosis (except that the cell has half the number of chromosome).
• Prophase II: This stage is often skipped. That is, the chromosomes to not elongate and then recondense. When telophase I is complete, the two daughter cells proceed directly to metaphase II.
• Metaphase II: Chromosomes align along the metaphase plate.
• Anaphase II: Sister chromatids disjoin.
• Telophase II: Cytokinesis produces 4 cells. (When did the cell go from 2n to 1n?
  

• Terminology: Tetrad, Bivalent, Dyad, Monad: The paired chromosomes at prophase I can be called a tetrad or bivalent. A chromosome consisting of just one chromatid is a monad. If it has two chromatids, it is a dyad.
  
• Variations: In males, the above pattern is followed: 2n spermatogonia become primary spermatocytes (cells in the middle of meiosis I) which become secondary spermatocytes (cells in the middle of meiosis II), which become 1n spermatids. These lose most of their cytoplasm and grow a flagellum and become sperm. In females, the same pattern is observed (oogonia and oocytes), but cytokinesis at telophase I and II is unequal producing a primary and a secondary polar body which do not survive. Timing of meiosis: Prophase I arrest in human females. Meiosis II not completed until after fertilization. (Eggs may be made throughout adulthood?)(Or may NOT?)
  
• So, what is the chance that any two human sperm will be identical??