I. How do cells divide or reproduce? Three major processes are common to all forms of cell division/reproduction. They are DNA replication (sometimes called division), separation of the duplicated chromosome(s), and division of the cytoplasm (cytokinesis).
1. Binary fission: This is the simplest type of cell division mechanistically. It is characteristic of all prokaryotes and some protists. It is a form of asexual reproduction.
a. Prokaryotes generally have just one circular chromosome, called a genophore.
b. As DNA replicates, each of the two resulting DNA molecules attaches to the plasma membrane. As the bacterium grows, new plasma membrane is added between the attachment points, causing the DNA molecules to move apart.
c. Cytokinesis, which is cell division, begins around 20 minutes after chromosome duplication is completed. In the middle of the cell a constricting ring forms in the plasma membrane. As this ring constricts it pinches or separates the cell into two daughter cells, each with a complete chromosome.
d. Binary fission is an asexual cloning process. Asexual because one parent cell divides to form two daughter cells. It is a form of cloning because the daughter cells are genetically identical to each other and to the original parent cell.
2. Mitosis: This type of nuclear division is characteristic of most eukaryotes. It functions for asexual reproduction and the multiplication and/or replacement of somatic cells. Mitosis generates two daughter cells with the same genetic information as the original parent cell, and is therefore functionally similar to binary fission. However the comparisons end here. Mitosis is mechanically more complicated than binary fission and virtually always involves the separation of two or more chromosomes. Mitosis will be outlined later in this chapter.
3. Meiosis: This type of nuclear division is characteristic of sexually reproducing eukaryotes. Its main function is to produce cells that contain only half the normal chromosome number for that species. Chromosome rearrangements and shuffling both occur during this process. Meiosis will be outlined in more detail in the following chapter.
1. Chromosome vs. genophore
2. Chromatin and chromosomes
3. Chromosome complement (ploidy): haploid, diploid, triploid, etc.
a. As a rule mature prokaryotes are haploid and adult eukaryotes are diploid.
4. chromosomes and chromatids: sister vs. non-sister chromatids
5. genes
II. The Cell Cycle: this represents the life of a cell. There are three major parts of the cell cycle; interphase, mitosis (M-phase) and cytokinesis
A.
Interphase:
A typical eukaryotic cell will spend most of its life in
interphase. Interphase consists of
three subphases
1. G1 phase: this is the first gap or growth phase. This phase includes all the normal metabolic functions of a cell. A cell that enters into a prolonged G1 phase is said to be in a Go phase.
2. S phase: This designates the time during which DNA replication occurs.
3. G2 phase: the second gap or growth phase: Cells only enter this phase if they are going to divide. As compared to G1, additional metabolic functions occur in this phase that help prepare the cell for mitosis.
B.
M-phase or mitosis: This phase can be further divided into a series of steps or
stages, each distinguished by characteristic features and processes. The following is an outline summary of the
stages of mitosis and some of their distinguishing featues:
1. prophase: chromosomes coil and condense into observable structures; Microtubule structures are assembled to form the spindle fibers, centrosomes, and kinetochores. Two centrosomes separate and migrate toward opposite cell poles.
2. prometaphase: Completion of the spindle apparatus with attachment of the spindle fibers to the kinetochores, and the dissolution of the nuclear membrane.
3. metaphase: Centrosomes are at opposite cell poles; centromeres are aligned along a single plane straddled by the sister chromatids of each chromosome.
4. anaphase: sister chromatids separate and move toward opposite cell poles
5. telophase: each set of sister chromatids, now referred to as chromosomes, come to rest at the opposite cell poles; the mitotic spindle dissolves; nuclear membranes form around each chromosome set; a single cell with two nuclei exists. Technically mitosis ends here, because it pertains only to the division of the nucleus. Cytokinesis may or may not occur, but when it does it typically occurs during telophase.
C.
Cytokinesis or cell division: this is the separation of the cytoplasm into
two cells. A cleavage furrow forms to
separate animal cells whereas in plants a cell plate forms to separate the
cells.
A. Transitions from G1 to S, S to G2,
and G2 to M depend on activation of proteins called cyclin-dependent
kinases, or Cdks.
1. Cdk concentrations are relatively constant throughout
the cell cycle.
2. The concentrations of another family of proteins,
called cyclins, fluctuate throughout
the cell cycle.
3. Cyclins activate Cdk proteins by binding to
them. The cyclin/Cdk complex regulates the activity of yet other proteins
involved in metabolic pathways necessary to produce the different phases of the
cell cycle.
4. The
cyclin/Cdk complex that triggers the transitions from the G2 to the M phase is
called MPF (mitosis, or maturation, promoting factor).
5. Roles of RB, p53 and p21 proteins in cell cycle
regulation and cancer.
B. Factors involved in cyclin regulation.
1. cytoplasmic volume
2. growth factors: PDGF, erythropoetin, interleukin.
3. density-dependent inhibition