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CELL DIVISION

A. OVERVIEW
Cell division is the process through which a cell reproduces itself.  Cell division creates duplicate offspring in unicellular organisms and provides for growth, development, and repair in multicellular organisms.  Cell division involves the distribution of identical genetic material, DNA, to two daughter cells.  A cell’s total hereditary donation, from both parents, is called its genome.

B. CELL DIVISION IN BACTERIA (PROKARYOTES)

  1. Bacteria reproduce by a process called binary fission, meaning “division in half”
  2. Most bacterial genes are carried on a single chromosome that consist of circular DNA molecule.  The single circular DNA molecule attach to the plasma membrane replicates. Growth of the membrane separates the duplicate chromosomes into two identical daughter cells.

C. CELL DIVISION IN EUKARYOTIC CELLS (MITOSIS & MEIOSIS)

  1. Eukaryotes have much larger genomes than prokaryotes. Copying and dividing the genes in eukaryotic cells involves a precise process.
  1. Each eukaryotic species has a characteristic number of chromosomes in each cell nucleus. The somatic cells, all the body’s cells except the reproductive cells, have 46 chromosomes and the gametes, the reproductive cells (sperm cells and egg cells) have 23 chromosomes 22 autosomes and 1 sex chromosome, which is half as many chromosomes of somatic cells.
  2. Each chromosome is composed of a long linear DNA molecule with various proteins and histones. This DNA-protein complex is called a chromatin.

 

SOMATIC CELL DIVISION: CELL CYCLE & MITOSIS

In preparation for cell division each chromosome is duplicated and produces 2 sister chromatids, during interphase, that attach together at a region called the centromere. Then sister chromatids separate during mitosis, the division of the nucleus.  Finally the cytoplasm divides to form genetically identical daughter cells during, cytokinesis.

  1. The cell cycle includes the mitotic (M) phase and Interphase.
  2. During the cell cycle, interphase, will occur first and is the longest portion of the cell cycle. In fact, interphase accounts for 90%, of the cell cycle. During interphase, there are 3 phases, G1, S and G2. During all 3 phase the cell grows by synthesizing proteins and producing cytoplasmic organelles. 
  3. Specifically, the G1 phase, is the growth of the cell before DNA replication occurs. Late in the G1 phase a crucial checkpoint occurs called the restriction point, which tells the cell to go/no go on dividing.  If “go” the cell moves on into the S phase if “no go” the cell moves on into the G0 (G zero), which is a non-dividing state.  The S phase, is the portion of interphase in which DNA is replicated, when each chromosome is duplicated to produce 2 sister chromatids. The G2 phase, is the second growth phase that occurs after DNA replication.
  4. Now that the cell has successfully copied it’s DNA into to sister chromatids that are attached by a centromere, the nucleus and cytoplasm will divide to produce two cells during the (M) mitotic phase.
  5. Part 1 of the (M) mitotic phase is Mitosis and part 2 is Cytokinesis. Mitosis is the process in which the nucleus divides.  It consist of prophase, prometaphase, metaphase, anaphase, and telophase. (see handout)
    1. Prophase: early formation of mitotic spindle, nucleoli disappears, the chromosomes are visible under a light microscope.
    2. Prometaphase: nuclear envelope disappears, the mitotic spindle invades the nucleus and interact with each chromosome.
    3. Metaphase: each chromosome lines up along the metaphase plate, an imaginary equator on the mitotic spindle.  The identical chromatids of each chromosome attach to the mitotic spindle at the centromeres.
    4. Anaphase: begins with each sister chromatids being separated at the centrosome region.  Now each chromatid moves to opposite sides of the mitotic spindle.  Each end of the mitotic spindle now has identical information called daughter chromosomes.
    5. Telophase:  at each end where the daughter chromosomes are, the daughter nuclei begin to form, the nuclear envelope forms from fragments of the parent cell’s nuclear envelope, and portions of the cytoplasmic organelles also begin to form.
    6. The equal division of the original nucleus into 2 identical nuclei is complete and Mitosis is over.  But how will the two nuclei become two separate cells.  This will occur during cytokinesis, the division of the cytoplasm.
    7. Cytokinesis: a cleavage furrow will form between each set of nuclei and pinch the one cell into two identical cells. This marks the end of the mitotic (M) phase.

GAMETE CELL DIVISION: (SEX CELL CYCLE) &MEIOSIS

Offspring acquire genes from their parents by inheriting chromosomes through the sex cell life cycle.  Fertilization and meiosis alternate during the sex cell life cycle. By means of sexual intercourse, a haploid cell (n), a cell with a single chromosome set, from both a male (sperm) and a female (ovum) combine via fertilization to produce a zygote.  The zygote has two haploid sets of chromosomes making it a diploid cell (2n). (n=23 and 2n=46) This zygote goes on to mature into a multicellular organism via mitosis.  At sexual maturity, ovaries and testes produce haploid gametes via meiosis.

  1. There are two cell divisions of meiosis, meiosis I and meiosis II, to  produce 4 haploid daughter cells each with half of the number of chromosomes as the parent cell.
  2. MEIOSIS I: Meiosis I is preceded by an interphase, during which each chromosome is replicated into two identical sister chromatids that remain attached at their centromere.  Prophase I: chromosomes begin to condense and form homologous chromosome pairs, via synapsis, that are visible, under a microscope. As well, the meiotic spindle begins to form and the nuclear envelope disappears. Typically the Prophase I can last 90% of time required for meiosis. Metaphase I: the spindle as formed the metaphase plate and the chromosomes align along this plate. Anaphase I: the sister chromatids remain attached and the homologous chromosomes separate from each other going to opposite ends of the plate. Telophase I and Cytokinesis: The spindle apparatus begins to disintegrate and the cleavage furrow forms (in animals) and  cell plate (in plants ) to divide the cytoplasm forming two daughter cells that have a haploid set of chromosomes.
  3. Now that meiosis I has ended, the two daughter cells now enter into meiosis II.  There is no interphase II !!!!
  4. MEIOSIS II:  Prophase II: a spindle apparatus forms and the chromosomes progress toward the metaphase plate II. Metaphase II: the chromosomes line up along the metaphase plate. Anaphase II: the centromeres of the sister chromatids separate and the individual chromosomes move towards opposite ends of the pole. Telophase II and Cytokinesis: the spindle has disappeared, the nuclear envelope begins to reappear at each end of the cell, and then cytokinesis occurs forming four daughter cells, each with a haploid number if unreplicated chromosomes.

CELL CYCLE CONTROL & CANCER CELLS

CANCER CELLS

  1. Cancer cells do not respond to the body’s indication to stop dividing and growing, these cells grow and divide excessively and invade other tissues and if unchecked could kill an organism.
  2. When a cancer cell develops the body’s immune system will often destroy it.  However if the cell proliferates, it will form a tumor, which is a mass of cancer cells within a tissue.  There are two types of tumors Benign tumors and Malignant tumors.  The Benign tumors remain at their original site and can be removed by surgery.  The Malignant tumors cause cancer as they invade and disrupt the functions of one or more organs.  Malignant tumors tend to have abnormal numbers of chromosomes and metabolisms.
  3. Malignant tumors are those that spread to surrounding tissues via the circulatory system to other parts of the body, this process is called metastasis.

CLICK LINK BELOW FOR CELL CYCLE CONTROL & CHECK POINTS

CELL CYCLE CONTROL & CHECK POINTS

WHY IS THE CELL CYCLE IMPORTANT?

The cell cycle is responsible for growth of cell, repair of cells that have been wounded, replacement of cells that have died.  For example, the red blood cells of the human body last about 120 days, therefore, replacement cells are produce by cell division in the bone marrow.

CELL DEATH

  1. APOPTOSIS is the programmed death of cells in the body.
  2. Mitosis (cell division) and apoptosis (cell death) are continuous processes that occur in a series of steps and are both initiated by signals in the extracellular environment.
  3. The balance between cell division and death maintains tissues during growth, development, and repair. In prenatal development these processes both contribute to the formation of  body parts.
  4. After birth, mitosis and apoptosis protect and maintain the body. Disruption of the balance between cell division and cell death can lead to cancer or other disorders.

 

MITOSIS VS. MEIOSIS

DNA REPLICATION

Mitosis: occurs during interphase, the longest phase.
Meiosis: occurs once, during interphase I prior to meiosis.

NUMBER OF DIVISION

Mitosis: one division including prophase, metaphase, anaphase, and telophase
Meiosis: two divisions, each including prophase, metaphase, anaphase, and telophase.

SYNAPSIS OF HOMOLOGOUS CHROMOSOMES

Mitosis: does not occur
Meiosis: synapsis is unique to  meiosis, it occurs during the prophase I

NUMBER OF DAUGHTER CELLS AND GENETIC COMPOSITION

Mitosis: two cells each diploid (2n) and genetically identical to the parent cell
Meiosis: four daughter cells each haploid (n) and genetically nonidentical to the parent or to each other

ROLE IN THE ANIMAL BODY

Mitosis: enables multicellular adults to rise from zygote (development), growth, and repair of tissues
Meiosis: produces gametes and introduces genetic variation amongst gametes.