CELL DIVISION (MITOSIS)
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
1. Eukaryotes have much larger genomes than prokaryotes. Copying and
dividing the genes in eukaryotic cells involves a precise process.
2. 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.
3. Each chromosome is composed of a long linear DNA molecule with
various proteins and histones. This DNA-protein complex is called a
chromatin.
OVERVIEW OF CELL CYCLE: 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.
D. CELL CYCLE
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)
a. Prophase: early formation of mitotic spindle, nucleoli disappears,
the chromosomes are visible under a light microscope.
b. Prometaphase: nuclear envelope disappears, the mitotic spindle
invades the nucleus and interact with each chromosome.
c. 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.
d. 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.
e. 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.
f. 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.
g. 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.
FOR A BETTER UNDERSTANDING SEE FIGURES IN TEXTBOOK
D. WHY IS THE CELL CYCLE IMPORTANT?
1. 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.
E. 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.
F. 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.
MEIOSIS AND SEX CELL LIFE CYCLE
A. OVERVIEW
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=46 and 2n=23) This zygote goes
on to mature into a multicellular organism via mitosis. At sexual maturity, ovaries
and testes produce haploid gametes via meiosis.
B. 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.
FOR A BETTER UNDERSTANDING SEE FIGURES IN TEXTBOOK.
B. 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.