CHAPTER 5: CELL DIVISION-B2
I. Cell Increase and Decrease
A.
1. Cell division increases the number of somatic or body cells.
2. Cell division consists of mitosis and cytokinesis.
B.
1. Apoptosis or programmed cell death decreases the number of cells.
2. Ex. Death of cells between your fingers and toes.
C.
1. Defined-The cell cycle is an orderly set of stages that takes place
between the time a cell divides and the time the resulting cells also
divide.
2. The Stages of Interphase (not part of mitosis or meiosis)
a.
b. This is when the cell carries on its usual functions.
c.
G1 stage occurs before DNA synthesis
S stage includes DNA synthesis,
G2 stage occurs after DNA synthesis.
3.
a. timing-following interphase, the cell enters the mitotic stage.
b. When complete, two daughter cells are present.
4. Apoptosis-During apoptosis, thecell progresses through a typical
series of events that brings about its destruction.
II.
A. Overview
1. Eukaryotic cells have evolved a complex system of both internal and ‘
external signals for regulation of the cell cycle.
B.
1. Carcinogenesis, the development of cancer, is a multistage process
involving disruption of normal cell division and behavior.
2. Proto-oncogenes encode protein that promote the cell cycle and
prevent apoptosis.
3. Tumor suppressor genes encodes proteins that stop the cell cycle
and promote apoptosis.
III. Maintaining the Chromosome Number
A.
1. At the time of cell division, the chromatin condenses to form highly
compacted structures called chromosomes.
2. Each species has a characteristic chromosome number called the
diploid or 2n number because it contains two (a pair) of each type
of chromosome.
a. One set from EACH parent
b. Two genes control each trait.
B.
1. Mitosis is nuclear division in which the chromosome number stays constant.
2. A 2n nucleus divides to produce two daughter nuclei that are also 2n.
3. Before nuclear division takes place, DNA replication occurs.
4. A duplicated chromosome is composed of two sister chromatids
held together at a region called the centromere.
C.
1. Principle-During mitosis, a spindle brings about an orderly distribution
of chromosomes to the daughter cell nuclei.
2.
Mitosis is arbitrarily divided into four phases:
a.
~The chromosomes are now visible.
~Spindle fibers appear and thenuclear envelope begins to disappear.
~
b. Metaphase
~The chromosomes are attached to spindle fibers and line up at the
metaphase plate. (equator of cell)
c.
~The sister chromatids separate, becoming daughter chromosomes that
move toward the opposite poles of the spindle.
d.
~The spindle disappears, the nuclear envelope re-forms, and
the chromosomes decondense.
3.
a. Exactly the same phases occur in plant cells as in animal cells.
b. Although plant cells have a centrosome and spindle, there are no
centrioles or asters during cell division.
4.
a. Cytokinesis usually accompanies mitosis, but is a separate process.
b.
~A cleavage furrow, an indentation of the membrane between the
two daughter nuclei, forms during anaphase.
~A contractile ring pinches the cleavage furrow separating
the two cells.
c.
~The rigid cell wall does not permit cytokinesis by furrowing.
~Instead, cytokinesis in plant cells involves building a new cell wall
between the daughter cells.
IV.
A. Meiosis reduces the chromosome number in such a way that the daughter nuclei receive
only one of each kind of chromosome.
B.
1. When a cell is diploid, the chromosomes occur in pairs.
2. The members of a pair are called homologous chromosomes.
3. Meiosis is divided into meiosis I and meiosis II.
a.
During meiosis I, the homologous chromosomes separate.
b.
~During meiosis II, the sister chromatids separate.
~The daughter cells mature into gametes or sex cells—sperm and egg that
fuse during fertilization.
~Fertilization restores the diploid number of chromosomes.
C.
1. Meiosis requires two nuclear divisions
2. Results in four daughter nuclei each with half the number of
chromosomes as the parental cell.
3. During meiosis I, genetic recombination of the parental genes occurs through
two key events:
a.
b. independent assortment.
4. First Division-Meiosis I has four phases called prophase I, metaphase I,
anaphase I, and telophase I.
a.
~synapsis occurs followed by crossing-over.
b.
~the homologues align at the metaphase plate.
c.
~Independent assortment occurs when these pairs
separate from each other.
d.
~In some species, the nuclear envelopes re-form and
nucleoli reappear.
~there are 2 cells with pairs of chromosomes present
e.
~The period of time between meiosis I and meiosis II
~No replication of DNA occurs.
5.
a. sister chromatids separatefrom each other.
b. Four daughter cells result.
D.
1. Meiosis produces haploid cells that are no longer identical to the diploid
parent cell.
2. Crossing-over and independent assortment result in different combinations of
chromosomes than the parental cell.
3. Upon fertilization, the combining of chromosomes from genetically different
gametes helps ensure that offspring are not identical to their parents.
V.
A. Figure 5.14 compares meiosis to mitosis.
B.
1. Meiosis occurs only at certain times in the life cycle of sexually reproducing
organisms.
2. Mitosis occurs almost continuously in all tissues during growth and repair.
C.
Tables 5.1 and 5.2 compare meiosis I and meiosis II to mitosis.
D.
During meiosis I, but not mitosis, homologous chromosomes pair, undergo crossingover, and separate.
E.
The events of meiosis II are just like mitosis except that the nuclei contain
the haploid number of chromosomes.
VI.
A. Overview
1. The human life cycle requires both meiosis and mitosis.
B. Spermatogenesis and Oogenesis in Humans
1.Spermatogenesis occurs in thetestes of males and produces sperm.
2. Oogenesis occurs in the ovaries of females and produces eggs.
3.
~ Primary spermatocytes, which are diploid, divide via meiosis to produce four
spermatids, which mature into sperm.
4.Oogenesis
a.Primary oocytes, which are diploid, divide in meiosis I to produce one secondary
oocyte, which receives almost all the cytoplasm and a polar body, a
nonfunctioning cell.
b. If the secondary oocyte is fertilized by a sperm, it completes the second
meiotic division in which it again divides unequally, forming the egg
and a second polar body.
c. If the secondary oocyte is not fertilized, it disintegrates and passes out of the
body.