THE CELL CYCLE
I.
INTRODUCTION
A. Chromosomes
1. At cell division, chromatin (long threads of DNA and protein) coils, folds, and twists
into a highly compact structure termed chromosomes
2. Each organism has a characteristic number of chromosomes (humans = 46).
3. In the human, the 23 pairs of chromosomes are divided into 22 pairs of autosomes
(nonsex chromosomes) and one pair of sex chromosomes: XX in females and XY in
males
4. Prior to cell division, each chromosome is composed of 2 genetically identical parts
termed sister chromatids held together at a region called the centromere
II. THE LIFE CYCLE OF HUMANS
A. Meiosis
1. Occurs in the gonads (testes and ovaries) to produce gametes (sperms, egg), which
are N, or haploid (1 set of chromosomes)
2. When the sperm fertilizes the egg, a zygote (diploid or 2N) is produced
B. Mitosis
1. The fertilized egg divides mitotically for growth and repair to form the adult male or
female individual with 46 chromosomes (23 pairs) in each cell
III. MITOSIS
A. Introduction
1. Each body cell is genetically identical as a result of mitosis
a) They all have the same number and kinds of chromosomes
2. Functions
a) Growth (baby in mother's womb)
b) Repair (of a wound) in multicellular organisms
c) Asexual reproduction in unicellular organisms
B. Cell cycle
1. Consists of mitosis followed by interphase.
2. Interphase
a) Cells have a distinct nuclear envelope, nucleoli and chromatin; metabolically
active
b) If a cell is to divide, it leaves the G1 phase and enters the S stage
(1) DNA replicates to form 2 genetically identical sister chromatids and the centrioles duplicate to form 2
pairs outside the nucleus
(2) The cell then enters the G2 stage
c) If a cell is not to divide, it remains in the GO stage
C. Overview of Mitosis
1. Mitosis produces 2 daughter cells, each with the same number and kinds of
chromosomes as the parent cell
a) Therefore, the parent cell and the daughter cells are genetically identical
2. Before cell division, each chromosome duplicates to form 2 sister chromatids
a) Sister chromatids separate during mitosis and go to daughter cells.
D. The Stages
1. Prophase
a) Two pairs of centrioles move away from each other and spindle fibers appear
between them
b) Nuclear envelope begin to disappear
c) Chromosomes (composed of sister chromatids) appear and are randomly placed
in the cell
2. Spindle fibers
a) Spindle fibers are composed of microtubules, capable of assembling and
disassembling that is associated with the centrioles at the poles
(1) Spindle fibers may be non-kinetochore (reaches from the poles to the equator and able to lengthen)
which elongate the cell
(2) Kinetochore microtubules attache to the chromosomes and able to shorten, thereby pulling the
chromosomes toward the poles
3. Metaphase
a) Chromosomes move to and align at the equator
b) At end of metaphase, the centromeres split
4. Anaphase
a) Chromatids separate and (now called daughter chromosomes) migrate to opposite
poles of the spindle
5. Telophase
a) Begins when chromosomes arrive at the poles
b) The spindle apparatus disappears (mictotubules disassemble)
c) The nuclear envelope and nucleoli reappear
d) Chromosomes untwist to form indistinct chromatin
e) Cytokinesis (cytoplasmic division) occurs in animal cells by furrowing (indentation
of cell membrane) to form 2 daughter cells
IV. MEIOSIS
A. Introduction
1. Used in sexual reproduction in animals (produces the egg and sperm) and plants
2. Importance
a) To keep the chromosome number constant generation after generation and to
ensure the next generation has a different genetic makeup
B. Overview of Meiosis
1. Consists of 2 cell divisions (Meiosis I and II) resulting in 4 daughter cells, each with 1/2
(haploid) the number of chromosomes (one of each kind of chromosome)
2. Homologous chromosomes separate during meiosis so that each daughter cell has
only 23 chromosomes, one from each homologous pair of chromosomes
3. Homologous chromosomes, each duplicated line up side by side, resulting in 4
chromatids in close proximity (called tetrads)
4. Crossing-over occurs between nonsister chromatids (but homologous chromosomes)
during synapsis and allows genetic exchange between chromosomes so as to provide
new combination of genes that are different from either parent
5. During meiosis I, homologous chromosomes separate (allows one chromosome of
every homologous pair to reach each gamete). During meiosis II, sister chromatids
separate
C. The Stages
1. The First Division
a) During prophase I, events are the same as in prophase of mitosis except
homologous chromosomes undergo synapsis and cross over
b) At metaphase I, tetrads line up at the equator
c) During anaphase I, homologous chromosomes separate
d) During telophase I, nuclear envelope reforms around the haploid number of
chromosomes
2. The Second Division
a) No duplication of chromosomes occurs during interphase
b) At prophase II, a spindle apparatus appears and attach to them independently
c) During metaphase II, chromosomes are lined up at equator and at end of
metaphase, the centromeres split
d) During anaphase II, sister chromatids separate and move toward each pole. In
telophase II, the spindle disappears, nuclear envelope reforms, and cytokinesis
occurs
D. Animal Meiosis
1. Spermatogenesis: (production of sperm in testes of males) via meiosis results in 4
viable haploid sperm from each original cell
2. Oogenesis: (production of eggs in ovaries of females) via meiosis results in unequal
cytoplasmic division producing only one functional egg and 3 nonfunctional polar
bodies that later degenerate
3. The sperm is small and flagellated while the egg is stationary and quite large (extra
cytoplasm provides nutrients for developing embyro)
V. COMPARISON OF MITOSIS TO MEIOSIS:
A. Meiosis only occurs in the gonads that give rise to the gametes. Mitosis occurs in
all types of cells in the body
B. Mitosis compared to Meiosis I: meiosis I has homologous chromosomes pairing in
Prophase I, tetrads at the equator in Metaphase I, and homologous chromosomes
separate in Anaphase
C. Mitosis compared to Meiosis II: both sister chromatids separate during anaphase,
but there are 4 haploid daughter cells in Telophase II compared to 2 diploid
daughter cells in mitosis