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The Cell Cycle
Cell Division Notes
• The cell cycle is an orderly sequence of
events that occurs from the time when a
cell is first formed until it divides into two
new cells
• Most of the cell cycle is spent in
interphase
• Following interphase, the mitotic stage of
cell division occurs
The Cell Cycle
Cell Increase and Decrease
•
Cell division increases the number of
somatic (body) cells, and consists of:
– Mitosis (division of nucleus)
– Cytokinesis (division of cytoplasm)
– Apoptosis (cell death) decreases the
number of cells.
•
Both cell increase and apoptosis occur
during normal development and
growth.
Mitotic Stage
Stages of Interphase
1. G0 stage – “normal” cell growth and
development, the period where there is no
reproduction of DNA.
2. G1 stage – cell growth, cell doubles its
organelles, accumulates materials for DNA
synthesis
3. S stage – DNA synthesis occurs, and DNA
replication results in duplicated chromosomes
4. G2 stage – cell synthesizes proteins needed for
cell division
• Recall S-phase - chromosomes reproduce.
– Makes a double set of 2N chromosomes for each cell
1 Chromosome
2 Chromatids = 1
chromosome that
has reproduced
• Following interphase is M stage
• Includes mitosis and cytokinesis
–During mitosis sister chromatids of each
chromosome separate, into two daughter cells
–The cell cycle ends with cytokinesis (the cleaving
of the cytoplasm)
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Mitosis in Detail
Mitosis Overview
• During mitosis, the spindle apparatus forms
from protein made in G2 of interphase
Chromosomes are
pulled apart by the
spindle to opposite
poles of the cell
– Spindle connects to the chromosomes and
distributes each of them into daughter cells
• Spindle contains fibers made of microtubules
that disassemble and assemble
– Aster - star shaped cluster of microtubules radiating
from the polar microtubule organizing center at the
start of mitosis
• Centrioles that divide during interphase
organize the spindle
• Mitosis has four phases: prophase, metaphase,
anaphase, and telophase
Mitosis Overview
Chromosomes are
decondensed and
nuclear membrane
desolves, spindle
forms and connects
to the centromeres
Chromosomes line
up at the center of
the cell
Chromosomes recondense, the
nuclear membrane forms and cell
membrane pinches in to form two
new cells
Overview of Mitosis
• Ploidy Number
– Diploid (2N) number chromosomes found in
somatic cells contain two chromosomes of
each kind (one from each parent)
– Haploid (N) number chromosomes contain
one chromosome of each kind
• Daughter cells from mitosis
– Genetically identical to parental cells
• Mitosis occurs in somatic cells for growth
and repair
Stem Cells
• Stem Cells - somatic tissue cells have capacity to replicate to cells
with similar properties to maintain pool of precursor cells
– Precursor cells - immature cells mature to functional adult cells
– Tissue stem cells - found in differentiated tissues
• Differentiate and/or divide to produce specialized cell types
• STEM CELL TYPES
– Adult stem cells - multipotent (capable of producing several
limited cell types)
– Fertilized Egg - considered totipotent (potential is total)
• Gives rise to all different types of cells in the body
– Embryonic Stem - totipotent or multipotent
Apoptosis
• Apoptosis is programmed cell death.
• Apoptosis occurs because of two sets of
enzymes (called capsases)
• The first set, the “initiators” receive a
signal to activate the second set, the
“executioners”
• The second set of capsases activate
enzymes that tear apart the cell and its
DNA
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Cell Division in Prokaryotes
Maintaining Chromosome Number
• The process of asexual reproduction in
prokaryotes (bacteria) is called binary
fission
• The two daughter cells are identical to the
original parent cell, each with a single
chromosome
• Following DNA replication, the two
resulting chromosomes separate as the
cell elongates
• Non dividing eukaryotic cells’ DNA stored
in proteins tangled mass called chromatin
• Each species has different number
Maintaining the Chromosome
Number
• In animals sperm and eggs have haploid
numbers of chromosomes
• Nuclei of somatic cells undergo mitosis
– Recall mitosis is nuclear division where
number of chromosomes stays constant.
• Before nuclear division occurs, DNA
replication takes place, duplicating the
chromosomes
– Storage protein called
histones (different for
different species)
– Starting cell division,
chromatin condenses to
form highly compacted
structures called
chromosomes
Overview of Meiosis
• Meiosis requires two nuclear divisions
and four haploid nuclei result
• Humans have 23 pairs of homologous
chromosomes, or 46 chromosomes total
• Prior to meiosis I, DNA replication occurs
• During meiosis I, synapsis occurs
– S phase of the cell cycle
Meiosis I in an animal cell
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Meiosis II
Maintaining the Chromosome
Number
• A duplicated chromosome is made of two
sister chromatids held together in a
region called the centromere
– Homologous chromosomes
– Sister chromatids genetically identical
• End of mitosis chromosomes consists of
a single chromatid
• During mitosis centromeres divide and
sister chromatids separate
Comparison of Meiosis with
Mitosis
• In mitosis and meiosis
– DNA replication occurs only once during
interphase (s phase)
• Mitosis requires one division, meiosis
requires two divisions
• Two diploid daughter cells result from
mitosis; four haploid daughter cells result
from meiosis
Reducing the Chromosome
Number
•
•
Meiosis reduces the chromosome
number such that each daughter cell
has only one of each kind of
chromosome
The process of meiosis ensures that the
next generation will have:
1. the diploid number of chromosomes
2. a combination of traits that differs from that
of either parent.
Comparison of Meiosis I to
Mitosis
Meiosis I:
Prophase I - pairing of
homologous
chromosomes
Metaphase I –
homologous pairs line up
at metaphase plate
Anaphase I – homologous
chromosomes separate
Telophase I – daughter
cells are haploid
Mitosis:
Prophase has no such
pairing
Metaphase –
chromosomes align at
metaphase plate
Anaphase – sister
chromatids separate
Telophase – diploid cells
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Comparison of Meiosis II to
Mitosis
Overview of Meiosis
• Events of meiosis II like those of mitosis
• Meiosis I separates homologous
pairs of chromosomes
• Daughter cells are haploid, but
chromosomes still in duplicated
condition
• No replication of DNA occurs
between the two divisions
– Meiosis II nuclei contain haploid number of
chromosomes
• End of telophase II of meiosis II
– four haploid daughter cells not genetically
identical
– End of mitosis two diploid daughter cells are
identical
Overview of Meiosis
Overview of Meiosis
• Meiosis II separates sister chromatids
• In many life cycles, haploid daughter
cells mature into gametes.
• Fertilization restores the diploid
number of chromosomes during
sexual reproduction
Sources of Genetic Variation
Meiosis in Detail
• The same four phases seen in mitosis –
prophase, metaphase, anaphase, and
telophase – occur during both meiosis I
and meiosis II
• The period of time between meiosis I and
meiosis II is called interkinesis
• No replication of DNA occurs during
interkinesis because the DNA is already
duplicated.
Meiosis and fertilization ensures sources of
genetic recombination:
•
1.
Independent assortment - alignment of paired
homologous chromosomes along metaphase I plate
2. Crossing-over - nonsister chromatids during
prophase I
3. Recombining - chromosomes of genetically different
gametes occurs during fertilization
These events assure new genetic combinations
in the offspring
•
•
Why is this important in living systems?
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Independent assortment
Synapsis and crossing-over
The Human Life Cycle
Life cycle of humans
• Human life cycle requires mitosis and
meiosis
• In males meiosis occurs as
spermatogenesis, produces sperm cells
• In females meiosis occurs as oogenesis,
produces egg cells
• Mitosis is involved in the growth and
development of a child and repair of
tissues during life
Human Sexual Characteristics
Male Reproductive System
• Primary Sexual Characteristics
– are those characteristics that deal with the physical
development of organs
– development off the male and female gonads and external
genitalia
• Secondary Sexual Characteristics
– are those characteristics that deal with the physical
development of body hair, voice intonation, body structure
and other characteristics that deal with gender, but not
organ development
• Controlled by the expression of the X and Y chromosomes
– These chromosomes have genes on them for the
expression (quantity and type) of testosterone and
estrogen (protein based hormones that control physical
development)
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Female Reproductive System
The Human Life Cycle
• In humans, both sperm cells and the egg
cell have 23 chromosomes each
• Following fertilization of the egg cell by a
single sperm, the zygote has 46
chromosomes, the diploid number found
in human somatic cells
• The 46 chromosomes represent 23 pairs
of homologous chromosomes
Fetal Development and Membranes
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