Biology 102 Chapter 6 - The Cell Cycle
Cell Cycle
● The continuity of life is based on the reproduction of cells, or cell division.
● In unicellular organisms, the division of one cell reproduces the entire organism
● Multicellular organisms depend on cell division for:
1. Development from a fertilized egg (zygote)
2. Growth
3. Injury repair
4. Asexual reproduction (in some)
● Cell cycle: the life of a cell from formation to its division.
The cell cycle is separated into several stages.
2 major stages:
Interphase: 90% of the cycle. Cell division: cells go through the steps necessary
● Cell grows, takes in nutrients, produces to divide into two.
biomolecules, and performs special functions
Interphase is further divided into 3 phases: Not all cells complete the cycle.
G1 (gap 1): first phase of a new cell ● Some cells may "step out" of the cell cycle
● Grows to full size during G1
● Normal cell functions ● The cells are said to be in G0 - a
S (synthesis): Cells begin preparations for division nondividing state
● DNA is copied (synthesized) ● Some cells never leave it
G2 (gap 2): Final preparation for division ○ Muscle cells
○ Neurons
Cell division and DNA
● Most cell divisions result in daughter cells with identical DNA.
● DNA is associated with special proteins to form chromatin.
● A eukaryotic cell’s chromatin is divided among several chromosomes.
● Prokaryotes have a single circular chromosome.
Eukaryotic Chromosomes
● Every Eukaryotic species has a characteristic # of chromosomes in each cell nucleus
● Humans have 46 (23 pairs).
● In preparation for cell division, DNA is replicated and the chromosomes condense.
● Each duplicated chromosome has 2 sister chromatids that separate during cell division.
● The centromere is the narrow “waist" of the duplicated chromosome, where the two
chromatids are attached.
1
, Biology 102 Chapter 6 - The Cell Cycle
Cell division
● Mitosis: division of the nucleus
○ 4 major steps:
1. Prophase: 2.Metaphase:
● Chromosomes begin to condense into distinct ● The nuclear envelope is gone.
units. ● Spindle fibers are attached to chromosomes.
● Each chromosome consists of two identical ● Chromosomes are lined up at the metaphase
“sister chromatids”. "plate".
● The mitotic spindle begins to form. ● For each chromosome, each sister chromatid
● The nuclear envelope breaks down. is attached to an opposite pole by spindle
fibers.
3. Anaphase: 4. Telophase:
● Sister chromatids separate and are now ● Nuclear envelopes reform around two new
considered daughter chromosomes. nuclei.
● Spindle fibers shorten-pulling the ● Chromosomes begin to decondense.
chromosomes toward the poles. ● Spindle disappears.
● Phase ends with a complete complement of ● Cytokinesis begins.
chromosomes at each pole.
● Cytokinesis: division of the cytoplasm and plasma membrane
○ Begins in late telophase.
○ Cytoplasm, including organelles, is divided equally.
○ The plasma membrane pinches in two.
○ Each cell is genetically identical to each other and the original parent cell.
Binary Fission
● Prokaryotes (bacteria) reproduce by a type of cell division called binary fission.
● In binary fission, the chromosome replicates and the two daughter chromosomes separate
as the cell divides.
Regulation of the Eukaryotic Cell Cycle
● The timing and rates of cell division in different parts of an organism are crucial for
normal growth, development, and maintenance.
● The frequency of cell division varies with cell type.
○ Some human cells divide frequently throughout life (skin cells)
○ Other human cells can divide, but keep it in reserve (liver cells)
○ Mature nerve and muscle cells do not appear to divide at all after maturity
2
Cell Cycle
● The continuity of life is based on the reproduction of cells, or cell division.
● In unicellular organisms, the division of one cell reproduces the entire organism
● Multicellular organisms depend on cell division for:
1. Development from a fertilized egg (zygote)
2. Growth
3. Injury repair
4. Asexual reproduction (in some)
● Cell cycle: the life of a cell from formation to its division.
The cell cycle is separated into several stages.
2 major stages:
Interphase: 90% of the cycle. Cell division: cells go through the steps necessary
● Cell grows, takes in nutrients, produces to divide into two.
biomolecules, and performs special functions
Interphase is further divided into 3 phases: Not all cells complete the cycle.
G1 (gap 1): first phase of a new cell ● Some cells may "step out" of the cell cycle
● Grows to full size during G1
● Normal cell functions ● The cells are said to be in G0 - a
S (synthesis): Cells begin preparations for division nondividing state
● DNA is copied (synthesized) ● Some cells never leave it
G2 (gap 2): Final preparation for division ○ Muscle cells
○ Neurons
Cell division and DNA
● Most cell divisions result in daughter cells with identical DNA.
● DNA is associated with special proteins to form chromatin.
● A eukaryotic cell’s chromatin is divided among several chromosomes.
● Prokaryotes have a single circular chromosome.
Eukaryotic Chromosomes
● Every Eukaryotic species has a characteristic # of chromosomes in each cell nucleus
● Humans have 46 (23 pairs).
● In preparation for cell division, DNA is replicated and the chromosomes condense.
● Each duplicated chromosome has 2 sister chromatids that separate during cell division.
● The centromere is the narrow “waist" of the duplicated chromosome, where the two
chromatids are attached.
1
, Biology 102 Chapter 6 - The Cell Cycle
Cell division
● Mitosis: division of the nucleus
○ 4 major steps:
1. Prophase: 2.Metaphase:
● Chromosomes begin to condense into distinct ● The nuclear envelope is gone.
units. ● Spindle fibers are attached to chromosomes.
● Each chromosome consists of two identical ● Chromosomes are lined up at the metaphase
“sister chromatids”. "plate".
● The mitotic spindle begins to form. ● For each chromosome, each sister chromatid
● The nuclear envelope breaks down. is attached to an opposite pole by spindle
fibers.
3. Anaphase: 4. Telophase:
● Sister chromatids separate and are now ● Nuclear envelopes reform around two new
considered daughter chromosomes. nuclei.
● Spindle fibers shorten-pulling the ● Chromosomes begin to decondense.
chromosomes toward the poles. ● Spindle disappears.
● Phase ends with a complete complement of ● Cytokinesis begins.
chromosomes at each pole.
● Cytokinesis: division of the cytoplasm and plasma membrane
○ Begins in late telophase.
○ Cytoplasm, including organelles, is divided equally.
○ The plasma membrane pinches in two.
○ Each cell is genetically identical to each other and the original parent cell.
Binary Fission
● Prokaryotes (bacteria) reproduce by a type of cell division called binary fission.
● In binary fission, the chromosome replicates and the two daughter chromosomes separate
as the cell divides.
Regulation of the Eukaryotic Cell Cycle
● The timing and rates of cell division in different parts of an organism are crucial for
normal growth, development, and maintenance.
● The frequency of cell division varies with cell type.
○ Some human cells divide frequently throughout life (skin cells)
○ Other human cells can divide, but keep it in reserve (liver cells)
○ Mature nerve and muscle cells do not appear to divide at all after maturity
2
