USC NURS 500 Bridge course section 3.1 Cell Cycle
The cell cycle is:
The series of changes a cell goes through from the time it is formed to the time that it reproduces.
Two major periods of the cell cycle:
– Interphase • Cell grows and carries on its usual activities. The period from cell formation to cell division.
– Cell division (mitotic phase) • The cell divides into two cells.
Cell division or the mitotic phase is when a cell divides into two cells.
Essential for body growth and tissue repair. Skin and intestinal lining cells reproduce continuously, while Liver, nervous tissue, skeletal muscle, and cardiac lose their ability to divide when fully mature, damaged cells are replaced with scar tissue.
Control of cell division is crucial, so cells divide when necessary, but do not divide unnecessarily. Stimulated by certain chemicals including growth factors, hormones and increasing cell size. Lack of space and inhibitory chemicals deter cell division. Cyclin-Cdk complex regulate cell division.
Events of cell division occur during M phase. Cell division consists of 2 distinct phases mitosis ( nuclear division) and cytokinesis ( division of the cytoplasm).
Subphases of interphase:
G0 phase: Cells that stop permanently dividing.
G1: the cell is metabolically active synthesizing proteins rapidly and growing vigorously. This phase is the most variable phase in terms of length can be several minutes to several hours. no cell division occurs, but the centrioles start tot replicate in preparation for cell division.
S phase: DNA is replicated, new histones are made and assembled into chromatin.
G2 phase: final brief phase of interphase. Enzymes and proteins needed for division are synthesized and moved to their proper sites. Centriole replication is complete. G2M checkpoint is when the cell checks the replicated DNA before continuing to cell division.
M phase – 4 stages:
Two distinct events: Mitosis ( division of the nucleus ) and cytokinesis ( division of the cytoplasm).
Mitosis- series of events that parcel out the replicated DNA of the mother cell to two daughter cells.
Cytokinesis begins in late anaphase and is completed after mitosis ends.
The chromosomes line up in the center of the cell during which phase of mitosis?
Interphase (metabolic or growth phase)
The period of a cells life when it carries out its normal metabolic activities and grows. From cell formation to cell division.
non-dividing phase of the cell cycle. Important checkpoints occur during interphase before proceeding to mitosis.
Consists of subphases: Cell grows by producing proteins and organelles.
DNA containing material is in the form of chromatin. The nuclear envelope and one or more nucleoli are intact and visable.
Tjree periods of this phase:
-G (gaps)1- the cell grows and centriole replication begins.
-S (synthetic) -DNA replicates, chromatin is reproduced.
-G2- Final preparations for mitosis are completed. Enzymes and other proteins are synthesized, centriole replication is completed.
One parental double stranded DNA molecule is converted to two identical off spring molecules. DNA base strands are complimentary. One DNA strand acts as a template for production of the other strand.
DNA must be replicated exactly before a cell can divide, this occurs in the S phase. Enzymes unwind the DNA double helix and break the hydrogen base bonds (replication fork). The old (parental) nucleotide strand acts as a template for constructing a complementary strand. 2 strands-leading and lagging- are synthesized in different directions. The DNA polymerase enzyme positions complementary free nucleotides along the template strands forming 2 new strands.
Synthesizes DNA, proofreads and repairs DNA.
Builds new strands of DNA
During DNA replication, which nucleotide will bind together to form the parental DNA?
Adenine binds with Thymine
Guanine binds with Cytosine
Checkpoint for DNA repair before proceeding to the next phase.
Check for: Cell size Nutrients Growth Factor DNA damage.
final preparations for cell division are made during the life cycle subphase called?
Early mitosis prophase
• Chromatin condenses, forming barlike visible chromosomes
• Each chromosome and its duplicate (called sister chromatids) are held together by a centromere, appear as two identical threads held together at the centromere.
• Centrosome and its duplicate begin synthesizing microtubules that push each centrosome to opposite poles of cell – Called the mitotic spindle – Other microtubules called asters radiate from centrosome
Late Mitosis prophase
• Nuclear envelope breaks up allowing the spindle to interact with the chromosomes.
• Special microtubules attach to specific area on centromeres called kinetochore and serve to pull chromosomes to center (equator) of cell
• Remaining nonkinetochore microtubules push against each other, causing poles of cell to move farther apart
The phase of mitosis during which centrioles reach the poles and chromosomes attach to the spindle
The second phase of Mitosis
– The two centrosomes are at opposite poles of the cell.
- The chromosomes cluster at the middle of the cell with their centrometes precisely aligned at equator of the spindle.
– The imaginary plane midway between poles is called metaphase plate
– The third and shortest phase of mitosis. Easy phase to recognize because the moving chromosomes look V shaped.
– Begins as the centromeres of chromosomes split simultaneously
- Each sister chromatid now becomes a separate chromosome
– Chromosomes are pulled toward their respective poles by motor proteins of kinetochores
- One chromosome of each original pair goes to opposite poles
– Nonkinetochore microtubules continue forcing poles apart
– Begins when chromosome movement stops
– Each set of chromosomes (at opposite ends of cell) uncoils to form chromatin
– New nuclear membranes form around each chromatin mass
– Nucleoli reappear – Spindle disappears
The division of the cytoplasm begins in late anaphase and is completed after mitosis ends.
Contractile ring made of actin filaments draw the plasma membrane inward to form a cleavage furrow over the cell which deepens until it pinches the cytoplamic mass into 2 parts yielding 2 daughter cells.
– Begins during late anaphase and continues through mitosis
– Ring of actin microfilaments contracts to form cleavage furrow
– Two daughter cells are pinched apart
Cell cycle is regulated by internal and external factors:
-ratio of cell surface area to cell volume- the greater the volume the more nutrients are needed. Surface volume relationship.
-chemical signals such as growth factors and hormones released by other cells.
-the availability of space- contact inhibition.
two groups of proteins are crucial to a cells ability to accomplish the s phase and enter mitosis:
cyclin proteins and Cdk initiate enzymatic cascades needed for cell division.
cyclin- dependent kinases (cdks)- activated or deactivaed by cyclins that function in a regulatory role.
enzymes destroy both at the end of mitosis
Loss of cell cycle controls in cancer cells:
do not respond normally to the body's control mechanisms and form tumors.