Cell Cycle

Number of Cellsand Cell Types

Humans have 100 trillion cells and 200 cell types at birth.

Single-celled Organisms

Organisms may be single-celled, such as bacteria (Amoeba).

Multicellular Organisms

Organisms may be complex multicellular, such as humans.

Cell Features

Cells have structure, function, reproduce, and have a cell cycle.

Purpose of Cell Division

Cell division is for growth and development, repair of tissues like skin cells, and reproduction to give more rise to more cells that are identical

Genetically Identical Daughter Cells

Cell division must generate genetically identical daughter cells.

Which Cells Undergoing Cell Division

Cells that divide frequently include skin, cheek, and intestinal cells. Cells that divide infrequently include liver cells. Nerve cells do not divide.

Regulation of Cell Cycle

The cell cycle must be regulated, cells cannot divide randomly as it leads to cancer. Cell division is under control of the “cell cycle”

Components of Cell Cycle

The cell cycle consists of G1, S, G2, M-phase, and interphase.

Gap Phases

Gap phases are preparation steps in the cell cycle.

Cell Cycling Times

The cell cycle takes around 24 hours, with G1 being the longest and M-phase being the shortest.

G0 Phase

G0 is a cell resting phase with no activity.

Regulation of Cell Cycle

The cell cycle is regulated by molecules in the cytoplasm that act as signals.

Signals in Cell Fusion

Signals in the higher phase can tell cells in a lower phase to reach that phase.

Mitosis-promoting Factor (MPF)

MPF is a protein kinase that activates the cell cycle when combined with cyclin. It is a cyclin-dependent kinase molecule which when combines with cyclin forms MPF. Levels of MPF are max as cells enters into mitosis

Checkpoints in the Cell Cycle

G1, G2, and M-phase are checkpoints that stop/start the cell cycle.

Purpose of Checkpoints

Checkpoints control traffic flow, determine cell division needs, repair damaged tissue, and determine when cells need to divide. If there are no checkpoints it leads to cancer

Platelet Derived Growth Factor (PDGF)

PDGF stimulates cell division for tissue repair. It is derived from blood cells. It stimulates cell division which results in production of connective tissue (fibroblasts) to repair the damage. Fibroblasts have receptors to bind tyrosine kinase that is present in PDGF

Mitosis

Mitosis is the process of cell division that results in genetically identical daughter cells. It is the division of the nucleus. DNA in the nucleus is replicated during the S-Phase and condenses with histone proteins to form chromosomes which consist of 2 sister chromatids joined at the centromere

Stages of Mitosis

Prophase, Prometaphase, Metaphase, Anaphase, Telophase.

Cytokinesis

Cytokinesis is the division of cytoplasm and formation of cell membrane around two new cells.

Kinetochore Microtubules

Kinetochore microtubules are proteins that attach to chromosomes during mitosis.

Stem Cells

Stem cells have the capability to divide and give rise to different cell types. They are from embryonic tissues from early stage of development (4-5 days old) which are called blastocysts . Also derived from umbilical cord or amniotic fluid around the fetus. Adult stem cells from blood cells or bone marrow cells can give rise to new cells

Pluripotent Stem Cells

Pluripotent stem cells can be re-programmed to become different tissue types. Skin cells can become heart cells

Benefits of Stem Cells Research

Stem cells have the potential to regenerate tissues that would not normally grow again like heart tissue or nerve cells, and treat degenerative diseases. There are not many examples of success but the potential for using stem cells is great for those who have spinal cord injury or heart damage.

Stem Cell Transplant

Stem cell transplants can replace damaged cells and treat certain diseases. Stem cells replace cells damaged by chemotherapy or disease or fight some types of cancer and blood-related diseases like leukemia and lymphoma. They are testing adult stem cells to treat other conditions that are degenerative like heart failure.

Potential Problems

Embryonic stem cells can grow irregularly into other types of cells and may be rejected by the immune system. Stem cells may fail to function as expected with unknown consequences.

Cell Cycle

Series of coordinated steps that results in cell division

G1, First gap phase

Cell growth occurs, cell is metabolically active and continually grows but does not replicate DNA

S Phase

Synthesis of DNA takes place

G2, Second Gap Phase

Cells are preparing for cell divisions, organelles and proteins necessary for cell division are produced.

M-phase

Mitosis

Interphase

G1+S+G2

Prophase

Chromosomes appear with sister chromatids The spindle apparatus forms (contains microtubule)

Prometaphase

Nuclear envelope breaks down The microtubules attach to chromosomes at the centromere (Kinetochores)

Metaphse

Sister chromatids migrate to the middle of the cells Spindle apparatus is complete Chromosomes line up

Anaphase

Chromatids (chromosomes) are pulled along by kinetochore microtubules

Telophase

Spindle apparatus breaks down. Nucleus envelope forms around chromosomes. Chromosomes decondenses

CDK, Cyclin and MPF during the cell cycle

Starts as CDK from G1 to S to G2 phase. At some point in the G2 checkpoint, cyclin comes in. Cyclin and CDK combine to create MPF and signals cell to enter mitosis. Cyclin is degraded and leaves and it returns to CDK in G1

G1 Checkpoint

It is the most important, If cells don’t pass G1 they go into G0 phase (non-dividing). Cell must have adequate size and sufficient nutrients and undamaged DNA to pass

G2 Checkpoint

Occurs after the S-phase. MPF complex interacts here. Cell must have chromosomes that have replicated successfully, undamaged DNA and activated MPF present to pass

M-Phase checkpoint

Stops the cell from dividing during mitosis, cell must have all chromosomes attached to spindle apparatus