Cell Cycle
Most cell division results in genetically identical daughter cells
A cell’s genetic information, packaged as DNA, is called its genome
DNA molecules are packaged into chromosomes
Every eukaryotic chromosome consists of one long, linear DNA molecule associated with many proteins. Together, the complex is referred to as chromatin
Human somatic cells (all body cells except sperm and egg) have 46 chromosomes, made up of two sets of 23 (one from each parent)
Human reproductive cells or gametes (sperm or eggs) have one set of 23 chromosomes, half the number in a somatic cell
Each duplicated chromosome consists of two sister chromatids, joined together, which contain identical copies of the chromosome’s DNA
As the chromosomes condense, the region where the chromatids connect shrinks to a narrow area, made up of two centromeres
Mitosis, the division of the nucleus, is usually followed by the division of the cytoplasm, cytokinesis
In contrast, gametes (eggs or sperm) are produced only in gonads (ovaries or testes) by a variation of cell division called meiosis
The mitotic phase alternates with interphase in the cell cycle
The mitotic (M) phase of the cell cycle, which includes mitosis and cytokinesis, alternates with the much longer interphase
Interphase has three subphases: the G1 phase (“first gap”), the S phase (“synthesis”), and the G2 phase (“second gap”). The daughter cells may then repeat the cycle
For convenience, mitosis is usually divided into five subphases: prophase, prometaphase, metaphase, anaphase, and telophase
Each of the two chromatids of a chromosome has a kinetochore, a specialized protein structure located at the centromere
The spindle fibers push the sister chromatids until they are all arranged at the metaphase plate, an imaginary plane equidistant from the poles, defining metaphase
The mitotic spindle, fibers composed of microtubules and associated proteins, is a major driving force in mitosis
Assembly of the spindle microtubules starts in the centrosome, or microtubule-organizing center, a subcellular region that organizes the cell’s microtubules
Each sister chromatid of a duplicated chromosome has a kinetochore of proteins and chromosomal DNA
In animal cells, cytokinesis occurs by a process called cleavage
The first sign of cleavage is the appearance of a cleavage furrow in the cell surface near the old metaphase plate
The first sign of cleavage is the appearance of a cleavage furrow in the cell surface near the old metaphase plate
Asexual reproduction of single-celled eukaryotes, such as an amoeba, includes mitosis and occurs by a type of cell division called binary fission, or “division in half.”
The eukaryotic cell cycle is regulated by a molecular control system
The sequential events of the cell cycle are directed by a distinct cell cycle control system
A checkpoint in the cell cycle is a control point where stop and go-ahead signals regulate the cycle
The kinases that drive the cell cycle are present at constant concentrations but require the attachment of a second protein, a cyclin, to become activated
Because of the requirement for binding of a cyclin, the kinases are called cyclin-dependent kinases, or Cdks
Peaks in the activity of one cyclin-Cdk complex, MPF, correspond to peaks in cyclin concentration
Particularly important for mammalian cells are growth factors, proteins released by certain cells that stimulate other cells to divide
The effect of an external physical factor on cell division can be seen in density-dependent inhibition of cell division
Most animal cells also exhibit anchorage dependence for cell division
The abnormal behavior of cancer cells begins when a single cell in a tissue undergoes a transformation that converts it from a normal cell to a cancer cell
If the abnormal cells remain at the originating site, the lump is called a benign tumor
A malignant tumor includes cells whose genetic and cellular changes enable them to spread to new tissues and impair the functions of one or more organs
In addition to chromosomal and metabolic abnormalities, cancer cells often lose their attachment to nearby cells, are carried by the blood and lymph system to other tissues, and start more tumors in an event called metastasis
In the bacterium Escherichia coli, the process of cell division begins when the DNA of the bacterial
chromosome starts to replicate at a specific place on the circular chromosome, the origin of replication, producing two origins