Cell Division Definition
Cell division is the process by which a single parent cell divides to form new cells, known as daughter cells. There are two types of cell division: mitosis and meiosis. Mitosis is the type of cell division that occurs most frequently in the body and is involved in the growth and repair of tissues.
Types of Cell Division
Prokaryotic Cell Division
Prokaryotic cells, such as bacteria, undergo a type of cell division known as binary fission. Binary fission is the process by which one cell elongates, duplicates its DNA and plasmids, and separates into two new cells using a Z-ring.
Before the cell divides, its DNA is copied in a process called DNA replication. The outcome of cell reproduction is a pair of daughter cells that are genetically identical to the parent cell.
Binary fission is less complex than mitosis because prokaryotic cells have a simpler structure than eukaryotic cells. Unlike eukaryotes, prokaryotes do not undergo karyokinesis and therefore have no need for a mitotic spindle.
However, the FtsZ protein plays a vital role in prokaryotic cytokinesis by forming a ring around the cell where it will divide. This ring contracts and pinches off the membrane to form two separate daughter cells.
Cell division is essential for unicellular organisms like bacteria to produce new individuals. In contrast to eukaryotes, where sexual reproduction produces unique offspring through meiosis and fertilization, binary fission produces genetically identical offspring.
Eukaryotic Cell Division: Mitosis
Mitosis is a process of nuclear division in eukaryotic cells that occurs when a parent cell divides to produce two identical daughter cells.
Mitosis is used to produce new body cells for growth and healing. The process of mitosis is conventionally divided into five stages known as prophase, prometaphase, metaphase, anaphase, and telophase.
During prophase, the replicated pairs of chromosomes condense and compact themselves. The pairs of chromosomes that have been replicated are called sister chromatids, and they remain joined at a central point called the centromere.
A large structure called the mitotic spindle also forms from long microtubules that extend from each centrosome. During metaphase, the microtubules pull the sister chromatids back and forth until they align in a plane along the center of the cell called the equatorial plane.
In anaphase, the sister chromatids separate at their centromeres and are pulled apart by motor proteins along the microtubules toward opposite poles of the cell. Finally, during telophase, two nuclei form around each set of chromosomes at opposite ends of the cell.
Although cytokinesis follows mitosis in all eukaryotes, it is quite different for eukaryotes that have cell walls such as plant cells compared to animal cells that lack cell walls.
In animal cells, cytokinesis follows anaphase where a contractile ring composed of actin filaments forms just inside the plasma membrane at the former metaphase plate. The actin filaments pull the equator of the cell inward forming a fissure or cleavage furrow which separates two daughter cells.
The division cycle of most eukaryotic cells is divided into four discrete phases: M (mitosis), G1 (gap 1), S (synthesis), and G2 (gap 2). Mitosis is usually followed by cytokinesis which completes one round of cell division.
Eukaryotic Cell Division: Meiosis
Eukaryotic cells undergo two types of cell division: mitosis and meiosis. Mitosis is used to produce new body cells for growth and healing, while meiosis is used to produce sex cells (eggs and sperm).
Meiosis is a type of cell division that reduces the number of chromosomes in half, resulting in four haploid daughter cells. The process of meiosis consists of two rounds of cell division: meiosis I and meiosis II.
Meiosis I begin with interphase, where the cell grows and replicates its DNA. The first stage of meiosis I is prophase I, where homologous chromosomes pair up and exchange genetic material through a process called crossing over.
This results in genetic diversity among the daughter cells. In metaphase I, the homologous pairs line up at the equator of the cell. During anaphase I, the homologous pairs separate and move towards opposite poles of the cell. Finally, during telophase I, two haploid daughter cells are formed.
Meiosis II is similar to mitosis in that it involves separating sister chromatids into individual chromosomes. However, unlike mitosis, there is no replication of DNA between meiosis I and II. Meiosis II begins with prophase II, where spindle fibers form and attach to each chromosome.
In metaphase II, the chromosomes line up at the equator of each cell. During anaphase II, sister chromatids separate and move towards opposite poles of each cell. Finally, during telophase II, four haploid daughter cells are formed.
Cell Division Stages
Depending upon which type of cell division an organism uses, the stages can be slightly different.
Mitosis is the process of cell division that results in two identical daughter cells. Mitosis is divided into five stages: interphase, prophase, metaphase, anaphase, and telophase. Interphase is the stage where the DNA in the cell is copied in preparation for cell division.
Prophase is the first stage of mitosis where chromosomes recruit condensin and begin to undergo a condensation process that will continue until metaphase. In most species, cohesin is largely removed from the arms of the sister chromatids during prophase, allowing individual sister chromatids to be visible.
Prometaphase occurs when microtubules of the developing spindle attach to kinetochores on chromosomes. Metaphase is when chromosomes line up neatly end-to-end along the center (equator) of the cell.
Anaphase occurs when sister chromatids separate and move toward opposite poles of the cell. Telophase occurs when a full set of chromosomes gather together at each pole of the cell and a membrane forms around each set of chromosomes.
Some textbooks list four stages of mitosis: prophase, metaphase, anaphase, and telophase. However, some textbooks break prophase into an early phase (called prophase) and a late phase (called prometaphase).
The six steps in order are prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis. Cytokinesis is not considered one of the stages of mitosis but rather a separate process that occurs after mitosis has been completed.
Mitosis allows for growth and replaces worn-out cells. Mistakes made during mitosis can result in changes in DNA that can potentially lead to genetic disorders if not corrected in time.
Meiosis is a type of cell division that produces four haploid cells from a single diploid cell. The process consists of two divisions, both of which follow the same stages as mitosis: prophase, metaphase, anaphase, and telophase. Before entering meiosis, a cell must first undergo interphase in which DNA is replicated to produce chromosomes consisting of two sister chromatids.
Meiosis I is a reduction division (diploid to haploid) in which homologous chromosomes are separated. During prophase I, chromosomes condense and form bivalents. Crossing over occurs during this stage.
In metaphase I, homologous pairs line up along the equator of the cell. In anaphase I, homologous pairs separate and move towards opposite poles of the cell. Telophase I marks the end of meiosis I when two haploid daughter cells are formed.
Meiosis II separates sister chromatids (these chromatids may not be identical due to crossing over in prophase). Prophase II begins with each daughter cell containing half the number of chromosomes as the original parent cell. Chromosomes condense again and become visible under a microscope.
Metaphase II involves lining up individual chromosomes along the equator of each daughter cell. Anaphase II marks the separation of sister chromatids as they move towards opposite poles of each daughter cell. Telophase II marks the end of meiosis when four haploid cells are produced.