Learn about mitosis and how chromosome structure aids in the cell division process.
Cell division produces new cells
All cells are produced through the process of cell division, during which one cell splits into two. Cell division is central to three biological processes: reproduction, organism growth, and cell replacement.
- Reproduction: Cell division is essential for reproduction, or the process by which parent organisms give rise to offspring. In sexual reproduction, two parents produce offspring through the fusion of sex cells, which are the product of cell divisions. In asexual reproduction a single parent produces offspring. This is common in unicellular organisms, where a single cell divides to produce a new, genetically identical organism. In some cases, multicellular organisms can also reproduce asexually through cell division.
Here are some examples of asexual reproduction in multicellular organisms:
Vegetative reproduction in plants: During vegetative reproduction, new plantlets grow from roots, stems, or leaves of a parent plant. Each plantlet is genetically identical to the parent plant, and capable of living independently.
Fragmentation: During fragmentation, an organism breaks into two or more parts, each of which develops into a new individual. This occurs in plants, as well as some animals (such as coral, sponges, and starfish).
Parthenogenesis: During parthenogenesis, a new organism develops from an unfertilized cell. This occurs in invertebrates, as well as in some species of fish, amphibians, reptiles, and birds.
Organism growth: Sexually reproducing, multicellular organisms start life as a single, fertilized egg. This single cell then grows into a mature organism, which can contain anywhere from thousands to trillions of cells! This increase in cell number is the result of repeated cell divisions.
Cell replacement: When cells are damaged, they are replaced via the division of healthy cells. This process is essential for healing wounds and regenerating tissues. In addition, some tissues require a continuous replacement of cells. For example, bone marrow continuously makes new blood cells to replace those that are naturally degraded or lost due to an injury or bleeding.
Mitosis is a type of cell division that produces genetically identical daughter cells
Mitosis is a form of cell division that produces two cells with identical genetic information. These cells are referred to as daughter cells. Prior to mitosis, a cell duplicates its DNA so that it can evenly distribute its genetic material to each daughter cell.
Mitosis involves the splitting of the nucleus, and so only occurs in eukaryotic cells. In unicellular eukaryotes, cell division by mitosis results in asexual reproduction. In multicellular eukaryotes, cell division by mitosis is responsible for organism growth, tissue repair, and (in some cases) asexual reproduction.
The accurate division of genetic material relies on chromosome structure
A eukaryotic organism’s genome is split into multiple DNA molecules that are organized into structures called chromosomes, which are found in the nucleus. Each chromosome consists of a single, long DNA molecule (that contains many genes) plus supporting proteins.
Together, the DNA and proteins that make up a chromosome are referred to as chromatin. Chromatin’s primary role is to tightly package the long strands of DNA into a dense structure that fits inside the nucleus. The basic structural units of chromatin are called nucleosomes, which consist of DNA coiled around proteins called histones.
Chromosomes during mitosis
Prior to mitosis, chromatin is loosely arranged in the nucleus, which means chromosomes cannot be seen individually under a light microscope. As a cell gets ready to divide, its chromatin condenses, making its duplicated chromosomes visible under a light microscope. Condensed, duplicated chromosomes are often depicted with an “X” shape in diagrams. The condensed form of chromatin ensures that replicated chromosomes are accurately distributed to each of the two daughter cells during mitosis.
While all chromosomes carry an organism’s genetic information and are made up of DNA, the chromosomes of prokaryotic cells differ from those in eukaryotic cells in several ways:
Prokaryotic chromosomes are found in the nucleoid region of the cytoplasm.
The cytoplasm of most prokaryotes contains a single, circular chromosome.
The proteins that prokaryotic cells use to organize their chromosomes are less complex, and structurally and functionally different from eukaryotic histones.
Mitosis is part of the cell cycle
When we use the term “mitosis,” we are often referring to the general process of cell division in eukaryotes. However, “mitosis” technically describes only one part of the cell division process—the splitting of replicated chromosomes into two nuclei.
In reality, mitosis is just one part of the cell cycle, a series of organized and regulated events through which cells grow, replicate their DNA, and ultimately divide. This cycle helps cells grow and reproduce properly, ensuring the accurate transmission of genetic material to daughter cells.
