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May 6, 2024 · Mitosis, a process of cell duplication, or reproduction, during which one cell gives rise to two genetically identical daughter cells. Strictly applied, the term is used to describe the duplication and distribution of chromosomes, the structures that carry the genetic information.
- Overview
- The concept of a chromosome
- The concept of mitosis
- The phases of mitosis
You started off as a fertilized cell inside your mom, called a zygote. Now, you’re a thriving community of hundreds of millions of cells, all working together towards a common purpose: to keep you alive. How did so many cells come from just one?
Generally speaking, the answer is straightforward: many cells come from just one by repeated cell division. Your first form as a zygote split to make two cells. Then those cells split, making four...and so on and so forth, until you became the living, functioning organism you are today.
A chromosome is a thread-like object (scientists literally called them threads or loops when they were first discovered) made of a material called chromatin. Chromatin is made of DNA and special structural proteins called histones. One way to think of a chromosome is as one very long strand of DNA, with a bunch of histone proteins stuck to it like beads on a string.
Chromosomes are stored in the nuclei of cells. If you compare the diameter of a cell nucleus (between 2 and 10 microns) to the length of a chromosome (between 1 and 10 centimeters, when fully extended!), you can see that a chromosome must be scrunched up into a very small package in order to fit inside a nucleus. Actually, the average chromosome is about a thousand times longer than a cell nucleus is wide. The situation is a bit like how a very long snake can coil up into a tight ball.
The basic construction of chromosomes (made of chromatin) and structure (long but scrunched up) is the same in all animals. The difference is that each species has its own set number of chromosomes. For instance, all human cells (except gametes) have 46 chromosomes. Cells of nematodes (worms), other than gametes, have 4 chromosomes. The number of total chromosomes in the non-gamete cells of a particular species is called the diploid number for that species. The diploid number of humans is 46, and the diploid number of nematodes is 4.
The total number of chromosomes in the gametes of a particular species is referred to as the haploid number of that species. This number is always half of the diploid number. For instance, the haploid number in humans is 23, and the haploid number in nematodes is 2.
The purpose of mitosis is to make more diploid cells. It works by copying each chromosome, and then separating the copies to different sides of the cell. That way, when the cell divides down the middle, each new cell gets its own copy of each chromosome.
In the first step, called interphase, the DNA strand of a chromosome is copied (the DNA strand is replicated) and this copied strand is attached to the original strand at a spot called the centromere. This new structure is called a bivalent chromosome. A bivalent chromosome consists of two sister chromatids (DNA strands that are replicas of each other). When a chromosome exists as just one chromatid, just one DNA strand and its associated proteins, it is called a monovalent chromosome. Here is a drawing of what happens in a nematode nucleus (diploid number 4) during interphase, with individual chromatids represented as numbers, sister chromatids as the same number, and the centromere represented as a “-”.
The second and third steps of mitosis organize the newly created bivalent chromosomes so that they they can be split in an orderly fashion. A lot of care has to be taken with this process, because unequal splitting of chromosomes creates malfunctioning cells. Down syndrome is one disease that results from unequal splitting of chromosomes.
In the second step, prophase, the bivalent chromosomes condense into tight packages, the mitotic spindle forms, and the nuclear envelope dissolves. Imagine the difference between a slinky fully stretched out, and a slinky that has been pressed back together. That's what happens to chromosomes during prophase: they get pressed together into tight packages.
In the third step of mitosis, called metaphase, each chromosome lines up in a single file line at the center of the cell. By this point in time, the membrane enclosing the nucleus has dissolved, and mitotic spindles have attached themselves to each chromatid in all the chromosomes. Here is a diagram of what a nematode cell nucleus looks like after prophase and metaphase.
In the fourth step, anaphase, the mitotic spindles pry each chromatid apart from its copy, and drag them to the opposite side of the cell. Four bivalent chromosomes become two groups of 4 monovalent chromosomes.
Once anaphase is over, the heavy lifting of mitosis is complete. In the final phase, telophase, membranes form around the two new groups of chromosomes, and the mitotic spindles that provided the power to create these groups are disassembled. Once mitosis is complete, the cell has two groups of 46 chromosomes, each enclosed with their own nuclear membrane. The cell then splits in two by a process called cytokinesis, creating two clones of the original cell, each with 46 monovalent chromosomes.
Mitosis is the process in which a eukaryotic cell nucleus splits in two, followed by division of the parent cell into two daughter cells. The word "mitosis" means "threads," and it refers to the...
Mitosis is the the point in the cell cycle associated with division or distribution of replicated genetic material to two daughter cells. During mitosis the cell nucleus breaks down and two new, fully functional, nuclei are formed.
Mitosis (/ m aɪ ˈ t oʊ s ɪ s /) is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis is an equational division which gives rise to genetically identical cells in which the total number of chromosomes is maintained.
People also ask
What is mitosis in eukaryotic cells?
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What is the process of mitosis called?
The cell cycle is defined as the events that enable cells to proceed from one cell division event to the next. Cell division itself consists of the overlapping processes of mitosis (nuclear division) and cytokinesis (division of the cytoplasm).
The process of mitosis, or cell division, is also known as the M phase. This is where the cell divides its previously-copied DNA and cytoplasm to make two new, identical daughter cells. Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase.
