Daughter cells are cells that result from the division of a single parent cell. They are produced by the division processes of mitosis and meiosis. Cell division is the reproductive mechanism whereby living organisms grow, develop, and produce offspring.
At the completion of the mitotic cell cycle, a single cell divides forming two daughter cells. A parent cell undergoing meiosis produces four daughter cells. While mitosis occurs in both prokaryotic and eukaryotic organisms, meiosis occurs in eukaryotic animal cells, plant cells, and fungi.
- Daughter cells are cells that are the result of a single dividing parent cell. Two daughter cells are the final result from the mitotic process while four cells are the final result from the meiotic process.
- For organisms that reproduce via sexual reproduction, daughter cells result from meiosis. It is a two-part cell division process that ultimately produces an organism's gametes. At the end of this process, the result is four haploid cells.
- Cells have an error-checking and correcting process that helps to ensure the proper regulation of mitosis. If errors occur, cancerous cells that continue to divide may be the result.
Mitosis is the stage of the cell cycle that involves the division of the cell nucleus and the separation of chromosomes. The division process is not complete until after cytokinesis, when the cytoplasm is divided and two distinct daughter cells are formed. Prior to mitosis, the cell prepares for division by replicating its DNA and increasing its mass and organelle numbers. Chromosome movement occurs in the different phases of mitosis:
- Prophase
- Metaphase
- Anaphase
- Telophase
During these phases, chromosomes are separated, moved to opposite poles of the cell, and contained within newly formed nuclei. At the end of the division process, duplicated chromosomes are divided equally between two cells. These daughter cells are genetically identical diploid cells that have the same chromosome number and chromosome type.
Somatic cells are examples of cells that divide by mitosis. Somatic cells consist of all body cell types, excluding sex cells. The somatic cell chromosome number in humans is 46, while the chromosome number for sex cells is 23.
In organisms that are capable of sexual reproduction, daughter cells are produced by meiosis. Meiosis is a two part division process that produces gametes. The dividing cell goes through prophase, metaphase, anaphase, and telophase twice. At the end of meiosis and cytokinesis, four haploid cells are produced from a single diploid cell. These haploid daughter cells have half the number of chromosomes as the parent cell and are not genetically identical to the parent cell.
In sexual reproduction, haploid gametes unite in fertilization and become a diploid zygote. The zygote continues to divide by mitosis and develops into a fully functioning new individual.
How do daughter cells end up with the appropriate number of chromosomes after cell division? The answer to this question involves the spindle apparatus. The spindle apparatus consists of microtubules and proteins that manipulate chromosomes during cell division. Spindle fibers attach to replicated chromosomes, moving and separating them when appropriate. The mitotic and meiotic spindles move chromosomes to opposite cell poles, ensuring that each daughter cell gets the correct number of chromosomes. The spindle also determines the location of the metaphase plate. This centrally localized site becomes the plane on which the cell eventually divides.
The final step in the process of cell division occurs in cytokinesis. This process begins during anaphase and ends after telophase in mitosis. In cytokinesis, the dividing cell is split into two daughter cells with the help of the spindle apparatus.
In animal cells, the spindle apparatus determines the location of an important structure in the cell division process called the contractile ring. The contractile ring is formed from actin microtubule filaments and proteins, including the motor protein myosin. Myosin contracts the ring of actin filaments forming a deep groove called a cleavage furrow. As the contractile ring continues to contract, it divides the cytoplasm and pinches the cell in two along the cleavage furrow.
Plant cells do not contain asters, star-shaped spindle apparatus microtubules, which help determine the site of the cleavage furrow in animal cells. In fact, no cleavage furrow is formed in plant cell cytokinesis. Instead, daughter cells are separated by a cell plate formed by vesicles that are released from Golgi apparatus organelles. The cell plate expands laterally and fuses with the plant cell wall forming a partition between the newly divided daughter cells. As the cell plate matures, it eventually develops into a cell wall.
The chromosomes within daughter cells are termed daughter chromosomes. Daughter chromosomes result from the separation of sister chromatids occuring in anaphase of mitosis and anaphase II of meiosis. Daughter chromosomes develop from the replication of single-stranded chromosomes during the synthesis phase (S phase) of the cell cycle. Following DNA replication, the single-stranded chromosomes become double-stranded chromosomes held together at a region called the centromere. Double-stranded chromosomes are known as sister chromatids. Sister chromatids are eventually separated during the division process and equally distributed among newly formed daughter cells. Each separated chromatid is known as a daughter chromosome.
Mitotic cell division is strictly regulated by cells to ensure that any errors are corrected and that cells divide properly with the correct number of chromosomes. Should mistakes occur in cell error checking systems, the resulting daughter cells may divide unevenly. While normal cells produce two daughter cells by mitotic division, cancer cells are distinguished for their ability to produce more than two daughter cells.
Three or more daughter cells may develop from dividing cancer cells and these cells are produced at a faster rate than normal cells. Due to the irregular division of cancer cells, daughter cells may also end up with too many or not enough chromosomes. Cancer cells often develop as a result of mutations in genes that control normal cell growth or that function to suppress cancer cell formation. These cells grow uncontrollably, exhausting the nutrients in the surrounding area. Some cancer cells even travel to other locations in the body via the circulatory system or lymphatic system.
- Reece, Jane B., and Neil A. Campbell. Campbell Biology. Benjamin Cummings, 2011.
All cells arise from other cells through the process of cell division. Meiosis is a specialized form of cell division that produces reproductive cells, such as plant and fungal spores and sperm and egg cells.
In general, this process involves a "parent" cell splitting into two or more "daughter" cells. In this way, the parent cell can pass on its genetic material from generation to generation.
Eukaryotic cells and their chromosomes
Based on the relative complexity of their cells, all living organisms are broadly classified as either prokaryotes or eukaryotes. Prokaryotes, such as bacteria, consist of a single cell with a simple internal structure. Their DNA floats freely within the cell in a twisted, thread-like mass called the nucleoid.
Animals, plants and fungi are all eukaryotes. Eukaryotic cells have specialized components called organelles, such as mitochondria, chloroplasts and the endoplasmic reticulum. Each of these performs a specific function. Unlike prokaryotes, eukaryotic DNA is packed within a central compartment called the nucleus.
Within the eukaryotic nucleus, long double-helical strands of DNA are wrapped tightly around proteins called histones. This forms a rod-like structure called the chromosome.
Cells in the human body have 23 pairs of chromosomes, or 46 in total. This includes two sex chromosomes: two X chromosomes for females and one X and one Y chromosome for males. Because each chromosome has a pair, these cells are called "diploid" cells.
On the other hand, human sperm and egg cells have only 23 chromosomes, or half the chromosomes of a diploid cell. Thus, they are called "haploid" cells.
When the sperm and egg combine during fertilization, the total chromosome number is restored. That's because sexually reproducing organisms receive a set of chromosomes from each parent: a maternal and paternal set. Each chromosome has a corresponding pair, orhomolog.
Mitosis vs. meiosis
Eukaryotes are capable of two types of cell division: mitosis and meiosis
Mitosis allows for cells to produce identical copies of themselves, which means the genetic material is duplicated from parent to daughter cells. Mitosis produces two daughter cells from one parent cell.
Single-celled eukaryotes, such as amoeba and yeast, use mitosis to reproduce asexuallyand increase their population. Multicellular eukaryotes, like humans, use mitosis to grow or heal injured tissues.
Meiosis, on the other hand, is a specialized form of cell division that occurs in organisms that reproduce sexually. As mentioned above, it produces reproductive cells, such as sperm cells, egg cells, and spores in plants and fungi.
In humans, special cells called germ cells undergo meiosis and ultimately give rise to sperm or eggs. Germ cells contain a complete set of 46 chromosomes (23 maternal chromosomes and 23 paternal chromosomes). By the end of meiosis, the resulting reproductive cells, or gametes, each have 23 genetically unique chromosomes.
The overall process of meiosis produces four daughter cells from one single parent cell. Each daughter cell is haploid, because it has half the number of chromosomes as the original parent cell.
"Meiosis is reductional," said M. Andrew Hoyt, a biologist and professor at Johns Hopkins University.
Unlike in mitosis, the daughter cells produced during meiosis are genetically diverse. Homologous chromosomes exchange bits of DNA to create genetically unique, hybrid chromosomes destined for each daughter cell.
A closer look at meiosis
Before meiosis begins, some important changes take place within the parent cells. First, each chromosome creates a copy of itself. These duplicated chromosomes are known as sister chromatids. They are fused together and the point where they are joined is known as the centromere. Fused sister chromatids roughly resemble the shape of the letter "X."
Meiosis occurs over the course of two rounds of nuclear divisions, called meiosis I and meiosis II, according to Nature Education's Scitable. Furthermore, meiosis I and II are each divided into four major stages: prophase, metaphase, anaphase and telophase.
Meiosis I is responsible for creating genetically unique chromosomes. Sister chromatids pair up with their homologs and exchange genetic material with one another. At the end of this division, one parent cell produces two daughter cells, each carrying one set of sister chromatids.
Meiosis II closely resembles mitosis. The two daughter cells move into this phase without any further chromosome duplication. The sister chromatids are pulled apart during this division. A total of four haploid daughter cells are produced during the course of meiosis II.
Meiosis I
The four stages of meiosis Iare as follows, according to "Molecular Biology of the Cell." (Garland Science, 2002):
Prophase I: At this stage, chromosomes become compact, dense structures and are easily visible under the microscope. The homologous chromosomes pair together. The two sets of sister chromatids resemble two X's lined up next to each other. Each set exchanges bits of DNA with the other and recombines, thus creating genetic variation. This process is known as crossing over, or recombination.
Even though in humans the male sex chromosomes (X and Y) are not exact homologs, they can still pair together and exchange DNA. Crossing over occurs within only a small region of the two chromosomes.
By the end of prophase I, the nuclear membrane breaks down.
Metaphase I: The meiotic spindle, a network of protein filaments, emerges from two structures called the centrioles, positioned at either end of the cell. The meiotic spindle latches onto the fused sister chromatids. By the end of metaphase I, all the fused sister chromatids are tethered at their centromeres and line up in the middle of the cell. The homologs still look like two X's sitting close together.
Anaphase I: The spindle fibers start to contract, pulling the fused sister chromatids with them. Each X-shaped complex moves away from the other, toward opposite ends of the cell.
Telophase I: The fused sister chromatids reach either end of the cell, and the cell body splits into two.
Meiosis I results in two daughter cells, each of which contains a set of fused sister chromatids. The genetic makeup of each daughter cell is distinct because of the DNA exchange between homologs during the crossing-over process.
Meiosis II
"Meiosis II looks like mitosis," Hoyt told Live Science. "It's an equational division."
In other words, by the end of the process, the chromosome number is unchanged between the cells that enter meiosis II and the resulting daughter cells.
The four stages of meiosis II are as follows, according to “Molecular Biology of the Cell, 4th edition.”
Prophase II: The nuclear membrane disintegrates, and meiotic spindles begin to form once again.
Metaphase II: The meiotic spindles latch onto the centromere of the sister chromatids, and they all line up at the center of the cell.
Anaphase II: The spindle fibers start to contract and pull the sister chromatids apart. Each individual chromosome now begins to moves to either end of the cell.
Telophase II: The chromosomes reach opposite ends of the cell. The nuclear membrane forms again, and the cell body splits into two
Meiosis II results in four haploid daughter cells, each with the same number of chromosomes. However, each chromosome is unique and contains a mix of genetic information from the maternal and paternal chromosomes in the original parent cell.
Why is meiosis important?
Proper “chromosomal segregation,” or the separation of sister chromatids during meiosis I and II is essential for generating healthy sperm and egg cells, and by extension, healthy embryos. If chromosomes fail to segregate completely, it's called nondisjunction and can result in the formation of gametes that have missing or extra chromosomes, according to "Molecular Biology of the Cell, 4th edition."
When gametes with abnormal chromosome numbers fertilize, most of the resulting embryos don't survive. However, not all chromosomal abnormalities are fatal to the embryo. For example, Down syndrome occurs as a result of having an extra copy of chromosome 21. And, people with Klinefelter syndrome are genetically male but have an extra X chromosome.
The most significant impact of meiosis is that it generates genetic diversity, and that's a major advantage for species survival.
"Shuffling the genetic information allows you to find new combinations which will perhaps be more fit in the real world," Hoyt said.