How many parent cells in meiosis

Living cells go through a series of stages known as the cell cycle. The cells grow, copy their chromosomes, and then divide to form new cells.

• G1 phase. The cell grows.
• S phase. The cell makes copies of its chromosomes. Each chromosome now consists of two sister chromatids.
• G2 phase. The cell checks the duplicated chromosomes and gets ready to divide.
• M phase. The cell separates the copied chromosomes to form two full sets (mitosis) and the cell divides into two new cells (cytokinesis).

The period between cell divisions is known as 'interphase'. Cells that are not dividing leave the cell cycle and stay in G0.

Mitosis and meiosis

Cells divide into two different ways to make new cells.

Mitosis

Mitosis is used to produce daughter cells that are genetically identical to the parent cells. The cell copies - or 'replicates' - its chromosomes, and then splits the copied chromosomes equally to make sure that each daughter cell has a full set.

Your body contains trillions of cells (thousands of millions). But you started life as a single cell - a fertilised egg cell. This cell then divided and divided to make more cells through a process called mitosis.

Mitosis is a way of making more cells that are genetically the same as the parent cell. It plays an important part in the development of embryos, and it is important for the growth and development of our bodies as well. Mitosis produces new cells, and replaces cells that are old, lost or damaged.

In mitosis a cell divides to form two identical daughter cells. It is important that the daughter cells have a copy of every chromosome, so the process involves copying the chromosomes first and then carefully separating the copies to give each new cell a full set.

Before mitosis, the chromosomes are copied. They then coil up, and each chromosome looks like a letter X in the nucleus of the cell. The chromosomes now consist of two sister chromatids. Mitosis separates these chromatids, so that each new cell has a copy of every chromosome. The copied chromosomes consist of two chromatids joined at the centromere.

The process of mitosis involves a number of different stages.

Meiosis

Meiosis is used to make special cells - sperm cells and egg cells - that have half the normal number of chromosomes. It reduces the number from 23 pairs of chromosomes to 23 single chromosomes. The cell copies its chromosomes, but then separates the 23 pairs to ensure that each daughter cell has only one copy of each chromosome. A second division that divides each daughter cell again to produce four daughter cells.

Some simple organisms - such as bacteria - can reproduce by simply dividing into two new individuals. Other organisms, including human beings, reproduce through sexual reproduction. New individuals are formed by the joining together of two special cells: a sperm cell and an egg cell.

The cells in our bodies contain 23 pairs of chromosomes - giving us 46 chromosomes in total. Sperm cells and egg cells contain 23 single chromosomes, half the normal number, and are made by a special form of cell division called meiosis.

Meiosis separates the pairs of matching (or 'homologous') chromosomes, so that sperm cells and egg cells have only one copy of each. That way, when an egg cell fuses with a sperm cell, the fertilised egg has a full set: that is, two copies of every chromosome.

Meiosis involves two cell divisions: Meiosis I and Meiosis II.

Meiosis I separates the matching - or 'homologous' - pairs of chromosomes.

Meiosis II divides each chromosome into two copies (much like mitosis).

In Meiosis I, each daughter cell receives a mix of chromosomes from the two sets in the parent cell. In addition, the chromosomes in each matching pair swap some genetic material before they are parted in a process called crossing over. These processes produce new combinations of genes in the sperm cells and egg cells.

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.