Every organism starts life as one cell, and most living beings have to multiply their cells to grow. Cell growth and division are part of the normal life cycle of organisms on Earth, including both prokaryotes and eukaryotes. Living organisms get energy from food or the environment to develop and grow.

Understanding cell division is critical to mastering cell biology.

Organisms need cell division to survive and multiply. The main goal of cell division is to make more cells. For example, most of the cells in the human body are somatic cells and divide regularly. This cell and tissue turnover is important for the organism's health and growth.

It allows a living being to replace dead, old or damaged cells, and it helps some organisms become bigger. Cell division is also a crucial part of reproduction and the production of gametes, which are the sex cells.

Three main types of cell division exist: mitosis, meiosis and binary fission.

Mitosis creates two identical cells from one parent cell. The main goal of mitosis is growth and the replacement of worn out or old cells. Most of the cells in the human body go through mitosis.

Meiosis creates four different daughter cells with half the chromosomes from one parent cell. The main goal of meiosis is to make sperm or egg cells.

Binary fission is how single-celled organisms divide and make a copy of their cells. Prokaryotes use binary fission to replicate their DNA and split the cell into two identical pieces: new cells.

The cell cycle is a series of steps and processes that describe the life of a cell. When cells divide, they do not do so constantly. Instead, it goes through periods of growth and DNA replication. Eukaryotic cells have two main parts in their cycles: interphase and the mitotic (M) phase.

Interphase is the part of the cycle that happens between cell divisions. It consists of the G1, S and G2 phases. During interphase, the cell grows and replicates its genetic material as it prepares for division. It makes copies of organelles, organizes its content and becomes larger.

Mitotic (M) phase is the actual division phase of the cells.

After cell division ends, the cell may go through quiescence, senescence, differentiation, apoptosis or necrosis.

If a cell enters a resting phase, it is called G₀ Phase. Quiescence is a state of inactivity for the cell and may happen because of a lack of nutrients or growth factors. The cell can leave the quiescence stage and become active again.

On the other hand, senescence is a state of inactivity for the cell that happens because of aging or damage. Senescence is not reversible and the cell can die.

Differentiation happens when a cell becomes specialized, such as becoming a blood cell in the human body. Terminal differentiation is a permanent stage, and the cell cannot go through the cell cycle again.

Apoptosis is cell death and is a normal part of the cycle. Cells are programmed to die after a certain period. Necrosis is cell death caused by injury or damage.

Sometimes things can go wrong during cell growth or cell division. Abnormal cell growth can cause diseases such as cancer. If old or damaged cells do not die, and the organism's cells keep dividing and cancer may develop.

Cancerous cells can grow out of control and form tumors. In addition, cancer cells are usually not specialized like other cells.

During mitosis, the parent cell divides into two, identical daughter cells. This type of cell division helps the organism grow and replace old or damaged cells.

The phases of mitosis include:

• Prophase: The parent cell's chromosomes condense and become compact. Spindle fibers form, and the nuclear membrane starts to dissolve. Some sources place another phase, called prometaphase, in between prophase and metaphase.
• Metaphase: The parent cell's chromosomes line up in the middle of the cell, and the mitotic spindles attach to the chromatids.
• Anaphase: The sister chromatids of the chromosomes separate and start to move to opposite poles of the parent cell.
• Telophase: Chromosomes reach the opposite poles, and new nuclear envelopes start to form around each set. The mitotic spindle starts to disintegrate.
• Cytokinesis: The two identical cells separate.

After mitosis ends, the cell can enter interphase until it is time to divide again.

The cell cycle explains the different stages in the life of a cell. Interphase includes G₁, S and G₂. During G₁ (gap phase one), the cell becomes bigger and starts to copy organelles. In the S phase, the cell makes copies of its DNA and centrosome.

During the G₂ (gap phase two), the cell grows more and makes more proteins or organelles. Mitosis happens during the M phase. When a cell exits the main phases, it can enter G₀, which is a resting phase.

Meiosis is a type of cell division that allows a parent cell to make four daughter cells with half of its DNA in them. The daughter cells are called haploid and they are sex cells. You can divide meiosis into two stages: meiosis I and meiosis II.

During meiosis I, the stages include:

• Prophase I: The cell's chromosomes condense, and crossing over happens as the chromosomes exchange pieces of DNA. The nuclear envelope starts to dissolve.
• Metaphase I: The chromosome pairs line up in the middle of the cell.
• Anaphase I: The chromosome pairs separate and start to move to opposite sides.
• Telophase I and Cytokinesis: The chromosomes reach the opposite poles of the cell, and the cell divides into two.

During meiosis II, the stages include:

• Prophase II: Each of the two daughter cells has its chromosomes condense, and the nuclear envelopes start to dissolve.
• Metaphase II: The chromosome pairs in each daughter cell line up in the middle of the cell.
• Anaphase II: The chromosome pairs in each daughter cell separate and start to move to opposite sides.
• Telophase II and Cytokinesis: The chromosomes in each daughter cell reach the opposite poles of the cell, and each cell divides into two. This results in four cells.

There are important differences between meiosis and mitosis. Mitosis creates two diploid daughter cells, but meiosis creates four haploid cells. Mitosis produces identical daughter cells, but meiosis makes genetically variable gametes like the egg and sperm cells.

Mitosis occurs in most cell types. Meiosis only happens in reproductive cells.

Cell cycle regulation is important for all organisms. Different genes control the cell cycle to make sure errors do not occur. If something goes wrong with regulation, cancer can develop.

For example, proto oncogenes usually help the cell grow normally. However, a mutation in a proto oncogene can turn it into an oncogene that leads to the cell growing out of control and cancer.

Tumor suppressor genes can make proteins that fix DNA errors and slow down division in cells. The TP53 gene codes for the tumor suppressor p53 protein in cells. However, mutations in tumor suppressor genes can cause cancer.

Most cells actively going through mitosis are precursor cells. They can become mature cells that form tissues through the process of cellular differentiation.

Cells have to become more specialized in complex organisms.