Cell Growth

Cell Growth Meaning

Let's start by looking at the meaning of cell growth.

Cell growth and DNA replication are two important processes that take place during cell division, and the sequence of events from the duplication of a cell genome to its division into two daughter cells is termed the cell cycle.

Cell growth is a continuous process, and during this time, cells become larger by creating more organelles and proteins and replicating DNA.

Cell division can also occur without cell growth, like during embryonic development. Similarly, cells such as neurons can grow without cell division during nervous system development and neuronal cell migration.

Cell Growth Examples

An example of how cells can grow is seen during tissue injury. When you fall and scrape your knee, cells need to divide and proliferate to close up the wound, so you do not get infected.

This is done through accelerated cell growth, cell migration, and cell division. The cells remaining on your skin after a wound has been inflected secrete specialized signals that encourage the nearby cells to grow and proliferate. This is why you see scabs on your knee after a few days.

Types of Cell Growth

There are many types of cell growth associated with different organisms. Eukaryotic cells divide via mitosis and meiosis, which we will discuss in detail below.

Other organisms, such as bacterial cells, divide via binary fission. During binary fission, DNA within the bacterial cell replicates and travels to opposite ends of the cell. Once this happens, cytokinesis takes place, and two identical daughter cells are formed.

Cell Growth and Division

As mentioned previously, cell growth occurs often occurs during the cell cycle. The cell cycle has many stages that are very important:

1. Interphase

2. Gap Phase

3. S phase

4. M phase

The first stage of the cell cycle is interphase, in which the cell prepares for mitotic cell division. When cells undergo interphase, they are constantly synthesizing RNA, generating proteins, and growing in size. Interphase is subdivided into four stages:

• Gap 0 (G₀)

• Gap 1 (G₁)

• Synthesis (S)

• Gap 2 (G₂)

These stages occur in sequential order and are extremely important to get the cell ready for division. Let's take a look at these phases in more detail.

G₀ Phase

The gap 0 phase (G₀) is technically not a part of the cell cycle but instead is characterized as a temporary or permanent resting phase in which the cell does not undergo cell division.

Usually, cells such as neurons that do not divide are said to be in the Gap 0 phase. The gap 0 phase can also occur when cells are senescent.

The number of senescent cells in the body increases as we age. Researchers are still investigating the cause of why senescent cells increase as we age, but they suspect that it could be due to decreased efficiency of autophagy.

G₁ Phase

During the gap 1 phase, the cell grows and produces numerous proteins which allows the cell to almost double in size. In this phase, the cells produce more organelles and increase their cytoplasmic volume, preparing for division.

Synthesis (S) Phase

During the synthesis (S) phase, the cell undergoes DNA replication (DNA synthesis), where the amount of cellular DNA is doubled.

During DNA replication, each chromatid from the G₁ phase is copied, doubling the amount of DNA. The two resulting chromatids are identical, and they join together in the center, in a region called the telomere.

G₂ Phase

The gap 2 phase is characterized by increased cellular growth as the cell prepares to enter the mitotic phase. The mitochondria, which are the cell's powerhouse, also divide in preparation for cell division.

• In this phase, the cell's organelles are also duplicated.

Cell Growth Phases

Now that the interphase is completed, let's move on to discuss the mitosis (M) phase, which is the second part of the cell cycle.

Mitosis consists of five stages:

1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase

Stage 1 - Prophase

During prophase, the DNA chromosomes condense into sister chromatids which makes them visible through a microscope. The centrosomes start to separate to opposite sides of the cell, producing long strands called spindle microtubules, or mitotic spindles, as they move through the cell. During prophase, the nuclear envelope surrounding the DNA begins to break down, allowing access to the chromosomes.

Stage 2 - Prometaphase

During this phase, the nuclear envelope fragments, and the nucleolus disappears. The centrosomes have reached the opposite sides of the cell, and the mitotic spindles begin to connect the centromeres of the chromatids at structures called kinetochores. This process allows the mitotic spindles to move the chromatids toward the center of the cell in preparation for metaphase.

Stage 3 - Metaphase

Metaphase is characterized by the sister chromatids lining up in the center of the cell in a straight line. This line is known as the metaphase plate. During metaphase, the centrosomes have fully separated to the opposite sides of the cell, and the mitotic spindles are fully formed.

Stage 4 - Anaphase

This stage is characterized by the separation of sister chromatids, which divides the DNA. During anaphase, the cohesion proteins holding together the sister chromatids break down while the mitotic spindles pull the sister chromatids apart to opposite ends of the cell.

Stage 5 - Telophase

During this stage, two nuclear envelopes begin to surround each set of chromosomes, and nucleoli begin to form within the new nuclei surrounding the chromosomes. At this point, the mitotic spindles break down and will be reused for the cytoskeleton of both daughter cells.

Cytokinesis

In this stage, the cytoplasm divides by forming a cleavage furrow and pinching off into two separate daughter cells with identical genetic codes.

Cell Growth and Reproduction

During reproduction, gametes from a mother and from a father come together to form a zygote. A special form of cell division known as meiosis occurs in the reproductive organs of males and females in order to produce gametes.

During meiosis, two rounds of cell division take place to produce four daughter cells with half the amount of genetic information as the parent cell.

The stage of meiosis is the same as those of mitosis, except they occur twice! After fertilization has taken place and the zygote is formed, the zygote grows and divides.

• At this point, cell growth is very important as during the early embryonic stage, cell growth and division take place rapidly until the fetus is formed.

During fetal development, cell growth also occurs in the nervous system and other bodily systems. Cell growth during this period is crucial for cell migration to their rightful spots within tissues. As the cells grow, their cytoskeletons change, which allows them to physically move throughout tissues until they get to their rightful places.

Cell Growth Curve

During biological experiments, cells are often cultured and used for experiments. Drug testing and a cell growth curve provide information about the cells' behavior during the culture process and during the experiments.

These characteristics are depicted in three phases:

• lag phase
• plateau phase
• death phase

During the lag phase, the cells recover from cultivation and start to attach to the surface of the container and divide. This then transitions into the log phase, where the cells grow exponentially and double.

At the plateau phase, the culture growth begins to slow and eventually stops. The plateau phase is followed by the death phase, where the cells begin to die and detach from the container.