Cell Growth and Division
Every organism is composed of one or more cells, which are units of life. A cell can grow larger to a certain extent. But at some point, instead of continuing to grow, the cell divides. The process by which one parent cell divides into two daughter cells is known as cell division, or cell reproduction.
The actual process of cell division depends on whether the cell is prokaryotic or eukaryotic. A prokaryotic cell lacks a nucleus or any other membrane bound organelles. A eukaryotic cell is organized into membrane-bound organelles, such as the nucleus and mitochondria. Cell division is relatively simple in prokaryotic cells such as bacteria. Prokaryotes reproduce through binary fission. During this process, a cell’s DNA is copied. The cell then splits into two parts. Each part receives one copy of the DNA. Binary fission produces two new daughter organisms out of one parent organism.
Eukaryotic cells undergo a more complex process that is part of a larger sequence of events related to cell growth and division called the cell cycle. The cell cycle forms new cells that enable an organism to grow and develop tissues, organs, and organ systems. The development of new cells also allows the organism to replace old or damaged cells.
The Cell Cycle
The cell cycle is divided into five major phases: G1, S, G2, mitosis, and cytokinesis. During the gap 1, or G1, phase, a cell increases in size and volume and performs normal cell functions. When the cell reaches a certain size, it may proceed to the synthesis phase, or S phase, during which the cell’s DNA is replicated. After successful replication, the cell proceeds to the gap 2 phase, or G2. In G2, the cell undergoes another phase of growth in which extra organelles and cytoplasm are produced in preparation for cell division.
The G1, S, and G2 phases are all considered part of a larger phase called interphase. After interphase, the cell is ready to divide.
During mitosis, or the M phase, the nucleus divides into two nuclei. Once mitosis is complete, the cell undergoes cytokinesis and divides into two daughter cells that are identical to the parent cell.
The cell must pass through a variety of checkpoints during the cell cycle. Each checkpoint is a stopping point that allows the cell to determine whether it is ready to move on to the next phase. These checkpoints occur during the G1 and S phases and at the end of the G2 and M phases. The checkpoint in the S phase, for example, evaluates whether the cell’s DNA has been replicated properly. If it has not, the cell may not continue to prepare for division.
Chromosomes and Cell Division
The nucleus of a eukaryotic cell divides during mitosis, or the M phase of the cell cycle. In other words, mitosis divides the genetic material in the cell. Genes are not located randomly in cells. Instead genes are lined up on structures called chromosomes. Each chromosome is made up of a combination of DNA wrapped with proteins and can contain thousands of genes.
Most of the time, a chromosome is a structure that looks like a thin thread. Just before a cell divides, the chromosome shortens and takes on an X shape. Each side of an X-shaped chromosome is called a chromatid. The two chromatids, called sister chromatids, are exact copies of each other and are held together at a point called the centromere. Chromosomes can be seen clearly in a microscope only when the cell is about to divide.
Chromosomes occur in pairs in most eukaryotic organisms. One chromosome in each pair comes from the male parent, and the other comes from the female parent. The chromosomes in each pair are known as homologous chromosomes. Homologous chromosomes have genes arranged in the same order. However, the genes may be different forms. Therefore the two chromosomes in a homologous pair are not genetically identical.
Before cells divide, they make a complete copy of their chromosomes. The nucleus of the cell then divides during mitosis. Scientists often describe mitosis as having the four stages as shown here. During prophase, the first stage, the chromosomes group together, and the nuclear envelope disappears. Spindle fibers begin to form, which are protein fibers that can attach to and provides a path for chromosomes to move along.
In metaphase, the second stage, the chromosomes line up across the center of the cell. During anaphase, the third stage, spindle fibers shorten and split the centromeres apart. As the fibers continue to shorten, sister chromatids move to opposite ends of the cell. In telophase, the final stage, the cell membrane pinches in at the center of the cell. A nuclear envelope reappears around each group of now single chromosomes. Simultaneously, in the process called cytokinesis, the cell splits into two identical cells called daughter cells. Each new daughter cell has a complete set of chromosomes.
During sexual reproduction, two sex cells combine to form a cell that can develop into a new organism. However, if sex cells were like normal body cells, the cell resulting from their combination would have twice the number of chromosomes needed.
A cell cannot function properly if it has too few or too many chromosomes. This is why sexual reproduction involves meiosis, a form of cell division in which the nucleus divides twice to reduce the number of chromosomes by half. Cell division that involves meiosis occurs only in the production of male gametes (sperm) and female gametes (eggs). When a sperm cell combines with an egg cell, the zygote formed has a nucleus with the correct number of chromosomes. The zygote then divides using mitosis to form the many cells of a multi-cellular organism. The diagram on page 130 contrasts, or shows some differences, between the processes of the two phases of meiosis.
Meiosis occurs in two phases: meiosis I and meiosis IL Each phase of meiosis has four stages.
Meiosis I separates homologous chromosomes.
- During prophase I, homologous chromosomes pair up. Each of these chromosomes consists of two chromatids-replicated during S phase producing chromatids. Crossing over between chromatids occurs during prophase I, and genetic material is exchanged between the homologous chromosomes. The nuclear envelope breaks down, and spindle fibers form.
During metaphase I, homologous chromosomes line up at the equator.
- During anaphase I, spindle fibers shorten and separate homologous chromosomes from each other, moving them to opposite sides of the cell.
- During telophase I, spindle fibers break down. Chromosomes uncoil, and nuclear envelopes form, creating two nuclei. The cell then divides. Meiosis II splits sister chromatids.
- During prophase II, chromosomes condense, and spindle fibers form once again within each new cell.
- During metaphase II, the centromeres of the chromosomes line up randomly along the center of each new cell.
- During anaphase II, the centromeres split, and sister chromatids separate. Spindle fibers shorten and move these chromatids to opposite sides of each cell.
- During telophase II, spindle fibers break down. Nuclear envelopes form around chromosomes to form four nuclei. Each cell divides. The end result of meiosis is four cells, each with half the number of chromosomes that are in a body cell of the organism.