Normal Cells

Cell Cycle.

The cell cycle, an essential sequence of events, promotes cellular division and tissue growth. The cell cycle consists of four phases, each of which is unique and vital for cellular survival. This four-phase process includes the synthesis (S), mitosis (M), GAP 1 (G₁), and GAP 2 (G₂) phases (see figure below). During the mitosis (cellular division) phase, which lasts about 1 hour, the chromatids (i.e., daughter strands of a duplicated chromosome) split to form two identical cells. Mitosis consists of a five-phase progression that leads to the final separation of all intracellular components and the formation of two new daughter cells. Each daughter cell consists of a complete identical set of chromosomes and the essential components of the original cell. On completion of mitosis, the cell either begins a new cycle by entering the GAP 1 phase or proceeds to the G₀ phase, also called resting the stage or quiescence.

Cell Cycle.

In GAP 1, which lasts 4 to 6 hours, the nucleus enlarges and the cell performs the processes of transcription and translation essential for the replication of DNA. The next phase is the S or synthesis phase, during which cells perform DNA replication and produce a set of chromatids. On completion of DNA replication, the cells enter the GAP 2 phase, lasting from 2 to 8 hours. GAP 2 is characterized by intense synthesis of protein and cellular organelles. It is during this phase that cells become committed to entering mitotic division.

Embryonic Cells

The concept of each normal, mature cell having a specific morphology and function (or set of functions) is astounding when one considers that all humans started life as a single cell. Although human embryonic cells, those present from conception until postconception day 8 are clearly normal, for a short period of time their behavior and characteristics are very different from the behavior and characteristics of mature differentiated cells.

Molecular Genetics

Genes, the smallest functional units of inherited information in living organisms, are the controlling factors for cellular development and function. Genes provide the instructions for making proteins, using different combinations of amino acids, and regulate when and where a protein will be produced (regulatory sequence). The process of making proteins is called protein synthesis. Proteins perform a host of biological functions. Some proteins, called enzymes, act to catalyze (speed up) chemical reactions within or between cells. Other proteins support the cytoskeleton of the cell, play a role in immune responses, or participate in the storage or transport of ligands. Some cells transport proteins that are used by other cells or used to direct the activity of other cells. In short, proteins play essential roles in cellular maintenance, growth, and function (see table below).

DNA Synthesis

DNA is a nucleic acid located in the nucleus of each cell as a condensed, compact structure that contains codes for the construction of every protein made in the body. The helix-shaped form of DNA is actually a pair of molecules consisting of polymers (long chains) of interlocking nucleotides called chromatin. The nucleotide molecules are chemical building blocks consisting of a phosphate, a five-carbon sugar, and a base (adenine [A], guanine [G], thymine [T], or cytosine [C]). The arrangement of the bases can be likened to a genetic alphabet that is used to create the language of intercellular and intracellular communication. (See figure on page 18).

The two strands of DNA are aligned as two complementary threads, held in close proximity by hydrogen bonds. These bonds are disrupted when the cell undergoes DNA or protein synthesis. The bases in the two adjoining strands can only pair up with one single predetermined base. Two nucleotides, paired together are termed a base pair. A pairs with thymine; T and G pair with C. The only possible combinations of the bases are A and T, T and A, C and G, and G and C. A on one strand of DNA can only pair up or mate with the T on the complementary strand. Therefore, if the sequence of nucleotides on one DNA strand is known, the order of the complementary DNA strand can be predicted accurately. One additional nucleotide, uracil (U) is rarely seen in DNA except in some viruses, where it can replace T (see figure on page 19). There are approximately three billion base pairs in the human genome. Base pairing ensures the structural integrity of DNA and is essential for the storage, retrieval, and transference of genetic information.

During mitosis, the DNA content of the dividing cell must first replicate through the process of DNA synthesis, which takes place within the nucleus of the cell. The original strands of DNA are the templates for the construction of new DNA. An enzyme, topoisomerase, is initially responsible for unwinding the double helix by cleaving a single strand of DNA. Then, other enzymes called helicases disassociate the hydrogen bonds that connect the two strands. The unwinding of the two strands of DNA, with each one acting as a template for a new strand, is known as semiconservative replication. The DNA relaxes, unwinds, slightly straightens, and separates the two strands.

The intertwined double strands of DNA are antiparallel (run in opposite directions) with the asymmetric ends (referred to as “five prime” [5′] and “three prime” [3′]) winding around a helix axis in a right-handed spiral. In a vertical double helix, 3′ is designated as “ascending” and 5′ is designated as “descending.” A polymerizing enzyme attaches itself to one strand and descends from the 5′ to the 3′ direction. As the enzyme moves along the strand, it “reads” the base sequence and forms a new string of DNA complementary to the template strand. After the strands of DNA are replicated, they condense into supercoiled chromosome sets that split to become part of two new daughter cells.

Common Terms Used in Molecular Genetics

DNA molecule. From the National Institutes of Health: National Human Genome Research Institute: www.genome.gov.

Protein Synthesis

Protein synthesis is the process by which DNA genes serve as codes for the production of amino acids and proteins. This activity occurs in the ribosomes, which are ribonucleoprotein complexes found in the cytoplasm of the cell. Proteins are formed by peptide bonds joined to individual amino acids together in a linear strand. To accomplish protein synthesis, a particular DNA gene sequence for a specific protein is transcribed into a piece of ribonucleic acid (RNA). RNA consists of a sugar-phosphate backbone with a nucleotide attached. Although RNA is similar to DNA, it uses the nucleotide base U instead of T and has a hydroxyl group attached to its ribose sugar.

Protein synthesis consists of two phases: transcription and translation. Initially, the cells receive a message to produce a specific protein. During transcription, the portion of the DNA that contains the genetic code for producing the protein uncoils (“unzips”) as occurs during cell division. Nucleotides then move along the strand of exposed gene and transcribe the subunits of DNA. The new strand is an RNA molecule referred to as messenger RNA (mRNA). The exposed DNA “zips” closed and remains in the nucleus of the cell. Before the transcripted mRNA moves into the cytoplasm, nuclear enzymes remove introns (noncoding sections, also called “junk DNA”) and splice together exons (the working sequences that code for proteins) (see figure on page 20).

Next, the genetic copies (i.e., mRNA) enter the cytoplasm through channels (pores) in the nucleus, bind to ribosomal RNA (rRNA) found in the cytoplasm, and are decoded. The mRNA is encoded with information on the particular arrangement of amino acids that will make up each protein. During translation, a molecule of transfer RNA (tRNA) matches up to the strand of mRNA and carries the correct amino acids to the ribosome (see figure on page 20). Three nucleotide bases (codons) are read at a time, each representing a specific amino acid (see figure on page 20). As each codon is decoded, the corresponding amino acid is activated. The amino acids are brought into the proper sequence once the entire message is read, and the newly formed polypeptide chain folds into its final three-dimensional shape. The total number of amino acids in a specific protein and the exact code that links them together determines the nature and activity of the protein. Different sequences of amino acids change the shape of the proteins and therefore their functions (see figure on page 21).

Neoplasia

Neoplasia is a term designated to signify a new growth of cells in the body. Although all neoplasms are considered to be abnormal, they are designated as either benign (noncancerous) or malignant (cancerous). All tumors, whether benign or malignant, share two things in common: a parenchyma (functional part) that contains proliferating neoplastic cells and a stroma (supportive structure) consisting of connective tissue and blood vessels (Kumar, Abbas, & Fausto, 2005).

Benign Neoplasia.

Benign neoplastic cells enter and progress through the phases of the cell cycle in the same fashion as normal cells. Benign neoplastic cells arise from normal cells and tend to retain most of the properties of the cells from which they arose. At the microscopic level, benign cells are euploid, containing the normal chromosome complement. However, because their behavior is not completely normal, it is likely that some alteration in gene regulation is present. Benign neoplastic cells are characterized by increased proliferation or decreased apoptosis (programmed cell death). A major difference in growth characteristics between benign neoplastic cells and normal cells is that density-dependent contact inhibition has been lost. Therefore, benign neoplastic cells continue to grow by expansion into the surrounding tissue.

DNA replication.

From the National Institutes of Health: National Human Genome Institute: www.genome.gov.

Although benign neoplastic cells have lost some degree of growth control, they have not developed the ability to metastasize (spread to other organs), which is the hallmark of a malignant neoplasm. The suffix “-oma” is generally assigned to designate a benign neoplasm on the basis of the cell of origin (e.g., fibroma [fibroblastic cell], osteoma [bone cell]). Additionally, the type of pattern that it displays may be used to name a benign neoplasm. For example, an adenoma generally displays a glandular pattern, but it may arise from several different types of cells (e.g., epithelium, glandular).

Benign neoplastic tissues are typically well differentiated. In essence, they are morphologically and functionally similar to the normal tissues from which they arose. They retain a small nuclear/cytoplasmic ratio and continue to synthesize fibronectin, as do normal cells. Like normal cells, benign neoplastic cells adhere tightly and do not metastasize. Even so, benign neoplastic growth can cause physiological dysfunction and death. For example, a benign tumor can exert pressure on nerves and blood vessels or obstruct lumens (e.g., trachea, colon) (Kumar, Abbas, & Fausto, 2005).

Malignant Neoplasia.

Proteins play an important role in the development of malignant disease. The normal cell cycle is regulated by proteins called cyclins, cyclin-dependent kinases, and cyclin-dependent kinase inhibitors. The role of these proteins is to provide cell-cycle checkpoints to control the signals that coordinate cell reproduction. Checkpoints occur in late G1 before DNA synthesis, during the S phase when DNA content is duplicated, and in G2 before cells become completely committed to mitosis. These checkpoints occur at times during the cell cycle when cells are most vulnerable to the harmful effects of DNA (genetic) damage. The checkpoints allow cells to make DNA repairs and remove damaged molecules before they threaten the survival of the organism (Reed, 2005; Rettig & Sawicki, 2001; Skaar & DeCaprio, 2006).

Protein synthesis—transcription and translation.

From the National Institutes of Health: National Human Genome Research Institute: www.genome.org.

Normal cells have multiple copies of specific DNA sequences and proteins at the ends of their chromosomes, called telomeres. Each time a cell divides, it loses a telomere, thus shortening the chromosome. When the chromosome is shortened to a predetermined length, the cell is signaled to enter the resting stage, also known as senescence. The cell remains viable but stops reproducing (Kashima et al., 2006). Normal cells in a culture plate have a limited life span and a specific number of doubling times before they enter senescence. In addition, they display contact inhibition. Once the cells reach the boundary of the culture dish, they stop proliferating.

Normal cells are anchorage dependent. With the exception of hematopoietic cells, they must be attached to a surface (substratum) to proliferate. Malignant (cancer) cells, by contrast, are immortal. In a culture medium, growth is not confined to the surface of the culture dish. When malignant cells reach the edges of the culture dish, they continue to grow on top of each other. Malignant neoplastic cells are also capable of growing without a supporting substratum. They grow easily in suspension in an agarose or other medium, whereas normal cells must adhere to the surface of the culture plate to grow. Thus, malignant cells have a loss of anchorage dependence.

Protein synthesis within the cell.

From the National Institutes of health: National Genome Institute: www.genome.gov.

Codones.

From the National Institutes of Health: National Human Genome Institute: www.genome.gov.

Protein structures.

From the National Institutes of Health: National Human Genome Institute: www.genome.gov.

Cancer cells range from well differentiated to anaplastic (undifferentiated). Anaplasia, or lack of differentiation, is a classic sign of malignant transformation. As cancer progresses, the cells become increasingly anaplastic until they no longer resemble the parent tissue and most differentiated functions are lost. Cancer cells continually undergo mitosis and perform fewer and fewer differentiated functions. As a result, cancer cells have a large nuclear/cytoplasmic ratio, whereas the reverse is true for normal cells.

The nomenclature for malignant neoplasms is the same as that used for benign tumors. Cancerous tumors of epithelial cell origin are called carcinomas (e.g., bronchogenic carcinoma [respiratory], renal cell carcinoma [kidney], hepatocellular carcinoma [liver]). Adenocarcinoma is a term reserved for tumors that display a glandular pattern. Cancers arising from smooth muscle are called leiomyosarcomas, whereas the term leukemia refers to a hematopoietic malignancy.

Oncogenes

Two types of genes are responsible for the development of cancer: oncogenes and tumor suppressor genes. Oncogenes are mutations of normal genes called proto-oncogenes that govern growth factors (GFs), GF receptors (GFRs), signal transducers, transcription factors, apoptosis regulators, and cell cycle regulators (see table on page 22). Defects in any of these can provide a “gain of function” and cause a cell to grow out of control. For example, GFs serve as ligands that bind to receptors on the surface of cells. If a GF, or GFR, is overproduced by an oncogene, the target cells may experience uncontrolled proliferation (Carpenter & Cantley, 2005; Connolly et al., 2006; Smith, Marks, & Lieberman, 2005). Other oncogenes can affect signal transduction in cells and activate a series of protein kinases, leading to uncontrolled cell proliferation. However, the mutation only occurs in one copy of a proto-oncogene.

Gene mutations.

From the National Institutes of Health: National Human Genome Institute: www.genome.gov.

Subsequent mutations are required for cancer to develop. This phenomenon is referred to as the multihit concept of tumor development. Examples of oncogenes that are altered in many types of cancers include myc (leukemia, lymphoma, lung cancer), ras (bladder, lung, breast, ovarian cancer), sis (glioma), and HER-2/neu (breast cancer, cervical cancer). GFs and GFRs function as ligands and regulate cell proliferation by binding to receptors on the plasma membrane. If excessive GF is produced because of genetic mutation, cells may proliferate inappropriately. In essence, the signal for cellular reproduction is constantly “on.”

Selected Oncogenes