04-Cell Growth and Reproduction
Proteins determine the structure and function of cells. Protein synthesis is directed by two nucleic acids: deoxyribonucleic acid, or DNA, and ribonucleic acid, or RNA.
DNA makes up the 46 chromosomes contained in the cell nucleus. DNA is a large molecule that is shaped like a spiral staircase. The molecule is made of many smaller units called nucleotides. Each nucleotide contains a sugar, a phosphate, and a base. Sugar, specifically deoxyribose, and phosphate units make up the sides of the spiral staircase. The “steps” of the staircase are made up of base pairs: adenine paired with thymine or guanine paired with cytosine. Base pairings are always the same, a concept called complementary base pairing, but the sequence of base pairs differs in different DNA molecules. This sequence of base pairs is what makes up a gene. Genes dictate formation of enzymes and other proteins by ribosomes, thereby indirectly determining a cell’s structure and functions.
RNA molecules are made from genes that do not code directly for proteins. Instead, they regulate cell processes, such as protein synthesis. Like DNA, RNA is also made of units called nucleotides, but the sugar is ribose, and the base uracil replaces thymine. There are different types of RNA. Messenger RNA, or mRNA, is the transcribed working copy of one gene. Ribosomal RNA, or rRNA, is a component of ribosome. Transfer RNA, or tRNA, carries specific amino acids to their locations on ribosomes during translation.
Protein synthesis occurs in the cytoplasm, thus genetic information must pass from the nucleus to the cytoplasm. Protein synthesis includes the processes of transcription and translation.
During the process of transcription, double-stranded DNA separates, and one strand is copied to form mRNA. Each strand of mRNA is a copy, or transcript, of a particular gene from a segment of DNA. Messenger RNA molecules pass from the nucleus to the cytoplasm, where they direct protein synthesis in ribosomes and endoplasmic reticulum.
Translation of the code in the mRNA transcript allows ribosomes to synthesize proteins. A codon is a series of three nucleotide bases in mRNA that acts as a code for a specific amino acid. Each tRNA has an anticodon, which is a three base sequence that complements the codon. tRNA with the complementary anticodon carries the amino acid and puts it into place along the mRNA strand. As amino acids are added in sequence along the mRNA, a strand of amino acids is formed. This strand will fold in on itself, and possibly combine with other strands, ultimately forming a complete protein molecule.
Abnormal DNA that is inherited or results from damage is often the basis of disease. The damage can lead to the formation of dysfunctional proteins or can prevent proteins from being formed at all. Factors that cause damage to DNA molecules include chemical or mechanical irritants, radiation, bacteria, and viruses.
The cell life cycle includes the reproduction of a cell involving the division of the nucleus and the cytoplasm. Two daughter cells result from the division. DNA replication is the process of copying DNA by way of the separation of strands and the synthesizing of a new complementary strand to make two whole DNA molecules identical to the original DNA molecule. DNA replication precedes mitosis.
Mitosis is the process in cell division that distributes identical nuclear chromosomes to each new cell formed when the original cell divides. Mitosis involves four stages—prophase, metaphase, anaphase, and telophase—and is separated by interphase, which is the period when the cell is not actively dividing.
Prophase is the first stage, when chromatin granules become organized. Chromosomes, referred to as pairs of linked chromatids at this stage, appear and centrioles move away from the nucleus. The nuclear envelope disappears, which frees genetic material, and the spindle fibers appear.
Metaphase is the second stage, in which chromosomes align across the center of the cell and spindle fibers attach themselves to each of the chromatids.
Anaphase is the third stage, when centromeres break apart. The separated chromatids are then called chromosomes, and these chromosomes are pulled to opposite ends of the cell. A cleavage furrow develops at the end of anaphase.
Telophase is the final stage, when cell division is completed. Nuclei appear in daughter cells, the nuclear envelope and nucleoli appear, and the cytoplasm is divided in a process called cytokinesis.
Mitosis ends as daughter cells become fully functional and enter interphase. The results of cell division are two identical cells, which will grow tissues or replace old or damaged cells. During periods of growth, mitosis allows certain groups of similar cells to differentiate into different tissues. Cells have the ability to adapt to changing conditions. Hypertrophy is the increase in the size of individual cells and can increase the size of tissue when multiple cells undergo hypertrophy. On the other hand, atrophy is the decrease in the size of individual cells, which can decrease the size of tissues. Hyperplasia is an increase in cell reproduction and, therefore, can cause an increase in tissue size by adding more cells. Anaplasia is the production of abnormal, undifferentiated cells. Uncontrolled cell reproduction results in formation of a benign or malignant neoplasm, commonly called a tumor.