Cells through a Microscope (3D Render)
A cell is the most basic, functioning unit of organisms. Cells perform the functions necessary to the survival and reproduction of the organism. They divide to make identical cells or reproductive cells; make proteins that build, protect, and regulate; generate energy; and provide structure. Cells are made up of defined parts called organelles that have specific functions. The nucleus is a membrane-bound organelle that houses and protects the genetic material or DNA (deoxyribonucleic acid.) Ribosomes are organelles that are found outside the nucleus. They assemble amino acids to make proteins.
A cell has all the instructions or genetic information needed to divide and grow into a unique organism. These instructions are contained within the chemical makeup of DNA. A gene is a segment of DNA that serves as the blueprint for the production of a protein. Proteins are working molecules made up of amino acids. Proteins provide structure and control the cellular processes within tissues and organs.
A Double Helix
The structure of a DNA molecule is called a double helix. It resembles a spiral staircase. The basic unit of DNA is called a nucleotide. A nucleotide is made up of a sugar group, a phosphate group, and a nitrogen base. The steps of the spiral staircase are composed of a nitrogen base pair. The sides of the ladder are strands of alternating sugar and phosphate groups. The two sides of the ladder are joined by hydrogen bonds between the two nitrogen bases. There are four nitrogen bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Each nitrogen base has a specific affinity to another nitrogen base. Adenine will only bind to thymine and guanine will only bind to thymine. The order of A, T, C, and G along the DNA strand is the coding information for the production of proteins.
DNA Replication
In addition to serving as the “master plans” for the cell, DNA also has the ability to copy itself. This process is called DNA replication. DNA replication is necessary to preserve the quality and quantity of DNA during cell division. Before a cell divides, DNA must be copied to provide each new cell with an exact copy of DNA. DNA replication begins when the spiral staircase unwinds and the hydrogen bonds joining the two base pairs are broken. The result is two half staircases. These unwound strands of DNA now have exposed bases that serve as the templates for the addition of complimentary nucleotides to each side of the DNA strand. This process continues until the entire length of the DNA molecule has been copied, producing two identical DNA molecules, one from each side of the parent DNA molecule.
Each cell has about 6 feet of DNA. To package the large amount of DNA found in each tiny cell, DNA is wound into tight configurations called chromosomes. Chromosomes are thread-like structures that occur in pairs in most cells, one originating from the female parent and one from the male parent. The number of chromosomes that an organism has is specific to a species. Most human cells have 23 pairs of chromosomes for a total of 46 chromosomes. Human sex cells have only 23 chromosomes. When the egg (female sex cell) and the sperm (male sex cell) unite, the resulting first cell of the new organism will have 46 chromosomes. This cell will contain all the information needed to divide, grow, and differentiate into the many different cell types that make up a complex, multicellular organism.
Cell Division
3D Model Showing Mitosis
After the replication Of DNA, a cell will divide and distribute half of the chromosomes into each new cell. Division of a cell is called mitosis. When a cell divides, two identical cells are formed called daughter cells. Mitosis occurs in four distinct phases: prophase, anaphase, metaphase, and telophase. During prophase, chromosomes form condensed, distinct structures that are visible under a microscope. In anaphase, the mitotic spindle forms from the two poles of the cell to the center or equator; the membrane around the nucleus starts to break down. Metaphase marks the event when chromosomes line up on the equator of the cell. The chromosomes separate during anaphase and are pulled to opposite poles by the mitotic spindle. During telophase, two new nuclei are formed, chromosomes uncoil, and the remaining contents of the cell divide producing two cells.
Protein Synthesis
DNA replication is only one of the activities that occur during interphase. The cell also grows, processes nutrients for energy, and makes proteins. The production of proteins is called protein synthesis. Protein synthesis occurs in two parts: translation and transcription. During translation, DNA unwinds and is decoded three nitrogen bases at a time producing a mRNA (messenger RNA) molecule that exits the nucleus. During transcription, ribosomes bind to the mRNA. A specific sequence of three bases along the mRNA strand, ATA, for example, is a codon. Amino acids are carried to the ribosomes by tRNA molecules that have complimentary bases to the mRNA. Amino acids are added as each codon on the mRNA molecule is read by tRNA, resulting in elongation of the protein chain. There are 20 amino acids. A specific sequence of amino acids forms a specific protein. Some codons also serve as commands during protein synthesis, indicating the beginning or end of a protein sequence. The proteins that result from a cell’s DNA determine the defining traits of an organism, such as blue eyes or brown hair.
