What are genes? What are chromosomes?
Everything that is alive has genes that determine what kind of creature the living being is. Genes are tiny strings of chemicals that are the microscopic building blocks of life. Genes are, in essence, instruction manuals for each life form.
Genes determine if a living being will be a plant or an animal, and what type of plant or animal the life form will be, down to what color it is and what kind of skin, fur or feathers are on its body. Genes also determine the individual, unique characteristics a living thing has, as well as the personality traits it will possess.
Genes are combinations of four different chemicals called nucleotide bases. The four nucleotides are called adenine, cytosine, guanine and thymine. Scientists simplify these into four initials, A,C, G and T, when they discuss genetic code. The different characteristics of a living thing are determined by the specific combinations of these four chemicals.
Chromosomes are long strings of gene codes. Some of these coding strings consist of thousands, or hundreds of thousands, of individual genes, all strung together into a string of DNA. Human beings have 23 pairs of chromosomes, but some animals have more. A donkey actually has more pairs of chromosomes than a human being, with a total of 31.
Gene expression refers to how your genetic makeup shows through in your health, personality or character traits. For example, if you have blue eyes, your eye color is the gene expression of that specific genetic code. You might have a particular medical condition because of particular genes, and that medical condition is a result of gene expression.
Your body is composed of more than a billion cells. Cells are continually dying, and new cells are continually being formed. An identical copy of your hereditary material is found in the nucleus of each and every somatic cell. A somatic cell is any cell in the body except for the reproductive cells in the reproductive system.
This genetic blueprint is organized into 46 chapters or parts known as chromosomes. It is estimated that, on average, each chromosome contains between one and two thousand genes. A gene contains the information for making a single protein or RNA product.
Every time a cell divides, each chromosome must be carefully replicated (copied) and then distributed to assure that each daughter cell gets a complete and accurate set of information. Thus, nuclear division includes successive processes of chromosome replication, separation, and distribution (Figure 1).
Figure 1: Chromosome Replication & Division
DNA synthesis occurs in the nucleus, producing an exact replica of every chromosome. A chromosome can be thought of as a very long DNA double helix. During replication, the double helix opens up and a new complementary strand is synthesized along each parent strand (Figure 2). This results in two identical DNA helices, each containing one original parent strand and one newly synthesized strand.
Figure 2: DNA Replicating
DNA synthesis occurs during the S phase of interphase. Each cell goes through a regular life cycle, similar to the cycle of life in humans. Where we might call our stages infancy, childhood, adolescence, young adult, adult, and senior, the major cell stages are interphase, mitosis, and cytokinesis. Interphase is subdivided into G1 (growth 1), S (synthesis), and G2 (growth 2), and mitosis is divided into P (prophase), PM (prometaphase), M (metaphase), A (anaphase), and T (telophase). This is shown in Figure 3.
Figure 3: Cell Cycle
Another way to illustrate this cycle is shown in Figure 4.
Figure 4: Cell Division
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