GENES

image courtesy of U.S. Department of Energy Human Genome Program

The human body contains about 30,000 to 40,000 genes, which carry information that determines traits like color of eyes, height etc. Genes hang out all lined up on thread-like things called chromosomes. Chromosomes come in pairs, and there are hundreds, sometimes thousands, of genes in one chromosome. Together, all of the chromosomes and genes make up DNA, which is short for deoxyribonucleic acid.

A gene consists of a promoter, the codons for an enzyme and a stop codon. Two genes are shown on the left picture. The long strand of DNA in an E. coli bacterium encodes about 4,000 genes, and at any time those genes specify about 1,000 enzymes in the cytoplasm of an E. coli cell. Many of the genes are duplicates.

image courtesy of HowStuffWorks

Chromosomes are found inside cells, the very small units that make up all living things. A cell is so tiny that can be only seen through the lens of a strong microscope, and there are billions of cells in a human body (You can stack about 100 cells within the thickness of a dime). Most cells have one nucleus. The nucleus, which is sort of egg-shaped, is like the brain of the cell. It tells every part of the cell what to do. How does the nucleus know so much? As tiny as it is, the nucleus has more information in it than the biggest dictionary.

Chromosomes

In humans, a cell nucleus contains 46 individual chromosomes or 23 pairs of chromosomes (chromosomes come in pairs, 23 X 2 = 46). Half of these chromosomes come from one parent and half come from the other parent. But not every living thing has 46 chromosomes inside of its cells. For instance, a fruit fly cell only has four chromosomes!

DNA from all organisms is made up of the same chemical and physical components. The DNA sequence is the particular side-by-side arrangement of bases along the DNA strand (e.g., ATTCCGGA). This order spells out the exact instructions required to create a particular organism with its own unique traits.

The genome is an organism’s complete set of DNA. Genomes vary widely in size: the smallest known genome for a free-living organism (a bacterium) contains about 600,000 DNA base pairs, while human and mouse genomes have some 3 billion. Except for mature red blood cells, all human cells contain a complete genome.

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image courtesy of U.S. Department of Energy Human Genome Program

DNA in the human genome is arranged into 24 distinct chromosomes--physically separate molecules that range in length from about 50 million to 250 million base pairs. A few types of major chromosomal abnormalities, including missing or extra copies or gross breaks and rejoinings (translocations), can be detected by microscopic examination. Most changes in DNA, however, are more subtle and require a closer analysis of the DNA molecule to find perhaps single-base differences.

Each chromosome contains many genes, the basic physical and functional units of heredity. Genes are specific sequences of bases that encode instructions on how to make proteins. Genes comprise only about 2% of the human genome; the remainder consists of noncoding regions, whose functions may include providing chromosomal s structural integrity and regulating where, when, and in what quantity proteins are made. The human genome is estimated to contain 30,000 to 40,000 genes.

image courtesy of U.S. Department of Energy Human Genome Program

When genes are expressed, the genetic information (base sequence) on DNA is first transcribed (copied) to a molecule of messenger RNA in a process similar to DNA replication. The mRNA molecules then leave the cell nucleus and enter the cytoplasm, where triplets of DNA bases (codons) forming the genetic code specify the particular amino acids that make up an individual protein. This process, called translation, is accomplished by ribosomes (cellular components composed of proteins and another class of RNA) that read the genetic code from the mRNA, and transfer RNAs (tRNAs) that transport amino acids to the ribosomes for attachment to the growing protein.

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image courtesy of U.S. Department of Energy Human Genome Program

Although genes get a lot of attention, it’s the proteins that perform most life functions and even make up the majority of cellular structures. Proteins are large, complex molecules made up of smaller subunits called amino acids. Chemical properties that distinguish the 20 different amino acids cause the protein chains to fold up into specific three-dimensional structures that define their particular functions in the cell.

Web Author: Tug Sezen