Chromosomes, Genes and DNA

Chromosomes

Every cell's nucleus contains chromosomes, a thread-like structure containing DNA molecules. The structure of a chromosome is supported by proteins called histones that are tightly coiled around DNA. A cell's nucleus cannot be seen under a microscope when it is not dividing because its chromosomes are not present. Chromosome DNA, however, becomes more closely packed during cell division, so it is visible underneath a microscope.

Research on chromosomes has been mostly based on observations made during cell division. The centromere, or the point where chromosomes are separated into their arms, is found at the center of each of the chromosomes. P-arms are considered the short arms of chromosomes. There is a long arm on a chromosome called the "q arm." It is the centromere that gives each chromosome its distinctive shape and helps identify specific gene locations.

Genes

The gene occupies a fixed location on each chromosome and contains genetic information. Genetic information is translated into proteins by genes. Eukaryotes (including plants and animals) contain DNA in their nucleus. Similarly, there are certain subsets of genes within the mitochondria and chloroplasts of animals and plants, which are dissimilar in function to those found in the nucleus. Prokaryotes (organisms without a nucleus, like bacteria) contain genes in a free-floating chromosome within the cytoplasm of the cell.

Plasmids, small extrachromosomal genetic elements with a limited number of genes, are also found in many bacteria. An individual species' genome contains many genes (all chromosomes added together). Compared to the human genome, the Escherichia coli genome contains only 5,416 genes, while the human genome has been estimated to contain between 20,000 and 25,000 genes. Arabidopsis thaliana has the smallest genome of any plant and was the first plant genome to be sequenced in full. The bacterium Mycoplasma genitalium, which can replicate independently, has the fewest genes, just 517.

DNA

Most organisms and humans inherit DNA as their inherited material. A person's DNA is found in nearly every cell of their body. Within the mitochondria (also known as mitochondrial DNA or mtDNA), there is a small amount of DNA. Most DNA is found in nuclear DNA; however, it can also be found in mitochondrial DNA. A mitochondrion is a structure within a cell that converts food into energy the cell can use. Essentially, DNA consists of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Over 99 percent of each person's DNA consists of the same 3 billion bases. Based on the order in which these bases appear, an organism can build and maintain itself, just as the letters of the alphabet form words and sentences based on the order in which they appear.

As DNA bases pair up, A and C form base pairs, and C and G form base triplets. Sugar molecules and phosphate molecules are also attached to each base. Bases, sugars, and phosphates are combined into nucleotides. Two nucleotide strands form a double helix in the cellular nucleus. A ladder, in which the base pairs represent the ladder's rungs and the sugar and phosphate molecules embody its vertical part, describes the structure of the double helix. One of DNA's most important properties is its ability to replicate itself. In the double helix, each strand of DNA serves as a template for replicating the sequence of the bases. For one new cell to contain the same DNA as an old one, a cell's DNA has to be precisely copied when it divides.

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