A chromosome is a compactly folded complex of DNA and proteins containing many genes, found in the nuclei of eukaryotic organisms and in the nucleoids of prokaryotic organisms.
Each cell in an organism has a complete set of genetic information called a genome. Different organisms have different numbers of chromosomes in their genomes, ranging from a single chromosome in most bacteria to seventy-eight chromosomes in chickens. Humans cells have forty-six chromosomes, but these represent two sets of information as humans are diploid organisms; each cell has one information set inherited from the organism's mother and a second set inherited from its father. If an offspring inherits one X chromosome and one Y chromosome, he will be genetically male. If an offspring inherits X chromosomes from both parents, she will be genetically female.
Chromosomes consist of two kinds of molecules, deoxyribonucleic acid (DNA) strands and proteins. Chromosomes from eukaryotic organisms have linear DNA strands containing approximately fifty genes per millimeter, compared to 2,500 genes per millimeter in bacteria. Some of the noncoding DNA (DNA that does not code for proteins) is found in special structures at the ends of the chromosomes called telomeres. Much of the noncoding DNA in eukaryotic chromosomes may be involved in compacting the DNA into the highly organized chromosome structure. Some of this DNA has highly repetitive sequences and has been useful in forensic analysis.
Proteins help to compact DNA: this is important because the DNA in a chromosome could not fit inside its cell if it were not compacted. Histones are positively charged proteins that neutralize negative DNA strands when they wrap around and form complexes with the DNA. This wrapped structure, called "beads on a string," represents the first level of compaction. The "beads" are condensed to form fibers, fibers fold into loops, loops combine with nuclear scaffold proteins to form rosettes, and rosettes condense to form coils. Finally, a chromatid with ten or more coils is formed. Nonhistone proteins within chromosomes are also important. These proteins have varied functions, including assisting in the unwinding of DNA and in the repairing of DNA.
Chromosomes from prokaryotic organisms have DNA strands that loop and form circles. The DNA in prokaryotic chromosomes forms complexes with histone like proteins that help to compact the DNA, link it to the cell membrane, and localize it in the nucleoid region of the cell. Some bacteria have extra chromosomal DNA—a mini-chromosome called a plasmid. Plasmids contain only a few genes but are rapidly exchanged among cells of a bacteria population. Plasmids have become useful tools in biotechnology and genetic engineering.
Recombination is a natural process of exchange of fragments of DNA strands between paired chromosomes, which happens occasionally during cell division. Genetic engineering techniques allow scientists to cut and paste DNA fragments from one source to another to produce recombinant chromosomes and transgenic organisms.
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