Natural Chromosome Function

DNA, which constitutes the genome of a cell, is always packaged with a variety of proteins, and together these make up the chromosomes. A chromosome serves to compact the DNA and protect it from the damage, while at the same time allowing the genes it contains to be available for transcription into RNA. In addition to these functions, extra ones are necessary when the cell divides. Prior to cell division the DNA must be copied and these copies separated (segregated) and delivered to different parts of the cell, ensuring that each of the new cells receives only a single copy.

To ensure correct segregation, chromosomes have to have distinct components that are composed of specific DNA sequences and associated proteins. Bacterial chromosomes, (plasmids) which are circular, have a single site at which DNA replication originates, and attachment to the cell membrane results in segregation. Artificial bacterial chromosomes (BACs) mimic this using appropriate origin sequences.

In organisms with multiple linear chromosomes (eukaryotic organisms) the process is more complicated. The ends of the chromosomes must be protected from degradation and from the mechanisms that the cell uses to protect itself against broken DNA. Telomeres, which provide these functions, are arrays of short, repeated sequences with complexes of specific proteins attached. To ensure segregation complexes of other proteins, DNA sequences known as kinetochores form at sites known as centromeres. These contain molecular motors, systems to monitor correct segregation, and sites for attachment of microtubules. Chromosomes will contain one or more origins of replication.

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