Related topics: genes · genome · cells · dna · cell division

New repair mechanism for DNA breaks

Chromosomal breaks are the most harmful damage for cells. If they are not repaired, they block the duplication and segregation of chromosomes, stop the growth cycle and cause cell death. These breaks appear frequently in ...

Scientists explore how females shut off their second X chromosome

Researchers at the European Molecular Biology Laboratory (EMBL) in Heidelberg and Institut Curie in Paris have shown that the protein SPEN plays a crucial role in the process of X-chromosome inactivation, whereby female mammalian ...

Unique centromere type discovered in the European dodder

Whenever the European dodder, Cuscuta europaea, is under scientific scrutiny, it usually is due to its lack of chloroplasts and its concomitant parasitic lifestyle. However, since the beginning of this year its chromosomes ...

Record-size sex chromosome found in two bird species

Researchers in Sweden and the UK have discovered the largest known avian sex chromosome. The giant chromosome was created when four chromosomes fused together into one, and has been found in two species of lark.

Genes linked to sex ratio and male fertility in mice

One of the more recent trends among parents-to-be is the so-called gender reveal, a party complete with pink or blue cake to answer the burning question, "Is it a boy or girl?" After all, it's presumed that there's a 50-50 ...

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Chromosome

A chromosome is an organized structure of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. The word chromosome comes from the Greek χρῶμα (chroma, color) and σῶμα (soma, body) due to their property of being very strongly stained by particular dyes. Chromosomes vary widely between different organisms. The DNA molecule may be circular or linear, and can be composed of 10,000 to 1,000,000,000 nucleotides in a long chain. Typically eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have smaller circular chromosomes, although there are many exceptions to this rule. Furthermore, cells may contain more than one type of chromosome; for example, mitochondria in most eukaryotes and chloroplasts in plants have their own small chromosomes.

In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin. This allows the very long DNA molecules to fit into the cell nucleus. The structure of chromosomes and chromatin varies through the cell cycle. Chromosomes are the essential unit for cellular division and must be replicated, divided, and passed successfully to their daughter cells so as to ensure the genetic diversity and survival of their progeny. Chromosomes may exist as either duplicated or unduplicated—unduplicated chromosomes are single linear strands, whereas duplicated chromosomes (copied during synthesis phase) contain two copies joined by a centromere. Compaction of the duplicated chromosomes during mitosis and meiosis results in the classic four-arm structure (pictured to the right). Chromosomal recombination plays a vital role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe and die, or it may aberrantly evade apoptosis leading to the progression of cancer.

However, in practice "chromosome" is a rather loosely defined term. In prokaryotes, a small circular DNA molecule may be called either a plasmid or a small chromosome. These small circular genomes are also found in mitochondria and chloroplasts, reflecting their bacterial origins. The simplest chromosomes are found in viruses: these DNA or RNA molecules are short linear or circular chromosomes that often lack any structural proteins.

This text uses material from Wikipedia, licensed under CC BY-SA