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

Scientists achieve first complete assembly of human X chromosome

Although the current human reference genome is the most accurate and complete vertebrate genome ever produced, there are still gaps in the DNA sequence, even after two decades of improvements. Now, for the first time, scientists ...

Researchers suspend, image single DNA molecules

(PhysOrg.com) -- Studying chemical modifications in the chromosomes of cells is akin to searching for changes in coiled spaghetti. Scientists at Cornell have figured out how to stretch out tangled strands of DNA from chromosomes, ...

Sex, genes, the Y chromosome and the future of men

The Y chromosome, that little chain of genes that determines the sex of humans, is not as tough as you might think. In fact, if we look at the Y chromosome over the course of our evolution we've seen it shrink at an alarming ...

New steps in the meiosis chromosome dance

Where would we be without meiosis and recombination? For a start, none of us sexually reproducing organisms would be here, because that's how sperm and eggs are made. And when meiosis doesn't work properly, it can lead to ...

Grandparents favor genetically close grandchildren

(PhysOrg.com) -- New research suggests that grandparents naturally and subconsciously favor the grandchildren who are most closely related to them genetically. The phenomenon is called "sexually antagonistic grandparental ...

Chromosomes look different than you think

In high school textbooks, human chromosomes are pictured as wonky Xs like two hotdogs jammed together. But those images are far from accurate. "For 90 percent of the time," said Jun-Han Su, "chromosomes don't exist like that."

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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