What is a chromosome?
Quick Answer
A chromosome is a tightly packed, thread-like structure of DNA and protein that carries genes inside the nucleus of a cell.
The Full Story
Chromosomes allow the enormously long DNA in each cell to be coiled and packaged neatly around proteins called histones. Most human cells contain 46 chromosomes arranged in 23 pairs, with one of each pair inherited from each parent. One pair, the sex chromosomes, usually determines biological sex: XX in females and XY in males. The correct human chromosome count of 46 was only established in 1956 by Joe Hin Tjio and Albert Levan; before that, scientists believed it was 48. Changes in chromosome number can cause conditions such as Down syndrome, which involves an extra copy of chromosome 21.
Key Facts
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Suggested video title for this topic:
"How 2 metres of DNA fits inside every cell"