What chromosomes occurs with scids4/14/2024 Most bacteria have one or two circular chromosomes. For example, people with Down syndrome have three copies of chromosome 21, instead of the two copies found in other people.Ĭhromosomes vary in number and shape among living things. If not, the resulting offspring may fail to develop properly. It is also crucial that reproductive cells, such as eggs and sperm, contain the right number of chromosomes and that those chromosomes have the correct structure. For example, in humans, one type of leukemia and some other cancers are caused by defective chromosomes made up of joined pieces of broken chromosomes. Still, mistakes do occur on rare occasions.Ĭhanges in the number or structure of chromosomes in new cells may lead to serious problems. Chromosomes are a key part of the process that ensures DNA is accurately copied and distributed in the vast majority of cell divisions. During cell division, it is essential that DNA remains intact and evenly distributed among cells. For example, if all of the DNA molecules in a single human cell were unwound from their histones and placed end-to-end, they would stretch 6 feet.įor an organism to grow and function properly, cells must constantly divide to produce new cells to replace old, worn-out cells. Without such packaging, DNA molecules would be too long to fit inside cells. The unique structure of chromosomes keeps DNA tightly wrapped around spool-like proteins, called histones.
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