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Ms. Sewsalot packed her spools of thread into moving boxes, so she had manageable packages to work with. DNA is packaged into chromosomes for the same reason. When a cell divides, it copies all of its DNA and segregates one complete copy of the genome to each cell. Chromosomes are manageable packages of DNA for the cell to work with.
During cell division, the entire chromosome is tightly packaged, or condensed. The DNA that isn't used to make proteins remains tightly packaged even when the cell isn't dividing. This highly condensed chromatin is called heterochromatin. Heterochromatin makes up about 10-25% of a eukaryotic genome. It contains highly repetitive DNA sequences. Heterochromatin is sometimes referred to as "junk DNA" because it contains only a few genes. The name is misleading because heterochromatin is important for proper chromosome packaging and movement during cell division.
Since there are few genes in heterochromatin, the DNA doesn't need to be accessible for transcription. These regions of chromatin remain highly condensed all the time, just as the threads Ms. Sewsalot doesn't use very often remain packed up in the moving boxes.
The regions of DNA that contain genes must be accessible for transcription, so they are unpackaged, or decondensed, after cell division. This type of chromatin is called euchromatin. Most genes are found in euchromatic regions. During gene expression, euchromatic regions decondense into looped domains. This makes the DNA accessible for transcription, but still keeps it organized, just as Ms. Sewsalot stacks thread boxes to keep things organized but accessible.
In addition to heterochomatin and euchromatin, chromosomes have several specialized regions. Each chromosome has one centromere, which is necessary for proper cell division. When a cell divides, long fibers pull the chromosomes to the daughter cells. The centromere acts as a handle for the fibers to grab onto the chromatin.The centromere is found in heterochromatin.
The ends of a chromosome are called telomeres. During DNA replication, DNA polymerase is unable to access the ends of the chromosome. Telomeres keep the chromosome from getting shorter with each round of replication. Telomeres are composed of short repeats of DNA. An enzyme called telomerase adds copies of the repeats to the telomere to keep the chromosome from shrinking.
Match the chromosome region with its function. Drag the labels and functions to the appropriate locations on the chromosome.
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The structures of the regions are related to their functions. The DNA must be accessible for transcription to occur, so most genes are located in euchromatin. Heterochromatin contains few genes and remains condensed all the time. Telomeres protect the chromosome ends during DNA replication. The centromere is where the fibers attach to the chromosome during cell division.
Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education