Eukaryotic Chromosome Structure

Chromosome*s are long strands of DNA* in cells that carry genetic information. Most prokaryotic cells contain a single circular chromosome. Eukaryotic cells, with their much larger genomes, have multiple, linear chromosomes. The length and linear nature of eukaryotic chromosomes increase the challenge of keeping the genetic material organized and passing the proper amount of DNA to each daughter cell during mitosis.

During cell division, eukaryotic chromosomes condense into highly coiled structures. The tight coiling and high degree of organization in this supercoiled DNA facilitate proper segregation during mitosis and cell division. The following illustration explores the shape, classification, and features of a eukaryotic chromosome.

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The earliest efforts to describe chromosomes were based on visible characteristics. The most prominent features are the central constriction point, and the branches stemming from this constriction point. The constriction point is called the centropmere, The branches are called arms. Each chromosome has either two or four arms extending from the centromere*.

Late in the cell cycle, before the cell has divided, the DNA supercoils. At this point in the cell cycle, the cell contains two complete copies of each chromosome. Each supercoiled copy of a chromosome is called a chromatid*. The pairs of matching chromosomes are called sister chromatids.

Towards the end of the cell cycle, sister chromatids come together and attach at the centromere forming the characteristic x-shaped structure. When cells divide during mitosis, each daughter cell receives one sister chromatid from each chromosome.

The centromere’s position is the basis of a classification system for chromosomes. The different classes are:

  • Metacentric - the centromere is in the middle of the chromosome. The arms on either side of the centromere are equal in length.
  • Submetacentric - The centromere is closer to the middle of the chromosome than it is to either end, but the arms differ in length.
  • Acrocentric - The centromere is closer to one end than it is to the middle. There is a large difference in the length of the arms, but each chromatid has two visible arms.
  • Telocentric - The centromere is at the very end of the chromosome. Each chromatid has only one arm.
The centromere is more than the connection point for the chromatids. It plays an essential role in cell division as the site of the kinetochore. The kinetochore is the point of attachment of the fibers that pull the sister chromatids apart during mitosis.

The linear nature of eukaryotic chromosomes presents challenges for replication because DNA polymerase** requires a priming sequence to elongate a strand of DNA. Without some mechanisms of preserving the ends, the linear strands of DNA in eukaryotic chromosomes would become shorter every time a cell divided, resulting in the loss of genetic information. Preventing the loss of genetic material is accomplished by having lengths of repetitive, non-coding DNA at either end of the chromosome. These repetitive sequences are called telomere**s and they protect the rest of the strand from degradation during replication.

The DNA of eukaryotic cells become supercoiled during cell division and need to be uncoiled (or relaxed) for transcription and replication to occur. For large period**s of the cell cycle the chromosomes exist in an uncoiled, diffuse state.

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