Eukaryotic Chromosome Structure
Chromosome*s contain the long strands of DNA* that carry genetic information. They are the unit of DNA replication in living cells. Typical 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 increases the challenge of keeping the genetic material organized and of passing the proper amount of DNA to each daughter cell during mitosis.
During cell division, eukaryotic chromosomes condense into highly coiled 4 armed structures. The tight coiling and high degree of organization in this supercoiled DNA facilitates proper segregation during mitosis and cell division. The following illustration explores the structure, classification and features of a eukaryotic chromosome.
The earliest efforts to describe chromosomes were based on visible characteristics. The most obvious features are the centromere* which is the central constriction point and the branches stemming from this constriction point. These branches are called arms. Each chromosome has either two or four arms extending from the centromere.
Late in the cell cycle when the process of mitosis has begun but before the cell has divided, cells contain two complete copies of their genome. When the DNA is supercoiled in preparation for cell division, the pair of copies of each chromosome come together to form the characteristic x-shaped structure. The point of attachment of the two complete copies occurs at the centromere. Each copy of the chromosome is called a chromatid*. The two copies of each chromosome are called sister chromatids. When cells divide, one sister chromatid is delivered to each of the daughter cells.
One common way to classify different types of chromosomes is based on the position of the centromere along the length of the chromosome. The classes in this system 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 that 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 important 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 ends of linear chromosomes:
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.