Chromosome Structure

The DNA-histone complex contained in the nucleus is called chromatin and condenses to form chromosomes that consist of a single chromatid or sister chromatids – depending upon the cell cycle stage. A functional eukaryotic chromosome must have a centromere, a DNA sequence that links sister chromatids. The centromere is also where the kinetochores are constructed after the chromosome has replicated. These protein complexes allow the spindle microtubules to move the chromosomes around during cell division. Depending upon the location of the centromere, chromosomes can exist in four major configurations. In the metacentric configuration, the centromere is centered, resulting in arms of similar lengths. Whereas, in the submetacentric configuration, the centromere is off-center, resulting in arms of different lengths. In the telocentric configuration, the centromere is at the very end of the chromosome, resulting in long, single arms.  In acrocentric chromosomes, the centromere is located near the end, giving the appearance of a ‘stalk’ and ‘bulb’. These various configurations occur naturally, making them useful in identifying specific chromosomes. For example, the human Y chromosome is acrocentric.      Each chromatid must also have telomeres, which consist of non-coding repetitive nucleotide sequences, at their tips.  The telomeres protect and stabilize the ends of chromosomes. If a chromosome breaks, it will begin to degrade at the newly created end, which lacks a telomere.  Finally, a chromosome must have multiple origins of replication, sequences of nucleotides that determine where DNA replication begins.  Human chromosomes contain approximately 30,000 origins of replication in order to expedite the replication process. If a human chromosome only contained one origin of replication, it would take over a month to replicate a single chromosome.  When each chromosome replicates, beginning at multiple origins of replication, the resulting sister chromatids are held together at the centromere with telomeres at their tips.  Right before cell division, chromosomes are in their most condensed state. This is why observations of chromosomes are often made at this point in the cell cycle.