I. Introduction

A DNA molecule that forms a linear chromosome must be able to carry out certain crucial functions in order to be functional. These functions include replication and the subsequent separation of replicated copies into daughter cells during cell division.

II. The Cell Cycle

• The process of replication and segregation of chromosomes into daughter cells is known as the cell cycle.
• During interphase, genes are expressed and chromosomes are replicated, remaining as a pair of sister chromatids.
• The chromosomes condense into mitotic chromosomes during a brief period of mitosis. This is the form in which chromosomes are easily visualized.
• Each chromosome must be able to replicate and be correctly partitioned into daughter cells for the process to occur successfully.

III. Specialized Nucleotide Sequences

• Three types of specialized nucleotide sequences control the replication and segregation of chromosomes.
• The first type acts as a DNA replication origin, the location at which duplication begins.
• The second type is a centromere, which allows one copy of each duplicated chromosome to be pulled into each daughter cell.
• The third type forms telomeres, the ends of a chromosome, which enable efficient replication and protect the end of the chromosome.

IV. DNA Sequences in Yeast Cells

• Experiments in yeast cells have identified the minimal DNA sequence elements required for each of the three functions.
• The sequences are relatively short and make up only a tiny fraction of the information-carrying capacity of a chromosome.

V. DNA Sequences in Complex Organisms

• In more complex organisms, the DNA sequences that form centromeres and replication origins are much longer than their yeast counterparts.
• For example, experiments suggest that a human centromere can contain up to a million nucleotide pairs and may consist of a large, regularly repeating protein–nucleic acid structure.

VI. Conclusion

The structure of chromosomes is controlled by three types of specialized nucleotide sequences. These sequences ensure the successful replication and segregation of chromosomes into daughter cells. The DNA sequences in yeast cells are relatively short while those in more complex organisms are much longer.