One more interesting feature of the genome is the typical size of a gene in the human species, which is approximately 27,000 nucleotide pairs in length. As was said earlier, the information necessary to determine the linear sequence of amino acids found in a protein is stored in the normal gene in the form of a linear sequence of nucleotides. To encode a protein of typical size requires just about 1300 pairs of nucleotides, which is a relatively small quantity. (approximately 430 different amino acids can be found in humans) The majority of the sequence that constitutes a gene is comprised of relatively lengthy pieces of DNA that do not code for proteins. These relatively lengthy sections of DNA are separated by relatively short sections of DNA that do code for proteins. Therefore, the most majority of human genes are constructed using a lengthy sequence of alternating exons and introns, with the introns being the greater portion of the gene. In contrast, the vast majority of genes in species whose genomes are relatively small do not have introns. This helps to explain both the far higher fraction of coding DNA in their chromosomes as well as the significantly smaller size of their genes (approximately one-twentieth the size of human genes).

Each gene has regulatory DNA sequences, which are responsible for ensuring that the gene is generated at the appropriate level, at the appropriate time, and only in the appropriate kind of cell. Genes are composed of introns and exons, and each gene also contains regulatory DNA sequences. In humans, the regulatory sequence of a typical gene is composed of tens of thousands of pairs of nucleotides. As one might anticipate, these regulatory sequences take up a far smaller amount of space in organisms that have genomes that are relatively small.

Researchers were taken aback by the finding that in addition to the 21,000 genes that code for proteins, the human genome also contains many thousands of genes that encode RNA molecules. This revelation has led to a new understanding of a number of essential biological processes. According to the nucleotide sequence of the human genome, which is not the least significant discovery, the information warehouse that is necessary to make a person looks to be in an alarming state of anarchy. This is one of the most important discoveries in recent history. An individual who left a comment stated that our genome "may in some ways resemble your garage/bedroom/refrigerator/life: highly individualistic, but unkempt; little evidence of organization; much accumulated clutter (referred to by the uninitiated as "junk"); virtually nothing ever discarded; and the few patently valuable items scattered throughout carelessly, indiscriminately."

In order for a DNA molecule to be able to create a functional chromosome, it must be able to copy itself and be capable of consistently dividing into daughter cells during each round of cell division. In addition to this, it needs to be able to carry genes. It is possible for there to be a period of time that elapses between the duplication of chromosomes and their separation into two daughter cells thanks to the cell cycle, which is an ordered progression of phases that together make up this process. In a nutshell, during a prolonged interphase, genes are expressed and chromosomes are replicated, with the two copies remaining together as a pair of sister chromatids. This process takes place in eukaryotic cells. Because the chromosomes are stretched out during this time, and a significant portion of their chromatin is present in the nucleus as long threads, it is difficult to distinguish between individual chromosomes during this phase. During mitosis, each chromosome only condenses for a significantly shorter period of time, which makes it possible for its two sister chromatids to be separated and distributed to the two daughter nuclei. The densely packed chromosomes that can be discovered in cells that are in the process of dividing are referred to as "mitotic chromosomes." It is much simpler to imagine chromosomes in this state; in fact, all of the images of chromosomes that have been presented in this chapter have been of mitotic chromosomes.

Each chromosome serves as a distinct structural unit; during cell division, in order for a copy of each chromosome to be transferred to each daughter cell, each chromosome must first be able to replicate, and then the newly duplicated copies must be appropriately divided and partitioned into the two daughter cells. Only then can a copy of each chromosome be transferred. These fundamental processes are regulated by three distinct types of specialized nucleotide sequences in the DNA. These nucleotide sequences bind to different proteins, which in turn direct the machinery that duplicates and separates chromosomes.

Experiments conducted on yeasts, whose chromosomes are relatively compact and easy to manipulate, were used to determine the DNA sequence components that are absolutely necessary for each of these tasks. The beginning of each round of DNA replication is marked by the presence of a particular nucleotide sequence, also known as the origin of the process. Chromosomes in eukaryotic organisms typically have multiple replication origins, which enables the complete chromosome to be replicated more quickly.

After DNA replication, the two sister chromatids that make up each chromosome remain joined to one another and are gradually compressed as the cell cycle progresses to become mitotic chromosomes. This process takes place during the mitotic phase of the cell cycle. Because there is a second specialized DNA sequence that is known as a centromere, when a cell divides, one copy of each duplicated and condensed chromosome is pushed into each daughter cell. This is accomplished thanks to the existence of a centromere. A protein complex that is known as a kinetochore develops at the centromere. It is responsible for connecting the duplicated chromosomes to the mitotic spindle, which in turn enables the chromosomes to separate.

The third unique DNA sequence is responsible for the creation of telomeres, which are found at the ends of chromosomes. Telomeres are characterized by repeated nucleotide sequences, which facilitate the efficient duplication of chromosomal ends. In addition, telomeres perform another job in the cell by generating structures that prevent the cell from incorrectly perceiving the end of the chromosome as a broken DNA molecule that requires repair. This is accomplished by preventing the telomeres from being damaged themselves. Repeated telomere DNA sequences and the areas around them are the building blocks of these structures.

Because each of the three types of sequences required to replicate a chromosome in yeast cells is often fairly short (less than 1000 base pairs), the information-carrying capacity of the chromosome is only partially utilized. Although the telomere DNA sequences of all eukaryotic creatures are generally short and straightforward, the centromere and replication origin DNA sequences of more sophisticated organisms like yeast are significantly longer. For instance, research indicates that a human centromere may not require a length of DNA with a specified nucleotide sequence and that it may contain as much as a million nucleotide pairs. This is because a human centromere can contain such a large number of nucleotide pairs. As we will see in the following section of this chapter, a human centromere is thought to consist of a sizeable, protein-nucleic acid structure that has a regular repeating pattern and can be passed down from generation to generation through the process of chromosome replication.