The haploid human genome consists of 22 autosomes and one of two sex chromosomes. Each chromosome is a single DNA molecule. The human genome was estimated to include more than 120,000 genes but with publication of the sequence of the human genome in Science and Nature the number has been revised downward to 26,000-38,000, but the final count is not in. In addition to the coding regions of genes and the non-coding regions between them, chromosomes include three regions with characteristic nucleotide sequences that are required for chromosome replication and movement through mitosis and meiosis. These are the origins of replication , centromere and telomeres .
Origins of replication are the sites at which DNA replication is initiated.
Most bacterial chromosomes have a single origin of replication while eukaryotic genomes often have many. Centromeres hold sister chromatids together and provide attachment sites for microtubules. Since DNA polymerase on the lagging strand requires an RNA primer it is never able to synthesize the last few bases of the strand. Since the filler between these three elements can be any DNA, YACs can be used to clone large regions of genomes. YAC libraries can be generated in which the regions cloned span the whole genome of the organism of interest.