The constricted region on the chromosome where the two DNA molecules are attached is called the centromere. Much of the sequence information available on centromere is based on comparison of centromeric DNA sequences among the chromosomes of Saccharomyces cerevisiae or the budding yeast. Each of the yeast chromosome carries contains a conserved centromeric region called CEN, which is similar among all the 16 chromosomes. The CEN region consists of about 160-220 nucleotide pairs that have been divided into three elements called CDEI, CDEII and CDEIII.
CDEI is an 8-nucleotide-pair sequence with the following consensus sequence:
CDEI, although important for proper centromere function, is not essential. The deletion of this sequence does not eliminate mitosis; however, chromosome stability declines during mitosis.
CDEII is a sequence of 78-86 nucleotide pairs which are A-T rich: They contain about 90% A-T pairs. CDEII is important for centromere as its deletion causes a loss of some chromosomes during cell division.
CDEIII is a 26-nucleotide-pair element with the consensus sequence
where the sequence highlighted in red (bold) is palindromic. CDEIII is essential for centromere function. If a substitution mutation alters the cytosine (highlighted in green) in the center of the palindrome, the centromere no longer functions.