Eukaryotic Chromosome Structure

Centromeres and Telomeres

Centromeres - condensed regions within the chromosome that are responsible for the accurate segregation of the replicated chromosome during mitosis and meiosis

Kinetochore - the location where spindle fiber attachment occurs, consists of protein and DNA

CEN DNA - DNA sequences within the kinetochore

Telomeres - the region of DNA at the end of linear eukaryotic chromosome; required for the replication and stability of the chromosome

McClintock recognized several features about the ends of chromosomes

• If two chromosomes were broken in a cell, the end of one could attach to the other and vice versa
• She never observed was the attachment of the broken end to the end of an unbroken chromosome.
• Thus the ends of broken chromosomes are sticky, whereas the normal end is not sticky,
• This suggests the ends of chromosomes have unique features.
• Usually, but not always, the telomeric DNA is heterochromatic and contains direct tandemly repeated sequences; the sequence TTAGGG is often the tandemly repeated sequence.

Replication of Telomeres

• Because DNA synthesis requires an RNA template (that provides the free 3'-OH group) to prime the synthesis, then a few bases at the end of a lagging strand would not be replicated during each round of replication
• This would shorten the length of the chromosome after each division.  But this is not seen.
• Apparently telomerases, enzymes that replicate the terminal end of the chromosome exist.
• These enzymes have a mode of action different than a normal DNA polymerase, but the end result is that sequences at the end of the chromosome do not go unreplicated during each replication round.