Molecular Genetics

Centromeres are important for chromosome segregation.

Facts

Historically, the finding that chromosomes condense prior to fertilization was followed rapidly by the observation that the same transformation occurs prior to cell division (left). The chromosomes line up in the middle of the cell. The two sister chromatids are then evenly distributed to opposite halves of the cell undergoing division (mitosis).

• At about the same time, it was also discovered that during the formation of egg and sperm cells, a cell division occurs without prior doubling of the number of chromosomes. In meiosis (right), two cell divisions follow a single round of chromosome replication. In the first division (meiosis I), homologous chromosomes pair and segregate to daughter cells. In the, second division (meiosis II), sister chromatids segregate.
• In the segregation of chromosomes to daughter cells, the chromosomes appear to be pulled via a structure attached to chromosome centromeres. This structure is now called the kinetochore and centromeres are formally defined as the kinetochore attachment sites. In many organisms, centromeres are located in regions of condensed chromosomes that have a smaller diameter than other regions. These regions are called primary constrictions.
• In metaphase chromosomes, the homologous arms of sister chromatids are apposed to one another almost always.

Interpretations

• The morphological observations of chromatin segregation led to important concepts in heredity.
• The consistent equal segregation of chromosomes to daughter cells suggested that the chromosomes were the carriers of heredity.
• Gametes contain a single complement of a species' chromosomes (n, haploid) and are formed by reductive division of a 2n (diploid) cell.
• Attachment of chromosomes at centromeres to kinetochores is essential for segregation.
• In meiosis I, sister chromatids must remain attached to one another. A protein factor may be required to prevent their premature dissociation.
• Sister chromatids remain connected to one another through replication and chromatin condensation. The cohesion must be destroyed at anaphase of mitosis and of meiosis II.
• Daughter DNA strand intertwining can be one explanation of sister chromatid cohesion. Other factors may also be responsible for cohesion. Topoisomerases play important roles in chromosome dynamics.

Further information

• Animation of meiosis illustrates the integration of replication, crossing over and segregation. A series of diagrams assists in understanding meiosis. 
• Photomicrographs and drawings of mitosis are also helpful.
• Centromeres can be identified by molecular genetic techniques.
• Proteins essential for cohesion are known. They are celled cohesins.  A cohesin-containing complex binds specifically to Alu containing elements (ref), a dispersed family of highly repeated DNA.
• Other proteins are required for condensation of chromatin. They are called condensins.
• Chromosome condensation and decondensation may be regulated by regulating the activity of cohesins and condensins.

This is page 1361 of Molecular Genetics by Ulrich Melcher, © 1997-9, 2001-2, 2004, 2009