On the left: Image of the mitotic spindle in a human cell showing microtubules in green, chromosomes (DNA) in blue, and kinetochores in red. On the right: Gli1 protein expression labeled in green, DNA labeled red. Gli1 is shown bound to the microtubules during cell division (image courtesy of the Iannaccone Lab, Lurie Children’s Research Center). Nucleus diameters are approximately 10µm, or about four 100,000ths of an inch.
September 13th, 2013
Mom’s genes load proteins and RNAs into the oocyte and control DNA replication in the one-cell zygote as well as the first cleavage which leads to the two-cell stage. The two-cell stage is where the mouse embryo switches from being controlled by maternal genes to being controlled by the embryo’s genome. In humans this transition happens at the 4-8 cell stage. This changing of the guard is where exclusive expression of genes from Mom prior to fertilization transition to expression of genes from the new embryo. Genes that are now uniquely the product of fertilization.
In other mammals the change in control varies from the 8-cell stage to the 16-cell stage called morulae [pronounced “more-you-lee” and the plural of morula]. Derived from the Latin for mulberries because the morulae are clumps of cells that sort of resemble mulberries.
On the left: Mulberries (adapted from the 1911 Encyclopædia Britannica). On the right: Two rat morulae at the 8-cell stage - morulae are 100 µm in diameter, or about four 10,000ths of an inch (image courtesy of Greg Taborn, Lurie Children’s Research Center).
One example of this shift is RNA polymerase II. The activity of RNA polymerase II has not been found in the one-cell zygote, while it is present in two-cell embryos. [RNA polymerase II is an enzyme (a protein that makes chemical reactions go faster) which is the main machine for turning the genetic code in our DNA into RNA that directs the production of specific proteins (see our previous post Genes, DNA, and RNA)].