A good example is of the neurons of our brain. In an adult most of the neurons are in Go state
and won’t normally divide again. [It is a big area of research to figure out how to get these cells
to divide if the person’s brain has been damaged.]
So now let’s say that a cell has grown enough to be ready to divide, and the cell does not have
DNA damage, and the cell asked not received a signal to undergo apotosis, and the cell has not
received a signal to enter G0, what can it do?
If a cell that is large enough, and has not received any of the negative signals it can pass “the
restriction checkpoint’ an important checkpoint in the cell cycle.
Once a cell gets past this restriction point it will go ahead with preparations to divide.
Since the DNA is the hereditary material, before a cell can successfully divide it must make a
copy of its DNA for the new cell. We say the cell must undergo DNA replication.
DNA REPLICATION IS WHAT HAPPENS IN THE NEXT STAGE OF THE CELL CYCLE - S PHASE. S stands
for synthesis. The DNA is synthesized (replicated) during S phase.
RNA and protein synthesis continues into S phase, but this synthesis usually relates to the
production of the proteins that are needed for DNA replication and for packaging the DNA into
ONCE THE DNA IS SYNTHESIZED, THE CELL ENTERS THE NEXT PHASE KNOWN AS CALLED G2.
Some cells actually grow in size during G2, but the main activity of most cells in G2 is to make
preparations to divide into two cells. Thus the cells are again busy making RNA, protein, lipids
and carbohydrates. Once these preparations are completed, the cell can enter M phase.
During M phase all of the essential components of the parental cell are divided into two shares
between the two newly forming cells. M phase ends with the creation of two daughter cells.
These two daughter cells then each move into G1 phase.
Now I want to talk about genome organization and DNA packaging and chromosomes.
EUKARYOTIC GENOME ORGANIZATION
EACH TRANSCRIPTION UNIT OF A EUKARYOTE IS KNOWN AS A GENE. Each gene is transcribed
into an RNA and the RNA might be for ribosomal RNA, tRNA or an mRNA that encodes for
All eukaryotic organisms have tens of thousands of genes. These genes are part of much larger
double stranded DNA molecules.