We commonly think of dividing cells passing through a series
of four stages, called the cell cycle, as they undergo sequential rounds of
proliferation. These four stages
G1 - Pre-synthesis Gap. Day to day function of the cell,
- Synthesis. DNA replication
G2 - Post-synthesis Gap. Preparation for cell division
- Mitosis or cell division
When our focus is on mitosis, we may consider G1, S and G2 as a single unit called Interphase. Although very different events are
happening in the cell during these three stages, there is little to
differentiate them microscopically.
Before shifting our focus to mitosis, we do need to make
sure we understand what is happening in S stage. During S, the DNA is replicated making two identical
copies of each double stranded DNA molecule. The two daughter strands, however, remain attached by a
centromere making a single large structure. Because of this attachment, even though the amount of DNA in
the cell has doubled, the number of chromosomes remains the same. Each chromosome now has two identical
chromatids attached by the centromere.
Interphase and Mitosis
When you examine a population of rapidly dividing cells,
such as those at the tip of an onion, you will be able to see cells that are in
Interphase or one of the four stages of mitosis. Following are descriptions of cells in these various stages
and brief descriptions of what is happening in the cell to cause the changes
that you are seeing.
Distinct nucleus. Although the nuclear membrane is too thin to see with a compound
microscope, you should be able to easily distinguish the nucleus
(stained blue) from the more pinkish color of the cytoplasm. The colors you observe are due to the
nucleic acids staining blue while proteins stain red
Distinct nucleolus. There are high levels of transcription going on in Interphase
cells, especially of rRNA and tRNA. The high concentration of nucleic acids make this portion of
the nucleus stain more darkly.
When chromosomes are not condensed, it is called chromatin. Because chromatin is very thin, you can
not see individual DNA strands.
Therefore the blue of the nucleus is due to indistinct
swirls of chromatin dispersed throughout the nucleus.
In early prophase,
the chromosomes will appear as thin strands dispersed about the
entire nucleus. The spindle apparatus will be forming
but the thinness of the individual microtubule will probably make
the spindle impossible to see at this early stage.
By middle prophase,
the chromosomes strands will get thicker and shorter.
The nucleolus will be gone as transcription must stop as
the DNA condenses.
In late prophase,
the chromosomes will be very short and will be clearly moving
towards the center of the spindle apparatus.
Short, thick chromosomes, The
chromosomes reach their maximum level of condensation.
Chromosomes align on the metaphase plate. The chromosomes are lined up in the middle
of the spindle apparatus.
Although the chromatids may be pointing in various directions,
the centromeres will be clearly all in a row.
Centromeres break. The centromeres break releasing the sister chromatids as individual
Chromosomes separate. Kinetochore microtubules shorten while polar microtubules elongate
which pulls/pushes the chromosome towards the opposite poles of
the spindle, Recall
that due to DNA replication in S phase that each new nucleus which
forms will have precisely the same DNA content.
Chromosome separation completed. The chromosomes have reached opposite
ends of the spindle.
Chromosomes unwind. The chromosomes unwind and return to the chromatin conformation.
may be able to observe new cell wall forming in the middle of
the old spindle apparatus.