11.3: What Happens during Mitosis?
Simple answer: segregation of replicated DNA.
Prior to mitosis, eukaryotic DNAis packed into very compact chromosomes
• Chromosome: one or two long, linear double stranded DNAmolecules bound with many
proteins (DNA/Proteins = chromatin)
• Before S phase each chromosome contains only one double-stranded DNAmolecule.
After it replicates during S phase there are two double stranded DNAmolecules: the
• Sister chromatids are held together by protein complex called cohesin.
Cohesin is removed at the end of mitosis. But some remains at the centromere
where the chromatids remain held together
• Condesins coat the DNAmolecules and make them more compact after G2
• Packing is achieved by histones, which are positively charged (amino acids) that bind to
negatively phosphate groups on DNA
DNA-Histone interactions and histone-histone interactions result in nucleosomes
During this phase of the cell cycle, DNAis accessible for proteins involved in
replication and transcription
However, once a mitotic chromosome is formed, its compactness makes it
inaccessible to replication and transcription factors
Overview: Mitosis segregates copies of genetic information
A single nucleus gives rise to two nuclei that are genetically identical to each other and the
Separate chromatids visible
Nuclear envelope breaks down
Chromosomes (two chromatids) attach to spindle apparatus
Chromosomes line up at the midline
Anaphase: Chromatids separate and move away from each other toward opposite poles
The centrosomes determine the plane of cell division
SpindleApparatus: (mitotic spindle) moves sister chromatids apart. Made of microtubules. Its
orientation is determined by the centrosome
The centrosome consists of a pair of centrioles, each one is a hollow tube formed by microtubule
triplets. These centrioles are at right angles to each other.
During S phase the centrosome doubles. Then, during prophase they move to opposite ends of
the nuclear envelope. This identifies the “poles” toward which chromosomes move during
Note: plants and fungi lack centrosomes.
The events of the centrosomes contribute to division in this way: determine the plane at which
the cell will divide. i.e. the spatial relationship between the two new cells.
The spindle begins to form during prophase
At this point, most of the cohesin that has held the replicated DNAtogether is removed, so
individual chromosomes become visible.
However, there is a small amount of cohesion at the centromere. Later in prophase,
kinetochores develop on the centromere, one on each chromosome.
Each of the two