Mitosis

DNA Replication

Cell Division

Inheritance

DNA Replication

Gene Linkage

Biotechnology

D2.1.7
Phases of mitosis

Mitosis is a process of nuclear division that involves four distinct stages and forms two genetically identical nuclei

It is preceded by interphase, during which the chromosomal DNA is replicated to form sister chromatids
The physical splitting of the cell into two (cytokinesis) occurs concurrently with the final stages of mitosis

Mitosis involves a sequence of events that can be organised into four stages: prophase, metaphase, anaphase and telophase

Mitotic division is a fluid process whereby one stage evolves into the next, meaning the identification of stages can be somewhat subjective
Nonetheless, each mitotic stage is characterised by certain key events that must occur within a consistent order

DNA supercoils and chromosomes condense (becoming visible under microscopy)
The chromosomes are composed of genetically identical sister chromatids held together via cohesions
Paired centrosomes move to opposite poles of the cell and produce microtubule spindle fibres
The nuclear membrane breaks down and the nucleus dissolves

Protein complexes called kinetochores attach to the centromere (one complex per sister chromatid)
The kinetochores mediate a connection between the centromere and the microtubule spindle fibres
The lengthening or shortening of spindle fibres cause the chromosomes to move and align at the centre of the cell
The movement of the chromosomes is controlled by microtubule motor proteins – kinesin and dynein

The cohesins that were holding the sister chromatids together are removed at the beginning of anaphase
Continued contraction of the spindle fibres cause the sister chromatids to separate
Once the chromatids are pulled apart, they are considered to be separate chromosomes
The genetically identical chromosomes are pulled by the spindle fibres towards the opposite poles of the cell

Once the two chromosome sets arrive at the opposite poles, the spindle fibres dissolve
The chromosomes decondense (into chromatin) and are no longer visible under a microscope
A nuclear membrane will reform around each of the two sets of chromosomes, completing nuclear division

Cytokinesis will occur concurrently with the later stages of mitosis, splitting the original cell into two (each with one nucleus)

D2.1.8
Identification of phases of mitosis

Cells undergoing mitosis will lack a clearly defined nucleus and possess visibly condensed chromosomes

In prophase, the chromosomes will be condensed but still appear confined to a nuclear region (centrally clustered)
In metaphase, the chromosomes will be aligned along the equator of the cell (forming a central line)
In anaphase, the chromatids have separated and will appear as two distinct clusters at the poles of the cell
In telophase, two distinct nuclei will be present in a single cell (can be difficult to identify as cytokinesis occurs concurrently)

Phases of Mitosis

The mitotic index of a cell population is the ratio between the number of cells in mitosis and the total number of cells

It can be used as a clinical tool to determine the proliferation status of a tissue (e.g. cancer identification)
In simple terms:
- Mitotic Index = Cells in mitosis ÷ Total number of cells