• fine radiating microtubules appear around each pair of
centrioles to form the astral rays.
• a pair of centrioles and astral rays constitute a star like aster
body.
• Cells at the end of prophase, when viewed under the
microscope, do not show Golgi complexes, endoplasmic
reticulum, nucleolus and the nuclear envelope.
- Metaphase
• The complete disintegration of the nuclear envelope
marks the start of the second phase of mitosis, hence the
chromosomes are spread through the cytoplasm of the cell.
• by this stage, condensation of chromosomes is completed
and they can be observed clearly under the microscope.
• This is the stage at which number and morphology of
chromosome most easily studied. Chromosomes can be
counted at this stage.
• Small disc-shaped structures at the surface of the centromeres
called kinetochores serve as the sites of attachment of
spindle fibres formed by the centrioles.
• Due to attachment of spindle fibres at the centromeres of
chromosomes, the chromosomes are arranged in the centre
or at equator or equatorial plate or metaphase plate.
• Hence, metaphase is characterised by all the chromosomes
lying at the equator with one chromatid of each chromosome
connected by its kinetochore to spindle fibres from one pole
and its sister chromatid connected by its kinetochore to
spindle fibres from the opposite pole.
• In animal cells and lower plants, centrioles are present
and asters, radiate from the centriole, which form spindle
fibres. This type of mitosis is called astral or amphiastral
mitosis. In higher plants, there are no centrioles and
hence no aster are formed, so this type of mitosis is called
anastral mitosis.
• Spindle fibres are chemically made of 97% tubulin protein
(protein of microtubules) and 3% rNa.
• Spindle fibres which connect the pole to the chromosomes
at the kinetochores are called chromosomal fibres and
those which extend without interruption from one pole to
other are the continuous fibres.
3. Anaphase
• This is the shortest stage of mitosis.
• Chromosomes divide at the point of centromere of
kinetochore and thus two sister chromatids, now referred to
as chromosomes of the future daughter nuclei, are formed.
• Interzonal fibres appear between the daughter centromeres
(i.e., formed after division of centromere). These sister
chromatids of future chromosomes or daughter chromosomes
now move towards the opposite poles of spindle and
this movement is due to repulsion between centromeres,
contraction of spindle fibres and relaxation of interzonal
fibres.
• Different shapes of chromosomes are observed at anaphase
(V or J or L or I-shaped), depending upon position of
kinetochore or centromere.
Table : Types of chromosomes
Types of
chromosomesShape Centromere
position
Metacentric V Median
Submetacentric L Submedian
acrocentric J Sub-terminal
Telocentric I Terminal- Telophase
• Chromosomes cluster at opposite spindle poles and their
identity is lost as discrete elements. Disintegration of spindle
fibres occurs and nuclear envelope assembles around the
chromosome clusters. Nucleolus, Golgi complex and er
reform.
cytokinesis
• Mitosis ends with division of cytoplasm known as cytokinesis.
It is derived from Greek word “cytos” means cell, “kinesis”
means movement.
• It starts towards the middle of anaphase and is completed
with the telophase.
• It is different in animals and plants. If nuclear division takes
place without cytoplasmic division, a syncytium is formed.- In which phase of cell cycle chromosomes get arranged at the equatorial plate?
- Discuss the drawbacks of amitosis.