Lecture Delivered at KMC – IC
4 October 2011
Format of Lecture
 Mitosis : Concept
 Phases of Cell Cycle in Mitosis
 Learning Outcome
Mitosis: Concept
 In 1882, a German anatomist named Walther
Flemming made an attempt in terms of understanding
the behaviour of chromosomes.
 It was in the context of understanding the patterns of
chromosomes during Mitosis.
 In Mitosis, the daughter cells have same chromosome
numbers like the parent cell
 Flemming coined the terms mitosis and chromatin.
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Continued:
 During the period between one cell division and the
next it appeared to Flemming that the cell was simply
growing larger.
 Many critical events occur during this stage in the life
of a cell.
Hypothesis
 Cell division results in genetically identical daughter
cells
 Explanation: Cell division involves the distribution of
identical genetic material – DNA – to two daughter
cells.
-- The most remarkable factor in the cell division is that
the DNA is passed along from one generation of cells
to the next
-- A dividing cell duplicates its DNA – allocates the two
copies to opposite ends of the cell and then only it
splits into daughter cells.
Phases of Mitosis
 Cell Cycle in Mitosis includes following two phases:
--- Inter Phase
--- Mitotic Phase
• Inter Phase is the longest Phase. It is further divided
into G 1 (Gap 1) Phase , the S Phase (Synthesis) and the
G 2 Phase (Second Gap)
• During all three sub phases, the cell grows by
producing proteins and cytoplasmic organelles such as
Mitochondria and endoplasmic reticulum.
Continued:
 During interphase, the single centrosomes replicates
and forms two centrosomes, which remain together
near the nucleus
 The two centrosomes move apart from each other
during prophase and prometaphase of mitosis as
spindle microtubules grow out of them.
Continued:
 Chromosomes are duplicated only during the
Synthesis phase
 S Phase is important because at this point the cell
becomes committed to divide and subsequently enters
the Mitotic (M)Phase
 A typical human cell might undergo one division in 24
hours. Of this time, the M phase would occupy less
than one hour.
 S phase might occupy 10-12 hours or about half the
cycle.
M – Phase
 Mitosis undergoes several phases:
-- Prophase
-- Prometaphase
-- Metaphase
-- Anaphase
-- Telophase
Features of Prophase
 It is usually of the longest duration in comparison to
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the other phases of mitosis
The chromosomes appear as long slender thread better
known as chromonemata which are longitudinally
double from the very beginning , each having two
chromatids
Each chromosomes possesses several bead like
structures – the chromomeres
As prophase proceeds the chromosomes become short
At late prophase, nuclear membrane and nucleolus
disappear and the organisation of spindle starts.
The Mitotic Spindle
 Many of the events of mitosis depend on the mitotic
spindle – which basically are formed in the cytoplasm
of the prophase.
 Structure of mitotic spindle consists of fibers made of
microtubules and associated proteins
 On the one hand mitotic spindle assembles and on the
other the microtubules of the cytoskeleton partially
disassemble.
 The process in turn provides the material, which are
used to construct the spindle.
Assembly of Mitotic Spindle
 Centrosome plays a dominant role in the assembly of
mitotic spindle
 Centrosome is a nonmembranous organelle that
functions throughout the cell cycle to organise the
cell’s microtubules
 In animal cells, a pair of centrioles is located at the
centre of the centrosome but the centrioles are not
essential for cell division
e
Mitotic
spindle
Continued:
 The centrioles of an animal cell are destroyed with a
laser microbeam and henceforth a spindle is formed
during mitosis
 Each of the two sister chromatids of a chromosome
has a kinetochore – a structure of proteins associated
with specific sections of chromosomal DNA at the
centromere
Features of Prometaphase
 The nuclear envelope fragments
 The microtubules of the spindle can now invade the
nuclear area and interact with the chromosomes,
which have become even more condensed
 Microtubules extend from each centrosome toward
the middle of the cell.
 Each of the two chromatids of a chromosome now has
a kinetochore, which is a specialised protein located at
the centromere
Continued:
 Some of the microtubules attached to the
kinetochores, which ultimately becomes kinetochore
microtubules. These Kinetochore microtubules jerk
the chromosomes back and forth
 Nonkinetochore microtubules interact with those
from the opposite pole of the spindle
Features of Metaphase
 Metaphase is the longest stage of mitosis lasting about
20 minutes
 The centrosomes are now at opposite ends of the cell.
 The chromosomes convene on the metaphase plate, an
imaginary plane that is equidistant between the
spindle’s two poles. The chromosomes centromeres lie
on the metaphase plate.
 For each chromosome, the kinetochores of the sister
chromatids are attached to kinetochore microtubules
coming from opposite poles
metaphase
Features of Anaphase
 Aanaphase is the shortest stage of mitosis lasting only
a few minutes
 Centromere divides and chromatids move to their
respective poles
 Each chromatid appears as V-shaped structure
 By the end of anaphase, the two end of the cells have
equivalent and complete collections of chromosomes
Anaphase
 Shortest phase
Features of Telophase
 The chromosomes are at the poles in telophase and
become less distinguishable as individual stands
 It is as long as prophase and is characterised by
reappearance of nuclear membrane and nucleolus
 It is some sort of a reversal of prophase
 Mitosis, the division of one nucleus into two
genetically identical nuclei is now complete.
Telophase
Telophase