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 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 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