Question

1. Summarize the events in each phase of mitosis.

2. Compare cytokinesis in a plant cell and an animal cell.

Answer 1

MITOSIS IS ALSO KNOWN AS EQUATIONAL DIVISION. SINCE THE NUMBER OF CHROMOSOSMES IN THE PARENT CELL AND PROGENY CELLS ARE SAME IN THIS DIVISION. MITOSIS OCCURS ONLY  IN EUKARYOTIC CELLS. MITOSIS IS ABSENT IN PROKARYOTIC CELL WHICH LACK A WELL DEFINED NUCLEUS.FOR CONVENIENCE MITOSIS IS DIVIDED INTO FOUR STAGES OF NUCLEAR DIVISION THEY ARE:

1.PROPHASE

2.METAPHASE

3.ANAPHASE

4.TELOPHASE

1.PROPHASE.: PROPHASE IS THE FIRST STAGE OF MITOSIS .PROPHASE IS MARKED BY INITIATION OF CONDENSATION OF CHROMOSOMAL MATERIAL. THE CHROMOSOMAL MATERIAL BECOMES UNTANGLED DURING THE PROCESS OF CHROMATIN CONDENSATION. THE CENTRIOLES MOVES TOWARDS THE OPPOSITE POLES OF THE CELL. CHROMOSOMAL MATERIAL CONDENSES TO FORM COMPACT MITOTIS CHROMOSOSMES . CELLS AT THE END OF PROPHASE WHEN VIEWED UNDER MICROSCPE DONOT SHOW GOLGI COMPLEXES , ENDOPLASMIC RETICULUM , NUCLEOLUS, AND NUCLEAR ENVELOPE.

2.METAPHASE:THE COMPLETE DISINTEGRATION OF THE NUCLEAR ENVELOPE MARKS THE START OF THE SECOND STAGE OF MITOSIS METAPHASE. IN THIS STAGE CONDENSATION OF CHROMOSOMES IS COMPLETED AND THEY CAN BE OBSERVED CLEARLY UNDER THE MICROSCOPE. OTHER MAJOR EVENTS DURING METAPHASE ARE-

SPINDLE FIBRES ATTACH TO KINETOCHORES OF CHROMOSOSMES.

CHROMOSOSMES ARE MOVED THROUGH SPINDLE EQUATOR AND GET ALIGNED ALONG METAPHASE PLATE THROUGH SPINDLE FIBRES TO BOTH POLES.

3.ANAPHASE: AT THE ONSET OF ANAPHASE , EACH CHROMOSOME ARRANGED AT THE METAPHASE PLATE IS SPLIT SIMULTANEOUSLY AND THE TWO DAUGHTER CHROMATIDS BEGIN THEIR MIGRATION TOWARDS THE TWO OPPOSITE POLES. AS EACH CROMOSOSME MOVES AWAY FROM THE EQUATORIAL PLATE , THE CENTROMERE OF EACH CHROMOSOSME IS TOWARDS THE POLE AND HENCE AT THE LEADING EDGE , WITH THE ARMS OF THE CHROMOSOSMES TRAILING BEHIND.

4.TELOPHASE: THIS IS THE LAST STAGE OF MITOSIS. AT THE BEGINIG OF THIS STAGE THE CHROMOSOMES THAT HAVE REACHED THE OPPOSITE POLES DECONDENSE AND LOSE THEIR INDIVIDUALITY. THE INDIVIDUAL CHROMOSOSMES CAN NO LONGER CAN BE SEEN AND THE CHROMATIN MATERIAL TENDS TO COLLECT IN A MASS IN THE TWO POLES . OTHER KEY EVENETS DURING TELOPHASE ARE:

NUCLEAR ENVELOPE ASSEMBLES AROUND THE CHROMOSOME CLUSTERS.

NUCLEOLUS , GOLGI COMPLEX AND ENDOPLASMIC RETICULUM REFORM.

THE DIFFERNCE BETWEEN THE CYTOKINESIS OF THE PLANT CELL AND ANIMAL CELL ARE:

CYTOKINESIS IS THE DIVISION OF CELLS INTO ITS DAUGHTER CELLS. IN ANIMAL CELL CYTOKINESIS STARTS BY THE APPEARENCE OF A FURROW IN THE PLASMA MEMBRANE . THE FURROW GRADUALLY DEEPENS AND ULTIMATELY JOINS IN THE CENTRE DIVIDING THE CELL CYTOPLASM INTO TWO. IN PLANTS , CELLS ARE ENCLOSED WITHIN CELL WALL WHICH IS ABSENT IN ANIMAL CELL. SO PLANT CELL UNDERGO CYTOKINESISI BY A DIFFERENT MECHANISM. IN PLANT CELLS , CELL WALL FORMATION STARTS AT THE CENTRE OF THE CELL AND GROWS OUTWARD TO MEET THE EXISTING LATERAL WALLS . THE FORMATION OF CELL WALL BEGINS WITH FORMATION OF A PRECURSER KNOWN AS CELL PLATE AT THE CENTRE OF THE CELL.. CELL PLATE REPRESENTS THE MIDDLE LAMELLA BETWEEN THE WALLS OF THE TWO ADJASCENT CELLS