More specifically, in the first part of anaphase - sometimes called anaphase A - the kinetochore microtubules shorten and draw the chromosomes toward the spindle poles. Meanwhile, changes in microtubule length provide the mechanism for chromosome movement. Upon separation, every chromatid becomes an independent chromosome.
Interphase Prophase Prometaphase The cell prepares for. In the upper diagram two tetrads are represented as two x-shaped chromosomes associated side by side. Enzymatic breakdown of cohesin - which linked the sister chromatids together during prophase - causes this separation to occur. During prophase I the two members of each homolog pair become intimately associated along their entire lengths (that is, they 'synapse') to form a structure known as a tetrad (or bivalent). Metaphase leads to anaphase, during which each chromosome's sister chromatids separate and move to opposite poles of the cell. Kinetochore microtubules attach the chromosomes to the spindle pole interpolar microtubules extend from the spindle pole across the equator, almost to the opposite spindle pole and astral microtubules extend from the spindle pole to the cell membrane.
In addition, the spindle is now complete, and three groups of spindle microtubules are apparent. At this point, the tension within the cell becomes balanced, and the chromosomes no longer move back and forth. Every chromosome has at least two microtubules extending from its kinetochore - with at least one microtubule connected to each pole. This process ensures that each daughter cell will contain one exact copy of the parent cell DNA.Īs prometaphase ends and metaphase begins, the chromosomes align along the cell equator. Condensation makes chromosomes become distinct and compact. As they move, they pull the one copy of each chromosome with them to opposite poles of the cell. Prophase I is the longest phase of meiosis and is further sub divided into 5 phases: Leptotene. The spindle tubules then shorten and move toward the poles of the cell. As mitosis progresses, the microtubules attach to the chromosomes, which have already duplicated their DNA and aligned across the center of the cell. These tubules, collectively known as the spindle, extend from structures called centrosomes - with one centrosome located at each of the opposite ends, or poles, of a cell. Early microscopists were the first to observe these structures, and they also noted the appearance of a specialized network of microtubules during mitosis. The word "mitosis" means "threads," and it refers to the threadlike appearance of chromosomes as the cell prepares to divide. V) Diakinesis : It consists of the following eventsī) Complete disappearance of nucleoli and nuclear membrane.Mitosis is the process in which a eukaryotic cell nucleus splits in two, followed by division of the parent cell into two daughter cells. It involves the following events:Ī) Nucleoli and nuclear membrane start disappearing.ī) The homologous chromosomes are joined at several points, called chiasmata. Iv) Diplotene: It is called double-threaded stage.
It involves the following events:ī) Chromosomes of a homologous pair appear as tetrads.Ĭ) Crossing over occurs between non-sister chromatids of homologous chromosomes. (iii) Pachytene : It is called thick-threaded stage. It is called synapsis.ī) Paired homologous chromosomes are called bivalents. It is characterized by the following events :Ī) Pairing of homologous chromosomes, takes place. Ii) Zygotene : It is called yoked-threaded stage. It involves following events :ī) Condensation of nuclear chromatin into chromosomes.Ĭ) The end of chromosomes are attached to nuclear membrane through attachment plate. I) Leptotene : It is called thin-threaded stage. It may be divided into five substages : leptotene, zygotene, pachytene, diplotene and diakinesis. Prophase-I of Meiosis-I: It is the first stage of karyokinesis of meiosis-I. It involves the reduction of chromosome number to half (haploid) So, meiosis is called reductional division.
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