Meiosis I - It results in the reduction of chromosome number to half thus, known as reductional division
PROPHASE-I
It is long, complex and divisible into following five substages on the basis of chromosome behaviour
Leptotene
Following changes are observed during this stage
Chromosomes become gradually visible under light microscope
Centrioles start moving towards opposite ends or poles and each centriole develops astral rays
Each chromosome is attached to the nuclear envelope through the attachment plate at both of its ends
Zygotene
Following changes are observed during this stage
During this stage chromosomes start pairing together called synapsis formation
Such paired chromosomes are called Homologous chromosomes
Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex, tripartite, Proteinaceous structure called synaptonemal complex
The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad
Pachytene
Following changes are observed during this stage
Bivalent chromosomes are clearly seen as tetrads
Each chromosome of a bivalent consists of two chromatids and are called sister chromatids
The two sister chromatids of one chromosome in a homologous pair with regard to the other chromosomal pair are called non-sister chromatids
This stage is characterised by the appearance of recombination nodules, the site at which crossing- over occurs between non-sister chromatids of the homologous chromosomes
Crossing- over this is the exchange of genetic material between two homologous chromosomes, it is an enzyme mediated process and the enzyme involved is called recombinase
Diplotene
Following changes are observed during this stage
Longest phase of prophase -I
There is dissolution of synaptonemal complex, the recombined homologous chromosomes start separating – De-synapsis
The separation of the homologous chromosomes is not completed
They remain attached at one or more point where crossing-over has occurred
The x shaped structures are called chiasmata
Diakinesis
Following changes are observed during this stage
This stage is marked by the terminalisation due to chiasmata, shifting
During this phase the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation
The nucleoli and the nuclear membrane disintegrate and disappear
Diakinesis represents transition to metaphase-1
Metaphase-I
It is the stage that follows by prophase-I
Following changes are observed during this stage
The bivalents arrange themselves on the two parallel equatorial plates
The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes
Anaphase-I
Next phase after metaphase-I
Following changes are observed during this stage
Homologous chromosomes break their connection with each other and get separated
This process of separation of homologous chromosomes is known as Disjunction
The separated chromosomes are univalent and are also called dyads
On reaching at the end of the anaphase, the two groups of chromosomes are produced with each having half number of chromosomes
Telophase- I
This is the last stage of meiosis-I
Following changes are observed during this stage
The chromatids at each pole of the spindle usually remains uncoiled and get elongated
Homologous chromosomes reach at their respective poles
Reappearance of nuclear membrane and nucleolus takes place
Cytokinesis
It is the stage during which the cytoplasm and other organelles get divided in to two equal halves of cells
