Different Phases of Meiosis I - Practice Questions & MCQ

Stages of Meiosis Ⅰ - Prophase I

• In meiosis I, the actual reduction in the number of chromosomes occurs.
• It starts after the homologous chromosomes have replicated during S-phase and the proteins required for division have been synthesized in G2 phase.
• Therefore, it occurs after the interphase of cell cycle.
• It is studied under Prophase I, Metaphase I, Anaphase I and Telophase I.

Prophase I

• It is the longest phase of meiosis I. 
• It is divided into five substages:  leptotene, zygotene, pachytene, diplotene and diakinesis.

LEPTOTENE:

• The chromatin begins to condense to form chromosomes.
• Chromosomes appear as thin and long threads.
• By the end of this phase, chromosomes become visible under microscope.

ZYGOTENE:

• The pairing of the homologous chromosomes initiate in this phase.
• The pairing of homologous chromosomes is called synapsis.
• The synapsed homologous chromosomes appear in the form of bivalent of chromosomes or tetrad of chromatids.
• In the tetrad, two similar chromatids of the same chromosome are called sister chromatids and those of two homologous chromosomes are termed non-sister chromatids.
• A filamentous ladder like nucleoproteins complex, called synaptonemal complex appears between the homologous chromosomes. It holds the homologous chromosomes together.

PACHYTENE:

• The exchange of parts between non-sister chromatids occurs during this phase.
• It is called the crossing over.
• Crossing over occurs through breakage and reunion of chromatids segments.
• Breakage is called nicking. It is assisted by an enzyme endonuclease.
• Reunion termed annealing. It is aided by an enzyme ligase. 

DIPLOTENE:

• This phase involves pulling away of the synapsed homologous chromosomes.
• The point of attachment of the homologous chromosomes where crossing over occurred is called chiasma.
• Homologous chromosomes remain attached only at chiasma.
• There can be more than one chiasmata.

DIAKINESIS:

• It marks the terminalization of chiasma.
• Nuclear membrane and nucleolus degenerates.
• Chromosome recondense and tetrad moves to the metaphase plate.
• Spindle fibres begin to form.
• When the diakinesis of prophase-I is completed than cell enters into the metaphase-I.

Stages of Meiosis Ⅰ - Metaphase I and Anaphase I

Metaphase I

• During this phase, bivalents arrange themselves on the metaphase plate.
• Hence, a fully formed spindle and equatorial alignment of the chromosomes are seen during this phase.
• The alignment of homologous chromosomes is independent of each other.
• This is responsible for generating genetic variability.

Anaphase I

• The homologous chromosomes of each bivalent separate from each other.
• The separated homologous chromosomes move to opposite poles
• Therefore, in this phase, the chromosomes separate and not the chromatids.
• So, each chromosome will still have two sister chromatids.
• Hence, anaphase I involve a reduction in the number of chromosomes.

Homologous Chromosomes separating from each other during Anaphase I

Stages of Meiosis Ⅰ - Telophase I, Cytokinesis I and Interkinesis

Telophase I

• Two daughter nuclei are formed but the chromosome number is half than the chromosome number of mother cell.
• This phase does not necessarily completes wholly.
• The spindle disappears, but new nuclear envelopes need not form before the onset of meiosis II.

Cytokinesis I

• It may or may not follow the telophase I.
• When it occurs, it forms the dyad of cells.

Interkinesis

• Following cytokinesis I, the cells enter interkinesis.
• It is also known as intermeiotic interphase.
• During this phase, there is no duplication or replication of DNA as the chromosomes are already duplicated.