What are the various stages of meiotic prophase-l? Enumerate the chromosomal events during each stage.
Prophase-I is the first stage of meiosis-I. It is considered to be the most complicated and prolonged phase as if compared to the similar stage in mitosis.
It is further sub-divided into five sub-phases on the basis of chromosomal behaviour, i.e., leptotene, zygotene, pachytene, diplotene and diakinesis.
(i)Leptotene : It is known to be the very first stage of meiotic division following the interphase. 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.
(ii)Zygotene : This is the next sub-stage that takes place after the completion of the leptotene. In this stage homologous chromosomes pair up. This pairing is done in a such a way that the genes of the same character present on the two chromosomes lie exactly opposite to each other. This process of association is known as synapsis. Formation of synaptonemal complex takes place by a pair of homologous chromosomes that show synapsis. The complex so formed, on account of synapsis forms a bivalent or a tetrad.
(iii)Pachytene: It is the stage which
immediately follows zygotene where the pair of chromosomes become twisted spirally around each other and cannot be distinguished separately. Bivalent chromosomes are clearly seen as tetrads. In this stage, sometimes exchange of genes or crossing over between the two non-sister chromatids of homologous chromosomes occurs at the points called recombination nodules, which appear at intervals on synaptonemal complex. By the end of pachytene recombination gets completed leaving the chromosomes linked at the sites of crossing over.
(iv)Diplotene : It is the stage of longest duration of all. In this the synaptonemal complex appears to get dissolve while, the chromatids of each tetrad remain clearly visible. Recombined homologous chromosomes of thebi valents get separated and form chiasmata (X-shaped structures). Chiasmata formation is necessary for the separation of homologous chromosome which have undergone the process of crossing-over.
(v) Diakinesis : This is known to be the final stage of meiotic prophase-I. Also known as terminalisation, due to the shifting of chiasmata towards the end of the chromosomes. Chromosomes become fully condensed, nucleolus degenerates, nuclear envelope breakdown into vesicles and there is formation of meiotic spindle (as in mitosis) in order to prepare the homologous chromosomes for separation.
Name a cell that is found arrested in diplotene stage for months and years. Comment in 2-3 lines how it completes cell cycle.
Given that the average duplication time of E.coli is 20 minutes, how much time will two E.coli cells take to become 32 cells?
A cell has 32 chromosomes. It undergoes mitotic division. What will be the chromosome number (n) during metaphase? What would be the DNA content (C) during anaphase?
Two key events take place during S phase in animal cells, DNA replication and duplication of centriole. In which parts of the cell do events occur?
lt is said that the one cycle of cell division in human cells (eukaryotic cells) takes 24 hours. Which phase of the cycle, do you think occupies the maximum part of cell cycle?
Mitosis results in producing two cells which are similar to each other. What would be the consequence if each of the following irregularities occur during mitosis?
(a) Nuclear membrane fails to disintegrate
(b) Duplication of DNA does not occur
(c) Centromeres do not divide
(d) Cytokinesis does not occur
In which phase of meiosis are the following formed? Choose the answers from hint points given below.
(a) Synaptonemal complex________
(b) Recombination nodule________
(c) Appearance/activation of________
enzyme recombinase
(d) Termination of chiasmata________
(e) Interkinesis________
(f) Formation of dyad of cells________
Hints: (1) Zygotene, (2) Pachytene, (3) Pachytene, (4) Diakinesis, (5) After Telophase-l/before Meiosis-ll, (6) Telophase-I /After Meiosis-I.
Mitochondria and plastids have their own DNA (genetic material). What is known about their fate during nuclear division like mitosis?
The following events occur during the various phases of the cell cycle, name the phase against each of the events.
(a) Disintegration of nuclear membrane________
(b) Appearance of nucleolus________
(c) Division of centromere________
(d) Replication of DNA________
lt is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit______ phase to enter an inactive stage called_______of cell cycle. Fill in the blanks.
If a tissue has at a given time 1024 cells, how many cycles of mitosis had the original parental single cell undergone?
The diagram shows a bivalent at prophase-l of meiosis. Which of the four chromatids can crossover?
Both unicellular and multicellular organisms undergo mitosis. What are the differences, if any, observed in the process between the two?
While examining the mitotic stage in a tissue, one finds some cells withl 6 chromosomes and some with 32 chromosomes. What possible reasons could you assign to this difference in chromosome number. Do you think cells with 16 chromosomes could have arisen from cells with 32 chromosomes or vice versal.
An organism has two pair of chromosomes (i.e., chromosome number = 4). Diagrammatically represent the chromosomal arrangement during different phases of meiosis-ll.
Comment on the statement – Meiosis enables the conservation of specific chromosome number of each species even though the process per se, results in reduction of chromo-some number.
An anther has 1200 pollen grains. How many pollen mother cells must have been there to produce them?
What are the various stages of meiotic prophase-l? Enumerate the chromosomal events during each stage.
