In this article, we shall learn the term “MEIOSIS” and phases of meiosis.
CONTENTS OF THE TOPIC
- INTRODUCTION TO MEIOSIS
- MEIOSIS PHASES
- SIGNIFICANCE OF MEIOSIS
Let’s first start with the introduction to meiosis
INTRODUCTION TO MEIOSIS
The term ‘Meiosis’ was first described by German biologist Oscar Hertwig in 1876. The interphase of meiosis divided into the same three phases like mitosis i.e. G1, S phase, and G2. Interphase is followed by meiosis 1 and meiosis 2.
Meiosis is a special type of cell division in which the number of chromosomes in daughter cells reduced to half, as compared to the parent cell. It takes place in diploid cells only in animal cells at the time of gamete formation, while in plants cell when spores are produced.
Each diploid (2n) cell after meiosis produced four haploid (1n) cells. It is because it involves two consecutive divisions after single replication of DNA. Two divisions are meiosis 1 and meiosis 2. The first meiotic division is called the reduction division, while the second division is the same as mitosis. Both divisions can further be divided into sub-stages like prophase 1, metaphase 1, anaphase 1, telophase 1, and the same names are used for meiosis 2 also.
This is a very prolonged phase and differs from the prophase of mitosis because chromosomes behave as homologous pairs. Each diploid cell has two chromosomes of each type, one member from each parent, because of the fusion of male and female gametes. Each chromosome has two chromatids because chromosomes have replicated during interphase. The interphase of meiosis lack G2 stage. These similar but not necessarily identical chromosomes are called homologous chromosomes. Prophase 1 further consists of the following stages:
Leptotene: The chromosomes become visible, short, and thick. The size of the nucleus increases and homologous chromosomes start getting closer to each other.
Zygotene: First essential process of meiosis .i.e. paring of homologous chromosomes called synapsis starts. This paring is highly specific and exact point for point, but with no definite starting point(s).
Pachytene: The pairing of homologous chromosomes is completed. Chromosomes become thicker and thicker. Each bivalent consists of four chromatids, which wrap around each other. Non-sister chromatids of homologous chromosomes exchange their segments due to chiasmata formation during the process called crossing over. In this way reshuffling of genetic material occurs which produces recombination. Pachytene is a large stage that may last for days, weeks, or even many years, whereas leptotene and zygotene are both short stages and may last only for few hours.
The paired chromosomes start to separate from each other during Diplotene. Separation, however, is not finish, because homologous chromosomes remain intact by their point of the interchange (chiasmata).
Diakinesis: During this phase, the condensation of chromosomes reaches its maximum. At the same time, separation of the homologous chromosomes started during diplotene is completed, but still, they are joined at one point, more often at ends. Nucleoli disappear.
Each bivalent is now ready for metaphase. In diakinesis, the nuclear envelope becomes fragmenting and nucleolus dispersing.
Spindle fibers originate and the kinetochore fibers attach to the kinetochore of homologous chromosomes from each pole and arrange bivalents at the equator.
The kinetochore fibers contract and the spindle or pole fibers elongate, which pull the individual chromosomes each having two chromatids towards their respective poles. It should be noted here that in contrast to anaphase of mitosis, sister chromatids remained combined. This phase is called the reduction phase, it is because each has half of the number of chromosomes.
At two poles, the nuclear membrane reorganizes, the chromosomes remain enclosed in the nuclei, nucleoli reappear thus two nuclei each with half number of chromosomes are formed, now cytoplasm divides thus terminating the first meiotic division. Chromosomes become de-condense during this state.
After telophase 1 two daughter cells face small interphase. There is no replication of chromosomes during the interphase of meiosis 2.
Prophase 2, metaphase 2, anaphase 2, and telophase 2 are just like the respective phases of mitosis. During which the chromosomes, mitotic apparatus forms, chromosomes are arranged at the equator, individual/sister chromatids move apart, and ultimately four nuclei at the respective pole of two daughter cells (formed after meiosis 1) are formed. Cytokinesis takes place and four haploid cells, with half number of chromosomes (chromatids), are formed.
SIGNIFICANCE OF MEIOSIS
Crossing over and a random assortment of chromosomes are two important happenings of meiosis. During crossing over parental chromosomes exchange their segments with each other which results in a large number of recombinations. Thus new generation produced has adapted to its environment. At the same time during anaphase, the separation of homologous chromosomes is random, which gives a very wide range of variety of gametes. Both the processes may cause variations and modifications in the genome. These variations are not only the basis of evolution but also make every individual-specific, particular and unique in its characteristics. Even the progeny of the same parent i.e. brother and sister are not the same.
Meiosis usually takes place at the time of sexual cells (gamete formation) in the case of animals and spore formation in the case of plants, thus halving the number of chromosomes in each, which is restored after fertilization and maintains a number of chromosomes constant generation after generation.
SOME MEIOTIC ERRORS
Non-disjunctions also called meiotic errors may occur in autosome and sex chromosomes. Two examples of each type discussed below.
Down’s Syndrome (Mongolism)
It is one of the problems of autosomal non-disjunction in man, during which the 21st fails to segregate, resulting in gamete with 24 chromosomes. This gamete fertilizes the normal gamete the new offspring will have 47 (2n+1) chromosomes.
The affected individuals have a flat, broad face, squint eyes with the skin fold in the corner and protruding tongue, mental retardation, and defective development on C.N.S.
Autosomal non-disjunction may occur other than the 21st chromosome which results in abortion, or death at a very early age.
These individuals have additional sex chromosome .e.g. 47 chromosomes (44 autosomes+ XXY). They are phenotypically male but have frequently enlarged breasts, a tendency to tallness, and underdevelopment of secondary sex characters.
Males with 48 chromosomes (44 autosomes+ XXXY) with 49 chromosomes (44 autosomes + XXXXY) and males with 47 chromosomes (44 autosomes +XYY) are also observed.
THE END ALLAH HAFIZ