Meiosis

MEIOSIS :DEFINITION 

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A two-part cell division process in organisms that sexually reproduce, which results in gametes with one-half the number of chromosomes of the parent cell. The diploid (2n) parent cell is reduced to form the haploid daughter cells (n). Hence, also called the reduction division.

STAGES  

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Meiosis comprises two successive nuclear divisions with only one round of DNA replication. Four stages can be described for each nuclear division. Meiosis I is the true reduction division. MEIOSIS I PROPHASE I: Duplicated chromatin condenses. Each

chromosome consists of 2 closely associated sister chromatids. CROSSING OVER occurs during latter part of this stage.

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METAPHASE I: Homologous chromosomes align at the equatorial plate.

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ANAPHASE I: Homologous pairs separate with sister chromatids remaining together.

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TELOPHASE I: 2 daughter cells are formed with each daughter containing only 1 chromosome of the homologous pair.

MEIOSIS II  

PROPHASE II: DNA does not replicate. METAPHASE II: Chromosomes align at the equatorial plate.

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ANAPHASE II: Centromeres divide and sister chromatids migrate separately to each pole.

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TELOPHASE II: Cell division is complete. Four haploid daughter cells are obtained.

OVERVIEW

EXPLANATION 

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Meiosis comes after the G-2 phase when the DNA replication (S phase) is already concluded so the cells bear 2n and 4c at the beginning of meiosis. During the two-step meiotic division not only the number of chromosomes and thus the amount of DNA is reduced, but also DNA is exchanged between the homolog chromosomes of the original diploid cell.( crossing- over and recombination)

PROPHASE I Unique event- pairing of homologous chromosomes  Linking of replicated homologous chromosomes  Results in formation of a TETRAD  Tetrad- composed of 2 chromatids from each chromosome- a 4-strand structure.  It is further divided into 5 sub-stages: a) Leptotene b) Zygotene c) Pachytene d) Diplotene e) Diakinesis 

1. LEPTOTENE 

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Chromosomes become visible shorten thick The nucleus increase in size The homologous chromosomes start getting closer to each other.

ZYGOTENE .2    

Synapses i.e. pairing occurs This pairing of homologous chromosomes is highly specific and exact point to point It begins from attachment point to nuclear envelope. Hence, corresponding regions lie in contact.

PACHYTENE.3  Pairing is completed.  Chromosomes shorten and thicken further  A TETRAD is formed-consisting of 4

chromatids, which wrap around each other  CROSSING OVER- exchange of chromosomal segment between non-sister chromatids facilitated by chiasmata  May lead to exchange of DNA

Chiasma -site where the exchange of chromosome .segments between homologous chromosomes takes place

4.DIPLOTENE  Homologous pair moves apart except at

chiasmata region.  Hence, separation not complete.  1-5 chiasmata formation evident in each homologous pair.

DIAKINESIS.5  Condensation of chromosomes reaches

its maximum.  Separation of chromosomes is completed but at one point.  Become spread out in nuclear envelope.  Nucleolus disappears  Spindle apparatus forms.

SUMMARY OF PROPHASE I  Chromatin condenses into chromosomes  Nucleolus dissolves  Nuclear membrane is disassembled  Spindle apparatus is formed

METAPHASE I  Paired homologous chromosomes move

to the equator  Spindle fibers are attached to each homolog’s centromere  So that they lie parallel to the equatorial plate  Centrioles of homologous pair point opposite to the poles

ANAPHASE I 

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Spindle fibers elongate and pull the homologous chromosomes away from each other to opposite poles. Sister chromatids are attached at centromere. Since positioning of the pairs is random hence, random assortment of genetic material occurs True reduction division as each side has 2 chromosomes instead of 4

TELOPHASE I  Spindle fibers disappear  Nuclear envelope and nucleoli reappear  Equator of the cell pinches in to

create 2

new cells  Each daughter cell has haploid number of chromosomes  But diploid amount of DNA as each chromosome has 2 chromatids

MEIOSIS II  The second division starts almost

immediately  No replication of DNA occurs  In this there is haploid number of chromosomes (23) and diploid amount of DNA  Four daughter cells formed at the end of MEIOSIS

SUMMARY OF EVENTS  23 chromosomes on equatorial plate  Division at centromere level  Chromatids get separated and move to

opposite poles  THEY ARE GENETICALLY DISSIMILAR as a result of random assortment

PROPHASE II 

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Nuclear membrane and nucleoli disperse (if reformed in Telophase I) Centrioles if present move to opposite poles Spindle fibers reappear

METAPHASE II  Kinetochores of sister chromatids of each

chromosome point towards opposite poles  Each of the chromosome attaches itself by its centromere to a spindle fiber  Spindle fibers and the chromosomes align singly on metaphase plate

ANAPHASE II  Centromeres of sister chromatids

separate  Sister chromatids of each pair move towards the opposite poles of the cell

TELOPHASE II  Chromosomes uncoil and become less

visible  Spindle fibers disassemble  Nuclear envelops and nuclei form at opposite poles of the cell  Cytokinesis occurs producing 4 haploid cells that are GENETICALLY NONIDENTICAL.

IMPORTANCE OF MEIOSIS 1. Crossing-over:  Parental chromosomes exchange segments with each other  Results in large number of recombinations 2.RANDOM ASSORTMENT: Occurs during anaphase Results in large variety of gametes

3. GAMETE FORMATION: Takes place at the time of sexual cell formation Halves the number of chromosomes which is restored after fertilization Maintains chromosome number in generation after generation 4. VARIATIONS AND MODIFICATIONS: Make every person different and unique in his characteristics-causes specificity.

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