- Blood Circulatory System
- Blood
- Bones of The Legs
- Bones of The Foot
- Bones of The Ankle
- Bones of Pelvis
- Blood Groups
- Scientific Name of Human Being
- Largest Organ In Human Body
- Largest Internal Organ in The Human Body
- Human Respiratory System
- Human Population
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- Human Impact on the Environment
- Human Heart
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- Human Genome Project Goals Significance
- Human Excretory System
- Human Evolution Progress
- Human Ear
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- Anatomy and Physiology of the Human Body
- Effects of Air Pollution on Human Health
Difference between
- Difference between Turner Syndrome and Klinefelter Syndrome
- Difference Between Transpiration and Guttation
- Difference Between Transpiration and Evaporation
- Difference Between Tracheids and Vessels
- Difference Between Thorns and Spines
- Difference Between T Cells and B Cells
- Difference Between Sympathetic and Parasympathetic
- Difference Between Sporophyte and Gametophyte
- Difference Between Spermatogenesis and Oogenesis
- Difference Between Sperm and Ovum
- Difference between Species, Population, and Community
- Difference Between Sleep and Hibernation
- Difference Between Saturated and Unsaturated Fats
- Difference Between Rust and Smut
- Difference Between Right and Left Lung
- Difference Between Replication and Transcription
- Difference Between Renewable and Non Renewable Resources
- Difference Between Red and White Muscle
- Difference Between Radicle and Plumule
- Difference Between Prokaryotic and Eukaryotic Transcription
- Difference Between Plasma and Serum
- Difference Between Pharynx and Larynx
- Difference Between Organs and Organelles
- Difference Between Open and Closed Circulatory Systems
- Difference Between Ocean and Sea
- Difference Between Monocytes and Lymphocytes
- Difference Between Mitochondria and Chloroplast
- Difference Between Lytic and Lysogenic Cycle
- Arteries and Veins Difference
Cell
- Growth and Development of an organism
- Meiosis Cell Division
- Cellular Respiration Concept Map
- Cell Signalling
- Cell Organelles
- Cell Cycle and Cell Division
- Cell Biology
Energy, Enzymes and membrane
Plant
- Scientific Names of Animals and Plants
- Plant Respiration
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- Plant Life Cycle and Alternation of Generations
- Plant Kingdom Plantae
- Plant Growth Regulators
- Plant Fibres
- Mendelian Inheritance Using Seeds of Different Colours Sizes of Any Plant
- Grassland Dominant Plants
- Effects of Air Pollution on Plants
- Biodiversity In Plants and Animals
Botanical Name
- Mustard Botanical Name
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- Botanical Name of Tea
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- Botanical Name of Rice
- Botanical Name of Pea
- Botanical Name of Lady Finger
- Botanical Name of Groundnut
- Botanical Name of Grapes
- Botanical Name of Coffee
- Botanical Name of Cabbage
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Biodiversity
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Symptoms, diseases
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- Appendicitis - Formation, Symptoms, Treatment
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Causes
Other Topics
Bio Articles (Alphabetic order)
- Antigens and Immunology
- Scientific Name of Vitamins
- Scientific Name of Neem
- Schistosomiasis Life Cycle
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- Salient Features of The Kingdom Monera
- Saddle Joints
- Root Modifications
- Role of Microbes In Food Processing
- RNA: World Genetic Material
- Rna Interference
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- Adolescence Secondary sexual characteristics
- Prolactin Hormone
- Productivity In Ecosystem
- Prions
- Principles of Treatment
- Principles of Prevention
- Principles of Inheritance and Variation
- Principles of Genetics
- Primary Ciliary Dyskinesia
- Prepare Pedigree Charts of Any One of the Genetic Traits Such as Rolling Of Tongue, Blood Groups, Ear Lobes, Widow’s Peak and Colour Blindness
- Prepare A Temporary Mount of The onion Root Tip To Study Mitosis
- Preparation and Study of Transverse Section of Dicot and Monocot Roots and Stems
- Pregnancy Parturition Lactation
- Neural Control and Coordination
- Nervous Tissue
- Nervous System Definition
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- Natural Selection Biological Evolution
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- Muscle Contraction Proteins
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- Morphology and Anatomy of Cockroach
- Monohybrid Cross - Inheritance of One Gene
- Molecular Basis of Inheritance
- MOET Technology - Multiple Ovulation Embryo Transfer Technology
- Modern Synthetic Theory of Evolution
- Miller Urey Experiment
- Micturition - Urination Process
- Microfilaments
- Microbodies
- Metabolism Metabolic Pathways
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- Mendelian Disorders
- Melatonin Hormone
- Meiosis Phases
- Meiosis I - Stages and Process
- Megafauna
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- Mass Flow Hypothesis
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- Life Cycle of Chicken
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- Centromere
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- Cartilaginous Joints
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- Can a Community Contain Two Populations of the Same Species?
- Bt Crops
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- Biotechnology Jobs
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- Biomes of The World
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- Asteraceae Brassicaceae Poaceae
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- Artificial Pollination
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- Apoplast and symplast pathway
- AntiMullerian Hormone (AMH)
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- Antibiotics
- Ant Life Cycle
- Annelida Meaning, Classification, Types, and FAQs
- Animal Nervous System
- Animal Kingdom Concept Map
- Animal Kingdom : Animalia Subphylum
- Animal Kingdom
- Animal Husbandry: Types and Advantages
- Animal Husbandry and Poultry Farming & Management
- Angina Pectoris
- Anatomy and Morphology of Animal Tissues
- Anagenesis
- An overview of Anatomy, its types and their applications
- Amphibolic Pathway
- Amphibia
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- Amino acids Properties, Functions, Sources
- Amensalism
- Alternatives To Dams
- Allergies Autoimmunity
- Allee Effect
- Alimentary Canal Anatomy
- Algae Definition, Characteristics, Types, and Examples
- Alcohol and Drug Abuse Measures for Prevention and Control
- Air Pollution Definition, Causes, Effect and Control
- Agriculture Seeds Selection Sowing
- Agriculture Practices - Organic Farming & Irrigation
- Agriculture Fertilizers
- Agricultural Implements and Tools
- Aerobic and Anaerobic Respiration Major Differences
- Advantages of Dams
- Adolescence and Drug Abuse
- Adh Hormone
- Adaptive Radiation Evolution
- Acth Hormone
- Acromegaly Causes, Symptoms, Treatment
- Acquired and Inherited Traits
- Acoustic Neuroma Symptoms, Causes, Diagnosis
Introduction to Cell Division
Reproduction and growth are two of the most defining characteristics of a pving organism. Be it a unicellular bacterium or a complex multicellular human being, the cells of all organisms undergo spanision, giving rise to two daughter cells. The eukaryotic cell cycle is an ordered and tightly regulated series of events that cause duppcation of a cell’s genome and its spanision into two daughter cells. In mitosis, a parent cell spanides and results in two daughter cells. Each daughter cell receives the same number of chromosomes as were present in the parent cell. However, sexually-reproducing organisms undergo a second spanision, known as meiosis, in which the chromosomal number in the daughter cells is reduced to half of what was originally present in the parent cells.
What is Meiosis?
Meiosis is described as a reductional spanision, referring to the reduction in the chromosomal number of the daughter cells.
Meiosis occurs to produce the sex cells aka gametes (i.e., eggs and sperms).
This special form of cell spanision occurs in diploid organisms, leading to the formation of haploid gametes.
When these haploid gametes fuse during fertipzation, the original ploidy of the organism is restored.
Process of Meiosis
The meiotic process comprises two stages or phases, namely, Meiosis I and Meiosis II.
While meiosis I produce two daughter cells, meiosis II terminates with the formation of four daughter cells, which are the gametes (i.e., the sperms or the eggs) of a sexually reproducing organism.
Each gamete consists of half the original amount of chromosomes that were in the parent cell, at the beginning of meiosis I.
Meiosis I and II proceed through 4 stages each, these stages are named the same as in mitosis, albeit they carry the roman numerals I or II, signifying the phase of meiosis.
Meiosis I is often referred to as reduction spanision, while meiosis II is called equational spanision.
Meiosis I
Meiosis I is characterised by the processes of synapsis, recombination and desynapsis.
During meiosis, I, the parent cell (2n), containing homologous sets of chromosomes spanides into two daughter cells. By the end of meiosis I, each daughter cell consists of a haploid set of chromosomes.
Meiosis I comprises four stages: prophase I, metaphase I, anaphase I and finally, telophase I.
Before meiosis begins, the chromosomes undergo reppcation, similar to mitosis. The meiotic interphase includes the G1, S and G2 phases, just pke mitosis.
Phases of Meiosis I
The following stages define meiosis I:
Prophase I
Prophase I further comprises the following sub-stages:
Leptotene: The reppcated chromosomes condense into long threads and become visible.
Zygotene
Chromosomes condense, and homologous chromosomes pair up.
Synapsis, i.e., the formation of the synaptonemal complex between two homologous chromosomes.
The synapsed pair of homologous chromosomes are now referred to as tetrads or bivalents.
Pachytene
Non-sister chromatids of the two homologous chromosomes undergo crossing over, at the synapse, i.e., the homologues exchange corresponding parts of their DNA.
Crossing over leads to the recombination of the DNA.
Diplotene: Characterised by desynapsis of the two homologues, i.e., the dissolution of the synaptonemal complex. However, each bivalent remains joined by chiasmata.
Diakinesis: Condensation of chromosomes, detachment from nuclear membrane, and the disintegration of the nuclear membrane takes place.
Images Coming soon
Metaphase I
Characterised by pning up of the homologous pairs of chromosomes at the equator or the metaphasic plate.
Spindle fibres from the polar centrosomes form and attach to the centromeres -one microtubule from each pole attaches to one centromere of each homologous pair.
Anaphase I
The homologous pairs of chromosomes are separated from each other.
Remember, only the homologous pairs are separated, and not the chromatids (compare this to the anaphase of mitosis). The centromere doesn t sppt, unpke mitosis.
Meiosis 1 is referred to as reductional spanision, manifested during anaphase I. This is because the original number of chromosomes (2n) has now been reduced to exactly half (n).
Telophase I
Chromosomes reach the poles at their ends.
At each pole, a new nuclear envelope starts to form, surrounding each set of chromosomes (haploid set).
Reformation of chromatin via decondensation of the chromosomes takes place.
Cytokinesis occurs, marking the completion of two daughter cells.
Each daughter cell now has exactly half the original amount of chromosomes that were present in the parent cell. Hence, each daughter cell contains a haploid set of DNA (n)
Marks the end of meiosis I.
Images Coming soon
Significance of Meiosis
One of the defining significance of meiosis is that this spanision leads to the generation of genetically variable cells. The event of crossing over is a manifestation of inheritance of variation by the offspring.
The reduction spanision that occurs during meiosis ensures that the ploidy of the sexually-reproducing organism is maintained.
In humans and other diploid organisms, the parent cell (2n) undergoes meiosis, eventually resulting in four haploid gametes (n).
The fusion of the male (n) and female gametes (n) during fertipzation restores the original number of chromosomes (2n).
Meiosis also represents the molecular basis of the law of segregation and the law of independent assortment. This is manifested in the formation of gametes and the random positioning of the homologous pairs of chromosomes during metaphase I.
Conclusion
Meiosis I is a cell spanision process that occurs in sexually reproducing organisms, resulting in a reductional spanision of chromosomes. The distinguishing processes in meiosis I are synapsis and recombination of homologous chromosomes, which results in genetic variation among sexually reproducing organisms. Meiosis I comprises prophase I, metaphase I, anaphase I and telophase I. At the end of meiosis I, each of the two daughter cells thus formed carries exactly half the original number of chromosomes.
FAQs
Q1. What is the difference between a bivalent and a tetrad?
Ans: The term bivalent refers to the pair of homologous chromosomes as a whole, during synapsis. Tetrad refers to all the four sister chromatids within a bivalent.
Q2. What exactly is the synaptonemal complex?
Ans: The synaptonemal complex is a proteinaceous structure that mediates the joining of the homologous chromosomes during zygotene.
Q3. Does crossing over occur in all organisms?
Ans: No, some organisms such as Drosophila (male) and Bombyx mori (female) do not undergo crossing over during meiosis I.
Q4. At which stage of meiosis 1 are the oocytes arrested?
Ans: The oocytes are arrested at the diplotene stage of meiosis I. The process is resumed just before ovulation, by the action of the luteinizing hormone.
Q5. How is it that the sister chromatids are prevented from separating from each other during the anaphase of meiosis I?
Ans: This is mediated via cohesin known as Rec8, specifically expressed only during meiosis I, which inhibits the separation of the sister chromatids of each homologue.