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The event that restores the normal chromosome number in the life cycle is called  fertilization. In sexually reproducing organisms, gametes such as sperm and egg are formed by a special type of cell division known as  meiosis.  Meiosis reduces the chromosome number from  diploid (2n) to haploid (n),  meaning the gametes (sperm in males and eggs in females) contain only  one set of chromosomes.  This reduction is very important because if both parents contributed diploid sets during reproduction, the chromosome number would double in every generation, which would lead to genetic imbalance and abnormalities. Fertilization is the process where the male gamete (n) fuses with the female gamete (n) to form a  zygote (2n).  This zygote contains a complete set of chromosomes, with one set coming from the mother and one from the father. In this way, the diploid number of chromosomes is restored, ensuring that the species maintains a constant chromosom...

Bouquet Stage The bouquet stage is a special arrangement of chromosomes during the early part of  zygotene stage of prophase I in meiosis.  In this stage, the ends of all chromosomes (called telomeres) gather together at one side of the nuclear envelope, making a shape that looks like a  bouquet of flowers.  This arrangement helps in bringing homologous chromosomes closer, so that they can easily pair with each other. The exact function of this stage is not fully understood, but it is believed to help in proper alignment for synapsis and crossing over. This stage is temporary and happens only during early meiosis. Chiasma (Plural: Chiasmata) A chiasma is the visible point where two homologous non-sister chromatids exchange genetic material during crossing over. It looks like an  X-shaped  structure under the microscope. Chiasmata are seen during the  diplotene stage of prophase I in meiosis.  At this point, homologous chromosomes are still attache...

DNA replication takes place during the  S phase,  which stands for Synthesis phase of the cell cycle. This phase is part of the Interphase, which is the period between two cell divisions when the cell prepares itself for the next mitotic or meiotic division. The Interphase has three main stages: G1 phase (Gap 1) –  The cell grows and carries out normal functions. S phase (Synthesis) –  This is the stage where DNA replication occurs. G2 phase (Gap 2) –  The cell prepares for division by synthesising proteins and other components. What Happens in the S Phase: In this phase, the entire genetic material (DNA) of the cell is copied. Each chromosome makes an exact duplicate of itself, forming two sister  chromatids  joined at the  centromere.  These chromatids will later be separated into daughter cells during mitosis or meiosis. This replication is semi-conservative, meaning each new DNA molecule contains one old strand and one newly synthesized s...

Aneuploidy Aneuploidy is a type of chromosomal abnormality in which the number of chromosomes in a cell is  not exactly 46,  which is the normal diploid number in humans. In this condition, the cell may have either one extra chromosome or one missing chromosome. So, the total number of chromosomes becomes either 45 or 47. This abnormality happens mostly due to  non-disjunction  during meiosis, which means that chromosomes fail to separate properly during the formation of gametes (sperm or egg). Because of this, the gamete may carry an extra or missing chromosome. When such a gamete fuses with a normal gamete, the resulting zygote has an abnormal chromosome number. There are two major types of aneuploidy: Trisomy –  where one extra chromosome is present (total becomes 2n + 1, that is 47) Monosomy –  where one chromosome is missing (total becomes 2n − 1, that is 45) Down's Syndrome Down's syndrome is a genetic disorder that occurs due to  trisomy  o...

Homologous chromosomes separate during the first division of meiosis, which is called  meiosis I.  More specifically, this separation happens in the  anaphase I stage of meiosis I. Before this stage, in the earlier phase of meiosis I called  prophase I,  homologous chromosomes come close to each other and form pairs. This process is called  synapsis.  Each pair has two chromosomes (one from the mother and one from the father) that carry similar types of genes. These pairs are called  bivalents or tetrads,  because they have four chromatids. Then, in  metaphase I,  these homologous chromosome pairs align themselves at the centre of the cell (equator). They are attached to spindle fibres from opposite poles of the cell. Now comes the most important step i.e.,  anaphase I.  In this stage, the  spindle fibres  pull the homologous chromosomes apart. One chromosome from each pair is pulled to one side of the cell and t...

Mitosis is a type of cell division that occurs in somatic cells of eukaryotic organisms. It is known as equational division because the number of chromosomes remains unchanged from the parent cell to the daughter cells. This process is very important for the growth of the body, replacement of old or damaged cells and asexual reproduction in many organisms. In humans and most animals, mitosis helps in increasing cell numbers without changing the genetic material. Each daughter cell formed after mitosis has the same number and type of chromosomes as the mother cell. The whole mitotic process happens in a stepwise manner and is completed in two main stages: Karyokinesis  (division of the nucleus) Cytokinesis  (division of the cytoplasm) 1. Karyokinesis (Division of the Nucleus) Karyokinesis is the first stage of mitosis in which the nucleus of the cell divides properly to ensure that each daughter cell receives an exact copy of the genetic material. The word karyokinesis is made ...

The non-dividing cells are most likely in the  G₀ phase  of the cell cycle. The G₀ phase (Gap zero phase) is a  resting or quiescent  stage where the cell exits the active cell cycle. In this phase, the cell does not prepare for division and stops progressing through the cycle. It is a reversible or permanent resting stage, depending on the cell type. Cells enter G₀ from the  G₁ phase  when conditions are not favorable for division or when the cell has reached full maturity. Some cells can re-enter the cell cycle from G₀ when needed, while others remain permanently in G₀ and never divide again. For example,  nerve cells  and  cardiac muscle cells  stay permanently in G₀ and do not divide after maturation. On the other hand,  liver cells  can re-enter the cell cycle from G₀ when there is damage or a need for regeneration. This phase is very important for controlling unnecessary cell division, especially in multicellular organism...

Crossing over is a very important process that takes place during  prophase-I  of meiosis, especially during the  pachytene sub-stage,  where homologous chromosomes exchange genetic material. This process increases genetic variation and is a key part of sexual reproduction. To explain how crossing over happens, many scientists have proposed different theories over time. These theories developed step-by-step, with later theories giving more accurate and detailed explanations. There are mainly four important theories that explain the process of crossing over. These are: 1. Copy Choice Theory Copy choice theory was proposed by  J. Belling  in 1931. This is one of the oldest theories. It suggested that new chromosomes are formed by copying some parts from one chromosome and some from the other during DNA replication. According to this theory, the copying machinery switches templates while making the new DNA strand. However, this theory was later rejected becaus...

In meiosis I, the process of chromosome segregation requires the pairing of homologous chromosomes. This pairing forms a structure known as a  bivalent.  A bivalent is essentially a pair of homologous chromosomes, each consisting of two sister chromatids. The formation of bivalents occurs during  prophase I,  a crucial phase of meiosis. These structures are key to ensuring the correct distribution of chromosomes into daughter cells, as well as contributing to genetic diversity. Formation of Bivalent During prophase I of meiosis, homologous chromosomes come together and pair up through a process called  synapsis.  These paired chromosomes are held together by the synaptonemal complex, a protein structure that stabilizes the connection. The bivalent consists of two homologous chromosomes, each with two sister chromatids, making a total of  four  chromatids in the structure. This pairing is crucial for crossing over, a process where genetic material ...

The cell cycle refers to a series of events that take place in a cell as it grows and divides to form two daughter cells. The cycle is crucial for the growth, development and maintenance of all living organisms, ensuring that the cells replicate and divide properly. The concept of the cell cycle was first systematically described by  Howard and Pelc  in 1953. They observed the different stages of cell division and growth, which led to the understanding of the continuous process of the cell cycle. The cycle is highly regulated, and any errors in this process can lead to diseases such as cancer. Phases of the Cell Cycle The cell cycle is divided into two main stages: Interphase:  The preparatory phase where the cell grows and DNA is replicated. Mitotic Phase (M phase):  The phase where the actual division of the cell occurs. 1. Interphase: Interphase is the  longest phase  of the cell cycle and is primarily involved in preparing the cell for division. It is d...

Chromosomes are most easily identified during  Metaphase I  and  Metaphase II  of meiosis. These are the two stages where chromosomes appear most condensed, structured and clearly visible under a microscope. Reasons Why Chromosomes Are Visible at These Stages 1. Chromosomes are Visible in Metaphase I In Metaphase I of meiosis, chromosomes are most clearly visible because of maximum  condensation.  During this stage, homologous chromosomes, which have already paired up during prophase I, align themselves at the equatorial plane of the cell. Each homologous pair, also called a  bivalent or tetrad,  contains four  chromatids  (two chromosomes). At this point, chromosomes are thick, short and highly condensed. Due to this condensed nature, they become clearly distinguishable under the microscope. Also, since the homologous pairs are arranged in a well-organised line along the metaphase plate, it becomes easy to identify individual chromosome...

Meiosis is a type of cell division that occurs only in  diploid germ  cells of sexually reproducing organisms. This division is responsible for producing haploid daughter cells (with half the number of chromosomes) from a diploid parent cell. Meiosis occurs in two main stages:  Meiosis I and Meiosis II.  One round of DNA replication is followed by two successive cell divisions. The major goal of meiosis is to reduce the chromosome number from diploid (2n) to haploid (n), so that the chromosome number remains constant after fertilization. Meiosis is also responsible for genetic recombination through processes like crossing over and independent assortment. The most important thing is that meiosis produces  gametes  (in animals) and  spores  (in plants) which are essential for sexual reproduction. Product of Meiosis: The final product of meiosis is  four haploid cells,  each containing half the chromosome number of the original diploid cell...

Synapsis is a special event that takes place  only during meiosis,  not mitosis. It refers to the pairing of homologous chromosomes i.e., one chromosome from the mother and one from the father that carry genes for the same traits. These two chromosomes come and lie side by side with each other during the early stages of meiosis. This process of exact alignment is called  synapsis.  It is very important for the proper distribution of chromosomes in gametes and also for allowing genetic recombination or crossing over to occur. Synapsis happens only once, during  prophase I of meiosis I  and does not occur again in meiosis II or mitosis. Without synapsis, homologous chromosomes cannot properly exchange genetic material or segregate correctly, which may lead to genetic disorders. Process of Synapsis in Meiosis Synapsis is the pairing of two homologous chromosomes during the early stage of meiosis I, specifically in the zygotene substage of prophase I. It is a h...

Independent assortment is a key genetic principle that explains how genes are inherited independently from one generation to the next. This concept was first clearly explained by  Gregor Johann Mendel  in 1865 through his work on  pea plants.  He described it in his  Second Law of Inheritance,  called the  Law of Independent Assortment.  It explains how the alleles of different genes get distributed into gametes separately and randomly, producing genetic variation in offspring. The actual biological basis of independent assortment lies in the behavior of chromosomes during meiosis, particularly during  metaphase I and anaphase I of meiosis I. Cellular Basis of Independent Assortment: During meiosis I, homologous chromosomes line up at the metaphase plate in a random orientation. Each homologous pair consists of  one maternal  and  one paternal  chromosome. The orientation of one pair is completely independent of the orient...

Mitosis is a very important process in eukaryotic organisms. It is a type of cell division in which a diploid (2n) mother cell divides to form two genetically identical diploid daughter cells. It occurs in somatic (body) cells and also in diploid germ cells (before meiosis begins). Mitosis maintains the same number of chromosomes in daughter cells as in the parent cell. This division is highly regulated and accurate so that each new cell gets a full and equal set of genetic material. The significance of mitosis can be explained under the following points: 1. Genetic Stability and Chromosome Number Maintenance: This is the most basic and important function of mitosis. It ensures that every daughter cell receives the same number and exact copy of chromosomes as the parent cell. For example, if a human somatic cell with 46 chromosomes divides by mitosis, both daughter cells will also have 46 chromosomes. This helps maintain genetic stability across tissues and organs and ensures organisma...

Mitosis is a type of equational cell division in which a single parent cell divides to produce two genetically identical daughter cells. It usually occurs in diploid cells and maintains the same number of chromosomes (2n) in daughter cells as present in the parent cell. Mitosis is commonly found in somatic cells and also in diploid germ cells like spermatogonia and oogonia, where it helps in producing more cells before meiosis begins. It is also seen during growth, repair, regeneration and asexual reproduction in some lower organisms. This division helps in tissue formation, wound healing, and replacement of old or damaged cells. It is a part of the cell cycle and occurs during the M phase. Mitosis has two main stages. The first stage is Karyokinesis which means division of the nucleus, and the second stage is Cytokinesis which means division of the cytoplasm. 1. Karyokinesis Karyokinesis is the nuclear division part of mitosis. It is a complex process where the chromosomes get separat...

The G1 phase, also known as the  first gap phase,  is the first phase of interphase in the cell cycle. It occurs after  mitosis (M phase)  and before the  S phase,  where DNA replication happens. This phase plays a crucial role in preparing the cell for DNA synthesis and mitosis. The main functions of the G1 phase are: 1. Cell Growth: During the G1 phase, the cell increases in size and mass. This phase is critical for the cell to accumulate enough resources, such as proteins, lipids and other macromolecules, needed to proceed with DNA replication and cell division. The cell prepares itself physically by expanding its cytoplasm and increasing the number of organelles, particularly mitochondria and ribosomes, to meet the demands of the next phases. 2. Protein and RNA Synthesis: One of the key activities during the G1 phase is the synthesis of proteins and RNA molecules that will be needed for subsequent cell cycle events. Proteins involved in DNA replication,...

Mitosis, also called the  M phase,  is the most active and visible part of the cell cycle. In this phase, a single mother cell divides into two daughter cells and each new cell receives the exact copy of genetic material as present in the original cell. This phase is mainly responsible for growth, repair of damaged tissues and maintaining genetic continuity. Mitosis ensures that the chromosome number remains the same in both daughter cells. The entire process is divided into two main stages:  karyokinesis,  which is the division of the nucleus and  cytokinesis,  the division of the cytoplasm. These two stages occur in a highly coordinated sequence to ensure the proper distribution of cellular components and genetic material. 1. Karyokinesis (Division of Nucleus) Karyokinesis is the first major step of mitosis, in which the nucleus of the parent cell divides. It ensures that each daughter cell will receive an equal and exact set of chromosomes. This process ...