CELL DIVISION, & THEORY OF Cell Cycle

• The cell cycle is the series of events that take place in the cell leading to its division and duplication that produces two daughter cells.

• Periods of cell cycle: • Interphase • Cell division • Cytokinesis Interphase

• It is also called the Interkinesis / Resting phase.

• During this stage, the nucleus is not dividing but the DNA in the nucleus is duplicated in preparation for the next division.

• Phases of interphase:

• Gap 0 (G0)

• Gap 1 (G1) • Synthesis (S)

• Gap 2 (G2) • Gap 1(G1) • The cell grows and functions normally. • A high amount of protein synthesis occurs and the cell grows to double its original size. • More organelles are produced. • The volume of cytoplasm increases and mitochondria and chloroplasts divide.

• If the cell is not to divide again, it will enter G0.

• Synthesis (S) • The cell duplicates its DNA. • This is also known as The Swanson Phase.

• Gap 2 (G2) • The cell resumes its growth in preparation for division.

• Gap Zero (G0) • Some cells do not further divided means their cell cycle is arrested so it is termed as Gap Zero (G0). Cell Division

• Cell division is the process by which a parent cell divides into two or more daughter cells.

• Eukaryotes undergo two type of cell division : • Mitotis •

• Prokaryotes undergo cell division : • Binary fission (Karyokinesis)

• It is the process in which the parental cell divides into two daughter cells which are genetically identical to the parent cell.

• All somatic (body) cells multiply by the mitosis cell division.

• Stages of Mitosis: 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase Prophase

• This process is also called as Chromatin condensation.

• The chromatin condenses into double rod-shaped structures ().

• Centrioles move to the opposite sides of the nucleus.

• Nucleolus disappears.

• Nuclear membrane disintegrates. Prometaphase

• It is the phase of mitosis following prophase and preceding metaphase.

• Kinetochore (the protein structure) on chromatids where the spindle fibers attach, during cell division to pull sister chromatids apart. Metaphase

• Chromosomes meet in the middle.

• Chromosomes arrange at equator of cell.

• Become attached to spindle fibers by centromeres.

• Homologous chromosomes do not associate. Anaphase

• Chromosomes get pulled apart.

• Spindle fibers contract pulling chromatids to the opposite poles of the cell. Telophase

• Chromosomes uncoil.

• Spindle fibres disintegrate.

• Centrioles replicate.

• Nuclear membrane forms.

• Cell divides. Cytokinesis

• Cytokinesis begins shortly after the onset of sister chromatid separation in the anaphase of mitosis.

• In this stage of cell division, there is invagination of cell wall at the equator toward the center of the parental cell which results into equal division of the cytoplasm and two daughter cells are formed.

• After cytokinesis, each daughter cell is in the interphase portion of the cell cycle.

Meiosis

• During this cell division, a nucleus divides into four daughter nuclei having half of the number to the parent nucleus.

• (sex cells) divided by this type of cell division.

• Stages of meiosis: 1) Meiosis I 2) Meiosis II Meiosis I

• Meiosis I segregates homologous chromosomes, producing two haploid cells from a diploid cell, so it is referred to as a Reduction division.

• Phases of Meiosis I : 1. Prophase I a) Leptotene b) Zygotene c) Pachytene d) Diplotene e) Diakinesis 2. Metaphase I 3. Anaphase I 4. Telophase I Prophase I

• Prophase I is the longest phase of meiosis.

• During prophase I, DNA is exchanged between homologous chromosomes in a process called Homologous Recombination.

• It can be subdivided into; • Leptotene • Zygotene • Pachytene • Diplotene • Diakinesis Leptotene(Leptonema)

• Greek words meaning "Thin threads".

• Leptotene is of very short duration.

• During this phase the chromosomes become visible.

• Progressive condensation and coiling of chromosome fibers takes place. Zygotene(Zygonema)

• Greek words meaning "Paired threads“.

• chromosomes line up with each other into homologous chromosome pairs.

• Pairing is brought about in a zipper-like fashion and may start at the centromere (Procentric) or at the chromosome ends (Proterminal) or at any other portion (Intermediate).

• The paired chromosomes are called Bivalent Or Tetrad Chromosomes. Pachytene(Pachynema)

• Greek words meaning "Thick threads“.

• At this point a tetrad of the chromosomes has formed known as a bivalent.

• Here chromosomal crossover occurs called as Crossing Over.

• Nonsister chromatids of homologous chromosomes may exchange segments over regions of homology.

• At this site they become attached, this point is called as Chiasmata. Diplotene(Diplonema)

• Greek words meaning "Two threads“.

• In mammalian and human fetal all developing develop to this stage and are arrested before birth.

• This suspended state is referred to as the Dictyotene Stage or . • • It lasts until meiosis is resumed to prepare the for ovulation, which happens at or even later. Diakinesis

• Greek words meaning "Moving through".

• This is the first point in meiosis where the four parts of the tetrads are actually visible.

• Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible.

• Other than this observation, the rest of the stage closely resembles prophase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form. Metaphase I

• The nuclear membrane disappears.

• Spindle is formed as in mitosis.

• Chromosomes are attached by centromeres.

• Bivalents gather on metaphase plate. Anaphase I

• Differs from that in mitosis. There is no splitting of the centromeres.

• One entire chromosome of each pair moves to each pole of the spindle.

• The resulting daughter cells therefore have half chromosomes, each made up of 2 chromatids. Telophase I

• Karyokinesis takes place or without it cell enters in 2nd division of meiosis, which is similar to mitosis.

• Cells may enter a period of rest known as Interkinesis or Interphase II. Meiosis II

• Also known as Equational division.

• Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different.

• The end result is production of four haploid cells from the two haploid cells, produced in Meiosis I. Phases of Meiosis II

1. Prophase II 2. Metaphase II 3. Anaphase II 4. Telophase II Prophase II

• The disappearance of the nucleoli and the nuclear envelope.

• The shortening and thickening of the chromatids.

• Centrosomes move to the polar regions and arrange spindle fibers for the second meiotic division. Metaphase II

• The centromeres contain kinetochores that attach to spindle fibers from the centrosomes at opposite poles. Anaphase II

• Chromosomes get pulled apart.

• Spindle fibers contract pulling chromatids to the opposite poles of the cell. Telophase II

• It is marked by decondensation and lengthening of the chromosomes.

• The disassembly of the spindle occurs.

• Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells.

• Meiosis is now complete and ends up with four new daughter cells. Gametogenesis

• Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes.

• Animals produce gametes directly through meiosis in organs called gonads (testicle in males and ovary in females). • Males and females of a species that reproduces sexually have different forms of gametogenesis.

(In male)  oogenesis (In female) Spermatogenesis

• Spermatogenesis is the process in which spermatozoa are produced from male primordial germ cells by way of mitosis and meiosis.

• It comprises: i. Spermatocytogenesis- Process of formation of from . ii. Spermiogenesis(Metamorphosis)- Process of transformation of into Spermatozoa. • A Spermatogonium is an undifferentiated male , originating in a seminiferous tubule and dividing into two primary in the production of spermatozoa.

• There are three subtypes: Type A(d) cells (with dark nuclei): These cells replicate to ensure a constant supply of spermatogonia to fuel spermatogenesis. Type A(p) cells (with pale nuclei): These cells divide by mitosis to produce Type B cells. Type B cells: which divide to give rise to Primary spermatocytes. • Each Primary Spermatocyte duplicates its DNA and subsequently undergoes meiosis I to produce two haploid Secondary Spermatocytes.

• Each of the two Secondary Spermatocytes further undergo meiosis II to produce two Spermatids (haploid).

• So 1 primary spermatocyte => 4 spermatids. • The process of spermatogenesis is divided into the following phases

• Multiplication phase • Growth phase • Maturation phase • Metamorphosis of Multiplication phase

• Also known as Spermatocytogenesis.

• The mother cells present in the germinal epithelium of the seminiferous tubules divide repeatedly by mitosis to form large number of diploid rounded sperm mother cells which are called as spermatogonia.

• Some of these sex cells move towards the lumen of seminiferous tubules and enter the growth phase. These cells are called primary spermatocytes.

• The primary spermatocytes are diploid and contain chromosomes.

• Some of the sex cells produced by the division of spermatogonia remain in their original condition and continue to divide giving rise to primary spermatocytes. Such cells are known as stem cells. Growth Phase

• During this phase the spermatocyte as well as its nucleus enlarges in size. It gets ready to undergo maturation division. Maturation Phase

• Each diploid primary spermatocyte undergoes meiosis I, which is a reduction division. Two daughter cells are formed each with 'n' number of chromosomes.

• The daughter cells are called secondary spermaotcytes.

• The secondary spermatocytes are haploid and much smaller comparitively, containing chromosomes.

• The secondary spermaotcytes undergo the second meiotic division (equational). This results in the formation of four daughter cells known as spermatids. Spermiogenesis/Metamorphosis

• The spermatids formed as a result of maturation division is a typical animal cell with all the cell organelles present in it.

• In this form it cannot function as a male .

• So many changes take place to change the non-motile spermatid into motile spermatozoan.

• The main aim of the changes is to increase the motility of the sperm. Phases

• Golgi phase • Cap phase • Acrosome phase • Maturation phase Golgi phase

• During this phase there is formation of pro-acrosomal granules by golgi apparatus. • These granules coalesces to form acrosomal granules which get attached at the tip of the nucleus of the spermatid. Cap phase

• Acrosomal granules remain stationary and its limiting membrane extends outward to form a head cap. Acrosomal phase

• During the phase the acrosomal material spread into head cap to form acrosomal cap & spermatid starts elongation. Maturation phase • The chromatin material transforms into homogenous mass. • The centrioles and cytoplasmic mass migrate caudally and the distal centriole give rise to flagella. • The mitochondria gather in the middle piece. • The formation of microfilament and the fibrous sheath takes place. • The excessive cytoplasm forms the protoplasmic droplet which is later cast off. • The spermatid disjoints and the spermatozoa is formed. • They were either attached to sertoli cells or release by a mechanism of apocrine budding. • They are stored in a epididymis for 6-14 days. oThe changes are;

• The nucleus shrinks by losing water and DNA becomes closely packed.

• An acrosome is formed from the golgi complex.

• An axial filament of the tail of the spermatozoan is formed from the distal centriole of the spermatid. • A mitochondrial ring is formed from the mitochondria around the distal centriole and is called as Nebenkern.  Much of the cytoplasm of the spermatid is lost and the remaining cytoplasm forms a sheath around the mitochondrial spiral.

 This sheath is known as Manchette.

• During the process of differentiation the developing have their head embedded in the sertoli cells (nurse cells) from which they obtain their nourishment.

• Spermatogenesis is controlled by FSH (follicle stimulating hormone) and the gonadotropins.

• In male, there are two types of sperms produced. 50% of the sperms have the X chromosome and are called as Gynosperms. 50% of the sperms have the Y chromosome and are called as Androsperms. Spermatogenesis Structure of Sperm

• A mature sperm cell has snake like structure.

• It has following parts; • Head • Neck • Middle Piece • Tail

• Head: • It is spherical in shape consisting of large nucleus and a dome shaped acrosome present on the nucleus.

• Function: • Nucleus contain genetic information and half number of chromosomes. • The acrosome releases a hyaluronidase enzyme which destroys the hyaluronic acid of the ovum and enters into the ovum.

• Neck: • It contains centrioles which are proximal centriole and distal centriole.

• Function: • Distal centriole gives rise to axial filament of the sperm which runs up to the end of the tail. • Middle piece: • It is tubular structure in which mitochondria are spirally arranged.

• Function: • Middle piece is called power house of sperm because it gives energy to the sperm to swim in the female genital tract.

• Tail: • It arises from middle piece and it is the end part of the sperm. It contains axial filaments.

• Function: • Tail helps the sperm to swim in the female genital tract. It is the main part of sperm to move. Oogenesis

• Oogenesis, Ovogenesis or Oögenesis is the creation of an ovum ( cell).

• It involves the development of the various stages of the .

• The first part of oogenesis starts in the germinal epithelium, which gives rise to the development of Ovarian Follicles (the functional unit of the ovary). • Oogenesis consists of several sub-processes:   Oocytogenesis  Ootidogenesis  Oogenesis proper (finally maturation to form an ovum) Folliculogenesis • Folliculogenesis is the maturation of the ovarian follicle.

• It describes the progression of a number of small primordial follicles into large preovulatory follicles i.e Graffian follicles that enter the . Oocytogenesis

• Oogenesis starts with the process of developing oogonia, which occurs via the transformation of Primordial Follicles into Primary Oocytes, a process called oocytogenesis.

• Oocytogenesis is complete either before or shortly after birth. Ootidogenesis

• The succeeding phase of ootidogenesis occurs when the Primary Oocyte develops into an Ootid.

• This is achieved by the process of meiosis. Oogenesis proper

• Maturation & transformation of Ootid to Ovum. Spermatogenesis Oogenesis SpermatogenesisOogenesis: Oogenesis: 1. It occurs in the testes. 1. It occurs in the ovaries. 2. Spermatogonia change to primary 2. Oogonia change to primary oocytes. spermatocytes. 3. A primary spermatocyte divides to 3. A primary oocyte divides to form one form two secondary spermatocytes. secondary oocyte and one . 4. A secondary spermatocyte divides to 4. A secondary oocyte divides to form form four spermatids. one ootid and one polar body. 5. No polar body is formed. 5. Polar bodies are formed. 6. A spermatogonium forms four 6. An forms one ovum. spermatozoa. 7. Sperms are minute yolkless and 7. Ova are much larger often with yolk motile. and nonmotile. 8. It is generally completed in the testes 8. It is often completed in the female and thus mature sperms are released reproductive tract or in many animals in from the testes. water because oocytes are released from the ovaries.

Theory of ovulation • The process by which there is releasing of ovum from the ovary is termed as Ovulation.

• There are 5 theories of ovulation: • Muscular contraction • Turgor theory • Stigma theory • Enzyme action • Hormonal action • Muscular contraction: • In frog and rat, Ovum is released because of rupture of mature follicle by involuntary muscles fiber contraction. • But, histological structure of ovary reveals that there is no presence of muscle fibers in all animals.

• Turgor theory: • In mammals, there is accumulation of follicular fluid secreted in the cavity of follicle surrounding the egg, resulting into great increasing of internal pressure (turgor) as well as a stretching and thinning of follicle wall which finally rupture and liberating egg. • In birds, there is accumulation of large quantities of yolk resulting into ovulation. • Stigma theory: • In some vertebrates , notably in birds, there is whitish linear weaker area on the surface of follicle which devoid of blood vessels. • From this site, the ovum is liberated.

• Enzyme action: • When the follicle comes on the surface of the ovary, then enzyme act upon the follicle. • The enzyme digests and weakens the outer wall of follicle resulting into ovulation.

• Hormonal action: • The hormone (FSH) from anterior pituitary gland act upon the ovary and under influence of this hormone the ovum is released. • Rugh (1935) has demonstrated that in the frog, the hormones contract the muscle fiber in ovary and with help of enzyme action which weakens the follicle wall causing ovulation.