L1 - Gametogenesis_Shalini PDF

Gametogenesis HUMAN DEVELOPMENT HELEN PAGE Edited by: Dr Shalini Ratnasingam Outline Primordial germ cell Gametogenesis Meiosis I Meiosis II Birth defects Oogenesis Spermatogenesis Primordial Germ Cell (PGC) Gametes (sex cells) are derived from primordial germ cells (PGCs). Formed in epiblast during...

Gametogenesis HUMAN DEVELOPMENT HELEN PAGE Edited by: Dr Shalini Ratnasingam Outline Primordial germ cell Gametogenesis Meiosis I Meiosis II Birth defects Oogenesis Spermatogenesis Primordial Germ Cell (PGC) Gametes (sex cells) are derived from primordial germ cells (PGCs). Formed in epiblast during 2nd wk. Move to wall of yolk sac during 4th wk to developing gonads. Loading… Maturation process = gametogenesis + cytodifferentiation. ↓ gamete making. ↓ cell differentiation sell Chromosome Theory of Inheritance Human traits are determined by specific genes on chromosomes inherited from father and mother. 46 chromosomes & ~ 35, 000 genes. 22 pairs of matching chromosomes – autosomes 1 pair of sex chromosomes XX = female XY = male G diploid - haploid >. O Jerell Meiosis Meiosis has just one purpose in the human body: the production of gametes. Its goal is to make daughter cells with exactly half as many chromosomes as the starting cell. Loading… -2n > n - Meiosis is hence known as reductive division, a process that takes us from a diploid cell (2n)—one with two sets of chromosomes—to haploid cells (n)— ones with a single set of chromosomes. - of naploid. throughout Meiosis Chromosome number must be reduced prior to sexual reproduction to ensure that the new organism has the same number of chromosomes as the parents. Fertilization of the haploid egg and haploid sperm restores the diploid number of chromosomes. Meiosis Meiosis I Involves pairing of homologous chromosomes and separation of homologues into 2 cells which halves the chromosome number. Prophase I, Metaphase I, Anaphase I, Telophase I Meiosis II Involves separation of the two sister chromatids. Prophase II, Metaphase II, Anaphase II, Telophase II if we don't specify which cycle -accoming you're ring No. of chromosomes per nucleus = 46 (2n) No. of DNA molecules per nucleus = 92 (2x) Meiosis I – Prophase Early prophase I Chromatin starts to condense. Synapsis occurs where homologous chromosomes pair up, forming bivalents. - Tetrad = Bivalent 4 chromatids of homologous chromosomes 2 homologous chromosomes bivalent tetrad No. of chromosomes per nucleus = 46 (2n) No. of DNA molecules per nucleus = 92 (2x) Meiosis I – Prophase Late prophase I Chromosomes further condense (sister chromatids become visible). Chiasmata form (singular – chiasma) crossing over that occurs between non-sister chromatids of homologous chromosomes. No. of chromosomes per nucleus = 46 (2n) No. of DNA molecules per nucleus = 92 (2x) Meiosis I – Prophase ↑ of alt forms As a result of this crossing over, there is an exchange of corresponding alleles between non-sister chromatids (allelic recombination) genetic variation. -. genee No. of chromosomes per nucleus = 46 (2n) No. of DNA molecules per nucleus = 92 (2x) Meiosis I – Prophase By the end of prophase Nucleolus disappears. Nuclear envelope starts to fragment and disintegrate into vesicles. Centrosomes move to opposite poles of the cell. Spindle fibres form and attach to the kinetochore of each homologue. Loading… No. of chromosomes per nucleus = 46 (2n) No. of DNA molecules per nucleus = 92 (2x) Meiosis I – Metaphase Spindle formation is complete. Kinetochore microtubules direct homologous chromosome pairs towards the metaphase plate. - imaginary plate atthe cell centre of Both sister chromatids of each chromosome attach to kinetochore microtubules from just one pole of the spindle, and the two homologues of a pair bind to microtubules from opposite poles. No. of chromosomes per nucleus = 46 (2n) No. of DNA molecules per nucleus = 92 (2x) Meiosis I – Metaphase The arrangement of chromosomes of each homologous pair is random relative to the orientation of other homologous chromosomes (either the paternal or maternal part of the homologue pair can face either pole) Independent assortment of homologous chromosomes. No. of chromosomes per nucleus = 46 (2n) No. of DNA molecules per nucleus = 92 (2x) Meiosis I – Anaphase Kinetochore spindle fibres contract (micro tubule depolymerisation) and pull the homologous chromosome pairs towards opposite poles. > losing - over are taking place. Sister chromatids move as a single unit towards the same pole (this pair of sister chromatids is considered 1 chromosome) unlike in mitosis where sister chromatids separate. Non-kinetochore spindle fibres guide the movement of the chromosomes. No. of chromosomes per nucleus = 46 (2n) No. of DNA molecules per nucleus = 92 (2x) Meiosis I – Telophase Chromosomes (each consisting of 2 sister chromatids) reach opposite poles of the cell. Each pole has a haploid set of chromosomes (n). Spindle fibres disintegrate. In some species, the nuclear envelope re-forms and the chromosomes decondense, although in others, this step is skipped—since cells will soon go through another round of division, meiosis II. No. of chromosomes per nucleus = 23 (n) No. of DNA molecules per nucleus = 46 (x) - Meiosis I – Cytokinesis everything is halted. Cytokinesis usually occurs simultaneously with telophase I, forming two haploid daughter cells. In animal cells formation of cleavage furrow In plant cells formation of cell plate. furrow llevage In some species, cells go straight from anaphase I to prophase II. some special skip telephace * There is no interphase following meiosis I. (DNA replication does not happen twice!) cell plate Form from golg - - - No. of chromosomes per nucleus = 23 (n) No. of DNA molecules per nucleus = 46 (x) - begins with 2 haploid daughter cells and involves separation of sister chromatids (like mitosis) forming haploid daughter cells Meiosis II – Prophase Nucleolus disappears. Nuclear envelope starts to fragment and disintegrate into vesicles. Chromatin starts to condense to form chromosomes. Centrosomes duplicate and migrate to opposite poles. Spindle fibres start forming. (this time, with their axes perpendicular to the spindle axis is meiosis I) No. of chromosomes per nucleus = 23 (n) No. of DNA molecules per nucleus = 46 (x) Meiosis II – Metaphase Spindle formation is complete. Kinetochore microtubules direct sister chromatids towards the metaphase plate and align them in a single file. (sister chromatids are not genetically identical due to crossing over in prophase I of meiosisI) o Sister chromatids identical are not due to genetically crossing over No. of chromosomes per nucleus = 23 (n) No. of DNA molecules per nucleus = 46 (x) Meiosis II – Anaphase Centromere of chromosomes divide into 2. Cohesin proteins between sister chromatids are cleaved (by separase) allowing them to part. Kinetochore spindle fibres contract (via micro tubule depolymerisation) and pull sister chromatids apart towards opposite poles. - Non-kinetochore spindles elongate and lengthen the cell. Each sister chromatid is now a daughter chromosome. > - Mormocome. No. of chromosomes per nucleus = 23 (n) No. of DNA molecules per nucleus = 46 (x) Meiosis II – Anaphase Daughter chromosomes reach the poles of the cell. Nucleoli reappear in each daughter nucleus. Nuclear envelope reforms around chromosomes at each pole. Daughter chromosomes decondense and form chromatin. Spindle fibres disintegrate. No. of chromosomes per nucleus = 23 (n) No. of DNA molecules per nucleus = 23 (x/2) always cnromatin in from before angning Meiosis II - Cytokinesis Cytokinesis results in 4 genetically different haploid daughter cells. In animal cells formation of cleavage furrow. In plant cells formation of cell plate. Meiosis I vs Meiosis II Stages Meiosis I Meiosis II Interphase DNA replication in S phase of interphase Interphase absent Prophase Crossing over between non-sister chromatids of homologous chromosomes occur in prophase I No crossing over in prophase II allelic recombination results in genetic variation in the population Metaphase Homologous chromosomes align at the metaphase plate in pairs (XX) Chromosomes (genetically non-identical sister chromatids) align at the metaphase plate in a single file (X) Kinetochore microtubules from opposite poles attach to each homologue. Kinetochore microtubules from opposite poles attach to chromosomes (instead of homologues) Meiosis I vs Meiosis II Stages Meiosis I Meiosis II Anaphase Homologous chromosomes are pulled apart and move to opposite poles due to depolymerisation of kinetochore microtubules (centromeres do not divide) Sister chromatids are pulled apart when centromeres divide and they move to opposite poles due to depolymerisation of kinetochore microtubules Telophase Chromosomes may not decondense to form chromatin if cells go from anaphase I straight into prophase II. Chromosomes decondense to form chromatin Nucleolus and nuclear envelope reform. Cytokinesis 2 haploid daughter cells are formed 4 haploid daughter cells are formed (gametes) Each chromosome is a duplicate one (X) Each chromosome is a single copy ( I ) ( I ) Meiosis I vs Meiosis II > DNA ↳ sel mo has not divided. Meiosis I Meiosis II Significance of meiosis 1. 1. Formation of haploid gametes for sexual reproduction At fertilization, the fusion of the haploid spermatozoon and haploid oocyte produces a zygote which restores the diploid condition of the cell. Ensures that ploidy is kept constant with each successive generation. Genetic variation in gametes and in population through: I. Crossing over between homologous chromosomes in Prophase I II. Independent assortment of chromosomes (Mendel’s law) in Metaphase I and II III. Random fusion of haploid gametes during fertilisation - Birth defects – Numerical Abnormalities Euploid – exact multiple of n chromosomes (haploid = n, diploid = 2n) Aneuploid – any chromosome number that is not euploid (e.g. trisomy or monosomy) Loading… Translocations – chromosome breaks and attaches to another. QUESTION – What genetic defects do you know of which are due to alterations in chromosome number? Birth defects – Numerical Abnormalities Down’s syndrome – trisomy 21 lower-than-normal ratio of the skull's length to its width 1 in 2000 conceptuses for women < 25, 1 in 300 for women at 35, 1 in 100 for those who are 40. Growth retardation, varying degrees of mental retardation, craniofacial abnormalities, cardiac defects. Curved fingers skin fold of the upper eyelid covering the inn corner of the ey Birth defects – Numerical Abnormalities Trisomy 18 / Edwards syndrome Approx. 1 in 6000 newborns. ~10% may survive past 1st year. Mental retardation, congenital heart defects, low-set ears, flexion of fingers/hands. Lower jaw undersized Birth defects – Numerical Abnormalities Trisomy 13 (Patau syndrome) Approx. 1 in 3000 and 1 in 8000 incidence rate. 3:1 female:male predominance. > 80% don’t survive past 1 month. Mental retardation, congenital heart defects, deafness, cleft lip/palate, eye defects, failure of forebrain to develop properly. absence of the epidermis, dermis, and occasionally subcutaneous tissue Birth defects – Numerical Abnormalities Klinefelter syndrome Cells have 47 chromosomes – XXY. Approx. 1 in 500-600 males. Symptoms detected at puberty – sterility, testicular atrophy, excess breast growth. Birth defects – Numerical Abnormalities as Turner syndrome cosmon Only monosomy compatible with life. chromosome Cells have 45 chromosomes – one X. Approx. 50 in 100, 000 live births (reported 98% of foetuses are spontaneously aborted). Individuals are female in appearance – no ovaries, short stature, broad chest. -female ~ has mental comprimise. Birth defects – Structural Abnormalities Usually involve one/more chromosomes. Usually chromosome breakage – caused by environmental factors including viruses, drugs, radiation. Examples include: Angelman syndrome or Prada-Willi syndrome Miller-Dieker syndrome Velocardiofacial syndrome Fragile X syndrome Oogenesis e99) PGCs reach gonad of genetic female differentiate into oogonia. Multiplication of cells (end of 3rd month) – clusters surrounded by layer of flat epithelial cells (follicular cells). Majority continue to multiply (by 5th month, max. total of ~7 million). Some enter meiosis, but pause at prophase I I multiply & surrounded by primary oocyte. folicular cells. Oogenesis After 5th month, cell death begins majority degenerate. Those that survive are near surface of the cluster and have entered prophase I. Primordial follicle = primary oocyte + follicular cells. Oogenesis – Maturation At birth – primary oocytes enter resting stage (diplotene stage). Held in this state by oocyte maturation inhibitor (OMI) produced by follicular cells. At puberty – pool of growing follicles created and continuously maintained from the primordial follicles. ~15-20 follicles mature each month, but usually only one reaches full maturity. 1) primary / preantral 2) secondary / antral / vesicular / Graafian 3) preovulatory Oogenesis – Maturation Primary oocyte grows, follicular cells change shape to cubes and multiply primary follicle. ↳ epithel to cubridal shape. Secretion of glycoproteins create zona pellucida. - I Fluid-filled spaces appears to form antrum cocyte secondary follicle. > ovulation isdre hy - self it of - Surge in lutenizing hormone (LH) induces preovulatory growth stage. increase LH will mature much facter becoming > oocytes - to Meiosis I is completed. thicken. Oogenesis Meiosis I complete BUT daughter cells are unequal. One cell receives most of the cytoplasm secondary oocyte Second cell receives practically none polar body meced having more cytoplane than the other - Oogenesis Meiosis II – creation of more polar bodies. Meiosis II paused at metaphase (~3 hours before ovulation). Meiosis II only complete if oocyte is fertilised. Degeneration of cell ~ 24 hours after ovulation. Oogenesis Spermatogenesis Spermatogenesis maturation of spermatogonia into spermatozoa Begins at puberty Takes place in seminiferous tubules Regulated by luteinizing hormone (LH) Spermatogenesis 1) Spermatocytogenesis Diploid spermatogonium multiplies via mitosis Two primary spermatocytes produced (2n) Go on to meiosis I secondary spermatocyte (n) 2) Spermatidogenesis Meiosis II – creates spermatids (n) Cells remain connected by cytoplasmic bridges Allows further synchronous development > - fromgogi Spermatogenesis 3) Spermiogenesis Microtubules grow on one centriole tail begins to form Anterior part of tail (midpiece) thickens mitochondria concentrated here. DNA is condensed / packaged Acrosome formed – derived from Golgi and covers half of spermatozoa head. otility Testosterone causes maturation moving by ring your Cytoplasm and organelles removed energy MATURE BUT NO MOTILITY mubility. help you ning Released into lumen of seminiferous tubule (spermiation) -and. oar to Spermatogenesis Transported to epididymis in testicular fluid. Spermatozoa gain motility (& now fertile). Spermatozoa released by muscle contraction during ejaculation. Spermatogenesis