Meiosis Premedical- Biology

Reproduction The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms. Asexual reproduction is not limited to single-celled organisms - most plants - binary fission- Bacteria - budding - yeasts and Hydras - conjugation - bacteria may exchange genetic information - parthogenesis, fragmentation and spore formation - hermaphroditic species can self-fertilize

Parthenogenesis - lower plants (where it is called apomixis) - invertebrates - water fleas, aphids, some bees and parasitic wasps - vertebrates - some reptiles, fish, and very rarely birds and sharks baby hammerhead Wingless female giving birth

Sexual reproduction by combination of genetic material contributed from two different members of the species Each contributes half of the offspring's genetic material male produce sperm or microspore in anisogamous species and female produce ovum or megaspore In isogamous species the gametes are similar or identical in form, but may have separable properties and then may be given other different names.

Sexually reproducing organisms have two sets of genes for every trait called alleles, offspring inherit one allele from each parent the offspring is combination of parental genes diploid and haploid phases alternation in cell lines inbreeding = is the reproduction from the mating of two genetically related parents inbred line – origin of homozygote offsprings by sexual reproduction

Human life cycle

= division of germ cells - reduction of diploid chromosomal number to haploid - origin of haploid gamets Meoisis

n M I = reduction 2n = heterotypic n Prophase I: 1.leptotene – spiralisation of chromosomes 2.zygotene – pairing of homologous chromosomes – synapsis - bivalents synaptonemal complex First meiotic division

Synaptonemal komplex = three levels of protein structure

3. pachytene – each chromosome has two chromatids – tetrads crossing-over = reciprocal exchange of homologous parts of non-sister chromatids = recombination of maternal and paternal genetic material Recombination possibilities between non-sister chromatids and result products- two chiasmata on one bivalent a - recombination only between 2 chromatids; b - 4 chromatids in recombination; c - 3 chromatids in recombination.

4. diplotene – separation of homologs connection only in sites of chiasmata, prerequisite of crossing-over = chiasma formation 5. diakinesis – terminalisation of chiasmata maximal contraction of chromosomes Metaphase I – bivalents in equatorial plane of the cell without splitting of centromers

Anaphase I – migration of homologous chromosomes to the opposite poles - randomly according their parental origin Telophase I - chromosomes to the opposite sides Cytokinesis – division of cytoplasm- equal in spermiogenesis, unequal in oogenesis Interkinesis – without replication, abbreviated interphase

n M II – homeotypic n = mitotic division n in Prophase II division and separation of centriols - mitotic spindle in Metaphase II splitting of centromers in Anaphase II separation of chromatids Second meiotic division

– formation of gametes - migration of primordial germ cells to the gonads - during early fetal development number of mitotic division Gametogenesis

Spermatogenesis in the time of sexual maturity (top off testosteron threshold) – continuous process growth MI mitotic division - spermatogonia primary spermatocyte (dipl) M II 2 secondary spermatocytes (hapl) 4 spermatids diferentiation-spermiogenesis 4 sperm 1 cycle = about 10 weeks

Oogenesis M I to the end of prophase 3 month of fetal life diplotene=dictyotene growth at the time of birth Oogonia primary oocyte (dipl) mitotic division in the time of sexual maturity M II M I continued secondary oocyte ovulation in metaphase II +1st polar body anaphase II + telophase II only after fertilization + 2nd polar body oocyte

oogonia Mitotic division primary oocyte secondary oocyte growth M I M II meiosis 1 st polar body 2 nd polar body 3 rd month of fetal life - Dictyotene MI - division stops at time of birth Sexual maturity- Metaphase MII - ovulation Fertilization – pronucleus Ovum after MII - pronucleus Anaphase II, Telophase II only after fertilization prenatal zygote egg cell Sexual maturity Oogenesis

oogonia primary secondary oocyte polar body 2n n I. meiotic division fertilization a II. Meiotic division Oogenesis a fertilization

Differences in gametogenesis Male Female Initiation Puberty Early embryonic life Duration cca 72 days years Numbers of mitoses in gamete formation Gamete production 4 spermatids 1 ovum+3 polar bodies Gamete production million 1 ovum per menstrual per ejaculate cycle

Degeneration of germ cells in ovary 5 th month of fetal life 7 x 10 6 of cells time of birth 2 x 10 6 of cells puberty of cells ovulated 400 of cells Long period between M I and ovulation = factor of high probability of nondisjunction in older mothers increased risk of nondisjunctions

Consequences of meiosis 1.reduction of diploid chromosomal number to haploid 2. segregation of alleles in M I, M II (2st Mendel´s law) 3. random assortment of homologues – random combination maternal and paternal chromosomes in gametes (3nd Mendel´s law) 4. increase of genetic variability by crossing over (chromatids with segments of maternal and paternal origin)

Genetic determination of sex 23,X 23,Y 23,X

If you feel small, low-spirited and useless, remember: Thanks to perfect Ovum and the fastest Sperm and their genetic information, YOU ARE ABSOLUTELY UNIQUE and NO REPEATABLE ELEMENT THE WHOLE UNIVERSE

Campbell, Neil A., Reece, Jane B., Cain Michael L., Jackson, Robert B., Minorsky, Peter V., Biology, Benjamin-Cummings Publishing Company, 1996 – Thank you for your attention