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Sexual Reproduction and Meiosis. Meiosis ≠ sexual reproduction! Meiosis makes the cells that are responsible for sexual reproduction. Sexual Reproduction. Producing a new organism by combining chromosomes from 2 parents Specialized sex cells = _____________ Male = ___________
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Meiosis ≠ sexual reproduction! • Meiosis makes the cells that are responsible for sexual reproduction
Sexual Reproduction • Producing a new organism by combining chromosomes from 2 parents • Specialized sex cells = _____________ • Male = ___________ • Female = _________ • Union of gametes = _________________ which produces a ______________ gametes sperm egg fertilization zygote
Gametes • Each sex cell of a multicellular organism contains the ___________ (n) number of chromosomes characteristic of that species (in humans, n = 23) • These chromosomes are NOT present in pairs • Gametes are produced during _____________ in the ___________ mono/haploid gametogenesis gonads
Male gonads = _________ • Female gonads = _________ • The process that creates monoploid gametes is ____________ • Organisms that contain both male & female gonads are called _____________ • Ex. Earthworms testes ovaries meiosis hermaphrodites
Meiosis AKA “Reduction Division” • A cellular division in which the number of chromosomes is reduced by half (2n → n) • Why do gametes need to have a monoploid number of chromosomes? • In order for fertilization to produce a normal 2n zygote, the sperm & egg must have only half the # of chromosomes • n + n 2n haploid haploid diploid sperm egg zygote fertilization
mitosis Meiosis I Meiosis II • To establish this monoploid # of chromosomes, sperm and eggs undergo meiosis • Similar to mitosis, but meiosis has one additional ___________ division Laser clip
1st Meiotic Division • Interphase I: • Chromatin replicates • Single stranded → double stranded • Prophase I: • Chromatin coils → chromosomes • Homologous chromosomes pair up during synapsis & form tetrad (group of 4 chromatids) • Chromosomes may twist & exchange genes = crossing over
Metaphase I: • Tetrads align @ equator (different than mitosis!! No tetrads in mitosis) • Spindles attach to centromeres • Anaphase I: • Tetrads split • Homologous pairs move to opposite poles (disjunction) • New cells: monoploid & double stranded • Non-disjunction: when tetrads fail to separate leaving more/less chromosomes in each new cell) • Telophase I: • Cytoplasm divides • 2 new daughter cells that are monoploid w/ double stranded chromosomes
2nd Meiotic DivisionJust like mitosis w/o chromosome replication • Prophase II: • Spindle fibers form & attach to centromeres • Metaphase II: • Chromosomes line up on equator • Anaphase II: • Double stranded chromosomes split & chromatids move to opposite poles & cells are now single stranded • Telophase II: • Both daughter cells divide forming 4 monoploid cells
End Result of Meiosis • 1 diploid primary (1o) sex cell (2n) ↓ • 4 monoploid gametes (n)
Spermatogenesis: • The production of sperm in the testes • 4 monoploid sperm produced from each primary spermatocyte
Oogenesis: • The production of eggs (Latin: eggs = ova) in ovaries • 4 monoploid eggs produced from each primary oocyte BUT…only one is viable (survives) • The other 3 are very small & are called polar bodies. They degenerate in the female’s body.
Unlike spermatogenesis, oogenesis is NOT continuous: • Before birth, oogonia divide by mitosis to produce the limited lifetime supply of oogonia (which develop into oocytes) • In sexually mature females, one primary oocyte develops into a viable egg approximately every 28 days .
Egg Monoploid Female ovaries Non-motile (sessile) Fewer in # (1 per meiotic division) Large Yolk Sperm Monoploid Male testes Motile Many in # (4 per meiotic division) Small No yolk Egg vs. Sperm
After gametes are produced, the next step in reproduction is FERTILIZATION: • The union of a monoploid (n) sperm w/ a monoploid (n) egg resulting in a diploid (2n) ZYGOTE n + n = 2n • Requires fluid medium for sperm to swim to egg • When a sperm comes in contact with an egg, the acrosome (covers head of sperm) releases enzymes that dissolve an opening into the egg
External The union of sperm & egg OUTSIDE of the body of the female Occurs mainly in aquatic animals Requires large #’s of eggs to insure survival because there is little protection for the egg Ex. Amphibians Internal Union of sperm & egg INSIDE the body of the female Occurs in most terrestrial vertebrate animals Smaller # of offspring & fewer # eggs released because the egg is protected inside the mother Ex. Mammals Sites of Fertilization
In vitro fertilization: • Fertilization outside of the female’s body • Zygote implanted into female after fertilization • Vitro = glass
I say, thank goodness for Veterinarians & IVF: No more singles’ bars, no more dating!
Parthenogenesis: • The development of an egg into a mature organism WITHOUT fertilization by sperm • Therefore all offspring look identical to the mother • Ex. Bees, artificial stimulation of frog eggs, sea urchins, rabbits & turkeys • Parthenogenesis is the “asexual reproduction” in normally sexually reproducing organisms.
mitosis fertilization meiosis
diploid Monoploid/haploid Chromosome number