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 = ___________ 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

Similar to mitosis, but meiosis has one additional ___________ To establish this monoploid # of chromosomes, sperm and eggs undergo meiosis Similar to mitosis, but meiosis has one additional ___________ division mitosis Meiosis I Meiosis II Laser clip

1st Meiotic Division Interphase I: Prophase 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: Anaphase I: Telophase 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 Division Just 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)

Comparison of Mitosis & Meiosis Ist step of division Replication of chromosomes same Type of reproduction Asexual & growth Used to make gametes for sexual reproduction Type & # of daugher cells compared to parent cell 2; same as parent (2n) 4, 1/2 chromosomes of parent (n) # of divisions 1 2 synapsis? no yes

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 vs. Sperm 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

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

What type of asexual reproduction is this?

What’s wrong with this picture?

Sites of Fertilization 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

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