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Discovery
• Edouard Joseph Louis Marie Van Beneden– Observed cells in roundworms and noticed the gametes
had half the number of chromosomes as the somatic cells
– Fusion of gametes = fertilization/syngamy; forms zygote
• Even before discovery scientists knew there was some mechanism for halving the number of chromosomes– Otherwise, the next generations would have double the
chromosomes
• Meiosis is the process of reduction division
Sexual Life Cycle
• Diploid cells have two sets of each chromosome– Found in somatic cells of
adults• Haploid cells have only
one of each chromosome– Found in sex cells (egg &
sperm)• Sexual reproduction
– Process of producing offspring by the fusion of haploid cells produced by meiosis from diploid cells
Sexual Life Cycle
• Somatic tissues– After fertilization, the
single diploid cell divides by mitosis
• Single cell gives rise to all cells
– These cells called somatic meaning body (2n)
– Divide by mitosis• Germ-line tissues
– Cells predestined to be gametes are set aside early in development
– Divide by meiosis
Features of Meiosis
• Synapsis– Pairing of homologues early in
first nuclear division• Homologous recombination
– Crossing over• Exchange of genetic information
between homologues
• Reduction division– Chromosomes do not duplicate
between the two nuclear divisions of meiosis, thus each resulting cell has half the original number of chromosomes
Sequencing Meiosis
• The first division (Meiosis I)– Prophase I
• Leptotene– Chromosomes condense tight (4 chromatids for each type of chromosome)
• Zygotene– Protein lattice laid down between homologues (synapsis)– Called synaptonemal complex• Pachytene– Synaptonemal complex holds homologues
together gene to gene– DNA unwinds at certain sites allowing
bases to pair with complimentary bases from the other chromosome (crossing over)
– Recombination nodules assist in recombination
– Crossing over evidence = chiasmata (seen through microscope)
Sequencing Meiosis (cont)
• Diplotene– Synaptonemal complex breaks down– Chromosomes decondense/transcription occurs
• Diakinesis– Transcription stops/chromosomes condense
Sequencing Meiosis (cont)
– Metaphase I• Chiasmata move down the
chromosomes until it reaches the end holding the homologues together
• Forces one side of centromere outward
• Spindle is then only able to attach to one centromere per chromosome
• Homologues then line up on metaphase plate
• Orientation of pair is random (mom’s doesn’t have to stay with mom’s and visa versa) = independent assortment
Sequencing Meiosis (cont)
– Anaphase I• Spindles shorten and break chiasmata pulling
centromeres to opposite poles• One chromosome from the homologous pair is
now on opposite sides
– Telophase I• Nuclear envelope reforms• Because of crossing over, sister chromatids no
longer identical• Cytokinesis may occur
Sequencing Meiosis (cont)
• The second division (Meiosis II)• Brief interphase (no DNA synthesis)• Resembles normal mitotic division
– Prophase II• Nuclear envelope breaks down/spindle reforms
– Metaphase II• Spindle fibers bind to both sides of centromere
– Anaphase II• Fibers contract spiltting centromeres and moving
sister chromatids to opposite poles– Telophase II
• Nuclear envelope reforms
Result of Meiosis
• Four haploid cells• No two cells are alike because of crossing over
and independent assortment• These haploid cells can
– Directly become gametes (animals)– Divide mitotically producing more haploid cells making
even more gametes (fungi, plants, protists)
Origin of Sex
• Unknown– Asexual reproduction works
• Bacteria, protists, plants, some animals (budding)
– Parthenogenesis• Development of adult without fertilization• Arthropods (females diploid, males haploid)• some amphibians and lizards (gamete goes
through mitosis, but doesn’t divide forming a diploid cell)
Origins of Sex (cont)
• Some species benefit from variability generated from meiosis
*However, the more specifically adapted an organism is, the more likely its offspring will not benefit from genetic variability that comes from meiosis; the more likely that offspring will suffer and be less likely to survive.
• Then why do we have sexual reproduction?
Sex and Evolution
• Read– DNA Repair Hypothesis (pg 236)– The Contagion Hypothesis (pg 236)– The Red Queen Hypothesis (pg 237)– Miller’s Ratchet (pg 237)
• Sexual reproduction has a huge impact on evolution
• ***Test soon to follow***