Sexual Reproduction

Chapter 12

AP

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***