Meiosis Formation of gametes egg & sperm Occurs in ovaries and testes (germ cells) Produces cells with haploid chromosome number (n)
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Meiosis
Formation of gametes
egg & sperm
Occurs in ovaries and testes (germ cells)
Produces cells with haploid chromosome number (n)
Meiosis
Allows one random copy of chromosome to be placed in each egg and each sperm cell —> GENETIC DIVERSITY
Law of Independent Assortment
Meiosis
Replication of chromosomes occurs first
Series of two separate stages of cell division
Involves synapsis of homologous chromosomes
Joining of homologous chromosomes to form tetrads
Synapsis/Tetrad
Crossing Over
Occurs during Prophase I
At chiasmata, two non sister chromatids, bond, break and rejoin to each other
Allows different combinations of genes
Gene Linkage Map
Crossing over is random- Chance of crossing over is equal at all points on a chromosome
Farther apart genes = greater chance of crossing over
Greater recombination frequency
Distance between genes = map units
Linked Genes
Meiosis Simulation
You need: One blue string of beads, one orange string of beads, 4 centromere magnets
Work in partners
Follow steps in Exercise 3B
Optional: Answer questions for further review
Meiosis- Genetic Diversity
Results from crossing over and independent assortment of chromosomes
Enormous advantage for sexual reproduction —> EVOLUTION
Meiosis- Results
Produces four gametes
Each has a different genetic combination
Spermatogenesis
Continues throughout a male’s life
Forms four, equal-sized and diverse cells
Starts with spermatogonium
Prophase I- Primary spermatocyte
Prophae II- Secondary spermatocyte
Spermatogenesis
Sperm Cell
Oogenesis
Begins with oogonium but stops at prophase I
Cells now called primary oocytes (present at birth)
Primary oocytes remain quiet until puberty
Hormones (FSH) trigger meiosis to begin again, but stop at metaphase II = secondary oocyte
Each month, secondary oocyte is released from ovary (ovulation)
Meiosis completes ONLY if fertilization occurs
Differences
Females are born with all primary oocytes
Development is interrupted
Produces 3 polar bodies (disintegrate) due to uneven cytokinesis
Oogenesis
Comparison
Human Embryonic Development
Day 1- Fertilization (one cell)
Day 1.5- 2 cells
Day 2- 8 cells
Day 4- 64 cells
Morula
Leaving fallopian tube, moving to uterus
Human Embryonic Development
Day 6- Blastula (filled with salt water)
Most identical twins split on this day (1 out of 150 embryos split)
Implanting in uterus begins
Day 10- Implanted in uterus
Placenta is developing
Day 21- First heartbeat
Umbilical cord attaches
Human Embryonic Development
Day 26- Spinal tube is closing
Day 28- Kidneys process water & contribute to amniotic fluid
Days 32-35- Face forming
Karyotype
Pictomicrograph of chromosomes in metaphase
Can be used to diagnose chromosomal conditions
Usually taken from white blood cells
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