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