BIOL 111 Chapter 13
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Genetics: Meiosis
P. 248-60
Specialized process of mitosis
Review of Mitosis
G1 → S → G2 → Prophase → Metaphase → Anaphase → Telophase → Cytokinesis → G1 (repeat)
- Genetics
- Study of heredity and changes in DNA
- Asexual reproduction
- creating a clone of an organism
- Sexual reproduction
- reproduction with genetic variation
Life cycle [vocab 1]
Different basic stages in an organism's life:
- Birth
- Reproduction ← important step
- Death
- haploid [vocab 2] state (1n)
- one set of chromosomes (sperm and egg)
- some organisms (plants, algae, and fungi) have multicellular haploid stages
- diploid [vocab 3] state (2n)
- two sets of chromosomes (normally full grown)
- humans are diploid: 1 set from mom, 1 set from dad
- Mitosis
- n ⇒ n + n
- 2n ⇒ 2n + 2n
- Meiosis
- 2n ⇒ n + n
- Fertilization
- n + n ⇒ 2n
Differences among organisms
Animals have a short haploid state
Plants and some algae have equal haploid and diploid states
Most fungi and some protists have short diploid states
Terms to know
Homologous Chromosomes [vocab 4] are two copies of the same chromosome (not replicated sister chromatids)
Sex chromosomes [vocab 5] (1 pair in humans) determine the sex of the organism
Autosomes [vocab 6] (22 pairs in humans) are all the remaining chromosomes
Gametes [vocab 7] are haploid cells
Zygotes [vocab 8] are diploid (fertilized) cells
When a diploid cell is ready to go through mitosis, it will have four copies of the same type of chromosome: 2 sister chromatids of each of the 2 homologous chromosomes
Process
Interphase
Result: 2n, 4 copies
Primordial germ cell undergoes regular mitosis. One copy is sent into Meiosis I
Meiosis I
Result: 1n, 2 copies
Homologous chromosomes are separated and the germ cell divides into two haploid cells
Prophase I
- Synapsis [vocab 9]
- homologous chromosomes pair up
- Cross-over [vocab 10]
- homologous chromosomes exchange DNA at chiasmata [vocab 11]
Metaphase I
Anaphase I
Telophase I
No chromosome replication at this point
Meiosis II
Result: 1n, 1 copy
Sister chromatids separate, creating four total gamete cells.
Prophase II
Same as Prophase I, only no crossing over
Metaphase II
Anaphase II
Telophase II
Genetic variation probability
- Independent assortment during Metaphase I (random division of homologous chromosomes)
- 223 = 8,388,608 different variations
- Crossing over during Prophase I
- 1-3 crossings/chromosome for 2 chromosomes (3×2 = 6)
- Random fertilization for sperm and egg
- (223 × 6) × (223 × 6) = 2,533,274,790,395,904
How amazingly unlikely is your birth!
Vocabulary
- ↑ an organism's life cycle is its steps of reproductive development
- ↑ haploid cells have only one set of chromosomes (like sperm and egg cells)
- ↑ diploid cells have two sets of chromosomes (one from mom, one from dad)
- ↑ homologous chromosomes are a pair of chromosomes (one from mom, one from dad) with same length, centromere position, and genetic content.
- ↑ sex chromosomes determine whether the organism is male or female
- ↑ autosomes are all of the other chromosomes besides the sex chromosomes
- ↑ gametes are haploid cells (sperm and egg)
- ↑ zygotes are diploid cells that have been fertilized
- ↑ synapsis is the point during prophase I at which the homologous chromosomes pair up
- ↑ cross-over is the point during prophase I at which the genetic material is exchanged between homologous chromosomes
- ↑ chiasmata is the location at which the chromosomes cross over during prophase I