1. Life Cycles: Meiosis and the Alternation of Generations
Chapter 12

2. Life Cycles Transfer of genetic information from parent to offspring
Two types of reproduction
Asexual reproduction
Sexual reproduction

3. Alternation of Generations
Refers to creation of both diploid and haploid bodies
Cherry tree life cycle
Cherry tree
Diploid part of life cycle
Referred to as sporophyte
Makes reproductive units called spores
Spores – one celled reproductive unit that can develop into new plant without mating with another organism

4. Alternation of Generations
Forms two kinds of spores
One kind develops into male haploid plant that makes gametes called sperm cells
Other kind develops into female haploid plant that makes a gamete called an egg

5. Alternation of Generations
Male gametophyte formation occurs in pollen sacs of anthers
Meiospores (produced by meiosis) divide by mitosis to form male gametophyte → pollen grain
Pollen grains released from anther
Pollen reaches stigma of female flower part
Pollen grain grows pollen tube
Contains two sperm nuclei (male gametophyte is now mature)

6. Alternation of Generations
Female gametophyte formation occurs in ovary
Chambers of ovary lined with ovules
Single ovule undergoes meiosis, produces 4 haploid cells
3 of the 4 cells degenerate
1 remaining cell matures into female spore (meiospore)
Meiospore remains in ovule where it divides by mitosis
Resulting cells divide 2 more times by mitosis to make a 7-celled female gametophyte

7. Alternation of Generations
Pollination
Transfer of pollen to tip of pistil
Pollen tube reaches egg
One sperm fuses with egg to form zygote
Plasmogamy → fusion of cytoplasmic contents
Karyogamy → fusion of nuclei
Other sperm fuses with polar nuclei to form endosperm

8. Alternation of Generations
Zygote divides mitotically
Forms embryo within seed coat
Small sporophyte that will become cherry tree when seed germinates

10. Embryophytes Embryophytes
Plants that shelter their offspring as embryos within parental body

11. Sexual Cycles Sexual reproductive cycles can be of two types
Heterosporic
Makes two kinds of spores and gametophytes
One spore produced in large numbers
Small enough to be carried far away
One spore too heavy to travel
Contains plenty of food

12. Sexual Cycles Homosporic Makes one kind of spore and gametophyte
Spores too small to travel far
Most mosses and plants such as ferns
Plants not important in our food supply except as emergency foods

13. Comparison of Heterospory and Homospory
Makes 2 kinds of spores and gametophytes
1 spore produced in large numbers and small enough to be carried far away, other spore too heavy to travel far but contains plenty of food
Seeds produced are part of our basic food supply
Homospory
Makes 1 kind of spore and gametophyte
Spores too small to travel far
Only important in human food supply as emergency food

14. Types of Life Cycles Zygotic or gametic life cycle Sporic life cycle
Life cycle that lacks sporophyte
No multicellular 2n stage
Example: Chlamydomonas (green alga)
Sporic life cycle
Life cycle that includes alternating sporophyte and gametophyte bodies
All embryophytes, mosses

15. Zygotic of Gametic Life Cycle
Gametophytes
Single, motile cells with haploid nucleus
Genetically exist as plus or minus mating types
Gametophyte nucleus occasionally undergoes mitosis and produces haploid spores
Parent cell bursts
Releases spores that develop into new gametophyte generation cell

16. Zygotic of Gametic Life Cycle
Plus and minus mating types can mate
Plasmogamy and karyogamy occur
Results in 2n zygote
Zygote eventually undergoes meiosis
Releases haploid cells
Each cell matures into either a plus or minus gametophyte generation cell

17. Gametic Life Cycle Example: Fucus (brown alga)
Begins with multicellular sporophyte
Large and complex
Within body cavities of sporophyte
Cells enlarge, become sporangia, nuclei of cells undergo meiosis
1 type of sporangium produces large meiospores
Other type of sporangium produces small meiospores

18. Gametic Life Cycle Gametes released into surf in large numbers
Large meiospore differentiates into female gametophyte (egg)
Smaller meiospores differentiate into male gametophytes (sperm)
Gametes released into surf in large numbers
Eggs from one parent and sperm from another parent fuse
Egg and sperm from same plant not attracted to each other

19. Gametic Life Cycle Plasmogamy and karyogamy occur
Zygote begins to divide and grows into sporophyte
Sporophyte enlarges, sinks to bottom, attaches to rock, grows into maturity
Only haploid phase is a single-celled gamete
No multicellular gamete generation in a gametic life cycle

20. Dominant Diploid Generation
Gametic and zygotic life cycles common among algae but absent from any more advanced plants
Sporic life cycles are rule among complex terrestrial plants
Increasing dominance by sporophyte in groups more recent in fossil record

21. Dominant Diploid Generation
Diploid condition
Permits recessive genes to be carried along from generation to generation
Could be valuable to species future
No recessive genes in haploid cells of gametophytes
Only 1 set of chromosomes
Every gene’s expression shows through in this phase

22. Dominant Diploid Generation
Dominance
Means sporophyte lives longer, is larger, is more structurally complex, and is more independent than gametophyte