Life Cycles: Meiosis and the Alternation of Generations

Chapter 12

Life Cycles

• Transfer of genetic information from parent to offspring

• Two types of reproduction– Asexual reproduction– Sexual reproduction

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

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

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)

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

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

Alternation of Generations

• Zygote divides mitotically– Forms embryo within seed coat– Small sporophyte that will become cherry tree

when seed germinates

Embryophytes

• Embryophytes – Plants that shelter their offspring as embryos

within parental body

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

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

Comparison of Heterospory and Homospory

• Heterospory– 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

Types of Life Cycles

• Zygotic or gametic 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

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

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

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

Gametic Life Cycle

• 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

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

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

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

Dominant Diploid Generation

• Dominance– Means sporophyte lives longer, is larger, is

more structurally complex, and is more independent than gametophyte