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

Asexual Reproduction Each generation genetically identical to last generation Occurs in unicellular and multicellular organisms

Asexual Reproduction Requires only a single parent cell or parent organism –All progeny identical to parent –Collection of identical organisms → clone Examples –Strawberry plants → stolons or runners –Redwood tree → dormant buds under bark at base of tree are stimulated to grow when tree is killed by fire or removed by timber harvest –Trembling aspen → produce special roots that grow horizontally, roots periodically give rise to stems some distance from parent tree

Asexual Reproduction Advantages –Only single parent is requires –Produces offspring that may be just as successful in habitat as parent was –Generates offspring faster than sexual reproduction –Costs less in terms of metabolic energy than sexual reproduction

Asexual Reproduction Disadvantage –Genetic diversity remains fixed

Mitosis Asexual cell division Chromosomes duplicated prior to mitosis Sister chromatids joined at centromere

Review of Mitosis Stage of MitosisDescription Prophase Chromosomes condense, spindle apparatus forms Metaphase Cell pauses to check that chromatids are correctly linked, chromosomes move to cell’s equator Anaphase Spindle fibers pull one sister chromatid of each chromosome to each pole Telophase Chromosomes uncoil, new nuclear enveloped form Cytokinesis Cell divides into two cells

Sexual Reproduction Each generation is genetically different Problems associated with sexual reproduction –Ways to bring gametes together –Reduction in chromosome number Requires union of two cells called gametes –Union of gametes restores normal chromosome number

Sexual Reproduction Chromosome set –Consists of one chromosome of each kind Haploid –Any cell with just one set of chromosomes –1n Diploid –A cell with 2 chromosome sets –2n –Zygote formed by union of two gametes is diploid

Sexual Reproduction Homologous chromosomes –Two copies of a given chromosome –Make up pair –Each carries same information

Meiosis Type of cell division which reduces chromosome number by half Before entering meiosis, 2n cell duplicates its DNA –Each resulting chromosome has two connected copies (sister chromatids)

Meiosis Carries out two rounds of cell division –Meiosis I Converts original 2n cell to two 1n cells with different combinations of parental genes –Meiosis II Mitotic division that separates sister chromatids and converts two 1n (haploid) cells to four 1n (haploid) cells

Meiosis I StageDescription Prophase I Chromatids condense, synapsis occurs → each homologous pair of chromosomes comes together, pairing makes it easy for cell to divide in a way that it produces haploid cells, crossing over → allow homologous chromosomes to trade segments, synapsis and crossing over give chromosomes new combinations of parental genes, spindle forms Metaphase I Pause for checking for missing links between chromosomes and spindle, chromosome pairs move to cell’s equator Anaphase I Spindle pulls each chromosome with its two sister chromatids to one of the poles Telophase I and Cytokinesis Creates new nuclear envelopes, cells divide into 2 haploid cells, each cell has different combination of parental genes, each chromosome still has two sister chromatids

Meiosis II StageDescription Prophase II No synapsis, no crossing over, each cell forms new spindle that links each sister chromatid of each chromosome Metaphase II Chromosome moved separately to equator, cell pauses to check for spindle linkage Anaphase II In each cell, spindle pulls the two sister chromatids of each chromosome to opposite poles Telophase II and Cytokinesis Each cell divides into two cells, each cell is haploid with different combinations of parental genes

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 –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