Meiosis and the Alternation of Generations Life Cycles:

Life Cycles Transfer Genetic Information Plant reproduction can be either asexual or sexual reproduction. Both transfer genetic information from parent to offspring. Some species can do both. Each generation is genetically identical to the previous generation. Asexual Reproduction – Any type of reproduction NOT involving the union of gametes (sex cells) or meiosis. Each generation is genetically identical to the previous generation. This type of reproduction requires only one parent in a sense. Clone- a group of genetically identical individual organisms produced asexually. Plants can clone themselves or reproduce asexually by vegetative propagation. In this process, a piece of the vegetative part of a plant, the root, stem or leaf, produces an entirely new plant genetically identical to the parent plant. Trees can be clones of each other and share the same root system. Examples are grafting, cuttings, bulbs and runners. Plant Examples: Wandering Jew & Banana. Trembling Aspens probably connected underground.

Asexual Reproduction Transfers Unchanged Genetic Information through Mitosis. mitosis Asexual reproduction requires mitosis. An Individual cell divides into two cells, each with the same number of chromosomes as the parent cell. Similarly, one 2n cell (diploid) yields two 2n cells (diploid). During this type of cell division, the chromosomes duplicate during interphase, then mitosis (involving nuclear contents) begins. Advantages of Asexual Reproduction - Requires only a single parent. - Individuals are often adapted to local environment. - Asexual Reproduction can happen quickly. - No energy required for flower production. Disadvantages of Asexual Reproduction - Genetic diversity remains fixed (Can’t adapt to environment); Genetic diversity is essential for adaptation (genetic change).

Each generation is a combination of genetics provided by the previous generation. Sexual Reproduction – Reproduction that requires meiosis and fertilization for a complete life cycle. This involves the union of egg and sperm. Egg and sperm are gametes. Each generation is a combination of genetics provided by the previous generation. Before union of gametes, the number of chromosomes are reduced during meiosis so offspring have the same number of chromosome as parents. Meiosis Meiosis - a type of cell division occurring in sexual reproduction in which two rounds of division convert one diploid (2n) cell to four haploid (n) cells, effecting a segregation of homologous chromosomes.

Sexual Reproduction continued – Humans have 23 chromosomes in one set. Humans have 2 sets (46 chromosomes) in their body. An egg or sperm has 23 chromosomes (1 set). Cotton has 10 chromosome in a set.Cotton has 2 sets (20 chromosomes) in its plant body. An egg or sperm has 10 chromosomes (1 set). Haploid or n cells have one chromosome set. Gametes (Sex cells, which are egg and sperm) are haploid. When egg and sperm join, a zygote is formed. n + n = 2n Diploid or 2n cells have two chromosome sets. Zygotes and regular plant body cells are diploid. The two copies of a given chromosome in a diploid cell are said to be homologous. This pair carries the same information (same genes). Thus, a gamete of a cotton plant has 10 unpaired chromosomes and a zygote has 10 chromosome pairs.

Alternation of Generations Most plants alternate from n to 2n generations. Sporophyte is 2n & is the diploid plant body. Gametophyte is n & produces & holds the gametes. 2n n FertilizationMeiosis 2n Zygote Mitosis Diploid Sporophyte (2n) Haploid Gametophyte (n) Haploid Spores (n) Mitosis n n n n n n n Gametes

Life Cycle of a Cherry Tree (Typical Flowering Plant Alternation of Generations) embryo When the pollen tube reaches an egg, one sperms fuses with the egg to make a zygote, which divides mitotically to from an embryo within a seed coat. An embryo is a small sporophyte, and the large cherry tree is still a sporophyte.

Plants Vary in the Details of Their Life Cycles Cone-bearing plants, ferns, mosses and algae differ in their life cycles when compared to a cherry tree. Below is generalized life cycle of plants. It is basically the same as two slides ago but explained a little differently. Generalized life cycle of organisms in the plant kingdom & plant-related relatives. No single species has all the steps shown here. The terms used in this diagram are basic, & many synonyms exist for variations. Karyogamy is when nuclei fuse. Plasmogamy is when cytoplasmic contents fuse. Note: Gametangia are cells or organs that has haploid cells that differentiate into gametes.

Life Cycles multicellular SPORIC – life cycle which includes alternating sporophyte and gametophyte bodies. The sporophyte & gametophyte bodies are multicellular. All embryophytes (Trees, Flowers, Ferns, Mosses) have sporic life cycles. These are the more complex (advanced) plants. 2 Different Types of Sporic Life Cycles 1) Heterosporic – has 2 kinds of spores & 2 gametophytes. The male spore is often lightweight & can easily travel. The female spore often has much stored energy and is immobile. Example: flowering plants and conifers. 2) Homosporic – has 1 kind of spore & 1 gametophyte. Therefore, male and female spores are together in one structure. Example: Seedless plants such as moss. Note: Some ferns are homosporic & other ferns are heterosporic.

Life Cycles single- celled zygote ZYGOTIC – A life cycle in which the only diploid phase is the single- celled zygote. The zygote does NOT divide & does NOT proceed into a multicellular stage. NO sporophyte present. NO multicellular 2n phase. However, it is multicellular during the haploid stage. Only one multicellular generation. Note: Life Cycle of Freshwater Green Algae (Kingdom Protista)

Life Cycles only the gametes are haploid. GAMETIC – A life cycle in which only the gametes are haploid. All other phases are diploid. NO multicellular gametophyte generation is present. However, a multicellular sporophyte is present. Only one multicellular generation. Note: Life Cycle of Intertidal Brown Algae, Rockweed, (Kingdom Protista)

The Diploid Generation Has Become Dominant Over Evolutionary Time Dominant means larger, more complex, and having a longer life span.

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