Sexual Reproduction and Meiosis Flowers I: Sexual Reproduction and Meiosis

Sexual reproduction Fusion of two gametes to produce a zygote Sperm and egg are male and female gametes Sperm + Egg ---> Zygote Zygote will develop into a new genetically unique individual Introduces variation into a population, while offspring produced by asexual reproduction are genetic clones

Zygote and chromosomes Zygote receives an equal number of chromosomes from each gamete Gametes contain only one set of chromosomes - haploid All other body cells contain two complete sets of chromosomes - diploid

Diploid Cells Two complete sets of chromosomes Microscopically can see two of each kind of chromosome Chromosome pairs are know as homologous chromosomes

Homologous chromosomes Look alike and carry genes for the same traits Each chromosome in a pair is derived from one of the haploid gametes During fertilization the diploid number is restored in the zygote

Meiosis A specialized type of cell division Occurs in all sexually reproducing organisms Reduces the number of chromosomes from diploid to haploid Without meiosis the number of chromosomes would double with each generation

Meiosis Consists of two consecutive divisions Results in the formation of four haploid cells Both divisions divided into four stages: prophase I prophase II metaphase I metaphase II anaphase I anaphase II telophase I telophase II

Division I - Reduction Division Prophase I similar to mitosis except that homologous chromosomes pair up (each member of pair composed of two chromatids) Metaphase I - homologous chromosome pairs line across the cell Anaphase I - homologous chromosomes separates Telophase I - two daughter cells - each has half the number of chromosomes

Division II Prophase II - similar to mitosis except cells are haploid Metaphase II - chromosomes line up Anaphase II - chromatids separate Telophase II - four haploid daughter cells from original diploid parent cell The four cells have unique genetic combinations that differ from the parent cell

Meiosis In animals gametes are produced directly by meiosis In plants gamete formation is delayed In plants the products of meiosis are haploid cells called spores

Sporophytes and Gametophytes Sporophyte is the diploid plant that undergoes meiosis to form spores Spores develop into haploid plants called gametophytes that produce the gametes In angiosperms, the gametophytes are always microscopic

Alternation of generations Meiosis Spores Gametophyte (haploid) Sporophyte (diploid) Zygote Gametes

Meiosis in flowering plants Meiosis occurs in two parts of the flower Stamens in pollen chambers of anther Carpels in developing ovules in ovary

Flower Structure

Flowers

Flowers Sexual reproductive structure of angiosperms Flower is a modified branch bearing four sets of floral organs sepals petals stamens carpels. Floral organs inserted on the receptacle, expanded top of the pedicel or flower stalk

Sepals Leaf-like structures that cover the unopened flower bud Usually green and photosynthetic Whorl of sepals is called the calyx

Petals Often brightly colored and conspicuous Function to attract animal pollinators The whorls of petals called the corolla Calyx and corolla comprise the perianth

Stamen Male reproductive structure Pollen chamber Male reproductive structure Consists of a pollen producing anther supported on a filament Each anther has four pollen chambers All stamens in a flower called androecium Anther Filament

Pollen chamber in anther

Carpels Flowers can have one to many carpels Located in the middle of the flower Gynoecium is collective term for all carpels

Carpels Stigma - expanded top of the carpel modified to trap pollen Style - neck of the carpel Ovary - expanded basal part with one to many ovules (structures which will eventually become seeds)

Carpels Simple pistil - a gynoecium with just one carpel When multiple carpels are present: May be fused together to form one compound pistil May remain separate as many simple pistils

Simple and Compound Pistils Orange Green Bean

Monocots and dicots Monocots - floral parts in 3s or multiples of 3 Dicots - floral parts in 4s or 5s or multiples

Amaryllis a member of the Lily Family is a monocot. There are 3 petals and 3 sepals, 6 stamens, and one carpel

Azalea, a dicot Each flower has 5 sepals, 5 petals, 5 stamens, and one compound pistil composed of 5 carpels