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Asexual Reproduction •When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cells. •This is one form of asexual (or without sex) reproduction that occurs in mostly single celled organisms. •(Take notes on underlined words) Sex requires a mixing of DNA between two similar organisms. Every offspring will have some DNA from both parents. In higher organisms, DNA will be present in homologous chromosomes, one from mom and one from dad. Homologous chromosomes have the same type of genes but one copy is from each parent. • On homologous chromosomes, the genes are arranged in the same order, but because Axial there are different possible alleles for the same gene, the two Constricted chromosomes in a homologous pair are not Short always identical to each other. Homologous Chromosome 4 a A D d T t Terminal Inflated Tall •A cell with two of each kind of chromosome is called a diploid cell. •Why would it be called a diploid? • What would happen if a diploid cell from a male combined with a diploid cell from a female? Mom 46 + Dad 46 = Baby 92 chromosomes This is a problem! If two diploid cells combined, then each generation would have a double number of chromosomes. • Each pea plant parent, which has 14 chromosomes, would produce gametes that contained a complete set of 14 chromosomes. • The F1 pea plants would have cell nuclei with 28 chromosomes, and the F2 plants would have cell nuclei with 56 chromosomes. How can it work? Haploid cells In the sex organs, organisms produce gametes that contain one of each kind of chromosome. •A gamete contains one of each kind of chromosome or half the number of chromosomes is called a haploid cell. •Now, what would happen if two haploid cells got together? Another type of cell division is needed for sexual reproduction that would produce gametes containing half the number of chromosomes as a parent’s body cell. It is called meiosis. Diploid and haploid cells Chromosome Numbers of Common Organisms Organism Body Cell (2n) Gamete (n) 4 Fruit fly 8 Garden pea 14 7 10 Corn 20 12 Tomato 24 Leopard Frog 26 13 Apple 34 17 Human 46 23 Chimpanzee 24 48 Dog 78 39 Adder’s tongue fern 1260 630 • This fact supports Mendel’s conclusion that parent organisms give one allele for each trait to each of their offspring. What is meiosis? • Meiosis consists of two separate divisions, known as meiosis I and meiosis II. *Meiosis I begins with one diploid (2n) cell. *By the end of meiosis II, there are four haploid (n) cells. *Or from one cell, four gametes are made. Why meiosis? *These haploid cells are called sex cells— gametes. *Male gametes are called sperm. *Female gametes are called eggs. *When the egg is fertilized, the resulting zygote once again has the diploid number of chromosomes. Why meiosis? Meiosis Haploid gametes (n=23) Sperm Cell Meiosis Egg Cell Fertilization Diploid zygote (2n=46) Mitosis and Development Multicellular diploid adults (2n=46) • This pattern of reproduction, involving the production and subsequent fusion of haploid sex cells, is called sexual reproduction. The Phases of Meiosis Click image to view movie. Interphase *At the end of interphase, each chromosome replicates to form two identical sister chromatids, held together by a centromere. Interphase Prophase I • The chromosomes coil up and a spindle forms. *As the chromosomes coil, homologous chromosomes line up with each other gene by gene along their length, to form a four part structure called a tetrad. Prophase I Prophase I •*Homologous chromosomes can break and exchange genetic material in a process known as crossing over •Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time. Prophase I Crossing Over Sister chromatids Tetrad Nonsister chromatids Crossing over in tetrad Homologous chromosomes •Why? Gametes • It is estimated that during prophase I of meiosis in humans, there is an average of two to three crossovers for each pair of homologous chromosomes. Prophase I Sister chromatids Tetrad Nonsister chromatids Crossing over in tetrad Homologous chromosomes Gametes *Crossing over results in new combinations of alleles on a chromosome for more genetic variety. Metaphase I *During metaphase I, the centromere of each chromosome becomes attached to a spindle fiber. • The spindle fibers pull the tetrads into the middle, or equator, of the spindle. Metaphase I *This is where independent assortment occurs as different chromosomes are pulled apart. This and crossing over is why you look different from your siblings. Anaphase I *Anaphase I begins as homologous chromosomes, each with its two chromatids, separate and move to opposite ends of the cell. • This critical step ensures that each new cell will receive only one chromosome from each homologous pair. Anaphase I Telophase I • Events occur in the reverse order from the events of prophase I. *In telophase, the spindle is broken down, the chromosomes uncoil, and the cytoplasm divides to yield two new cells. Telophase I Telophase I • Each cell has half the genetic information of the original cell because it has only one chromosome from each homologous pair. Telophase I The phases of meiosis II * During prophase II, a spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes. Prophase II The phases of meiosis II * During metaphase II, the chromosomes, still made up of sister chromatids, are pulled to the center of the cell and line up randomly at the equator. Metaphase II The phases of meiosis II *Anaphase II begins as the centromere of each chromosome splits, allowing the sister chromatids to separate and move to opposite poles. Anaphase II The phases of meiosis II * In Telophase II, nuclei reform, the spindles break down, and the cytoplasm divides. Telophase II Meiosis Provides for Genetic Variation • Cells that are formed by mitosis are identical to each other and to the parent cell. In meiosis, reassortment of chromosomes and the genetic information they carry, either by crossing over or by independent segregation of homologous chromosomes, is called genetic recombination. • Thus, variability is increased. Meiosis video more life like Flower reproduction roles Asexual reproduction Some plants can make new plants from cuttings or buds from the original plant The new plants from asexual reproduction have the same genetic makeup as the original plant. (Clone) Sex for plants (sounds familiar) The female gamete is an egg and the male is a sperm. When a sperm fertilizes an egg, a diploid zygote (later embryo) forms. This is sexual reproduction. The embryo develops to maturity as a seed. Many seeds are surrounded by fruit (the ovary) to provided nourishment to develop the seed into a plant . The structure of a flower is for plant sex Petals are colorful structures at the top of a flower stem. Petals are to attract birds and insects that transfer pollen between plants. The flower stem is called peduncle and supports the flower. Sepals are usually leaf-like and encircle the peduncle below the petals. Sepals protect the flowers before blooming The structure of a flower – male parts Inside the petals are the stamens. A stamen is the male reproductive organ of a flower. It is made up of the filament that holds up the anther. At the tip of the stamen is the anther. The anther produces pollen that contain sperm. A flower makes millions of pollen grains. The structure of a flower Stigma Petals Style Ovary Anther Stamen Filament Sepal Peduncle Pistil The structure of a flower The pistil is the female organ of the flower. The pistil is made of the stigma which is sticky to catch the pollen and the style that hold up the stigma. At the bottom portion of the style is the ovary, a structure with one or more ovules, each usually contains one egg. A plant makes fewer eggs than sperm (‘cause most sperm get lost and never reach the eggs). Development of the female ovules In flowers, the female ovule forms within the ovary. In the ovule, a cell undergoes meiosis and produces haploid cell (half the genetic info). In most flowering plants, these haploid cells undergoes mitosis three times, producing eight haploid nuclei. Only three of the eggs cells will be fertilized. Development of the male parts (pollen) Haploid cells (sperm) are produced by meiosis within the pollen sac. The nucleus of each haploid cell undergoes mitosis. A thick, protective wall surrounds these cells. This structure is the immature male cells or pollen grain. Pollination In flowering plants, pollination (sex) is the transfer of the pollen grain from the anther (male part) to the stigma (top of female part). Depending on the type of flower, the pollen can be carried to the stigma by wind, water, or animals (birds and bees) that are attracted by nectar (sugar water). Two plants having artificial sex This is what Mendel did to control the cross pollination of his plants. Fertilization Inside each pollen grain are two haploid nuclei, the tube nucleus (grows tube) and the generative nucleus (to fertilize egg). Sperm is delivered by the pollen tube to the ovules (female). Stigma Style Pollen grain Two sperm nuclei Pollen tube Tube nucleus Ovary Central cell Ovule Egg cell Fertilization One of the sperm unites with the egg forming a diploid zygote which begins the new baby plant as the beginning of a seed. Double Fertilization The wall of the ovule becomes the seed coat, which can aid in seed dispersal and help protect the embryo until it begins growing. Seeds are alive just dormant! One sperm fertilizes the egg cell (2n) One sperm fertilizes the central cell (3n) Seed formation Click this to view movie. As the seeds develop, the surrounding ovary enlarges and becomes the fruit. Fruit Ovary Ovules Sepals Stamen Fused petals Seeds and Fruits Fruit formation Click this to view movie. Germination – the dormant seed wakes up Water is important because it activates the embryo’s metabolic system. The first part of the plant embryo to appear is the embryonic root that will grow toward the pull of gravity. A second shoot grows away from gravity to make leaves. It straightens, bringing with it the cotyledons and the plant’s first leaves.