Understanding Growth & Development

Meiosis Formation of Gametes

Reproduction Strategies One characteristic of all living things is the ability to reproduce This is important because it allows genes for common traits to pass from one generation to the next This ensures the survival of the species

Reproduction Strategies, cont. Two main categories of reproductive strategies: Asexual Sexual Many species use only one strategy Some species are able to use both strategies depending on availability of partners or environmental conditions

Reproduction Strategies, cont. Methods of asexual reproduction: Binary fission – bacteria Budding – yeast & hydra Mitosis – single celled eukaryotes Advantages: One organism can increase a population Energy is not used making sex cells No need to search for mates Disadvantage: No genetic diversity; one dies, they all die!!!

Reproduction Strategies, cont. Methods of sexual reproduction: External fertilization (sperm and egg have to make contact in body of water) – fish & frogs Internal fertilization (implantation of male gamete in moist environment of female) – plants, birds, reptiles & most mammals Advantage: genetic diversity Disadvantage: requires a mate

Formation of Gametes Gametes – sex cells; sperm or egg (ovum) Diploid (2n) cell – cell with complete set of chromosomes - All body cells are diploid Haploid (n) – cell with half the genetic material of parent cell - Gametes are haploid - Creation of haploid cells is called meiosis

Meiosis The creation of haploid cells requires 2 cycles of division: Meiosis I – genetic recombination Meiosis II – reduction division Somatic (body) cells do not undergo meiosis As in mitosis, meiosis is preceded by interphase and DNA replication

Meiosis I Prophase 1: 1. Matching (homologous) chromosomes pair up with each other to form a tetrad

Meiosis I, cont. Prophase I: 2. Homologous chromosomes exchange genetic information with each other – called “crossing over”

Meiosis I, cont. The cell goes through Metaphase, Anaphase and Telophase I At the end of Telophase I/Cytokinesis I, there are two new cells that are still diploid (2n) The genetic material has been recombined, so the new cells are no longer identical to the parent cell

Meiosis II From Cytokinesis I, the cells immediately go through P M A T (II) At the end of Cytokinesis II, there are four new daughter cells that are haploid (n) These are the gametes

Importance of Meiosis Allows for sexual reproduction which is the basis of genetic variation in offspring This variation allows the offspring a diversity that could be the difference between living or dying This gives the species as a whole a greater chance of surviving in a changing ecosystem Genetic variation is the foundation of evolution by natural selection

The Human Reproductive System

Species that reproduce sexually have specialized structures that produce gametes In humans, this process is influenced by the endocrine system This system produces hormones that play an important role in the development and management of reproductive capacity

Male Reproduction During childhood the body grows but does not change much Puberty begins between the ages of 11 & 16 This process is triggered when the brain sends signals to the hypothalamus that it is time

Male Reproduction, cont. Steps of puberty: Hypothalamus signals pituitary gland to release FSH (follicle stimulating hormone) FSH travels through circulatory system to testes FSH triggers luteinizing hormone (LH) to trigger specialized cells to make testosterone Testosterone triggers: Sperm production Development of external organs Facial/body hair Deepening of voice Increase in muscle/bone mass

Male Reproduction, cont. In human males (and many other organisms) gametes are called spermatozoa or sperm Formation of sperm from stem cells (called follicles) is called spermatogenesis 1 follicle produces 4 sperm Length of meiosis: 65 to 75 days

Male Reproduction, cont. Approximate number of stem cells that go through process each day: 3 million There are 300-500 million sperm that exit the body upon ejaculation Sperm are able to remain viable up to 48 hours inside the female reproductive system

Did You Know? The shape of the head of a sperm varies from rounded to more oblong or pointed depending on the species? Rat Human Fly

Female Reproduction Oogenesis – production of female gametes (ovum/ova) Begins as female fetus develops within the uterus of her mother As the fetus develops, follicles go through Prophase I of Meiosis and then lie dormant within the ovaries Each follicle will produce one egg and 3 non-viable polar bodies

Female Reproduction, cont. Steps of puberty (10-14 years old): Hypothalamus signals pituitary gland to release FSH (follicle stimulating hormone) FSH travels through circulatory system to ovaries FSH triggers luteinizing hormone (LH) to trigger the rest of meiosis in one follicle Estrogen and progesterone are also secreted to support growth of uterine lining If the ovum is not fertilized, it and the uterine lining are shed; this is menstruation

Female Reproduction, cont. Estrogen production is the beginning of puberty; also triggers: Breast development Broadening of hips Growth of body hair Slight deepening of voice

Female Reproduction, cont. The zygote: Once the egg is fertilized, it becomes a zygote The zygote secretes a hormone that allows it to implant in the uterine wall Estrogen and progesterone keep the zygote implanted and the uterine wall becomes a structure called the placenta

Female Reproduction, cont. Function of placenta – trading post between circulatory systems of mother and fetus Umbilical cord – attaches placenta to fetus (~ 60 cm long) Amniotic sac - Provides a place for fetus to grow and develop that is safe from outside pressures Filled with fluid Amniotic sac and fluid maintain a slightly higher temperature than the mother’s body, usually 99.7 F

Cell Differentiation Zygote goes through rapid mitosis and becomes an embryo This is possible because of cell differentiation During process, genes switch on and off that control production of chemicals These chemicals initiate differentiation in cells