CELL DIVISION: MEIOSIS
How do Organisms Reproduce? Option 1: Asexual Reproduction Can be done by a single organism without the involvement of gametes (sperm or egg) Offspring are clones of the parent, meaning they are genetically identical There are many different types of asexual reproduction. Example: Bacteria undergo binary fission
Types of Asexual Reproduction 1. Vegetative Propagation 2. Fragmentation 3. Budding
How do Organisms Reproduce? Option 2: Sexual Reproduction Requires two parents who produce gametes that unite to form an embryo or zygote Offspring may be somewhat similar to the parents, but are genetically unique Many eukaryotic organisms reproduce sexually Example: plants and animals
Chromosome Pairs An embryo receives one set of chromosomes from its mother and one set from its father This means that sexually reproducing organisms have 2 sets of chromosomes that are arranged in pairs. These chromosome pairs are called homologous They are similar but not identical
From Dad From Mom Homologous Pair
Two Basic Types of Cells 1. Diploid: Cells that are body cells such as skin, nerve, and bone that have the full number of chromosomes (2 sets = 2N) 2. Haploid: Sex cells such as egg and sperm that have half the number of chromosomes (1 set = 1N)
Gamete Production Gametes are produced by the process of meiosis A series of cell divisions (with similar phases as mitosis PMAT) Meiosis starts with one diploid cell and creates four haploid gametes. The term for creating gametes is gametogenesis The term for creating eggs is oogenesis The term for creating sperm is spermatogenesis
Where does Meiosis Occur? Meiosis occurs in the reproductive organs Spermatogenesis occurs in the testes Oogenesis occurs in the ovaries A human female is born with all of the primary oocytes she will ever have ~1-2 million Males continually produce primary spermatocytes throughout their lifetime.
What is Meiosis? The goal of meiosis is to reduce the chromosome number by half! Humans: 46 chromosomes à 23 Meiosis also creates genetic diversity through a process called crossing-over This is the reason why siblings have different DNA (except for identical twins)
What is Meiosis? Meiosis I Prophase I Metaphase I Anaphase I Telophase I Cytokinesis Prophase II Meiosis II Metaphase II Anaphase II Telophase II Cytokinesis
Meiosis I Begins with interphase: 1. Cells increase in size 2. Cells produce RNA 3. Cells synthesize (create) proteins 4. Cells replicate DNA
Phases of Meiosis I
Prophase I The nuclear envelope breaks down Spindle fibers & centrioles appear The chromosomes pair up with their homologous partner This form a tetrad Because the homologous chromosome pairs are in close proximity, an exchange of chromosome genetic material between the pairs often occurs This is called Crossing Over
Prophase I Crossing Over results in a new combination of alleles.
Metaphase I Spindle fibers attach to the centromeres Homologous chromosomes line up in pairs across the middle of the cell
Anaphase I The chromosome pairs separate, one chromosome to each side of the cell Each daughter cell will receive only one chromosome from each homologous chromosome pair Sister chromatids remain attached to each other There are more than 8 million different configurations for how our chromosomes can be independently assorted
Telophase I & Cytokinesis Spindle fibers disappear and the nuclear membrane reforms. The cell membrane pinches inward and two haploid cells are formed. These two cells do not enter interphase They will both proceed directly to Meiosis II
Phases of Meiosis II Meiosis II is just like Mitosis
Prophase II The nuclear envelope breaks down Spindle fibers & Centrioles appear
Metaphase II Spindle fibers attach to the centromeres The chromosomes line up singularly across the middle of the cell
Anaphase II Spindle fibers shorten and separate the chromosomes Sister chromatids are pulled apart and move to opposite ends of the cell
Telophase II & Cytokinesis Spindle fibers disappear and the nuclear membrane reforms. The cell membrane pinches inward and two cells are formed. 4 Haploid cells are produced 2 from each division
1 diploid cell Interphase Meiosis I Meiosis II 4 haploid cells
Meiosis Animation
Mitosis vs. Meiosis Mitosis Occurs in somatic cells Makes 2 diploid cells Daughter cells are genetically identical to parent Meiosis Occurs in gametic cells Makes 4 haploid cells Produces gametes that are genetically unique