Meiosis Biology Lecture Notes PDF

Meiosis Cell Types Two main cell types: 1. somatic cells – non-reproductive cells 2. germ cells – reproductive cells which form gametes Gametes are divided into two types: 1. male gamete – sperm 2. female gamete – ovum  Ploidy Ploidy (x): The number of chromosomes...

Meiosis Cell Types Two main cell types: 1. somatic cells – non-reproductive cells 2. germ cells – reproductive cells which form gametes Gametes are divided into two types: 1. male gamete – sperm 2. female gamete – ovum  Ploidy Ploidy (x): The number of chromosomes in a chromosome set for a particular species. Essential Haploid (n): Describes a cell with a single set of understanding chromosomes (gametes in humans) Diploid (2n): Describes a cell with two sets of chromosomes (somatic cells in humans) Polyploidy (_x): Multiple sets of chromosomes, > 2.  Humans Diploid species (two sets) Ploidy (x): 23 Gamete (n): 23 Somatic (2n): 46 Essential understanding Bread wheat Hexaploid species (six sets) Ploidy (x): 7 Gamete (3n): 21 Somatic (6n): 42 What happens in sexual reproduction? Meiosis Zygote Mitosis  Somatic cells = 38 chromosomes meiosis Gametes = 19 chromosomes What happens in sexual reproduction? Fertilization The sex chromosomes XY X XX Y X Reproduction What do you think would happen if mitosis were the only strategy for reproduction? In mitosis, there is no reduction of the genetic material → so sperm and ovum would both be diploid (2n) → Organisms will become increasingly polyploid with each generation → Sperm (2n) + Ovum (2n) → Zygote (4n)  Recall The two identical DNA molecules are identified as sister chromatids, and are held together by a single centromere Pairs of similar chromosomes are called homologous chromosomes (inherited from mother and father)  Meiosis: Basic Process Mitosis AND Meiosis are preceded by interphase, during which DNA is replicated (in the S phase) to produce two genetically identical copies  Meiosis: Basic Process The first meiotic division (meiosis I) is a reduction division in which homologous chromosomes are separated Each daughter cell has half the number of chromosomes as the original cell  Meiosis: Basic Process The second meiotic division (meiosis II) separate sister chromatids (which may no longer be identical due to crossing over in meiosis I)  Stages of Meiosis P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form tetrads (bivalents). Synapsis and crossing over occurs. M-I: Spindle fibres from opposing centrosomes connect to bivalents at centromeres (pro-metaphase) and align them along the middle of the cell. Random/independent assortment occurs. A-I: Spindle fibres split the tetrad, and move homologous chromosomes to opposite poles of the cell T-I: Chromosomes de-condense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells  Stages of Meiosis P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before) M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at the centromere) and align them along the cell equator A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called daughter chromosomes) move to opposite poles. T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells Benefits of sexual reproduction? The advantage of meiotic division and sexual reproduction is that it promotes genetic variation in offspring The three main sources of genetic variation arising from sexual reproduction are: 1. crossing over (in Prophase I) 2. random assortment of chromosomes (in Metaphase I) 3. random fusion of the products of meiosis (gametes) from different parents  Crossing Over PI: homologs become connected in a process known as synapsis—i.e. homologs pairing to form a tetrad (or bivalent) homologous chromosomes are held together at points called chiasmata (singular: chiasma)  Crossing Over Crossing over of genetic material between non-sister chromatids can occur at these chiasmata Recall: non-sister chromatids have the same genes, but different alleles  Crossing Over Chromatids that consist of a combination of DNA derived from both homologous chromosomes are called recombinant chromatids  Crossing Over After crossing over all four haploid daughter cells are genetically distinct (sister chromatids are no longer identical) Crossing over results in new combinations of alleles in haploid cells and thus increases the genetic diversity of potential offspring Random Assortment  Random Assortment When homologous chromosomes line up in metaphase I: The orientation towards the opposing poles is random The orientation of each tetrad occurs independently  Random Assortment The total number of combinations that can occur in gametes is 2n – where n = haploid number of chromosomes Humans have 46 chromosomes (n = 23) and thus can produce 8,388,608 different gametes by random orientation If crossing over also occurs, the number of different gamete combinations becomes immeasurable Random Fertilization Fusion of two haploid gametes results in the formation of a diploid zygote The sperm and egg are randomly “chosen” oogenesis spermatogenesis  How does Meiosis produce eggs and sperm (gametes)? Gametogenesis is the process by which diploid precursor cells undergo meiotic divisions to become haploid gametes (sex cells) In males, this process is called spermatogenesis and produce spermatozoa (sperm) In females, this process is called oogenesis and produce ova (eggs) The process of gametogenesis occurs in the gonads and involves the following steps: 1. Multiple mitotic divisions and cell growth of precursor germ cells 2. Two meiotic divisions (meiosis I and II) to produce haploid daughter cells 3. Differentiation of the haploid daughter cells to produce functional gametes How does Meiosis produce sperm and eggs (gametes)? Spermatogenesis in the Seminiferous Tubules of the Testis Spermatogenesis occurs in the testes and begins during puberty The resulting spermatid from meiosis mature into sperm Mature sperm cells have a head region containing the nucleus, but little cytoplasm, a midsection full of mitochondria, and a long tail-like flagellum for locomotion Oogenesis Oogenesis occurs in the ovaries Oogenesis begins before birth with the oogonium reproducing by mitosis and arrest during prophase I of meiosis (at birth, females have all of the oocytes they will ever have) Meiosis I continues for one oocyte each month beginning at puberty and ending at menopause (when there are no eggs left) Oogenesis Oogenesis involves the unequal division of cytoplasm The cells receiving less cytoplasm and nutrients are called polar bodies (they eventually degenerate) This unequal division creates a cell that is well prepared to support the zygote The final stages of meiosis II are not completed unless fertilization occurs Spermatogenesis vs Oogenesis Twins! Fraternal twins: two sperm fertilize two eggs (different paternal and maternal DNA) Identical twins: one sperm fertilize one egg, which then divides (same paternal and maternal DNA) Rare Twins  Meiosis: Outcomes Genetic reduction: produces daughter cells with half the number of chromosomes of the parent cell Genetic recombination: produces daughter cells with different combinations of alleles Sexual Reproduction: Why Meiosis? Most sexually reproducing organisms are diploid, meaning they have two copies of every chromosome (maternal / paternal copy) In order to reproduce, these organisms need to make gametes that are haploid (one copy of each chromosome) Fertilization of two haploid gametes (egg + sperm) will result in the formation of a diploid zygote that can grow via mitosis If chromosome number was not halved in gametes, total chromosome numbers would double each generation (polyploidy) How are meiosis and mitosis different? How should I draw the stages of Meiosis?