Mitosis and Meiosis

Chromosomes Key features of a chromosome: centromere (where spindle attaches), telomeres (special structures at the ends), arms (the bulk of the DNA). Chromosomes come in 2 forms, depending on the stage of the cell cycle. The monad form consists of a single chromatid, a single piece of DNA containing a centromere and telomeres at the ends. The dyad form consists of 2 identical chromatids (sister chromatids) attached together at the centromere. Chromosomes are in the dyad form before mitosis, and in the monad form after mitosis. The dyad form is the result of DNA replication: a single piece of DNA (the monad chromosome) replicated to form 2 identical DNA molecules (the 2 chromatids of the dyad chromosome).

More Chromosomes Diploid organisms have 2 copies of each chromosome, one from each parent. The two members of a pair of chromosomes are called homologues. Each species has a characteristic number of chromosomes, its haploid number n. Humans have n=23, that is, we have 23 pairs of chromosomes. Drosophila have n=4, 4 pairs of chromosomes.

Cell Cycle The cell cycle is a theoretical concept that defines the state of the cell relative to cell division. The 4 stages are: G1, S, G2, and M. M = mitosis, where the cell divides into 2 daughter cells. The chromosomes go from the dyad (2 chromatid) form to the monad (1 chromatid) form. That is, before mitosis there is 1 cell with dyad chromosomes, and after mitosis there are 2 cells with monad chromosomes in each. S = DNA synthesis. Chromosomes go from monad to dyad. G1 = “gap”. Nothing visible in the microscope, but this is where the cell spends most of its time, performing its tasks as a cell. Monad chromosomes. Cells not actively dividing are said to be in the G0 state, which is just like G1 with monad chromosomes G2 (also “gap”). Dyad chromosomes, cell getting ready for mitosis. G1, S, and G2 are collectively called “interphase”, the time between mitoses

Mitosis Mitosis is ordinary cell division among the cells of the body. During mitosis the chromosomes are divided evenly, so that each of the two daughter cells ends up with 1 copy of each chromosome. For humans: start with 46 dyad chromosomes in 1 cell, end with 46 monads in each of 2 cells. Stages: prophase, metaphase, anaphase, telophase.

Stages of Mitosis Prophase: --chromosomes condense --nuclear envelope disappears --centrioles move to opposite ends of the cell --spindle forms Metaphase: --chromosomes are lined up on cell equator, attached to the spindle at the centromeres

Stages of Mitosis, pt. 2 Anaphase: --centromeres divide. Now chromosomes are monads --the monad chromosomes are pulled to opposite poles by the spindle. Telophase: --cytokinesis: cytoplasm divided into 2 separate cells --chromosomes de-condense --nuclear envelope re-forms --spindle vanishes

Meiosis Meiosis is the special cell division that converts diploid body cells into the haploid gametes. Only occurs in specialized cells. Takes 2 cell divisions, M1 and M2, with no DNA synthesis between. In humans, start with 46 chromosomes (23 pairs) in dyad state. After M1, there are 2 cells with 23 dyad chromosomes each. After M2 there are 4 cells with 23 monad chromosomes each.

First Meiotic Division (M1) Prophase of M1 is very long, with a number of sub-stages. Main event in prophase of M1 is crossing over, also called recombination. In crossing over, homologous chromosomes pair up (this pairing is called synapsis), and exchange segments by breaking and rejoining at identical locations. Several crossovers per chromosome, with random positions. This is the basis for linkage mapping.

More M1 Metaphase of meiosis 1 is very different from metaphase in mitosis (or M2). In metaphase of M1, pairs of homologous chromosomes line up together. In mitosis and M2, chromosomes line up as single individuals. Anaphase of M1: the spindle pulls the two homologues to opposite poles. However, the centromeres don’t divide, and the chromosomes remain dyads. Telophase of M1: cytoplasm divided into 2 cells, each of which has 1 haploid set of dyad chromosomes

Second Meiotic Division (M2) Meiosis 2 is just like mitosis. In prophase, the chromosomes condense and the spindle forms. Metaphase of M2: dyad chromosomes line up singly on the cell equator. Anaphase of M2: centromeres divide, chromosomes are now monads which get pulled to opposite poles. Telophase: cytoplasm divided into 2 cells. After M2: total of 4 cells from the original cell. Each contains one haploid set of monad chromosomes

Gametogenesis in Mammals Gametogenesis is the creation of the sperm and egg cells from the products of meiosis, through changes in the cytoplasm. in male mammals, sperm production is continuous from puberty until death. All 4 meiotic products remodel their cytoplasm and grow a long flagellum to become spermatozoans, or sperm cells.

Gametogenesis in Female Mammals in female mammals, ovarian cells start meiosis 1 before birth, but the process is arrested in prophase of M1. Meiosis resumes after puberty, under hormonal control. A small number of oocytes (cells undergoing meiosis) are shed from the ovary during a human female’s menstrual cycle. Usually only 1 oocyte is shed in humans, but other mammals produce higher numbers. After ovulation, the oocyte finishes meiosis 1. Meiosis 2 only occurs after fertilization. During both meiotic divisions, the division of the cytoplasm is asymmetric: one cell gets nearly all of the cytoplasm. This cell becomes the egg. The other cell in both divisions is called a polar body. One polar body is created in M1, and another in M2. In some mammals, the first polar body divides so there are a total of 4 meiotic products, 1 egg plus 3 polar bodies. n humans, the first polar body never undergoes M2, so the final meiotic products in human females are the egg, the first polar body and the second polar body.

Angiosperm Life Cycle Angiosperms are flowering plants. All eukaryotes alternate between a diploid phase and a haploid phase. In animals, the haploid phase is a single cell, the sperm or the egg, and there is no haploid cell division. In plants, there is a distinct haploid organism which has cell divisions and a life of its own. The plant diploid phase is called the sporophyte. In angiosperms and most other land plants, the sporophyte is the large visible plant body that we see. The plant haploid phase is called the gametophyte. In lower plants, such as club mosses, this phase is prominent. But in angiosperms, the gametophyte stage is quite short and small.

Angiosperm Gametophytes Specifically, the male gametophyte, the pollen grain, consists of 3 haploid nuclei. These nuclei are derived from one haploid meiotic product, by mitosis. Two of the nuclei are “sperm nuclei” and the other controls the metabolism of the pollen grain. The female gametophyte, the ovule, consists of 8 haploid nuclei. These 8 nuclei are derived from one of the meiotic products.

Double Fertilization All angiosperms undergo “double fertilization”. It is a major defining characteristic of angiosperms. In double fertilization, 2 sperm (pollen) nuclei fertilize the ovule. When the pollen grain lands on the stigma of a flower, it germinates a long “pollen tube”, which grows down the style to the ovary, which contains the ovules. The 2 sperm nuclei migrate down the pollen tube into the ovule. Each ovule has a cell at one end that is pollinated by one of the sperm nuclei. This fertilized cell is diploid, and it grows into the embryo and ultimately into the sporophyte plant body. The ovule also has 2 nuclei in the center, which join with the other sperm nuclei to form a triploid tissue, the endosperm. The endosperm thus contains 2 maternal haploid genomes plus one paternal haploid genome. The endosperm develops into a nutritive tissue used by the germinating seed.

Seed Development After fertilization, both embryo and endosperm grow and develop into a seed. The seed is a resting stage that contains a multicellular embryo and a multicellular endosperm surrounded by a protective coat. When conditions are right, the seed germinates: the embryo eats the endosperm until photosynthesis begins.