Mitosis and Meiosis Chromosome a single DNA molecule

Mitosis and Meiosis

Chromosome: a single DNA molecule complexed with histone proteins § DNA is organized within the nucleus as chromosomes § Each chromosome is comprised of subunits called nucleosomes: DNA + proteins

Chromosome numbering/naming § Chromosome numbering starts with the “largest” chromosome § Karyotype: a picture containing all the numbered chromosomes of a species Above: Zea mays (maize/corn) karyotype Left: Human female karyotype

Chromosome landmarks § Centromere § Arms § “Long (q) vs. short (p)” § Telomeres § Sister chromatids (after DNA replication in mitosis and meiosis)

Mitosis and Meiosis Mitosis Meiosis § 2 identical daughter cells (each 2 n) § 4 daughter cells (each n), which may be genetically different § All cells (potentially), zygote until death § Specialized cells (megaspore mother cell, pollen mother cell) Vv Vv V V v v Vv Vv

The cell cycle 1. Interphase a) G 1 (Gap 1): RNA and protein synthesis, but no DNA replication b) S (Synthesis): DNA synthesis. The total DNA content goes from 2 n to “ 4 n” c) G 2 (Gap 2): At this point, a diploid cell contains two complete diploid sets of chromosomes 2. M (Mitosis or Meiosis)

Mitosis example: Maize (2 n=2 x=20) § Prophase: Chromatin contracts. 1 chromosome = 2 sister chromatids. One strand in each chromatid is from the "original" chromosome, the other strand was made during the S phase (semiconservative replication). Chromatids are held together at centromere. § 20 sets of 2 sister chromatids = 40 chromatids. § Metaphase: Orientation of paired chromatids on Metaphase "plate". § 20 sets of sister chromatids aligned on plate = 40 chromatids.

Mitosis example continued: Maize (2 n=2 x=20) § Anaphase: Sister chromatids are pulled apart. § 20 chromatids migrate to each pole. § Telophase: Single chromatids, or at this point, chromosomes, return to the relaxed Interphase state. § Cytokinesis divides original cell into two daughter cells. § Two cells, each with 10 pairs of chromosomes = 20 chromosomes per cell. Maize : 2 n=2 x=20

Mitosis stages § Mitosis involves the creation of identical daughter cells. § These stages are part of a continuum – there are no abrupt stops or starts between the mitotic stages.

Meiosis stages § Meiosis is divided into two main phases Meiosis I and Meiosis II

Meiosis I Prophase I (5 stages) 1. Leptonema 2. Zygonema 3. Pachynema 4. Diplonema 5. Diakinesis Centromeres Telomeres

Meiosis I Prophase I - Zygonema 2. Zygonema: § Pairing of homologous chromosomes. § Formation of synaptonemal complex and additional DNA synthesis

Meiosis I Prophase I - Pachynema 3. Pachynema: Crossing over occurs (chiasma; chiasmata) § Bivalent = 2 paired homologous chromosomes = 2 sets of 2 chromatids

Meiosis I Metaphase I – Telophase I § Metaphase I: § Bivalents appear on the Metaphase plate § The random alignment of non-homologous chromosomes is the basis of independent assortment § Anaphase I: § The physical separation of homologous chromosomes is the basis of segregation § Telophase I. § Each pole receives one-half of the original chromosome number of the meiocyte (i. e. one set of chromosomes in the case of diploidy)

Meiosis II § Prophase II: § Chromatin condensation § Metaphase II: § Chromosomes align on the Metaphase plate § Anaphase II: § Sister chromatids go to opposite poles § Telophase II: § Cytokinesis: Tetrad of n daughter cells

By now you should be able to… 1. Draw a figure showing how two specific sequences of double stranded DNA are organized in a pair of homologous chromosomes. 2. Differentiate between mitosis and meiosis with regards to: § Start (parent cell) and end (daughter cells) § Timing: when in the life cycle of a plant does mitosis/meiosis happen? § Location: in what kind of cells does mitosis/meiosis happen? 3. Describe what happens in the S phase of Interphase, and in each phase of mitosis and meiosis. Remember to relate: § Metaphase I (Meiosis) random alignment of non-homologous chromosomes independent assortment of alleles in daughter cells. § Anaphase I (Meiosis) separation of homologous chromosomes segregation of alleles in daughter cells. 4. Describe the role of spindle fibers. 5. Define the terms: chromatin, centromere, homologous chromosomes, sister and non-sister chromatids, crossing over (or chiasma).