Genetics: Chromosomes, Mitosis, Meiosis

Chromosomes, Mitosis, and Meiosis 

  • Mitosis vs Meiosis    * Mitosis: cell repair      * Makes two identical daughter cells
  •   * 2n 2n 2n 2n 4n 2n
  • \n   * Meiosis: creating gametes for reproduction, resulting in 4 different daughter cells   * Synapsis: when homologous chromosomes line up next to each other   * Crossing over: exchange of sections of DNA between non sister chromatids at the chiasma     * Chiasma: location of crossing over     * Happens during Meiosis I in Prophase I
  • Chromosomes
  • Centromere: the area where the chromosome shape is constricted
  • Telomeres: ends
  • Chromosome arms: “p” for the short arm and “q” for the long arm 
  • Types of Chromosomes (based on arm length)

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  • ii) Banding patterns caused by Giesma stain   *  Dark bands = DNA is tightly packed   * Light bands = DNA is loosely packed. 

  • iii) Size: chromosomes are named in order of descending size so that chromosome 1 is the largest in the genome. Note: sex chromosomes (X and Y) are always placed last. 

  • c) Using these characteristics, we can see that most eukaryotic genomes are made up of pairs of chromosomes, called homologous chromosomes. 

  • i) Homologous chromosomes: same size, shape, and banding pattern, and include the same genes in the same order.    * (1) Within this pair, one was inherited from each parent.    * (2) The homologous chromosomes are not identical at the sequence level, they have small differences in sequence (SNPs). SNPs within genes may result in alleles, which are versions of a  gene that affect the gene product’s function. 

  • ii) Genomes made up of homologous pairs are diploid genomes, meaning the genome is made up of two haploid sets of chromosomes.   * (3) Haploid number and chromosomal characteristics vary between species 

  • iii) Some variation in the chromosomes of the genome occurs within species, ie by sex or by chromosomal disorders such as trisomy.

  • 4) Mitosis and the Cell Cycle      * b) Mitosis is only one part of the cell cycle. The cell spends most of its time in interphase, which is divided  into three stages:    * i) G1 – Gap 1: growth and synthesis of new proteins.      * G1/S checkpoint, at which point the cell may either be sent into a non-dividing stage,  G0, or continue to the next stage.    * ii) S – Synthesis is when the genome is replicated, and the centrosome is duplicated.      * (1) Replicated DNA molecules are held together by rings of cohesin protein so that each chromosome is now made up of two identical sister chromatids, each is one DNA molecule.      * (2) Although the number of DNA molecules in the cell doubles at this stage, we count the number of chromosomes as unchanging at this stage.      * (a) Now, each chromosome is made up of two sister chromatids, each composed of a single  DNA molecule with identical sequence.    *      (iii) G2 – Gap 2:cell to grow; the      * G2/M checkpoint:  assess whether the genome has been duplicated completely and is undamaged. 

  • iii) Mitosis is the division of the nucleus, and is divided into 5 stages:    * (1) During prophase   * the nuclear envelope dissolves   * the chromosomes condense   * spindle structure (made of microtubules) forms   * (2) During prometaphase     * the spindle attaches to the chromosomes at the kinetochore   * (3) During metaphase   * the chromosomes are all attached to the spindle and align along with the metaphase plate.      * governed by the polarity of the spindle microtubules and the DNA, which is negatively charged.    * Spindle assembly checkpoint: where the connections between the spindle and each chromatid are checked for errors.    * (4) During anaphase    * the sister chromatids separate.      * separase breaks down the cohesin rings.    * (5) During telophase   * the chromosomes de-condense   * the spindle dissolves   * Nuclear envelope reforms.    * (6) Cytokinesis,   * usually occurs concurrently with telophase. 

  • iv) Errors in mitosis and cytokinesis

  • Endoreplication: DNA replication occurs in the S stage, but mitosis and cytokinesis do not  occur.    * As a result, the cell has twice as much DNA in the nucleus. 

  • Binucleated cells:  mitosis occurs without cytokinesis   * cells have two nuclei

  • Mosaicism:  cells within the body do not have identical DNA sequences    * due to mutations that arise during the cell divisions of embryonic development or growth     * all daughter cells descending from that cell will carry the same mutation. 

  • Chromosomal nondisjunction: a pair of sister chromatids don’t separate properly during anaphase, resulting in one daughter cell with an extra chromosome and one missing a  chromosome.

  • 5) Meiosis is the cell division that occurs only in eukaryotes as preparation for sexual reproduction.

  •  Meiosis in eukaryotes occurs in germline cells to produce four variable haploid gamete cells.    * Prophase I     * homologous chromosome pairs align with each other (synapsis)     * form chiasmata between non-sister chromatids     * cross over to exchange DNA sequences     * the  nuclear envelope dissolves     *  the spindle forms     *  the chromosomes condense,     * Condensed chromosome pairs start attaching to the spindle.    *  Metaphase I –     * the homologous pairs of chromosomes align along the metaphase plate     * both sister chromatids and homologous chromosomes are held together by cohesin rings   * Anaphase I     * separase selectively digests the cohesin rings holding homologous chromosomes together     * Cohesin holding together sister chromatids is protected from separase by the binding of the

  • Telophase I and Cytokinesis    * the cell divides once, each daughter cell is now  haploid

  • Interkinesis   * a stage of variable duration that falls between the two divisions   *  The centrosomes likely replicate at this stage. 

  • Prophase II    * Each chromosome will start to attach to the newly formed spindles. 

  •  Metaphase II   * chromosomes align on the metaphase plate 

  • Anaphase II   *  separase digests the cohesin rings holding the sister chromatids together   * Sister chromatids separate

  •  Telophase II and Cytokinesis   * the nuclear envelope reforms   * chromosomes de-condense   * the spindle dissolves

  • c) Gametogenesis – during and after meiosis, some modifications make sperm traits and egg traits.   *  i) In spermatogenesis, the products of meiosis are then further modified to make sperm (ie, adding  tails).    * ii) In oogenesis, each cytokinesis is unequal, so that meiosis results in one large useable egg and two or  three small polar bodies.    * \n   * ** **

  • d) Errors in Meiosis   * i) Nondisjunction can also occur during meiosis   * either in the first division or the second   * The consequences of this are similar to mitosis but vary slightly depending on the timing of the nondisjunction.    * Example: XY      * Nondisjunction in Meiosis I: 2XY: 2 0     * In Meiosis II: 2 XX (or YY): 1 0: 2 Y (or X)

e) The number of combinations can be calculated by 2n when n is the haploid number of the genome.