Mitosis and Meiosis (2)

Mitosis & Meiosis Overview

Course: BIOL 1134: Evolution, Ecology and Diversity

Instructor: Paul Klawinski

Learning Goals

• Understand and describe the differences between mitosis and meiosis, particularly in the context of gamete production and genetic variation.

• Perform monohybrid and dihybrid crosses to predict inheritance patterns of traits in offspring.

• Define and differentiate between dominant and recessive alleles, and their impact on phenotypic expression.

• Calculate allele frequencies and understand how to derive genotypic and phenotypic ratios based on Mendelian genetics.

Evolution via Natural Selection Preconditions

1. Overproduction of Offspring: More offspring are produced than can survive, leading to competition for resources.

2. Phenotypic Variation: There exists interindividual variation in characteristics (phenotypes) among individuals of a species.

3. Heritage of Variations: The variations in traits must be heritable, meaning they can be passed down from parents to offspring.

4. Survival Advantage: Variations must influence survival and/or reproductive success in the given environment, leading to differential survival.

Organization of the Genome

• Genes: Fundamental units of heredity, which are segments of DNA located on chromosomes.

• Chromosomes: Structures that organize genes; human cells typically contain 46 chromosomes arranged in 23 pairs, consisting of 22 pairs of autosomes and 1 pair of sex chromosomes (XX for females, XY for males).

DNA Replication

• Processes Involved: Prior to mitosis or meiosis, DNA must be replicated to ensure each daughter cell receives a complete set of genetic information.

• Enzymatic Actions:

  • Helicase: Unwinds the double helix structure of DNA to create single strands.

  • Primase: Synthesizes short RNA primers required for DNA polymerase to initiate replication.

  • DNA Polymerase III: Extends the RNA primers, synthesizing the new DNA strands by adding complementary nucleotides in the 5' to 3' direction.

  • DNA Polymerase I: Removes RNA primers and replaces them with DNA nucleotides.

  • DNA Ligase: Joins Okazaki fragments on the lagging strand, sealing nicks in the sugar-phosphate backbone.

The Cell Cycle

Phases of the Cell Cycle

1. G1 Phase: Cell growth occurs; the cell synthesizes proteins and organelles while monitoring the environment for signals to divide.

2. S Phase: DNA synthesis phase where the entire genetic material replicates, resulting in two copies for each chromosome.

3. G2 Phase: The cell undergoes final preparations for division, including the synthesis of proteins involved in mitosis or meiosis.

4. M Phase: The phase of cell division where mitosis or meiosis occurs, culminating in the separation of the cell’s genetic material and formation of new cells.

Mitosis vs. Meiosis

Detailed Mitosis Phases

• Interphase: The phase prior to division where the cell grows and DNA replication occurs. It is divided into G1, S, and G2 phases.

• Prophase: Chromatin condenses into visible chromosomes, and the mitotic spindle begins to form.

• Metaphase: Chromosomes align along the metaphase plate, ensuring each sister chromatid is attached to spindle fibers from opposite poles.

• Anaphase: Sister chromatids are pulled apart towards opposite poles of the cell by the spindle fibers.

• Telophase: Chromosomes decondense back into chromatin, and nuclear membranes reform around each set of chromosomes.

• Cytokinesis: The physical division of the cytoplasm occurs, resulting in two separate cells (in animal cells, a cleavage furrow forms; in plant cells, a cell plate forms).

Meiosis Overview

• Stages of Meiosis: Divided into two main stages:

  • Meiosis I: Consists of Prophase I (where synapsis and crossing over occur), Metaphase I, Anaphase I, and Telophase I.

  • Meiosis II: Similar to mitosis but starts with haploid cells, consisting of Prophase II, Metaphase II, Anaphase II, and Telophase II.

• Synapsis and Crossing Over: Critical processes in Prophase I that enhance genetic diversity by exchanging genetic material between homologous chromosomes.

• Key Points in Meiosis: Meiosis incorporates independent assortment of chromosomes and genetic recombination, producing four genetically unique gametes, essential for sexual reproduction.

Summary of Mitosis and Meiosis

• Both processes follow the duplication of genetic material before division.

• Mitosis: Results in two identical daughter cells through one division, crucial for growth and repair.

• Meiosis: Produces four unique gametes through two divisions, necessary for sexual reproduction and genetic diversity.