Chromosome Structure, Mitosis and The Cell Cycle
Reproduction is biology means that parents produce a new generation of cells or multicelled individuals like themselves. Parent cells must provide their daughter cells with hereditary instructions, encoded in DNA, and enough metabolic machinery to start up their own operation.

I. Dividing Cells: The Bridge Between Generations
  • Overview of Division Mechanisms
    • Mitosis and Meiosis are mechanisms for NUCLEAR division. These processes sort out and package a parent cell's DNA into new nuclei for the daughter cells
    • Multicelled organisms grow, and repair tissues by way of mitosis and division of body cells (a.k.a. somatic cells). This is known as asexual reproduction.
    • Germ cells (sex cells) are created by meiosis and used in sexual reproduction
  • Some Key Points about Chromosomes
    • Chromosomes are a single strand of DNA with all the attached proteins. These are usually stretched out and working during normal cell functions.
    • Before Mitosis, each chromosome duplicates. While these copies remain attached to each other they are known as sister chromatids.
    • The sister chromatids are connected at a point called the centromere. The location of the centromere depends on the chromosome.
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  • Mitosis and Chromosome Number
    • Chromosome number is specific to each species and is the sum total number of chromosomes found in the nucleus of a somatic cell. (Ex. Humans have 46, Gorillas have 48, Pea plants have 14)
    • These chromosomes are organized into pairs based on the information they carry. Half of the chromosomes were inherited from mom and the other from dad. Each chromosome in a pair contain the same basic instructions, just a different version. (Ex. Humans have 23 pair, Gorillas have 24 pair, Pea plants have 7 pair)
    • The chromosome number of a cell is diploid (2n) if it contains two of each type of chromosome specific to the species. Somatic cells are diploid.
    • The chromosome number of a cell is haploid (n) if it contains only one of each type of chromosome. Germ cells are haploid.
    • During mitosis a diploid parent cell produces two diploid daughter cells. The chromosomes are divided so that each new cell receives two of each type of chromosome.
II. The Cell Cycle
  • The cell cycle starts each time a new cell is produced and ends when it completes its own division.
  • Interphase - usually the longest phase of the cell cycle and has three parts. During this phase a cell increases its mass, doubles the number of its cytoplasmic components, and duplicates its DNA
    • G1 - "Gap 1" of cell growth before the onset of DNA replication. A cell will function normally during this phase until a critical level of signalling molecules is reached.
    • S - "Synthesis" during this phase DNA is replicated
    • G2 - "Gap 2" of cell growth after DNA replication. The cell prepares to divide.
  • M - Mitosis; nuclear division only. Ususally followed by cytoplasmic division (cytokinesis)
  • The length of a cell cycle depends on the type of cell. Some cells progress rapidly through the cell cycle while others may take years to divide or may never divide. Cells that are arrested in interphase are said to be in G0.
  • Environmental conditions may also play a role in the length of cell cycle. Cells that are deprived of enough nutrients may stay in interphase and never divide; however once a cell proceeds past a certain point in interphase cell division will continue regardless of outside conditions.
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III. The Stages of Mitosis - An Overview
When a cell gets the proper signals to divide, itl will copy its DNA, stockpile energy and cellular components, and progress through mitosis. Mitosis has four stages; prophase, metaphase, anaphase, and telophase.
  • Prophase: Mitosis Begins
    • Characterized by the coiling and condensing of the chromosomes
    • Construction of the spindle ( microtubules that attach to the centromeres of chromosomes during division and move them around the cell)
    • Nuclear membrane breaks apart
  • Metaphase
    • Chromosomes are pulled to the middle of the cell and align down the "metaphase plate"
  • Anaphase
    • Sister Chromatids separate from each other and are moved to opposite ends of the cell
      • microtubules attached to the centromeres shorten and pull chromatids apart
      • the spindle elongates as microtubules push the spindle poles farther apart
    • Once separated, chromatids are now called chromosomes
  • Telophase
    • Chromosomes arrive at a spindle pole and are no longer attached to microtubules
    • They unwind into threadlike strands
    • The nuclear membrane reforms around each bundle of chromosomes
    • There should be a diploid number of chromosomes in each new nucleus.
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IV. A Closer Look at the Cell Cycle
  • Interphase - the cell's working phase!! The longest phase of the cell cycle. It is composed of G1, S, and G2. All of these phases are governed by control mechanisms that determine the time spent in each phase and if a cell will continue to the next phase or stay arrested in the current phase.
  • Chromosomes, Microtubules, and the Precision of Mitosis
    • The ability of a cell to precisely divide out its DNA to daughter cells depends upon chromosome organization and interactions among microtubules and motor proteins
      • Organization of Chromosomes
        • Supercoiling of DNA during prophase produces highly compacted structures that are easy to move during Mitosis and Meiosis. DNA strands wrap around proteins called histones which act like a spool and get wrapped with DNA. One histone/DNA complex is called a nucleosome. Nucleosomes that coil up in a cylindrical fiber is known as a solenoid. Solenoids then coil up into the final puffy looking Chromosome.
        • DNA topoisomerase is an enzyme that untangles DNA if is gets caught up during supercoiling. Without this enzyme, sister chromatids fail to seperate during anaphase
        • Each centromere has a circular disk called a kinetochore where spindle microtubules attach to pull chromatids apart.
      • Spindles Formation
        • Each cell has a different number of microtubules in its mitotic spindle. These tubules are constantly being assembled and disassembled all the time. This building and breaking cycle can be chemically altered by adding the poison colchicine.
        • During anaphase microtubules shorten where kinetochores slide over them. You could think of the kinetochore as a train and the track is the microtubule. As the kinetochore passes over the microtubule the "track" is broken down behind the train as it moves forward.
        • Where microtubules overlap, motor proteins actively push them in opposite directions and simultaneously push the spindle poles apart.
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V. Division of the Cytoplasm (cytokinesis)- this process is different in different organisms because of structural differences.
  • Cell Plate Formation in Plants
    • Cytokinesis in plants requires the formation of a cell plate. This process involves building a new cell wall in pieces and slowly adding onto it. It grows from its margins until it fuses with the parent cell's plasma membrane.
  • Clevage of Animal Cells
    • animal cells are not confined by a cell wall so the process of cytokinesis is more of a pinching method known as cleavage
      • a shallow ringlike depression forms above the midsection of the cell known as a cleavage furrow. Microtubules attached to the plasma membrane slowly tighten like a drawstring and pinch the cytoplasm apart.

Asexual and Sexual Reproduction, Changing the Chromosome Number, and Meiosis

I. Comparison of Asexual and Sexual Reproduction
  • Offspring acquire genes from parents by inheriting chromosomes
    • genes - units of hereditary information that are made of DNA and are located on chromosomes
      • have specific sequences of nucleotides, the monomers of DNA
      • Most genes program cells to synthesize specific proteins; the action of these proteins produce an organism's inherited traits.
  • Inheritance is possible because;
    • DNA is precisely replicated producing copies of genes that can be passed from parents of offspring
    • Sperm and ova contain each parents chromosomes and these are combined in the nucleus of a fertilized egg.
    • Chromosomes contain hundreds of thousands of genes, each of which is a specific region of the DNA molecule

Asexual Reproduction

Sexual Reproduction

Single individual is the sole parent
Two parents give rise to offspring
Single parent passes on all its genes to its offspring
Each person passes on half of its genes to its offspring
Offspring are genetically identical to the parent.
Offspring have a unique combination of genes inherited from both parents.
Results in a clone, or genetically identical individual. Rarely, genetic differences occur as a result of mutation, a change in DNA
Results in greater genetic variation; offspring vary genetically from their siblings and parents.
II. How Meiosis Halves the Chromosome Number
  • Meiosis reduces the chromosome number from diploid to haploid
    • Diploid - Condition in which cells contain two sets of chromosomes (2n)
    • Haploid - Condition in which cells contain one set of chromosomes; it is the chromosome number of gametes and is abbreviated as n
      • gametes are haploid reproductive cells
        • sperm and ova are gametes and only have one set of chromosomes
        • human gametes contain a single set of 23 chromosomes; 22 autosomal (non-sex) chromosomes and 1 sex chromosome (either X or Y)
  • Meiosis and sexual reproduction significantly contribute to genetic variation among offspring
  • There are two division during Meiosis instead of one
    • Meiosis I separates Homologous Chromosomes but NOT sister chromatids
    • Meiosis II separates the sister chromatids just like in Mitosis.
III. A Tour of the Stages of Meiosis
  • Interphase I- this comes before meiosis
    • Chromosomes replicate just like in MItosis
    • Each chromosome now consists of two sister chromatids just like before Mitosis
  • Meiosis I - This cell division separates the two chromosomes of each homologous pair but not sister chromatids
    • Prophase I - this is longer and more complex than the Prophase of Mitosis
      • Chromosomes Condense.
      • Synapsis occurs - During this process, homologous chromosomes come together as pairs
      • Since each chromosome has two chromatids, each homologous pair in synapsis appears as a complex of four chromatids or a tetrad.
      • Crossing over of genetic information occurs during synapsis and genes from one homologue are exchanged with genes from the other homologue.
      • Centriole pairs move to opposite ends of the cell
      • Nuclear envelope breaks down
      • Spindle starts to form
      • Takes up 90% of meiosis
    • Metaphase I
      • Tetrads are aligned on the metaphase plate
    • Anaphase I
      • Homologous chromosomes separate and are moved towards the poles by the spindle fibers
      • Sister chromatids remain attached together at the centromere
    • Telophase and Cytokinesis
      • The spindle fibers continue to separate homologous chromosomes until the chromosomes reach the poles
      • Each pole now has a haploid set of chromosomes that are each still made of two sister chromatids
      • Usually cytokinesis occurs at the same time forming two complete haploid daughter cells.
      • No DNA replication takes place between Meiosis I and Meiosis II!!!
  • Meiosis II - this second meiotic division separates sister chromatids of each chromosome.
    • Prohpase II
      • Spindles start to form and attach to the centromeres of the chromosomes
    • Metaphase II
      • Chromosomes line up along the metaphase plate
    • Anaphase II
      • Centromeres of sister chromatids separate
      • Sister chromatids are pulled to opposite ends of the cell.
    • Telophase II and Cytokinesis
      • Nuclei form at opposite ends of the cell
      • Cytoplasm is divided producing four haploid daughter cells
        • In sperm the cytoplasm is divided equally producing 4 viable sperm
        • In ova the cytoplasm is divided unequally producing 1 egg and 3 polar bodies. The polar bodies are never fertilized... just the egg.
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this depicts the different cytoplasmic division seen in the creation of sperm and egg

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IV. Mitosis and Meiosis Compaired
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V. Genetic Variation - genetic Variation is produced through sexual reproduction

  • This variation is provided by: Independent assortment, Crossing over during prophase I of meiosis, and random fusion of gametes during fertilization
    • Independent Assortment of Chromosomes - the random distribution of maternal and paternal chromosomes to the gamete (keep in mind this means genes!!)
      • During Metaphase I of meiosis homologous pairs of chromosomes line up down the metaphase plate. Each pair is made of a mother's chromosome and a father's chromosome
      • The side they line up on is random. So the mother's chromosome may be on the side of one pole and the father's on the side of the other pole. Each new gamete gets a fifty-fifty chance of receiving either a mother's chromosome or a father's chromosome.
      • Each pair of homologous chromosomes line up independently of the other pairs; soooo.... the first division in meiosis results in independent assortment of maternal and paternal chromosomes.
    • Crossing Over - another mechanism that increases genetic variation. This takes place during synapsis of Prophase I in meiosis.
      • It is the exchange of genetic material between homologous chromosomes
      • Happens when homologous portions of two nonsister chromatids trade places
      • Produces chromosomes that contain genes from both parents
      • In humans, there is an average of two or three crossovers per pair of chromosomes
      • Synapsis during prophase I is precise, so that homologues align gene by gene so exchanges are correct.
    • Random Fertilization
      • in humans, an egg cell that is one of 8 million different possibilities will be fertilized by a sperm cell that is also one of 8 million possibilities. The resulting fertilized egg (zygote) can have one of 64 trillion different possible combinations of genes.