Chapter 14: How Eukaryotic Cells Sort and Transmit Chromosomes Mitosis and Meiosis

Life Continuum and Cell Theory

• Cell Division: Life continues through the formation of new living cells via the division of pre-existing cells.

• Omnis Cellula e Cellula: Latin phrase meaning “every cell originates from another cell,” proposed by Rudolf Virchow in 1858.

• Evolutionary Aspect: Cell division is ancient, dating back nearly 4 billion years, facilitating growth of all organisms from unicellular bacteria to multicellular organisms.

Eukaryotic Cell Cycle

• Cell Cycle Definition: Series of events in a cell that lead to division, requiring regulation to occur accurately.

• Bacterial vs. Eukaryotic Division: Bacterial cells reproduce via simple binary fission; eukaryotic cells have a complex cycle involving chromosome sorting.

Phases of the Eukaryotic Cell Cycle

• G1 Phase (First Gap):

  • Longest phase; cell grows and functions normally, preparing for DNA synthesis based on external signals like nutrients.

• S Phase (Synthesis):

  • Chromosomes replicate, forming sister chromatids; original count doubles (e.g., 46 → 92 chromatids).

• G2 Phase (Second Gap):

  • Preparation for mitosis; proteins for division are synthesized.

• M Phase (Mitosis and Cytokinesis):

  • Nucleus divides (mitosis) followed by division of the cytoplasm (cytokinesis).

Regulation of the Cell Cycle

• Cyclins and Cyclin-Dependent Kinases (cdks): Proteins advancing the cell through its cycle; cyclins fluctuate in concentration, while cdks bind to them to be functional.

• Checkpoints: Key regulatory points ensuring proper conditions for division. Include:

  • G1 Checkpoint: Checks for DNA damage and favorable conditions.

  • G2 Checkpoint: Verifies DNA replication and checks for damage.

  • Metaphase Checkpoint: Monitors spindle apparatus attachment for correct chromosome segregation.

Chromosome Basics

• Homologous Pairs: Set of chromosomes derived from both parents; similar but slightly different in genetic information. For instance, the Herc2 gene may have variations that determine traits such as eye color.

• Karyotype: A visual representation of a cell's chromosomes; humans have a karyotype of 46 chromosomes (23 pairs, including germ (sex) chromosomes XX in females and XY in males).

• Diploid vs. Haploid: Most human cells are diploid (2n), same as most eukaryotic organisms; gametes (sperm and egg) are haploid (1n).

Mitosis: Process of Cell Division

• **Mitosis Phases:

1. Prophase: ** Chromosomes condense and become visible; nuclear envelope begins to break down.

2. Prometaphase: Nuclear envelope fragments; spindle apparatus forms and attaches to kinetochores.

3. Metaphase: Chromosomes align along the metaphase plate.

4. Anaphase: Sister chromatids are pulled apart to opposite poles.

5. Telophase: Chromosomes reach poles, nuclear envelope reforms, and chromosomes decondense.

• Cytokinesis: Divides cytoplasm; animals form a cleavage furrow, while plants form a cell plate to separate daughter cells.

Meiosis: Generating Genetic Diversity

• Meiosis Definition: The process that halves the chromosome number from diploid to haploid, essential for sexual reproduction.

• Process: Involves two rounds of division (meiosis I and II), leading to four genetically diverse haploid cells (gametes).

• Genetic Diversity: Achieved through independent assortment during metaphase I and crossing over during prophase I.

• prokaryotic cell only has 1 chromosome

• prokaryotic reproduces asexually by binary fission

  **How does the cell know its completed?

  ~B/c of the origin of replication

  **Viruses don’t fall into eukaryotic or prokaryotic but they have genetic material, RNA, and DNA

  ** Viruses can’t reproduce on their own

• eukaryotic have more than 1 chromosomes

• humans have 46 chromosomes

• chromosomes have to be completed in eukaryotic is called karyotype

• 1 allele is considered 1 gene

• allele is a sequence variant meaning arising by mutation over time

• gamete=reproductive cell that has half the number of chromosomes of normal cell

• each chromatid is a half duplicated chromosomes

• centromere (DNA that is hidden beneath the kinetochore)

• mitosis: cell division that produces two identical danger cells, each with the same number of chromosomes as the original cell

• mitosis happens in somatic cells(body cells, Ex: cells in arm)

  -True or False:

  cells spend most of their time in the M phase

  ~False

  -True or False:

  cells keep dividing while they’re going through mitosis

  ~False

• S phase doubles the number of chromatids but the number of chromosomes doesn’t change

• a chromosome is considered a chromatid b/c its gone through the S phase

  Prokaryotic vs. Eukaryotic Cells

  • Prokaryotic Cells:

    • Lack a nucleus, contain circular DNA.

    • Size: Relatively small, between 1 and 10 μm.

    • Organelles: Do not contain membrane-bound organelles.

    • Example: Escherichia coli.

  • Eukaryotic Cells:

    • Contain a nucleus and linear DNA.

    • Size: Larger, measuring between 10 and 100 μm.

    • Organelles: Include membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus.

    • Example: Animal and plant cells.

  Structure of Prokaryotic Cells

  • Components of a bacterial cell:

    • Outer Membrane and Cell Wall: Protect the cell and maintain shape.

    • Nucleoid Region: Contains the bacterial chromosome.

    • Flagella: Aid in movement.

    • Plasma Membrane: Encloses the cytoplasm.

    • Ribosomes: Sites of protein synthesis.

  Chromosomes in Eukaryotic Cells

  • Eukaryotic Chromosomes:

    • Are linear and highly condensed within the nucleus.

    • DNA is complexed with histone proteins to form chromatin.

  • Human Chromosomes:

    • Diploid (2n) organism has 23 pairs of chromosomes (46 total), consisting of autosomes and sex (germ) chromosomes (XX or XY).

  Concepts of Genes and Alleles

  • Alleles: Variants of genes found at specific loci on homologous chromosomes.

    • Dominant (e.g., A) and recessive alleles (e.g., a) can code for different traits (e.g., hair color).

  Cellular Reproduction: Mitosis and Meiosis

  Mitosis

  • Responsible for asexual reproduction, producing identical somatic cells.

  • Stages include:

    • Interphase: Preparation phase where DNA is replicated.

    • M Phase: Includes mitosis and cytokinesis.

    • Subdivided into Prophase, Metaphase, Anaphase, and Telophase.

  • Cytokinesis: Division of the cytoplasm, resulting in two identical daughter cells.

  Meiosis

  • A process used in sexual reproduction to produce haploid gametes.

  • Consists of two rounds of division: Meiosis I and Meiosis II.

  • Meiosis I: Homologous chromosomes separate (reduction division).

    • Leads to a reduction in chromosome number from diploid (2n) to haploid (n).

  • Meiosis II: Similar to mitosis, sister chromatids separate without any reduction in chromosome number.

  • Crossing Over: Occurs during Prophase I, allowing genetic variation among gametes.

  Cell Cycle Regulation

  • Cell cycle includes G1, S (DNA synthesis), G2, and M phases.

  • Checkpoints ensure that cells are ready to proceed to the next phase, monitoring DNA integrity and cell size.

  • Proteins include cyclins and cyclin-dependent kinases (cdks) which control progression through the cycle.

  • G1, G2, and Metaphase Checkpoints: Ensure proper conditions for division.

  Genetic Variation and Its Importance

  • Variation introduced during meiosis (crossing over and independent assortment) promotes genetic diversity in gametes.

  • Sexual reproduction allows offspring to carry varied combinations of traits, giving a survival advantage in changing environments.

  Conclusion

  • Understanding cellular structures, chromosome function, and reproduction processes is critical in the study of biology. These processes are fundamental for life's continuity, genetic diversity, and evolution.

  • cytokinesis is a processes where cleavage furrow occurs

  • When do the chromosomes condense and become highly visible?

  • During what type stage does the nuclear envelope start what stage does it breakdown, what stage does it finish?

  • True or False: The number of chromosomes at the beginning of mitosis is the same at the end.

    ~True

Prokaryotic vs. Eukaryotic Cells and Chromosomes

Prokaryotic Cells

• Cell Features:

  • Cell wall.

  • Plasma membrane.

  • Ribosomes in cytoplasm.

  • DNA in the nucleoid region.

  • Flagellum.

• Cell Diameter: Relatively small, from 1 to 10 μm.

• Genome: Usually one circular DNA molecule.

• DNA: Not complexed with histones in eubacteria; some histones in archaea.

• Amount of DNA: Relatively small.

• Membrane-bounded Organelles: Absent.

• Cytoskeleton: Absent.

• Nucleus: Absent

• Reproduction: Asexual via binary fission. Escherichia coli can divide every 20 minutes. During binary fission, the bacterial cell replicates its chromosome, and the cell divides into two daughter cells.

• DNA state: Circular DNA is NOT complexed with histones.

Binary Fission

• The prokaryotic cell contains a single circular chromosome.

• As the chromosome replicates, the origins segregate to opposite sides.

• The origins are anchored to opposite sides of the cell.

• The cell divides. Each new cell has an identical copy of the original chromosome.

Eukaryotic Cells

• Cell Features:

  • Nucleus with a nuclear envelope.

  • Endoplasmic reticulum.

  • Ribosomes.

  • Mitochondrion.

  • Vacuole (in plant cells).

  • Chloroplast (in plant cells).

  • Golgi apparatus.

  • Plasma membrane.

  • Cell wall (in plant cells).

  • Lysosome

  • Centriole

  • Microtubule

  • Microfilament

• Cell Diameter: Relatively large, from 10 to 100 μm.

• Genome: Multiple linear DNA molecules.

• DNA: Complexed with histones to form chromatin.

• Amount of DNA: Relatively large.

• Membrane-bounded Organelles: Present.

• Cytoskeleton: Present.

• Chromosomes: Linear, highly condensed, and contained within a nucleus.

• Chromosome Sets: Diploid (2n) set of chromosomes and variant alleles. Humans have 23 pairs of chromosomes, including the sex chromosomes, X and Y. Males are XY, females are XX. A diploid organism has two sets of chromosomes organized as homologous pairs. One allele from each parent via the gamete.

• Genes, alleles, and loci: Genes exist in different versions called alleles. Different alleles for a particular gene occupy the same locus on homologous chromosomes.

Viruses

• Have DNA/RNA encapsulated by proteins.

• Consist of a protein coat surrounding a piece of nucleic acid (DNA or RNA).

• Not considered "living" and are dependent upon host cells for replication.

Eukaryotic Chromosomes

• A chromosome can consist of a single chromatid OR 2 sister chromatids joined by a shared centromere; both referred to as a single chromosome.

• Types:

  • Metacentric.

  • Submetacentric.

  • Acrocentric.

  • Telocentric.
    (based on location of centromere)

Cell Cycle (Interphase and M Phase)

• G1 Phase: The cell grows.

• Cells may enter G0, a non-dividing phase.

• G1/S Checkpoint: After this checkpoint, the cell is committed to dividing.

• S Phase: DNA duplicates.

• G2 Phase: The cell prepares for mitosis.

• G2/M Checkpoint: After this checkpoint, the cell can divide.

• M Phase: Mitosis and cytokinesis (cell division) take place.

• Checkpoints: Spindle-assembly checkpoint.

Cell Division Events

• For any cell to reproduce successfully, three fundamental events must take place:

  • (1) its genetic information must be copied;

  • (2) the copies of genetic information must be separated from each other;

  • (3) the cell must divide.

Asexual Reproduction via Mitosis

• Produces new somatic cells (from Greek mitos- thread).

• One purpose of cell division is asexual reproduction. This is the means by which some unicellular organisms produce new individuals. Examples: Bacteria, Amoeba, Yeast – Saccharomyces cerevisiae (Baker’s yeast)

• A second important reason for cell division is multicellularity. Plants, animals and certain fungi are derived from a single cell that has undergone repeated cell divisions. For example, Humans start out as a single fertilized egg and end up as an adult with several trillion cells

Mitosis Stages

• Interphase: The nuclear membrane is present, and chromosomes are relaxed.

• Prophase: Chromosomes condense. Each chromosome possesses two chromatids. The mitotic spindle forms.

• Prometaphase: The nuclear membrane disintegrates. Spindle microtubules attach to chromatids.

• Metaphase: Chromosomes line up on the metaphase plate. Single unpaired chromosomes line up, in random single-file fashion, during Metaphase.

• Anaphase: Sister chromatids separate and move toward opposite poles. Chromosome number transiently doubles during Anaphase.

• Telophase: Chromosomes arrive at spindle poles. The nuclear membrane re-forms, and the chromosomes relax. Chromosome number returns to original # during Telophase.

• Cytokinesis:
  (a) Cleavage of an animal cell: Cleavage furrow.
  (b) Formation of a cell plate in a plant cell: Cell plate and Phragmoplast.

Cell Cycle Summary

Mitosis Chromosome Count

• Mitosis results in no net gain/loss of chromosomes.

• The number of chromosomes per cell remains constant.

• The number of DNA molecules per cell doubles during the S phase, then halves during Anaphase,

Sexual Reproduction via Meiosis

• Produces germ cells that produce gametes. (evolved ~1.5-2 billion years ago)

• Meiosis reduces the number of chromosomes from 2n to n.

• MEIOSIS I: Reduction division.

• MEIOSIS II: Equational division.

Meiosis I

• Division of homologous chromosome pairs.

• "Reduction Division".

• # of chromosomes reduced by half.

• Homologous chromosomes separated.

• Middle Prophase I: Chromosomes begin to condense, and the spindle forms.

• Late Prophase I: Homologous chromosomes pair (Synapsis).

• Crossing over takes place, and the nuclear membrane breaks down. Chiasmata form during Late Prophase I of Meiosis I

• Metaphase I: Homologous pairs of chromosomes line up along the metaphase plate.

• Anaphase I: Homologous chromosomes separate and move toward opposite poles.

• Telophase I: Chromosomes arrive at the spindle poles and the cytoplasm divides.

Meiosis II

• Division of sister chromatids.

• "Equational Division".

• # of chromosomes unchanged.

• Sister chromatids separated.

• Resembles Mitosis.

• Prophase II: The chromosomes recondense.

• Metaphase II: Individual chromosomes line up on the equatorial plate.

• Anaphase II: Sister chromatids separate and move toward opposite poles.

• Telophase II: Chromosomes arrive at the spindle poles and the cytoplasm divides.

Meiosis Stage Summary

Chromosome Movement

• Polymerization/depolymerization of spindle microtubules "moves" chromosomes.

• Spindle microtubules are composed of tubulin subunits.

• Microtubules lengthen and shorten at either the + or the - end.

Spindle Microtubules

• Microtubules are formed by rapid polymerization of tubulin proteins.

• There are three types of spindle microtubules:

  • Aster microtubules: Important for positioning of the spindle apparatus.

  • Polar microtubules: Help to “push” the poles away from each other.

  • Kinetochore microtubules: Attach to the kinetochore, which is bound to the centromere of each individual chromosome.

Genetic Variation

• Genetic variation is introduced at 2 points during Meiosis I (not during Mitosis or Meiosis II).
  *Why do kids sometimes look just like dad, or just like mom, or a combination of mom and dad?

Variation via Crossing Over

• During late Prophase of Meiosis I.

• During crossing over in prophase I, segments of nonsister chromatids are exchanged.

Variation via Random Alignment

• Random alignment of chromosomes during Metaphase of Meiosis I.

• The # of possible combinations = 2^n, where n= the #of homologous pairs of chromosomes.

Mitosis and Meiosis Comparison