Cell Division and Chromosomes

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 have 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