CHAPTER 12 REVIEW-1

Chapter 12 Review: DNA Structure and Replication

Prokaryotic vs. Eukaryotic DNA

• Prokaryotic DNA:

  • Simple structure, circular shape, typically located in the nucleoid region of the cell.

  • Lacks membrane-bound organelles.

  • Contains fewer genes and smaller overall size compared to eukaryotic DNA.

• Eukaryotic DNA:

  • Complex structure, linear shape, found within the nucleus of the cell.

  • Contains multiple chromosomes, larger size, and more genes.

  • Associated with histone proteins, forming chromatin.

DNA Replication

• Before DNA Replication: Chromosomes exist as chromatids (single strands).

• After DNA Replication: Each chromosome consists of two sister chromatids, linked at a centromere.

Chromosomes and Chromatids

• Chromosomes:

  • Composed of DNA tightly coiled around histone proteins.

  • Present in the form of single chromatids before replication and as paired sister chromatids after replication.

• Chromatids:

  • Identical halves of a duplicated chromosome, joined at the centromere.

• Stage Identification:

  • Chromosomes are visible during prophase, metaphase during mitosis.

  • Chromatids are present in anaphase.

Centromeres

• Structure:

  • The constricted region of the chromosome where sister chromatids are joined.

• Purpose and Function:

  • Serves as the attachment point for spindle fibers during cell division.

  • Crucial for proper separation of chromatids during mitosis and meiosis.

Kinetochores

• Structure:

  • Protein complex that assembles at the centromere during cell division.

• Function:

  • Attaches to spindle fibers and helps facilitate the movement of chromosomes during mitosis and meiosis.

Haploid and Diploid Cells

• Haploid Cells (n):

  • Contain one set of chromosomes.

  • Example:

    • Humans: 23 chromosomes (gametes).

    • Fruit flies: 4 chromosomes.

• Diploid Cells (2n):

  • Contain two sets of chromosomes.

  • Example:

    • Humans: 46 chromosomes (somatic cells).

    • Dogs: 78 chromosomes.

Stages of Mitosis and Meiosis

• Mitosis:

  • Prophase: DNA condenses into chromosomes, spindle apparatus forms.

  • Metaphase: Chromosomes align at the cell's equatorial plate.

  • Anaphase: Sister chromatids are pulled apart.

  • Telophase: Chromosomes de-condense, nuclear envelope reforms.

• Meiosis:

  • Similar stages to mitosis but involves two rounds of division (Meiosis I and Meiosis II).

  • Prophase I: Homologous chromosomes undergo recombination.

  • Metaphase I: Paired homologous chromosomes align.

  • Anaphase I: Homologous chromosomes separate.

  • Telophase I: Two haploid cells form.

Amount of DNA and Chromosomes in Cell Cycle Phases

• G0 Phase:

  • Cells are in a resting state, not actively preparing to divide.

• G1 Phase:

  • Cells grow and synthesize proteins.

  • Diploid cells have full DNA content.

• S Phase:

  • DNA replication occurs, resulting in duplicated chromosomes (sister chromatids).

• G2 Phase:

  • Prepares for mitosis; checks DNA for errors.

  • Structure and number of chromosomes are still doubled.

Structures Involved in Mitosis and Meiosis

• Centrosome:

  • Organizes microtubules and important for spindle formation.

• Centrioles:

  • Located within the centrosome; aid in forming the spindle fibers during mitosis and meiosis.

Prokaryotic vs. Eukaryotic Cells

• Differences: Prokaryotic cells are generally smaller, lack a nucleus, and have simpler structures compared to eukaryotic cells, which have compartmentalized organelles and are typically larger.

• Plant vs. Animal Cells:

  • Plant cells have a cell wall and chloroplasts; animal cells have centrioles and lysosomes.