Chromosomes and Cellular Reproduction

Introduction

This chapter explores the structure and function of chromosomes, the differences between prokaryotic and eukaryotic cells, and the processes of cellular reproduction, including mitosis. Understanding these concepts is fundamental to the study of genetics and cell biology.

Prokaryote and Eukaryote Cells

Cellular Organization

• Prokaryotic cells are unicellular organisms lacking membrane-bound organelles. Their genetic material is not enclosed within a nucleus.

• Eukaryotic cells can be unicellular or multicellular and possess a compartmentalized structure with membrane-bound organelles, including a nucleus that houses genetic material.

• Viruses are not classified as prokaryotic or eukaryotic. They consist of a nucleic acid (DNA or RNA) surrounded by a protein coat and require a host cell for replication.

Comparison of Prokaryotic and Eukaryotic Cells

Cell Reproduction

General Requirements

• Genetic information must be accurately copied (DNA replication).

• Copies of genetic information must be separated into daughter cells.

• The cell must physically divide (cytokinesis).

• Processes differ between prokaryotes and eukaryotes due to structural differences.

Prokaryotic Cell Reproduction

• Occurs by binary fission, a simple division process.

• Replication begins at a single origin of replication and proceeds rapidly.

• Each daughter cell receives an identical copy of the circular chromosome.

Eukaryotic Cell Reproduction

• More complex due to multiple chromosomes and organelles.

• Involves the nuclear matrix to ensure proper chromosome segregation.

• Cell division includes both mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Chromosome Structure

Chromosome Number and Types

• Somatic (body) cells are typically diploid, containing two sets of chromosomes (homologous pairs).

• Gametes (sperm and egg) are haploid, containing one set of chromosomes.

• Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY).

Chromatin and Nucleosome Structure

• Chromatin is the complex of DNA and proteins (mainly histones) that forms chromosomes.

• The nucleosome is the basic unit of chromatin, consisting of DNA wrapped around an octamer of histone proteins (two each of H2A, H2B, H3, and H4).

• Linker DNA and histone H1 help organize nucleosomes into higher-order structures.

Key Chromosome Regions

• Centromere: The constricted region where spindle fibers attach during cell division; essential for proper chromosome segregation.

• Telomere: The ends of linear chromosomes, important for stability and limiting cell division; play a role in aging.

• Origins of replication: Specific sites where DNA synthesis begins.

Chromosome Morphology

• Classified by centromere position:

  • Metacentric: Centromere in the middle

  • Submetacentric: Centromere slightly off-center

  • Acrocentric: Centromere near one end

  • Telocentric: Centromere at the very end (not found in humans)

Human Chromosome Facts

• Gene density does not correlate with chromosome size.

• Chromosomes 19 and 22 are small but have high gene density (e.g., 27 and 23 genes/Mb, respectively).

• Chromosomes 4 and 5 are large but have low gene density (e.g., 8 and 9 genes/Mb, respectively).

The Cell Cycle and Mitosis

The Cell Cycle

• Interphase: Period between cell divisions; includes G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis).

• M phase (Mitotic phase): Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

• Checkpoints (G1, G2, and spindle assembly) ensure proper progression and fidelity of cell division.

Stages of Mitosis

• Prophase: Chromosomes condense and become visible; spindle apparatus begins to form.

• Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores at centromeres.

• Metaphase: Chromosomes align at the metaphase plate (center of the cell).

• Anaphase: Sister chromatids separate and move toward opposite poles; each chromatid is now considered an individual chromosome.

• Telophase: Nuclear envelope reforms around each set of chromosomes; chromosomes decondense; nucleoli reappear.

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

Genetic Consequences of the Cell Cycle

• Produces two cells genetically identical to each other and to the original parent cell.

• Each new cell receives a full set of chromosomes and approximately half the cytoplasm and organelles of the parent cell.

Summary Table: Stages of Mitosis

Key Terms and Definitions

• Chromatid: One of two identical halves of a replicated chromosome.

• Homologous chromosomes: Chromosome pairs, one from each parent, that are similar in length, gene position, and centromere location.

• Kinetochore: Protein structure on the centromere where spindle fibers attach during cell division.

• Spindle apparatus: Structure made of microtubules that segregates chromosomes during mitosis and meiosis.

Example: Human Karyotype

• Humans have 46 chromosomes (23 pairs) in somatic cells.

• Autosomes: 22 pairs; Sex chromosomes: 1 pair (XX or XY).

• Spectral karyotyping (SKY): A technique for visualizing all chromosomes in different colors to detect structural abnormalities.

Additional info: The notes reference videos and handouts for further study, which may provide visual reinforcement of mitosis and chromosome structure.