Cellular Basis of Reproduction and Inheritance: Study Guide

Cellular Basis of Reproduction and Inheritance

Chromosome Structure and Terminology

The structure and terminology of chromosomes are fundamental to understanding cell division and inheritance.

• Chromosome: A thread-like structure composed of DNA and proteins, carrying genetic information.

• Centromere: The constricted region of a chromosome where sister chromatids are joined and where spindle fibers attach during cell division.

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

• Sister Chromatids: Two identical chromatids joined at the centromere, formed during DNA replication.

• Sister Chromosome: Additional info: This term is less commonly used; typically refers to homologous chromosomes, which are pairs of chromosomes (one from each parent) with the same genes but possibly different alleles.

• Example: During mitosis, each chromosome is duplicated to form two sister chromatids.

Eukaryotic Cell Cycle

The eukaryotic cell cycle consists of a series of stages that prepare the cell for division and ensure accurate genetic transmission.

• Interphase: The period of cell growth and DNA replication, subdivided into G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis).

• M Phase (Mitotic Phase): Includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

• Diagram: Additional info: The cell cycle is often depicted as a circular flow: G1 → S → G2 → M.

• Key Events:

  • G1: Cell grows and prepares for DNA replication.

  • S: DNA is replicated, forming sister chromatids.

  • G2: Cell prepares for mitosis.

  • M: Chromosomes are separated and cell divides.

Mitosis: Stages and Structures

Mitosis is the process by which a eukaryotic cell divides its nucleus, ensuring each daughter cell receives an identical set of chromosomes.

• Prophase: Chromosomes condense, spindle fibers form, nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids are pulled apart toward opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

• Significant Structures: Spindle fibers, centrioles (in animal cells), chromosomes, centromeres.

• Example: Skin cells divide by mitosis to replace lost or damaged cells.

Cytokinesis in Animal vs. Plant Cells

Cytokinesis is the division of the cytoplasm, differing in mechanism between animal and plant cells.

• Animal Cells: Cytokinesis occurs via a cleavage furrow, where the cell membrane pinches inward.

• Plant Cells: Cytokinesis occurs via formation of a cell plate, which develops into a new cell wall.

• Comparison Table:

Cell Type	Cytokinesis Mechanism
Animal	Cleavage furrow (membrane pinching)
Plant	Cell plate formation (new cell wall)

Controls of Cell Division

Cell division is regulated by internal and external signals to ensure proper growth and development.

• Checkpoints: G1, G2, and M checkpoints monitor cell cycle progression.

• Growth Factors: Proteins that stimulate cell division.

• Contact Inhibition: Cells stop dividing when they touch neighboring cells.

• Example: Cancer cells bypass normal controls and divide uncontrollably.

Diploid and Haploid Definitions

Understanding ploidy is essential for grasping inheritance and reproduction.

• Diploid (2n): Cells with two sets of chromosomes, one from each parent.

• Haploid (n): Cells with one set of chromosomes, typical of gametes.

• Example: Human somatic cells are diploid (46 chromosomes), gametes are haploid (23 chromosomes).

Sexual Reproduction: Purpose and Advantages

Sexual reproduction involves the fusion of haploid gametes, increasing genetic diversity.

• Genetic Variation: Offspring inherit a mix of genes from both parents.

• Adaptation: Variation enhances survival in changing environments.

• Example: Most animals and plants reproduce sexually.

Meiosis: Stages and Structures

Meiosis is a specialized cell division that produces haploid gametes, essential for sexual reproduction.

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.

• Meiosis II: Sister chromatids separate, similar to mitosis.

• Significant Structures: Synaptonemal complex, spindle fibers, homologous chromosomes.

• Diagram: Additional info: Meiosis is often depicted as two consecutive divisions: Meiosis I and Meiosis II.

• Example: Formation of sperm and egg cells in animals.

Homologous Recombination

Homologous recombination is the exchange of genetic material between homologous chromosomes during meiosis.

• Crossing Over: Occurs during Prophase I of meiosis, increasing genetic diversity.

• Chiasmata: Sites where crossing over occurs.

• Example: Recombination creates new allele combinations in offspring.

Chromosomal Disorders: Un-standard Chromosome Numbers

Abnormal chromosome numbers can lead to genetic disorders.

• Aneuploidy: Presence of an abnormal number of chromosomes.

• Examples:

  • Down syndrome (trisomy 21)

  • Turner syndrome (monosomy X)

  • Klinefelter syndrome (XXY)

Sex Determination Systems

Different organisms use various systems to determine sex.

• XY System: Used by mammals; males are XY, females are XX.

• ZW System: Used by birds; males are ZZ, females are ZW.

• XO System: Used by some insects; males are XO, females are XX.

• Environmental Sex Determination: Some reptiles determine sex by temperature during egg incubation.

• Example: In humans, the presence of a Y chromosome determines maleness.