Chromosomes and Chromatin

Structure and Types of Chromosomes

Chromosomes are highly organized structures of DNA and protein found in the nucleus of eukaryotic cells. They carry genetic information essential for inheritance and cellular function.

• Homologous Chromosomes: Chromosomes that exist in pairs, one from each parent, and are similar in size, shape, and gene content.

• Autosomes: Non-sex chromosomes; humans have 22 pairs.

• Sex Chromosomes: Determine biological sex (XX in females, XY in males); X and Y are not homologous.

• Sister Chromatids: Identical copies of a chromosome connected by a centromere, formed during DNA replication.

Chromosomes vs. Chromatin

DNA exists in two forms within the cell, depending on the stage of the cell cycle:

• Chromatin: Loosely packed DNA, present during interphase, allowing access for transcription and replication.

• Chromosomes: Condensed, tightly packed DNA, visible during cell division, ensuring accurate segregation.

Gene Loci and Alleles

Genes are specific sequences of DNA located on chromosomes. Each gene has a specific position, or locus, and may exist in different forms called alleles.

• Locus: The physical location of a gene on a chromosome.

• Allele: A variant form of a gene; different alleles can result in different traits.

The Cell Cycle

Phases of the Eukaryotic Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (G1, S, G2) and the mitotic (M) phase.

• G1 Phase (Gap 1): Cell growth and normal metabolic roles.

• S Phase (Synthesis): DNA replication occurs, resulting in duplicated chromosomes.

• G2 Phase (Gap 2): Preparation for mitosis; further growth and protein synthesis.

• M Phase (Mitosis and Cytokinesis): Division of the nucleus and cytoplasm to form two daughter cells.

Cell Division: Mitosis

Purpose and Overview

Mitosis is the process by which a eukaryotic cell divides its nucleus and genetic material to produce two genetically identical daughter cells. It is essential for growth, tissue repair, and asexual reproduction.

• Prophase: Chromatin condenses into visible chromosomes; spindle apparatus forms; nucleolus disappears.

• Metaphase: Chromosomes align at the metaphase plate; spindle fibers attach to centromeres.

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

• Telophase: Chromosomes decondense; nuclear envelope reforms; cytokinesis begins.

Mitotic Spindle and Chromosome Movement

The mitotic spindle is a structure made of microtubules that segregates chromosomes during mitosis.

• Centrosomes: Organize spindle fibers at opposite poles of the cell.

• Kinetochores: Protein complexes on centromeres where spindle fibers attach.

• Asters: Radial arrays of microtubules that help position the spindle apparatus.

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two separate daughter cells. The process differs between animal and plant cells:

• Animal Cells: Formation of a cleavage furrow that pinches the cell in two.

• Plant Cells: Formation of a cell plate that develops into a new cell wall.

Cell Division: Meiosis

Overview and Purpose

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing four genetically unique haploid gametes. It is essential for sexual reproduction and genetic diversity.

• Meiosis I: Homologous chromosomes separate, reducing ploidy from diploid to haploid.

• Meiosis II: Sister chromatids separate, similar to mitosis, but no further DNA replication occurs.

Genetic Variation in Sexual Reproduction

Sexual reproduction introduces genetic variation through several mechanisms:

• Crossing Over: Exchange of genetic material between homologous chromosomes during prophase I of meiosis, resulting in new allele combinations.

• Independent Assortment: Random distribution of maternal and paternal chromosomes to gametes.

• Fertilization: Random fusion of gametes further increases genetic diversity.

Example: Each human can produce over 8.3 million different gametes by independent assortment alone; a couple can produce over 64 trillion different zygotes (not including variation from crossing over).

Comparison of Mitosis and Meiosis

Prokaryotic Cell Division

Binary Fission

Prokaryotes, such as bacteria, divide by a simpler process called binary fission. The single, circular chromosome is duplicated, and the cell splits into two genetically identical daughter cells.

Applications and Importance of Cell Division

• Reproduction: Unicellular organisms reproduce by cell division.

• Growth and Development: Multicellular organisms grow by increasing cell number.

• Tissue Renewal: Damaged or old cells are replaced through mitosis.

Key Terms and Definitions

• Centromere: Region where sister chromatids are joined and spindle fibers attach during cell division.

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

• Spindle Apparatus: Structure that separates chromosomes during cell division.

• Cytokinesis: Division of the cytoplasm to form two daughter cells.

• Ploidy: Number of sets of chromosomes in a cell (haploid = 1 set, diploid = 2 sets).

Summary Table: Stages of Mitosis

Key Equations

• Number of possible gametes by independent assortment:

$2^{n}$, where n = number of chromosome pairs

• Number of possible zygotes from two parents:

$(2^{n}) \times (2^{n}) = 2^{2n}$

Additional info: These equations do not account for genetic variation introduced by crossing over.