Chromosomes

Chromosomes are fundamental thread-like structures found within the nucleus of eukaryotic cells, responsible for carrying genetic information.

Chromosome Structure and Storage

• Composition: Chromosomes are made of a very long molecule of DNA (deoxyribonucleic acid) wound around proteins called histones. This DNA-protein complex is known as chromatin.

• Packaging: The long DNA molecule is extensively wound and coiled around histones to make a compact structure, allowing it to fit inside the nucleus.

• Eukaryotic vs. Prokaryotic DNA:

  • Eukaryotic cells contain linear DNA molecules organized into chromosomes within the nucleus. Mitochondria and chloroplasts in eukaryotic cells also have their own circular, shorter DNA, similar to prokaryotic DNA, and not associated with histone proteins.

  • Prokaryotic cells have shorter, circular DNA molecules that are not wound around histones; instead, they condense by supercoiling to fit in the cell.

Chromosomes and the Cell Cycle

• Duplication (Interphase): Before cell division (mitosis or meiosis), during the interphase S phase, each chromosome's DNA unravels and replicates, producing two identical copies called sister chromatids. These sister chromatids are joined in the middle by a centromere.

• Mitosis: This is a nuclear division that produces two genetically identical daughter cells from a parent cell, each with the same diploid (2n) number of chromosomes.

  • Prophase: Chromosomes condense, becoming shorter and fatter. The nuclear envelope breaks down, and the spindle begins to form.

  • Metaphase: Chromosomes (each with two chromatids) line up along the equator (middle) of the cell and attach to the spindle fibers at their centromeres.

  • Anaphase: Centromeres divide, separating sister chromatids. Spindle fibers contract, pulling chromatids (now considered individual chromosomes) to opposite poles.

  • Telophase: Chromatids reach poles, uncoil, and nuclear envelopes reform around each group, forming two new nuclei. Cytokinesis (cytoplasm division) typically finishes, resulting in two genetically identical daughter cells.

• Meiosis: This is a reductive nuclear division that occurs in reproductive organs, producing four genetically different haploid (n) gametes from a diploid (2n) parent cell. Meiosis involves two divisions (Meiosis I and Meiosis II) but only one round of DNA replication.

  • Meiosis I: Homologous chromosomes pair up and then separate, halving the chromosome number.

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

• Genetic Variation (Meiosis):

  • Crossing Over: Exchange of alleles between homologous chromosomes during Prophase I, resulting in new combinations of alleles on chromatids.

  • Independent Assortment (Random Orientation): Homologous pairs of chromosomes align and separate randomly during Meiosis I, leading to many possible combinations of maternal and paternal chromosomes in the gametes.

Chromosome Number and Types

• Fixed Number: The number of chromosomes is characteristic and fixed for each species (e.g., humans have 46, mice have 40, onions have 16).

• Haploid (n) and Diploid (2n):

  • Diploid (2n) cells contain two complete sets of chromosomes (a pair of each chromosome), one from each parent. Most normal body cells are diploid.

  • Haploid (n) cells contain one complete set of chromosomes, half the number of diploid cells. Gametes are haploid.

• Homologous Pairs: In diploid cells, chromosomes exist in homologous pairs, meaning they are the same size, have the same genes at the same locus (fixed position), but can have different alleles (versions of genes)