Chromosomes

Chromosomes

• Definition and nature

  • Rod-shaped or threadlike structures composed of condensed chromatin fibers.

  • Function as hereditary vehicles, storing and transmitting coded hereditary information.

• Discovery

  • First demonstrated in eukaryotic cells by E. Strasburger in 1875.

  • Termed "chromosomes" by W. Waldeyer in 1888.

• Behavior during Cell Division

  • Chromosomes become visible during karyokinesis for the distribution of genetic material.

  • The number of chromosomes is species-specific, with a single set in haploid (gametophytic) forms and two sets in diploid (sporophytic) forms.

• Structure and Form

  • Size and shape are distinct, more visible in metaphase and anaphase.

Chromosome Structure

• Chromatids

  • At metaphase, each chromosome consists of two symmetrical structures called sister chromatids, each containing one DNA molecule.

  • Sister chromatids are held together by the centromere and separate during anaphase.

• DNA Replication

  • DNA in each chromatid replicates during S phase to produce identical copies; hence, during G2, each chromosome has two chromatids.

• Chromonemata

  • Thin strands visible during prophase and early interphase, representing early chromatids.

  • Chromatid and chromonemata are essentially the same structure, with chromatid being the fundamental unit of chromosomes.

• Centromere

  • Area where sister chromatids are joined, appearing constricted; also known as primary constriction.

Kinetochore and Constrictions

• Kinetochore

  • Specialized structure associated with the centromere at each chromosome.

• Constriction Types

  • Secondary Constrictions: Non-staining gaps in certain chromosomes.

Types of Chromosomes

• Classification by Centromere Position

  1. Metacentric: Centromere in the median, arms are equal (V-shaped during anaphase).

  2. Sub-metacentric: Centromere situated sub-median, resulting in unequal arms (J-shaped).

  3. Acrocentric: Centromere sub-terminal, giving one short and one long arm.

  4. Telocentric: Centromere located at one end of the chromosome.

• Classification by Number of Centromeres

  • Acentric, Monocentric, Dicentric, Polycentric.

Chromatin Organization

• Nucleosome Model

  • Explains the organization of DNA and associated proteins in chromosomes, proposed by Roger Kornberg in 1974.

  • Confirmed by P. Oudet et al. in 1975.

• Structure of Nucleosomes

  • In eukaryotes, DNA is tightly bound to histones, forming nucleosomes, the fundamental unit of chromatin.

  • Each nucleosome consists of two copies of four core histones (H2A, H2B, H3, and H4).

  • DNA wraps around the histone octamer, with linker DNA acting as spacing between nucleosomes.

  • Nucleosomes give chromatin a "beads-on-a-string" appearance.

Chromatin Types

• Euchromatin vs. Heterochromatin

  • Euchromatin: Lightly stained during interphase, more accessible.

  • Heterochromatin: Stained heavily during interphase; may be constitutive or facultative.

    • Constitutive Heterochromatin: Remains permanently heterochromatic, found at centromeres.

    • Facultative Heterochromatin: Can switch between euchromatin and heterochromatin states.

Karyotype and Idiogram

• Karyotype

  • The complete set of chromosomes in a species, used to study evolutionary relationships.

  • Karyotypes can be symmetric or asymmetric, based on size variation.

• Idiogram

  • Diagrammatic representation showing chromosome morphology based on centromere position.

Giant Chromosomes

• Polytene Chromosomes

  • Large chromosomes found in the salivary glands of certain organisms; formed by longitudinal splitting of chromatids without cell division (endomitosis).

  • Drosophila polytene chromosomes show transverse bands and swellings (puffs) during growth.

• Lampbrush Chromosomes

  • Found in oocytes of some amphibians, visible without a microscope; exhibit loops during diplotene of meiotic prophase.

B Chromosomes

• Definition

  • Extra chromosomes not essential for the individual, found more commonly in plants.

  • Exhibit variable number, smaller size, and more heterochromatinization.

• Behavior

  • Often do not affect phenotype but may be detrimental in some plants.

  • May have adaptive roles, possibly originating from the gradual conversion of standard chromosomes.