L11 DNA packaging and histone code

Karyotype

The visual representation of the full chromosome set of an organism. It includes the number, size, and shape of chromosomes.

Centromere

The constricted region of a chromosome where sister chromatids attach and where spindle fibers bind during cell division.

Metacentric

Chromosomes with the centromere in the center, resulting in arms of equal length.

Submetacentric

Chromosomes with the centromere slightly off-center, creating arms of unequal length.

Acrocentric

• Chromosomes with the centromere close to one end, leading to a very short p arm and a long q arm.

Telocentric

Chromosomes with the centromere at the terminal end, possessing only one visible arm. Humans lack telocentric chromosomes.

Holocentric

Chromosomes with centromeric activity distributed along their entire length, common in certain species like nematodes

P5.5: Understand the process of nondisjunction and the resultant gamete products.

• Nondisjunction occurs when chromosomes fail to separate properly during meiosis I or II.

  • Meiosis I Nondisjunction: Homologous chromosomes fail to separate, resulting in two gametes with an extra chromosome and two with one missing chromosome.

  • Meiosis II Nondisjunction: Sister chromatids fail to separate, producing one gamete with an extra chromosome, one with one missing chromosome, and two normal gametes.

• The result is aneuploidy, which can lead to disorders like Down syndrome (trisomy 21)​

Kinetochore

A protein complex at the centromere that attaches chromosomes to the spindle microtubules, enabling their segregation during mitosis and meiosis.

Mitotic Spindle Formation

Composed of microtubules, the spindle organizes and separates chromosomes. It arises from centrosomes and attaches to kinetochores during metaphase.

Chromatin Structure

DNA and proteins form chromatin, which has two main types:

• Euchromatin: Loosely packed and transcriptionally active.

• Heterochromatin: Tightly packed and transcriptionally inactive. Constitutive heterochromatin surrounds the centromere​

Nucleosomes

The fundamental unit of chromatin, consisting of DNA wrapped around a histone octamer. Each nucleosome protects ~146 bp of DNA.

Histones

Basic proteins rich in lysine and arginine that form the nucleosome core. Types include H1, H2A, H2B, H3, and H4. H1 helps compact DNA by packing nucleosomes together.

DNA Compaction:

• DNA wraps around nucleosomes.

• Nucleosomes coil into a 30 nm fiber, facilitated by histone tails.

• Higher-order structures, including solenoid loops, attach to scaffolds.

• Scaffold attachment regions (SARs) on DNA anchor loops to maintain structure​