Eukaryotic Chromosomes Overview

Study Notes

Chromosomes are made of chromatin, a material consisting of DNA and proteins called histones.
Histones are highly conserved proteins, indicating their crucial role in nuclear biology.
Core histones (H2A, H2B, H3, H4) form an octamer around which DNA wraps.
H1 histone, a linker histone, helps package nucleosomes.
Chromatin fibers further compact into loop domains facilitated by CTCF and cohesin.
Chromosomes exist in an extended, partially unraveled form during interphase.
During cell division, chromatin condenses, making chromosomes visible.
Condensin complexes contribute to chromosome condensation during mitosis and meiosis.

Each species has a unique chromosome number in its somatic cells.
Human somatic cells have a diploid number (2n) of chromosomes, meaning two sets of 23 chromosomes, one from each parent.
The total number of chromosomes in humans is 46.
The number of chromosomes and DNA amount don't correlate with complexity or intelligence of a species.

The cell cycle is an ordered sequence of events where a cell duplicates its contents (interphase) and divides into two (M phase).
The number of cell divisions in a cell lineage is defined.
The timing of the cell cycle varies widely between actively growing cells.
Cells spend most of their time in interphase, but the duration varies between cell types.
Interphase has two gap phases (G1 and G2) which allow cells to grow, synthesize materials, and monitor their environment before committing to S and M phases.

A nuclear division mechanism occurring in somatic cells.
Produces two identical daughter nuclei with the same chromosome sets as the parent.
Divided into five phases: prophase, prometaphase, metaphase, anaphase, and telophase.

Interphase: The cell carries out its usual activities. DNA replication occurs if the cell is preparing to divide.
Prophase: Long chromatin condenses into mitotic chromosomes, each consisting of two sister chromatids attached at the centromere. The mitotic spindle forms between centrioles and moves to opposite poles. The nuclear envelope breaks down.
Prometaphase: Spindle microtubules attach to kinetochores on the centromeres, pulling chromosomes toward the cell's midplane.
Metaphase: Chromosomes align at the equator of the cell. Spindle microtubules attach each chromosome to both poles.
Anaphase: Sister chromatids separate at their centromeres, moving to opposite poles.
Telophase: Chromosomes cluster at the poles, decondense. Nuclear envelope reforms, creating two separate nuclei.

Mitosis is a precise process that ensures each daughter cell receives a complete set of chromosomes from the parent cell.
It is essential for growth, development, and repair in multicellular organisms.

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