Cell Biology & Macromolecules

Histones $\text{H}_3$ and $\text{H}_4$ are among the most highly conserved proteins in all of eukaryotic evolution, differing by only a couple of amino acids between organisms as distant as plants (peas) and mammals (cattle). Linker histone $\text{H}_1$ is much more variable.

Molecular geometry of methane ($\text{CH}_4$):

• Linear geometry ($180^\circ$) occurs in $sp$-hybridized molecules such as $\text{CO}_2$ or acetylene.
• In methane ($\text{CH}_4$), the central carbon atom undergoes $sp^3$ hybridization with four equivalent bonding electron pairs directed toward the corners of a regular tetrahedron, yielding a tetrahedral geometry with bond angles of $109.5^\circ$.
• Trigonal planar geometry ($120^\circ$) occurs in $sp^2$-hybridized molecules like boron trifluoride ($\text{BF}_3$).
• Trigonal pyramidal geometry ($107^\circ$) occurs when an $sp^3$-hybridized central atom possesses one lone pair, such as in ammonia ($\text{NH}_3$).

Structural organization of eukaryotic genome:

• Histochromosomes is a non-existent biological term.
• The fundamental repeating unit of eukaryotic DNA packaging is the nucleosome ("beads-on-a-string"). A continuous tandem assembly of repeating nucleosomes linked by linker DNA and folded into higher-order fibers ($10\text{ nm}$ fiber, $30\text{ nm}$ solenoid) constitutes chromatin, which condenses into visible chromosomes during mitosis.
• chromatosome refers to a single individual nucleosome core particle plus the linker histone H1 and adjacent linker DNA, not the collective macromolecular assembly.
• Histosomes is not a valid genetic packaging term.