Biology - Cell Cycle, Cell Division and Structure of Chromosomes

Chromosomes are highly coiled structures made of DNA ($40\%$) and Histone proteins ($60\%$). The repeating unit of a chromosome is the nucleosome, consisting of DNA wrapped around a histone octamer.

A DNA nucleotide consists of three components: a Phosphate group, a Pentose sugar (Deoxyribose), and a Nitrogenous base ($A$, $T$, $G$, or $C$).

Specific nitrogenous base pairing occurs via hydrogen bonds: Adenine ($A$) pairs with Thymine ($T$) with two bonds ($A=T$), and Guanine ($G$) pairs with Cytosine ($C$) with three bonds ($G \equiv C$).

The Cell Cycle is divided into Interphase ($G_1$, $S$, $G_2$) and the $M$-Phase (Mitosis).

During the $S$-phase (Synthesis phase), DNA replication occurs, doubling the DNA content from $2C$ to $4C$, although the chromosome number remains $2n$.

Mitosis consists of four main stages: Prophase (disappearance of nucleolus), Metaphase (alignment at the $equator$), Anaphase (splitting of centromeres), and Telophase (reappearance of nuclear membrane).

Karyokinesis refers to the division of the nucleus, while Cytokinesis refers to the division of the cytoplasm.

Meiosis occurs in germ cells to produce gametes. It involves two divisions, reducing the chromosome number from diploid ($2n$) to haploid ($n$), ensuring constant chromosome numbers across generations.