The Nucleus: DNA and Chromosome Structure

Comprehensive vocabulary flashcards reviewing DNA structure, chromosome organization, nucleosome assembly, and chromatin regulation mechanisms from Essential Cell Biology Chapter 5.

Codon

A group of three consecutive nucleotides in RNA that specifies one amino acid during protein synthesis.

Complementary Base-Pairing

The specific matching of bases in a DNA molecule where Adenine ($A$) pairs with Thymine ($T$), and Guanine ($G$) pairs with Cytosine ($C$).

Central Dogma

The framework describing the directional flow of genetic information: DNA replication (DNA synthesis), transcription (RNA synthesis), translation (protein synthesis), and reverse-transcription.

Chromatin

The complex of eukaryotic DNA together with histone and non-histone proteins found within the cell nucleus.

Histones

Positively charged basic proteins rich in lysine and arginine that bind negatively charged DNA to form chromatin subunits.

Nucleosomal Histones

The core histones consisting of H2A, H2B, H3, and H4, ranging in size from $102$ to $135$ amino acids, which associate to form an octamer.

Histone H1

A linker histone of approximately $220$ amino acids ($6$ subtypes) that associates with linker DNA to help pull nucleosomes together.

Nucleosome

The basic structural and functional unit of chromatin, composed of approximately $200$ nucleotide pairs of DNA wrapped around an octameric core of histone proteins.

Nucleosome Core Particle (NCP)

An $11\,\text{nm}$ particle released by nuclease digestion consisting of $147$ nucleotide pairs of DNA wrapped around an octamer formed by an H3-H4 tetramer and two H2A-H2B dimers.

Chromatosome

A chromatin sub-structure consisting of the nucleosome core particle ($147\,\text{bp}$ DNA wrapped around the core histone octamer), linker DNA, and linker histone H1.

Telomere

A specialized DNA sequence capping both ends of a eukaryotic chromosome that prevents degradation and protects chromosome integrity.

Centromere

A specialized chromosome region that allows attachment to the mitotic spindle during M phase to ensure proper distribution of duplicated chromosomes during cell division.

Replication Origin

A specialized sequence along the chromosome where the duplication of DNA initiates during interphase.

Chromosome Territories

Individual, non-overlapping spatial domains occupied by specific chromosomes within the interphase nucleus.

Heterochromatin

Highly condensed, transcriptionally inactive chromatin localized predominantly at telomeres and centromeres.

Euchromatin

Decondensed, open chromatin state that permits access to RNA polymerases and other proteins for active gene transcription.

Histone Tail Modifications

Covalent chemical modifications (such as acetylation, methylation, and phosphorylation) on histone N-termini that serve as docking sites for regulatory proteins to condense or decondense chromatin.

H3K9me3 (Histone H3 Lysine 9 Trimethylation)

A specific histone tail mark resulting in heterochromatin formation and gene silencing.

H3K4me3 / H3K9ac

A set of histone H3 tail modifications consisting of trimethylation on lysine 4 and acetylation on lysine 9 that promotes gene expression.

Histone Acetylation

A dynamic chromatin modification where 'writer' enzymes add acetyl groups to open chromatin ('Gene On'), 'readers' recognize them, and 'eraser' enzymes remove them to compact chromatin ('Gene Off').

DNA Methylation

An inhibitory epigenetic modification occurring at CpG sequences to produce 5-methylcytidine, catalyzed by DNA methyltransferases (DNMT) to maintain genetic memory.

Chromatin Remodeling Complexes

Protein complexes that mediate cyclic nucleosome disruption and re-formation to expose localized regions of DNA for transcription, replication, and repair.