DNA Packaging and Gene Regulation

Vocabulary practice flashcards covering chromatin packaging levels, eukaryotic transcription factors, enhancer-mediated gene regulation, and post-transcriptional control mechanisms.

Chromatin

A relatively loose complex composed of DNA and associated proteins inside the nucleus, representing an unpackaged state compared to a fully compacted chromosome.

Chromosome

A highly compacted structure composed of DNA and protein that forms fully during the metaphase stage of the cell cycle to allow proper separation during anaphase.

Nucleosome

The first level of DNA packaging, consisting of double-stranded DNA wrapped twice around a histone core protein, producing a 'beads on a string' appearance.

Histone octamer

The protein core of a nucleosome, composed of $8$ polypeptides: two molecules each of H2A, H2B, H3, and H4.

H1 histone

A type of histone protein that is not part of the nucleosome core but resides outside of it, functioning in higher levels of DNA packaging by connecting nucleosomes together.

Euchromatin

A loosely packaged form of chromatin in which nucleotide sequences of DNA remain accessible to RNA polymerase and other proteins for gene expression.

Heterochromatin

A tightly packaged form of chromatin in which DNA is completely sequestered by proteins, preventing RNA polymerase and enzymes from accessing the genes and keeping them turned off.

$10\,\text{nm}$ fiber

The first level of chromatin compaction, having a diameter of $10\,\text{nm}$ and consisting of nucleosome beads spaced along a DNA string.

$30\,\text{nm}$ fiber

A higher level of chromatin packaging with a $30\,\text{nm}$ diameter, formed when nucleosomes coil further and connect to one another through H1 histone proteins.

$300\,\text{nm}$ fiber

A level of chromatin packaging formed when the $30\,\text{nm}$ fiber is folded into looped domains with the assistance of specialized proteins.

Differential gene expression

The process by which cells containing the same identical genome express different subsets of genes to fulfill distinct cellular functions or developmental roles.

Histone acetylation

The addition of acetyl chemical groups to histone tails, which alters histone conformation, loosens DNA from the histones, and exposes promoter sequences for transcription.

General transcription factors

Ubiquitous proteins found in all cells that bind near the promoter at proximal control elements to weakly recruit RNA polymerase II, establishing basal background-level transcription.

Specific transcription factors

Regulatory proteins, specifically activators and repressors, that bind to distal control elements to either increase transcription multifold above basal levels or decrease it.

Basal level transcription

The low, background level of transcription driven by general transcription factors, representing the persistent ambient expression that exists in living cells rather than zero transcription.

Proximal control elements

DNA sequences located close to the gene near the promoter where general transcription factors bind to initiate basal transcription.

Distal control elements

Regulatory DNA sequences located far from the gene they control—either upstream, downstream, or hundreds to thousands of nucleotides away.

Enhancers

Distal DNA sequences to which activator or repressor proteins bind to stimulate high-level transcription or down-regulate expression.

DNA-bending proteins

Proteins that physically bend the DNA to loop it, bringing activator proteins bound at distant enhancers in direct proximity to the promoter and transcription machinery.

Combinatorial control

The regulatory mechanism where a specific combination of enhancer sequences and available activator proteins dictates which genes are transcribed in a given cell type.

MicroRNA (miRNA)

Small regulatory RNA molecules complementary to target mRNAs that regulate expression post-transcriptionally by degrading the mRNA (if at least $7$ nucleotides match) or blocking translation.

MyoD

A master transcription factor protein whose expression commits an embryonic precursor cell into a determined myoblast that subsequently activates muscle-specific genes such as myosin.