Chromatin and Chromosome Structure

This flashcard set covers the fundamental vocabulary of chromatin organization, ranging from basic nucleosome structure and histone proteins to advanced folding models, euchromatin, heterochromatin, telomeres, and centromeres.

Chromatin

The physiological combination of DNA and protein within a cell.

Histones

A type of proteins that associate with DNA to provide the first order or level of structure to chromatin.

Micrococcal Nuclease

An enzyme used to digest nuclear DNA; when proteins are eliminated, it typically produces DNA fragments in intervals of $200$ base pairs ($200$, $400$, $600$, $800$, $1000$).

Nucleosome

The basic unit of chromatin structure representing approximately $2001,bp$ of DNA.

Chromatosome

The structural unit consisting of a nucleosome core particle and Histone H1, encompassing approximately $166\,bp$ of DNA.

Nucleosome core particle

A structure consisting of approximately $147\,bp$ of DNA wrapped around an octamer of histones H2A, H2B, H3, and H4, resulting in a 6-fold shortening of DNA.

"Beads on a string"

The first order level of chromatin structure, appearing as $11\,nm$ (or $10\,nm$) fibers in electron microscopy.

Solenoid Form

The second order level of chromatin structure characterized as a $30\,nm$ chromatin fiber, which provides a 50-fold shortening of DNA.

Looped Form

The third order level of chromatin structure where further folding of the chromatin fiber is mediated by scaffolding proteins.

Euchromatin

The relatively decondensed form of chromatin distributed throughout the nucleus where genes are transcribed and DNA is replicated.

Heterochromatin

A very highly condensed, transcriptionally inactive state representing approximately $10\%$ of all chromatin, resembling the state of chromatin during mitosis.

Topologically Associated Domains (TADs)

Regions of the genome at an intermediate scale ($10\,kb$ to a few $Mb$) that regulate local contact frequency through chromatin loops.

Sister Chromatids

Exact copies of one another that are highly compact during metaphase and cannot be used as templates for transcription.

Telomeres

The ends of chromosomes containing telomeric repeats and T-loops; they do not encode for any gene product.

Shelterin

A protein complex containing Telomere Repeat Binding Factors (TRF1 and TRF2) that maintains the structure of telomeres.

Kinetochore

A specialized structure consisting of proteins attached to the centromere that serves as the attachment site for spindle fibers.

CENP-A

An $H3$-like histone found in all centromeres (formerly known as Centromeric H3 or CenH3) that is essential for centromeric chromatin inheritance.