Cytogenetics and Chromosome Structure

A set of vocabulary flashcards covering cytogenetics, chromosome structure, histone organization, chromatin types, chromosome numbers, and genetic principles based on the lecture material.

Cytogenetics

The branch of genetics dedicated to the study of chromosomes, including their structure, function, and numerical variations.

Chromosome

An organelle composed of DNA and proteins that contains genetic material and maintains a structure that is conserved across different organisms.

Unineme Model

The model of chromosome architecture establishing that each cytogenetically visible metaphase chromosome contains a single molecule of DNA organized into higher orders of packaging.

Histones

A group of basic proteins tightly bound to cellular DNA, categorized into five major types: H1, H2A, H2B, H3, and H4.

Histone Octamer

The central core block of eight histone proteins—comprising two molecules each of H2A, H2B, H3, and H4—around which DNA wraps to form a nucleosome.

Nucleosome

The fundamental unit of DNA packaging consisting of approximately $140$ to $146$ base pairs of DNA wound in roughly $1.7$ turns around an octamer core of histones.

Linker DNA

A short segment of spacer DNA, typically $20$ to $80$ base pairs long, associated with an H1 histone protein that links adjacent nucleosomes together.

Histone Acetylation

A chemical reaction in which acetyl groups are added to histones, reducing their positive charge and binding affinity to DNA, thereby opening chromatin to activate gene expression.

Euchromatin

Gene-bearing chromosomal material that constitutes the main body of the chromosome and is transcriptionally active.

Heterochromatin

Condensed chromosomal material that is variable in quantity, largely devoid of active genes, and categorized into constitutive and facultative types.

Constitutive Heterochromatin

Permanently condensed heterochromatin containing highly repetitive DNA sequences, situated near centromeres, in satellites, on short arms of acrocentric chromosomes, and on the long arm of the Y chromosome, stained by C-bands.

Facultative Heterochromatin

Chromatin that has been conditionally converted into a transcriptionally inactive state, such as the mammalian inactivated X chromosome.

Barr Body

A condensed, transcriptionally inactive X chromosome found in female mammalian cells, serving as an example of facultative heterochromatin.

Diploid Number ($2N$)

The full set of chromosomes present in the somatic (body) cells of an organism.

Haploid Number ($N$)

The half set of chromosomes found in the germ (gametic) cells of an organism.

Down Syndrome

A genetic condition where individuals possess a diploid count of $47$ chromosomes due to an extra copy of chromosome 21 (trisomy 21).

Turner Syndrome

A genetic condition in females characterized by a diploid count of $45$ chromosomes due to a missing X chromosome.

Klinefelter Syndrome

A genetic condition in males characterized by the presence of an extra X chromosome alongside the standard male sex chromosome complement.

Autosomes

The $22$ non-sex chromosome pairs found in human somatic cells.

Mendel's Law of Segregation

The principle stating that one homolog of each chromosome pair segregates randomly with equal probability into each daughter gamete during meiosis.

Mendel's Law of Independent Assortment

The principle stating that different chromosome pairs assort independently of one another during gamete formation.

Chromosome Packaging Hierarchy

The sequential condensation levels of genomic DNA, progressing from the $2\,\text{nm}$ DNA double helix to the $10\,\text{nm}$ nucleosome chain, the $30\,\text{nm}$ chromatin fiber, up to the ~$1400\,\text{nm}$ metaphase chromosome.