DNA Packaging in Prokaryotes vs. Eukaryotes and Chromosome Structure

Vocabulary flashcards covering prokaryotic and eukaryotic DNA packaging, histone interactions, levels of chromatin organization, and chromosome terminology based on the lecture material.

Prokaryotes

Single-celled organisms, such as bacteria and archaea, that lack membrane-bound organelles and package their DNA in the cytoplasm as one circular chromosome along with many small circular plasmids.

Eukaryotes

Single- or multi-celled organisms (protists, fungi, plants, animals) containing membrane-bound organelles and packaged DNA consisting of many linear chromosomes in the nucleus, plus circular DNA in mitochondria and chloroplasts.

Relaxed B-form DNA

The standard conformation of double-stranded DNA that contains 1 turn per $10\,bp$.

Positive Supercoiling

The over-twisting or over-rotation of the DNA double helix that causes it to coil upon itself.

Negative Supercoiling

The under-twisting or under-rotation of the DNA double helix that causes it to coil upon itself; this is the normal state of prokaryotic DNA.

Topoisomerases

Enzymes that alter DNA supercoiling by breaking phosphodiester bonds in DNA, rotating the ends to add or remove rotations, and rejoining the pieces.

Histones

Small, positively charged, lysine-rich proteins (H1, H2A, H2B, H3, and H4) that bind to negatively charged eukaryotic DNA.

Histone Core

An octamer composed of two copies each of the histone proteins H2A, H2B, H3, and H4.

Nucleosome

The fundamental repeating subunit of eukaryotic chromatin, consisting of a histone core wrapped approximately 1.7 times by roughly $146\,bp$ of DNA.

Linker DNA

A segment of DNA of approximately $50\,bp$ that connects adjacent nucleosomes in the 10 nm fiber.

Chromatin

The complex of eukaryotic DNA and histone proteins.

10 nm Fiber

The primary level of eukaryotic DNA packaging, often called 'beads on a string,' consisting of repeating nucleosomes joined by linker DNA.

30 nm Fiber

The second level of eukaryotic DNA compaction, formed when the 10 nm fiber coils on itself with six nucleosomes per turn.

300 nm Fiber

The third level of eukaryotic DNA compaction, in which the 30 nm chromatin fiber forms looped domains attached to a nonhistone protein chromosome scaffold.

Euchromatin

A less condensed form of chromatin (10 nm or 30 nm fiber) that contains actively expressed genes.

Heterochromatin

A highly condensed form of chromatin (such as 300 nm fiber) that contains few actively expressed genes.

Chromosome

A cellular structure composed of tightly packaged DNA and associated proteins.

Gene

A discrete unit of heredity consisting of a specific sequence of DNA that encodes instructions for a particular trait.

Karyotype

A visual display depicting all the chromosomes of an individual or cell.

Autosomes

Non-sex chromosomes; humans possess 22 pairs of autosomes.

Sex Chromosomes

Chromosomes involved in determining the sex of an organism; humans have 1 pair (X and Y).

Homologous Chromosomes

Chromosome pairs that pair during meiosis and carry identical sets of genes for the same traits, though they may carry different alleles.

Heterologous Chromosomes

Non-homologous chromosomes that carry different genes and do not pair during meiosis.

Homozygous

Having two identical alleles for a particular gene on homologous chromosomes.

Heterozygous

Having two different alleles for a particular gene on homologous chromosomes.

Sister Chromatids

The two identical duplicate copies of a single replicated chromosome that are joined at the centromere.

Non-sister Chromatids

Chromatids derived from different homologous or non-homologous chromosomes.