Levels of DNA Organization

Level 1 of DNA Structure is ___.

Double Helix

Each ___ consists of a sugar, a phosphate group, and one of four nitrogenous bases: adenine, thymine, cytosine, or guanine.

Nucleotide

Nitrogenous bases form ___, which are held together by hydrogen bonds, which stabilize the double helix.

Complementary Pairs

The most common form of DNA in biological systems, characterized by a right-handed twist and a regular, repeating structure.

B-Form Helix

The helix is stabilized by ___ between bases as well as ___ between stacked bases.

Hydrogen Bonding

Van der Waals Interactions

Refers to the over- or under-twisting of the DNA double helix beyond its relaxed state.

Supercoiling

___ is essential for regulating access to DNA, as it influence whether sections of DNA are tightly wound or loosely coiled

Supercoiling

This occurs when the DNA molecule is would more tightly in the direction of the DNA helix. Can happen ahead of the replication fork during DNA replication

Positive Supercoiling

This occurs when the DNA helix is made less tightly wound. Most cellular DNA exists in this state, which makes it easier for DNA strands to separate during replication and transcription.

Negative Supercoiling

Enzymes that help regulate the supercoiling of DNA, allowing it to be unwould or rewound to relieve supercoils, and then re-ligate the strands.

Topoisomerases

Small, positively charged proteins that bind to the negatively charged DNA, helping neutralize the DNA's charge and making it easier to compact

Histones

DNA wraps around the core of eight histone proteins to form a structure called a nucleosome.

Histone Octamer

A fundamental structural unit of DNA packaging. Consists of approximately 147 base pairs of DNA wrapped around the histone core in 1.65 turns.

Nucleosome

Between each nucleosome is a short segment of DNA, known as ___, which is not wrapped around histones.

Linker DNA

A fifth histone protein, ___, binds to the linker DNA, helping to further stabilize the nucleosome strucutre

H1

Nucleosomes compact the DNA, and condense the DNA length by a factor of about ___, but further compaction is still necessary.

Six

After being compacted into nucleosomes, the next level of DNA organization is the formation of the ___. This compacts the DNA further, shortening its length by another factor of 40.

30-nanometer fiber

In non-dividing cells, the 30-nanometer fiber is further organized into ___ and ___.

Larger Loops

Domains

Protein loop domains are anchored to a ___ of proteins, forming a more compact structure within the nucleus.

Scaffold

The ___ provides a scaffold for controlling gene expression, and it is dynamic, changing based on cell type, cell cycle stage, and environmental signals.

Loop Domain Structure

During cell division, the DNA is compacted into discrete structures called ___. This is achieved through further folding and coiling of the 30-nm fibers and loop domains into tightly packed chromatids.

Chromosome

In human cells, there are 46 chromosomes (23 pairs), and each chromosome contains one very long DNA molecule, which carries thousands of ___.

Genes

Each chromosome contains ___ and ___.

Centromere

Telomeres

The region where two sister chromatids are held together. Also serves as the attachment point for spindle fibers during cell division.

Centromere

Repetitive sequences at the ends of chromosomes that protect them from degradation and prevent them from being mistaken for broken DNA.

Telomeres

Within the nucleus, each chromosome occupies a specific region known as a ___. These are non-random and help organize the nucleus spatially.

Chromosome Territory

Chromatin in its loosely packed form, that is generally transcriptionally active, meaning genes in this region are more accessible to the cellular machinery that reads and transcribes DNA into RNA.

Euchromatin

A more densely packed chromatin and is generally transcriptionally inactive. It is found in regions of the chromosome containing repetitive sequences, such as centromeres and telomeres.

Heterochromatin

The position of chromosomes within the nucleus can influence gene expression and cellular function. For example, certain genes may be located near the nuclear periphery, where ___ is more abundant, leading to their silencing.

Heterochromatin