Name: chromatin and histones in dna packing
Uploaded: 2026-08-26T08:20:24.493Z
Duration: 1 min 10 s
Channel: JoVE

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Structure and Function

3.1.9-12.A

DNA Packaging

In eukaryotes, double-stranded DNA is organized inside a membrane-bound nucleus to fit within the cell&#x2019;s limited space.At the first level of compaction, DNA wraps tightly around specific proteins called histones.A histone core with DNA wrapped around it forms a nucleosome, the basic unit of DNA packaging. The linker histone binds at the DNA entry and exit points, helping to stabilize and compact the structure.Nucleosomes are connected by linker DNA, forming a structure that resembles beads on a string.At the next level of packaging, nucleosomes and linker DNA fold together to form chromatin fibers.Finally, additional fibrous proteins compact the chromatin even further. This allows long DNA molecules to remain structured as chromatin, which becomes highly condensed into visible chromosomes during cell division.

dna packaging

Eukaryotes have large genomes compared to prokaryotes. In order to fit their genomes into a cell, eukaryotes must pack their DNA tightly inside the nucleus. To do so, DNA is wound around proteins called histones to form nucleosomes, the main unit of DNA packaging. Nucleosomes then coil into compact fibers known as chromatin.<h4>You Have Enough DNA to Stretch to the Sun and Back Hundreds of Times</h4>Most cells in the human body contain about 3 billion base pairs of DNA packaged into 23 pairs of chromosomes. It is hard to imagine exactly how much DNA these numbers represent. So how much packing has to happen to fit the genome into a cell?We can gain some insight by expressing the genome in terms of length. If we were to arrange the DNA of a single human cell, like a skin cell, into a straight line, it would be two meters long&#x2013;over 6.5 feet. The human body contains around 50 trillion human cells. This means that each person has a total of about 100 trillion meters of DNA. In other words, each person has enough DNA to stretch from the Earth to the Sun 300 times!And humans do not have particularly large genomes&#x2013;those of many fish, amphibians, and flowering plants are much larger. For example, the genome of the flowering plant Paris japonica is 25 times larger than the human diploid genome. These figures emphasize the astonishing task that eukaryotes must accomplish to pack their DNA inside cells.<h4>Nucleosomes Are Central Players in DNA Packaging</h4>Each nucleosome consists of DNA wrapped around a core of eight histone proteins. Each core is composed of four different types of histones&#x2014;H2A, H2B, H3, and H4&#x2014;that are each present in two copies. Another type of histone&#x2014;H1&#x2014;binds to both the nucleosome and the linker DNA, stabilizing the structure.DNA becomes more compact as nucleosomes and linker DNA coil into chromatin fibers. Uncondensed chromatin fibers, or euchromatin, are approximately 10 nm in diameter. Nucleosomes resemble beads on a string in these fibers. As DNA continues to condense, the 10-nm fibers coil into strands that are approximately 30 nm thick, which in turn form loops that make 300-nm thick fibers. When chromatin is fully compacted it is known as heterochromatin.The loosely packed structure of euchromatin allows enzymes, such as RNA polymerase, access to the DNA. Transcription, therefore, tends to occur predominantly in euchromatic regions of the genome, which are rich in genes. By contrast, the tightly packed structure of heterochromatin blocks access to the DNA, preventing transcription. Heterochromatin predominates in the centromeres and telomeres of chromosomes, where highly repetitive DNA sequences are much more common than genes. Furthermore, organisms can dynamically adjust the level of DNA packing in response to cellular and external environmental cues, de-condensing DNA when genes need to be turned on, and re-condensing it to turn them off.

Overview

Eukaryotes have large genomes compared to prokaryotes. In order to fit their genomes into a cell, eukaryotes must pack their DNA tightly inside ...

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KEY TERMS AND CONCEPTS

chromatin and histones in dna packing

Eukaryotic cells must pack long DNA molecules into the nucleus, and chromatin makes that possible. DNA wraps around proteins called histones to form nucleosomes, which are the main unit of DNA packaging. Nucleosomes then coil into compact chromatin fibers.

The amount of DNA in a cell is much larger than it first seems. A single human skin cell has about 3 billion base pairs of DNA, and if that DNA were stretched out in a line, it would be about 2 meters long. With about 50 trillion cells in the human body, each person has roughly 100 trillion meters of DNA, which could stretch from Earth to the Sun 300 times.

Humans do not even have the largest genomes. Many fish, amphibians, and flowering plants have much larger genomes, and the flowering plant Paris japonica has a genome about 25 times larger than the human diploid genome. These examples show how much packing eukaryotes must do to fit DNA inside cells.

Each nucleosome contains DNA wrapped around a core of eight histone proteins. That core includes two copies each of H2A, H2B, H3, and H4. Another histone, H1, binds to the nucleosome and the linker DNA, which helps stabilize the structure.

Chromatin becomes more compact as it folds into higher levels of packing. Loose chromatin fibers, called euchromatin, are about 10 nm wide and look like beads on a string. These fibers can coil into 30-nm strands and then form loops that create 300-nm fibers.

Fully compacted chromatin is called heterochromatin. Euchromatin is open enough for enzymes such as RNA polymerase to reach the DNA, so transcription usually happens in gene-rich regions. Heterochromatin blocks access to the DNA, so it prevents transcription and is common in centromeres and telomeres, where repetitive sequences are more common than genes.

Cells can also change how tightly they pack DNA. When genes need to be turned on, DNA can de-condense. When those genes need to be turned off, the DNA can re-condense. This flexible packing helps cells respond to internal signals and environmental cues.

Chromatin and Histones in DNA Packing

the dna helix

How DNA Stores Genetic Information

DNA stores genetic information that is passed from one generation to the next in organisms and most viruses. It is built from two strands of nucleotides that twist together into a double helix. The story of how scientists uncovered this structure developed step by step over nearly a century. Each DNA strand is made of nucleotides. A nucleotide contains the sugar deoxyribose, a phosphate group, and one of four nitrogen bases: adenine (A), guanine (G), cytosine (C), or thymine (T). Adenine and...

Eukaryotic cells must pack long DNA molecules into the nucleus, and chromatin makes that possible. DNA wraps around proteins called histones to form nucleosomes, which are the main unit of DNA packaging. Nucleosomes then coil into compact chromatin fibers. The amount of DNA in a cell is much larger than it first seems. A single human skin cell has about 3 billion base pairs of DNA, and if that DNA were stretched out in a line, it would be about 2 meters long. With about 50 trillion cells in...

organization of genes

Genes, Introns, and Regulatory DNA

Eukaryotic genes are arranged within DNA along with non-coding regions that help control gene activity. A DNA strand includes genes and intergenic regions, which are the stretches between genes. These regions do not code for proteins, but they can contain important regulatory elements such as promoters, enhancers, and silencers. A gene itself is not a single continuous coding block. It usually contains protein-coding exons and non-coding introns. After DNA is transcribed into pre-mRNA in the...

karyotyping

Chromosome Stains for Detecting Disorders

Chromosome staining helps scientists find genetic disorders by showing the number and physical features of chromosomes. These features appear after DNA condenses into chromosomes during mitosis. Cytogeneticists then stop mitosis and stain the chromosomes to study their banding pattern. The pattern reveals details such as chromosome number, shape, and type. A chromosome description like this is called a karyotype. To create one, scientists take an image of the stained chromosomes and identify...

replication in prokaryotes

How Prokaryotic DNA Is Copied

Prokaryotic DNA is copied through a coordinated process with three main stages: initiation, elongation, and termination. It begins when initiator proteins bind to the single origin of replication, or ori, on the cell’s circular chromosome. The DNA then copies around the circle in both directions, starting from two replication forks, and this produces two DNA molecules. Several proteins work together to open and protect the DNA during replication. Topoisomerase cuts one side of the...

replication in eukaryotes

Eukaryotic DNA Copying and Telomeres

Eukaryotic DNA copying is tightly regulated and must duplicate many linear chromosomes before cell division. The process is highly conserved and uses many proteins with specialized roles. It moves through initiation, elongation, and termination, and it ends with two complete sets of chromosomes in the nucleus. Eukaryotic replication shares basic steps with prokaryotic DNA replication, but the larger genome and linear chromosomes require more proteins. Replication begins at multiple origins of...

proofreading

DNA Polymerase Fixes Replication Errors

DNA polymerase helps copy DNA and fix mistakes during replication. It adds nucleotides in a sequence that matches the template DNA strand. The enzyme also has a higher affinity for the correct base, which helps keep DNA replication accurate. As new genomic DNA is made, it is synthesized in the 5’ to 3’ direction. In many cells, DNA polymerases have different jobs in this process. DNA polymerase delta and DNA polymerase epsilon can proofread when nuclear DNA is being copied. These polymerases...

mismatch repair

DNA Mismatch Repair in Replication

DNA mismatch repair fixes copying errors that happen during DNA replication. When DNA polymerase makes a mistake, the cell can detect a wrong base pair, such as A with G or T with C, and replace it with the correct nucleotide. This repair system works in both prokaryotes and eukaryotes and depends on several proteins. The Mutator protein family plays a central role in this process. In bacteria such as Escherichia coli, MutS first binds the mismatch. MutL then helps identify which strand is the...

nucleotide excision repair

DNA Damage Repair by Excision

DNA damage repair by excision protects cells from mutagens that can change DNA. Exposure to environmental mutagens can create bulky lesions that bend the double helix or block transcription. If these changes are not repaired, they can lead to mutations and may contribute to cancer or disease, depending on which DNA sequences are affected. Nucleotide excision repair, or NER, is the process that detects and removes this kind of damage. Specialized proteins scan the DNA for a damaged region, then...

mutations

DNA Changes From Point Mutations to UV Damage

Mutations are changes in the DNA sequence. They can happen by chance during DNA copying, or they can be caused by environmental factors such as radiation and chemicals. Scientists also describe mutations by how much DNA they affect, how they change a protein, and whether they happen in body cells or germline cells. Some mutations affect only one nucleotide, which is a point mutation. When a point mutation falls inside a gene, the outcome depends on how the amino acid sequence changes. A silent...

transcription

Transcription and Gene Control

Transcription and gene control explain how cells make RNA from DNA and decide which genes are active. RNA polymerase copies a DNA sequence into RNA, and this is the first step in making a protein from a gene. Many other proteins and control sequences help make messenger RNA, or mRNA, correctly. In eukaryotes, the DNA is first copied into a primary RNA, also called pre-mRNA. This RNA can then be processed into mature mRNA, which serves as the template for protein synthesis. In prokaryotes such...

translation

Protein Synthesis and mRNA Decoding

Protein synthesis begins when ribosomes decode messenger RNA (mRNA) and build a chain of amino acids. This final step of gene expression is called translation. The finished chain is a polypeptide, which can later become a functional protein. Proteins are often called the building blocks of life because they make up much of the body, from muscle fibers to hairs and immune system parts. The instructions for each protein are encoded in genes found in the DNA of every cell. In biology, the central...

bacterial transformation

Griffith’s Mouse Experiment and DNA

Bacterial transformation explains how DNA can move into bacteria and change their traits. Frederick Griffith first noticed this in 1928 while studying pneumonia-causing Streptococcus pneumoniae in mice. He compared one pathogenic strain with one non-pathogenic strain and found that only the pathogenic strain killed the host mice. Griffith made an unexpected discovery when he killed the pathogenic bacteria and mixed their remains with live, non-pathogenic bacteria. The mixture killed the mice,...

key terms and concepts

Explore how histones, nucleosomes, euchromatin, and heterochromatin help eukaryotic cells pack DNA into the nucleus and control transcription.

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3.1.9-12.B

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3.1.9-12.C

Chromatin and Histones in DNA Packing

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