Lesson Explainer: DNA in Eukaryotes Biology

In this explainer, we will learn how to describe the structure of chromosomes in eukaryotic cells and explain the role of histones and nucleosomes in forming chromosomes.

How tall are you? Are you as tall as the DNA contained in a single diploid cell of your body?

The haploid human genome is about 3 billion nucleotides long. Since most cells in our body are diploid, this means the nucleus of most cells contains 6 billion base pairs. Each nucleotide is about 0.34 nanometres long, which means there is about 2 metres of DNA compacted inside diploid cells!

You will recall that eukaryotes, like humans, contain membrane-bound organelles including the nucleus to house the DNA. This DNA is highly organized and packed tightly into the nucleus in the form of chromosomes. In humans, there are 46 chromosomes and each of these comes from a single, extremely long, molecule of double-stranded DNA. This DNA is compacted to eventually give rise to the familiar X-shaped structure known as a chromosome as shown in the 3D rendered illustration below.

chromosomes 3d illustration

Figure1

Key Term: Eukaryote

A eukaryote is an organism consisting of cells that contain their genetic material within a membrane-bound nucleus.

Definition: DNA (Deoxyribonucleic Acid)

DNA is the molecule that carries the genetic instructions for life. It is composed of two strands of nucleotides that coil around each other to form a double helix.

Key Term: Chromosome

A chromosome is an organized structure of DNA and associated proteins that contains the genetic information of an organism in the form of genes.

Example 1: The Length of DNA in Human Cells

In a single human body cell, if the chromosomes were uncoiled, stretched out, and laid end to end, approximately, how long would this strand of DNA be?

  1. 2 centimetres
  2. 2 metres
  3. 2 millimetres
  4. 2 nanometres
  5. 20 metres

Answer

The DNA in eukaryotic cells is organized into structures called chromosomes. In humans, there are 23 pairs of chromosomes or 46 chromosomes in total. These chromosomes are highly compacted to give this structure, and they contain a lot of DNA.

The human genome (haploid) is about 3 billion nucleotides long. Most cells in our body are diploid, which means there is about 6 billion nucleotides worth of DNA in the 46 chromosomes. Each nucleotide is about 0.34 nanometres long. Multiplying 0.34 nanometres by 6 billion equals 2‎ ‎040‎ ‎000‎ ‎000 nanometres or about 2 metres of DNA. So, if all the DNA in the 46 chromosomes were laid out and uncompacted, it would be about 2 metres in length.

Therefore, the length of this strand of DNA is B: 2 metres.

DNA is not always compacted into condensed chromosomes as seen in the illustration above. In fact, DNA only takes on this appearance as the cell prepares to undergo division. At this point, this is the most compacted form that the DNA can take. When cells are not preparing for cell division, the 46 chromosomes are mixed together in a form called chromatin. This is still highly compacted, but less so compared to the condensed chromosome structure.

Besides the nucleus, there is DNA inside mitochondria and chloroplasts. These organelles perform important functions that provide the cell with energy; genes for these processes are located within the organelle’s DNA. Mitochondrial and chloroplast DNAs are circular in structure.

So, how is DNA compacted from essentially 2 metres of double helix into the nucleus of a tiny cell?

DNA goes through several levels of compaction. First, the double helix is coiled around specialized proteins called histones to form nucleosomes. Then, these nucleosomes are coiled again to form chromatin. As mentioned, cells that are not dividing will stop at this step, but if the cell is preparing for division, then another level of compaction happens to form the condensed chromosome structure. These steps are shown in Figure 2 and will be described in more detail in this explainer.

DNA compaction involves a group of proteins that can be divided into histone and nonhistone proteins.

Histones are specialized structural proteins that are able to interact with DNA very closely. This interaction is due to histones and DNA having opposite charges to each other. Histones carry a positive charge because they contain many positively charged amino acids like arginine and lysine. These positively charged amino acids in histones can bind to the negatively charged phosphate groups in the sugar phosphate backbone of DNA.

Definition: Histone

A histone is a specialized protein that is involved in the compaction of DNA.

There are also nonhistone proteins that include some structural proteins that are involved in the spatial organization of DNA inside the nucleus, as well as regulatory proteins that are involved in gene expression. These regulatory proteins determine which proteins and enzymes are produced from DNA.

DNA can be tightly coiled around a group of histone proteins to form a structure called a nucleosome. Each nucleosome contains 8 histone proteins and about 146 base pairs of DNA. Nucleosomes and DNA resemble beads on a string when looked at with a high-powered microscope.

Definition: Nucleosome

Nucleosomes are the subunits of chromatin. They are structures that contain compacted DNA wrapped around histone proteins.

Example 2: Understanding the Role of Proteins in DNA Compaction

In the initial stages of chromosome formation, double-stranded DNA wraps around specialized proteins. What are these proteins called?

  1. Hormones
  2. Isoforms
  3. Centromeres
  4. Histones
  5. Nucleosomes

Answer

In human cells, there are 46 chromosomes; if you were to lay them down and stretch them out, they would be about 2 metres in length! Obviously, this amount of DNA needs to be compacted to fit it into the nucleus of a cell.

The major protein involved in compacting DNA is called a histone. DNA is able to wrap tightly around groups of 8 histones to form a nucleosome. This nucleosome can then be further compacted into chromatin and finally into a chromosome.

Let’s look at the different answers to see which best describes the protein that DNA wraps around.

Answer A, “hormones,” is incorrect. Hormones are chemical messengers that are able to exert their function to various tissues in the body. These have nothing to do with DNA compaction.

Answer B, “isoforms,” is incorrect. Isoforms are different versions of a protein that are similar in function and amino acid sequence.

Answer C, “centromeres,” is incorrect. Centromeres are specialized parts of chromosomes where two sister chromatids are linked.

Answer D, “histones,” is correct. Histones are the specialized proteins that DNA can wrap around to eventually form chromosomes.

Answer E, “nucleosomes,” is incorrect. Nucleosomes can form after DNA wraps around histones, but a nucleosome itself is not a protein.

Therefore, we can conclude that the specialized proteins are called “histones.”

Nucleosomes can then be assembled further to form the next level of compaction called chromatin. Chromatin, with its histone and nonhistone proteins, contains about the same amount of DNA and proteins. In nondividing cells, chromatin is how DNA is compacted in the nucleus, and you can see this in the 3D-rendered illustration below, which shows chromatin inside a nucleus with each chromosome being in its decondensed state and having different multicolors. In this form, the chromosomes are not condensed and are mixed together with one another. Chromatin is relatively accessible to the proteins that need to physically interact with DNA, like those involved in DNA replication or gene expression.

Chromosomes, DNA, cells package the DNA into the nucleus 3d illustration

Figure3

Key Term: Chromatin

Chromatin is a highly organized complex formed when DNA associates with histone proteins.

Under the microscope, it is not possible to differentiate each chromosome when it is in the decondensed chromatin state. In the final level of DNA compaction, chromatin is further coiled and compacted to form the condensed chromosome structure. Only at this point is it possible to differentiate the individual chromosomes.

Chromosomes only condense during the prophase stage of mitosis. You can see an overview of mitosis in Figure 4 below. Notice the chromatin state of DNA during interphase (containing all 46 chromosomes, all mixed together) that then condenses to reveal the individual chromosomes during prophase. During cell division, the DNA must be divided equally into the two daughter cells, and it is much easier to do this when chromatin is condensed into organized chromosomes.

Relationship: Compaction of DNA in Eukaryotes

DNAnucleosomeschromatinchromosomes

Example 3: Understanding the Stages of DNA Compaction

DNA is wrapped around proteins and coiled into loops to form chromatin. At what point will chromatin condense to form visible chromosomes?

  1. Immediately after chromatin has formed
  2. When cells are stimulated by chemical messengers
  3. Shortly after cells have completed cell division
  4. When cells become fertilized
  5. As cells prepare for cell division

Answer

In human cells, there are 46 chromosomes; if you were to lay them down and stretch them out, they would be about 2 metres in length! Obviously, this amount of DNA needs to be compacted to fit it into the nucleus of a cell.

Each chromosome in the nucleus is a single molecule of DNA. To compact this DNA, it first needs to be associated with specialized proteins called histones. DNA is able to coil around a group of histones to form a nucleosome. This nucleosome can then be coiled around again to form chromatin. Chromatin is how DNA is compacted in nondividing cells. In cells that are preparing to divide, chromatin is condensed further to form chromosomes. You can see the overall process below.

Therefore, the answer is E: chromatin will condense into visible chromosomes as cells prepare for cell division.

DNA compaction can be used to regulate how DNA is replicated or how genes on a chromosome are expressed. Genes can be “turned off” by compacting the DNA around it so it is not accessible for the enzymes involved in expressing the gene. This DNA must be in a less compacted state before enzymes can act on it to allow gene expression.

A good example of this is the inactivation of the X chromosome in females. You will recall that males have one copy of the X and Y chromosomes, while females have two copies of the X chromosome. This second copy of X must be inactivated because having two active copies of X, with double the gene products, can be lethal to the cell. The cell is able to inactivate one copy of the X chromosome by tightly compacting it so it cannot be accessed for gene expression.

Let’s recap some of the key points we have covered in this explainer.

Key Points

  • Chromosomes are highly compacted forms of DNA.
  • DNA is compacted using histone proteins in nucleosomes, which are further compacted into chromatin, then condensed further to chromosomes.
  • Chromosomes are only visible when the cell prepares to divide.

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