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Euchromatin vs Heterochromatin MCAT
DNA Chromatin Guide

William Cohen
Published by William Cohen
Last Updated On: November 19, 2021

So, you’ve started getting ready for the MCAT. As you go over all the topics covered on the MCAT, you’ll also see DNA.

DNA can be complicated to understand, and I’ve had countless students come to me struggling with this topic.

I’ve successfully helped them understand the basic DNA info and pass the MCAT with high scores.

Today, I’ve decided to write my guide to heterochromatin and euchromatin and how it’s related to DNA.

Difference Between Euchromatin and Heterochromatin

  • DNA is a high-yield topic on the MCAT, so it’s extremely important to cover it in detail.
  • DNA has two chromatins: heterochromatin and euchromatin.
  • There are a number of key differences between these two chromatins.

DNA

DNA structure with virus

Deoxyribonucleic acid (DNA) is the chemical name for the hereditary material in humans and almost all living things. [1]

This is one of the two main molecules of nucleic acid. The other is RNA or ribonucleic acid.

Almost every cell in our body has the same DNA, which is located in the cell nucleus.

The DNA molecule is made of two strands that wind around one another in a shape called a double helix.

DNA information is stored in a code made up of four bases:

  • Adenine (A)
  • Guanine (G)
  • Cytosine (C)
  • Thymine (T)

Knowing DNA and its function is very important for the MCAT because DNA is the basis of life.

DNA is transferred from one generation of cells to another to avoid any errors.

Some DNA errors are minor, whereas others can be life-threatening, such as cancer.

What is Heterochromatin?

Heterochromatin figure

Chromosomes that have DNA-specific strains and are condensed are known as heterochromatin.

Heterochromatin is the tightly packed DNA in the nucleus.

Since heterochromatin is so tightly packed, it’s inaccessible to the protein engaged in gene expression, and it doesn’t allow chromosomal crossing. Because of this, heterochromatin is genetically inactive.

It’s highly compressed, which is good for fitting into the nucleus, but it cannot be expressed.

There are two kinds of heterochromatin:

  • Facultative heterochromatin — genes that are silenced through the process of Histone methylation. This isn’t a permanent part of every cell nucleus.
  • Constitutive heterochromatin — genes that are repetitive and structurally functional, such as telomeres or centromeres. These are part of the cell’s nucleus.

Because of its compact nature, heterochromatin’s main function is to keep the DNA safe from endonuclease damage.

If you deacetylate the histones, that favors heterochromatin and reduces the expression of DNA areas that are packed more tightly.

What is Euchromatin?

Euchromatin figure

Euchromatin is the unwound form, which can be transcribed and replicated.

This is the part of chromosomes that are rich in genes and loosely packed.

Euchromatin has about 90% of the human genome and covers the maximum part of the dynamic genome to the inner nucleus.

I’ve mentioned that euchromatin allows transcription. This is because some genome parts that have active genes are so loosely packed that the DNA is readily available.

“If you can control the degree to which DNA is wound or unwound, that allows you to control the expression of that DNA, be it by replication or transcription.” -PreMedHQ Science Academy, YouTube Channel

Euchromatin is active in DNA to RNA transcription. The active genes become transcribed to make mRNA.

If you add an acetyl group to a histone, that makes it less tightly wound. If you have acetylation, that favors euchromatin, and thus the gene can be transcribed and expressed as protein.

Heterochromatin and Euchromatin Differences

A model of Heterochromatin and Euchromatin

Here are the main differences between heterochromatin and euchromatin:

Heterochromatin Euchromatin
Tightly packed form of DNA Loosely packed form of DNA
Appears as small, dark staining in irregular particles [2] Isn’t stainable, and it’s dispersed
Appears in cells that are less active or not active at all Appears in cells that are active in the transcription of many of their genes
Found at nucleus periphery only in eukaryotic cells Found in the inner body of eukaryotic and prokaryotic cells
Genetically inactive, with little or no transcriptional activity Genetically active and has high transcriptional activity
Has structural integrity of the cell and allows gene expression regulation Results in genetic variations and allows genetic transcription
Compactly coiled Loosely coiled
Late replicative Early replicative

Heterochromatin vs Euchromatin on the MCAT: Final Thoughts

I hope you found this guide to heterochromatin and euchromatin helpful.

Before you dive into these chromatins, start with the DNA. Learn what it is and all the terms associated with it.

DNA is a high-yield topic, so expect to see a fair number of questions about it on the MCAT.

As for heterochromatin and euchromatin, go over the explanations for both and pay special attention to the differences.

Most importantly, know that heterochromatin doesn’t allow transcription, while euchromatin does.

Finally, go over as many AAMC practice tests until you can answer all DNA questions correctly.


References:

  1. https://www.britannica.com/science/DNA
  2. http://medcell.med.yale.edu/histology/cell_lab/euchromatin_and_heterochromatin.php

 

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