Histones, Chromatin, and Chromosome Coiling – MCAT Biology | MedSchoolCoach

Histones, Chromatin, and Chromosome Coiling

MCAT Biology - Chapter 1 - Section 3.1 - DNA - Histones, Chromatin, and Chromosome Coiling
  1. Home
  2. »
  3. MCAT Masterclass
  4. »
  5. Biological and Biochemical Foundations of Living Systems
  6. »
  7. Biology
  8. »
  9. Histones, Chromatin, and Chromosome Coiling – MCAT Biology
Play Video about MCAT Biology - DNA - Chromosome Organization - Histones, Chromatin, and Chromosome Coiling

Sample MCAT Question - Histones, Chromatin, and Chromosome Coiling

Chromosome supercoiling in DNA occurs with the help of which enzyme?

a) DNA polymerase

b) Helicase

c) DNA Ligase

d) Topoisomerase

Click to Reveal the Answer

D is correct. Topoisomerase.

There are several levels of eukaryotic chromosomal organization. When DNA wraps around histones, it forms structures called chromatin fibers. The chromatin fibers are coiled together to form a denser structure called a 30-nm fiber. The 30-nm fiber is further supercoiled using the enzyme topoisomerase to form chromosomes. During DNA replication, topoisomerase is responsible for removing supercoils, illustrating that one enzyme can catalyze both forward and reverse reactions. Answer choices A, B, and C are all enzymes involved in DNA replication, but none are involved in supercoiling.

Get 1-on-1 MCAT Tutoring From a Specialist

With MCAT tutoring from MedSchoolCoach, we are committed to help you prepare, excel, and optimize your ideal score on the MCAT exam.

 

For each student we work with, we learn about their learning style, content knowledge, and goals. We match them with the most suitable tutor and conduct online sessions that make them feel as if they are in the classroom. Each session is recorded, plus with access to whiteboard notes. We focus on high-yield topics if you’re pressed for time. If you have more time or high-score goals, we meticulously cover the entire MCAT syllabus.

Chromosomes Intro

Chromosomes are long strands of DNA molecules that contain the genetic information of an organism. Compared to prokaryotic chromosomes, eukaryotic chromosomes are larger and linear, rather than circular in shape. Moreover, they are located within the nucleus of the cell. There are many important proteins involved in eukaryotic chromosomal structure, but one main type to know for the MCAT exam are histones.

Histone Proteins

In eukaryotic chromosomes, histone proteins help to package and organize DNA. Figure 1 shows how histone proteins function. DNA wraps around histone proteins to form structures called nucleosomes. The binding interaction between histone proteins and DNA is an attractive electrostatic interaction, meaning it is between positive charges and negative charges. Recall that DNA has a sugar-phosphate backbone, which gives it a negative charge. Having a negative charge means that histone proteins, around which DNA wraps, have to have a positive charge. Histone proteins contain a large proportion of basic amino acids, meaning that they are positively charged at physiological pH.

Histones & Nucleosome Beads - MCAT Biology
Fig 1. Histones & Nucleosome Beads

Chromosome Organization: Chromatin and Supercoiling

Figure 2 shows many of the different levels of eukaryotic chromosome organization. At the top is the basic structure of DNA, the DNA double helix. Below the double helix is a depiction of DNA wrapped around histone proteins, often called octamers because there are two sets of 4 histone proteins bundled together. When DNA is wrapped around histones in this way, they form b, which are the bundles of DNA wrapped around histone proteins. In between the nucleosomes are DNA fibers. This formation of nucleosomes and DNA fibers is where the “beads on a string” phraseology originates. The beads are the nucleosomes, consisting of DNA wrapped around histones, and the string is the DNA fibers between the nucleosomes. A more formal name for this structure is chromatin.

Chromatin, Supercoiling, and Chromosome Organization - MCAT Biology
Fig 2. Chromatin, Supercoiling, and Chromosome Organization

 At the next level of organization are coiled up chromatin fibers, which are dense structures of 30-nanometer fibers. They are so named because the diameter of the fiber is 30-nanometers. Furthermore, through supercoiling, the 30 nm fibers can be further coiled to form chromosomes. It is important to understand that DNA supercoiling is done by the enzyme topoisomerase, which is the same enzyme involved in DNA replication. Recall that in DNA replication, in order to form the replication fork and unwind the DNA, an enzyme called helicase acts on the DNA strand first. The unwinding of the DNA into two distinct strands by helicase creates strain in the DNA through supercoiling. Topoisomerase will help to relieve the strain by removing the supercoiling. Remember that enzymes can catalyze both the forward and reverse reactions. In this way, topoisomerase is involved in both relieving and creating supercoils.

 

As a quick and fun aside, each human cell has around 6 feet of DNA. If each human has around 10 trillion cells in our bodies (this is actually a low-ball estimate), that would mean that each person has around 60 trillion feet or around 10 billion miles of DNA inside of them. The Earth is about 93 million miles away from the sun, meaning that the DNA in each of our bodies could make approximately 540 round trips between Earth and the sun. Clearly, our bodies need to do a fantastic job of packing DNA incredibly tightly, and they do!

Prokaryotic Chromosome Organization

Prokaryotic chromosome organization is more simple than eukaryotic chromosome organization because prokaryotes do not have histone proteins. Notice in Figure 3 how prokaryotes can pack their DNA using supercoiling alone. Essentially, DNA forms coils, and then those coils form more coils. This process repeats itself until supercoils are created.

Prokaryotic DNA Supercoiling - MCAT Biology
Fig 3. Prokaryotic DNA Supercoiling

Explore More MCAT Masterclass Chapters

Take a closer look at our entire MCAT Masterclass or explore our Biochemistry lessons below.

Interview - Job interview

One-on-One Tutoring

Are you ready to take your MCAT performance to a whole new level? Work with our 99th-percentile MCAT tutors to boost your score by 12 points or more!

See if MCAT Tutoring can help me

Talk to our enrollment team about MCAT Tutoring

Medical College Admission Test - MCAT Physics

MCAT Go Audio Course

Engaging audio learning to take your MCAT learning on the go, any time, any where. You'll be on the way to a higher MCAT score no matter where you are. Listen to over 200+ lessons.

Stock photography - Image

MCAT Practice Exams

Practice makes perfect! Our mock exams coupled with thorough explanations and in-depth analytics help students understand exactly where they stand.

MCAT Prep App Mobile

MCAT Prep App

Access hundreds of MCAT videos to help you study and raise your exam score. Augment your learning with expert-created flashcards and a question banks.