Organization of DNA in the Cell: Videos & Practice Problems

DNA organization is crucial for cell division. The genome is the complete set of DNA, which associates with histone proteins to form nucleosomes. In non-dividing cells, DNA exists as chromatin (loosely packed), while in dividing cells, it condenses into chromosomes (tightly packed). Before division, DNA replication occurs, producing replicated chromosomes with two identical sister chromatids joined at the centromere. Understanding these structures is essential for grasping cell division processes.

Organization of DNA in the Cell

So recall from our previous lesson video that before any cell can divide, it must first replicate or duplicate or make an extra copy of its DNA. And how the DNA is organized in the cell is very important. In this video, we're going to introduce the organization of DNA in the cell. What you'll notice is that throughout this video, we're going to introduce a whole bunch of different terms and notice that the terms that link directly to the image down below are labeled with these letters, and we have letters a, b, c, d, and e. Once again, these letters, labeling specific terms, are going to link directly to the letters that we have down below in our image. So, keep that in mind as we go through this video. The very first term we're going to introduce here is a term called the genome. The genome refers to the complete set of all of a cell's DNA, with "all" being the keyword. The genetic material is referring to molecules that determine the inherited traits of an organism, and usually when scientists use the term genetic material, they're referring to the DNA of the organism. Within a cell, the DNA is going to associate with proteins that are called histones. These histone proteins are going to form units that are called nucleosomes. Nucleosomes themselves can be defined as units of 8 histone proteins at the core with DNA wrapped around it.

To start to understand these terms, let's take a look at our image down below. We'll start here, with term A, the genetic material, DNA. Notice here we're showing you an image of DNA, as we've discussed in our previous lesson videos. A double-stranded helix of nucleotides as we see. Now within a cell, it turns out that the DNA is going to be organized into these units called nucleosomes. Nucleosomes form when DNA is wrapped around these histone proteins. Notice these little purple circles that you see throughout are histones, these histone proteins. The histone proteins will organize into these units of 8 histone proteins. The 8 histone proteins form the core of the nucleosome, and DNA will wrap itself around the histone protein, almost like taking the core of a pencil or a pen if that was the histone core, and taking a yarn and wrapping the yarn around the histone protein. Histone proteins at the core with DNA wrapped around it. Now, it turns out that within the cell, these nucleosomes can actually take different forms depending on if the cell is actually in a non-dividing state or if the cell is in a dividing state.

When the cell is in a non-dividing state, when the cell is not dividing, it turns out that the DNA, the nucleosomes, are going to be organized into what's known as chromatin. Chromatin represents loosely packed or coiled nucleosomes in non-dividing cells. But when the cell is about to divide in a dividing cell, the nucleosomes take a different form and they're no longer loosely packed or coiled. Instead, they're going to become tightly packed into what is known as chromosomes. Chromatin refers to loosely packed or coiled nucleosomes in a non-dividing cell. Chromosomes are referring to tightly packed or highly condensed nucleosomes in a dividing cell. Let's take a look at our image down below to get a better idea of this.

Notice on the left-hand side, what we're showing you is a non-dividing cell. Notice that within the nucleus of this non-dividing cell when you zoom in the DNA is organized in this format here, where the genetic material, the DNA, is wrapped around histone proteins to form these nucleosomes. These nucleosome subunits, these nucleosome units here, when the cell is in a non-dividing state, it's going to be in a loosely coiled state called chromatin. Over here in the non-dividing state, the DNA is in a chromatin form. The chromatin form, because it's so loosely coiled, you can really think of a ball of yarn that is really loosely coiled. But notice that, over here on the right-hand side, what we have is the dividing cell. The DNA is going to condense, and so we have condensing DNA here. The DNA condenses into highly condensed or tightly packed chromosomes. This term, chromosomes, you can think of a ball of yarn that is really, really tightly coiled, nicely wrapped around, unlike the ball of yarn that's over here.

Chromatin and chromosomes are both referring to DNA wrapped around protein. The difference is really that chromatin is going to be a more loose form of the DNA, found in non-dividing cells. Whereas chromosomes are going to be more tightly coiled, DNA found in dividing cells. This is going to be very important. This condensing of the DNA into chromosomes is going to be very important for moving the DNA around within a cell that is dividing. We'll get to talk a lot more about chromosomes and the dividing cell as we move forward in our course here. But for now, this here concludes our introduction to the organization of the DNA in the cell, and we'll be able to get some practice applying these concepts as we move forward in our course. See you all in our next video.

A collection of DNA and associated proteins in a cell defines the term:

• A. Sister Chromatids.
• B. Centromere.
• C. Chromatin.
• D. Chromosome.

Answer: (C)

Choose the option that correctly lists the following genetic material in a dividing cell from smallest to largest.

• A. DNA, Chromatin, Nucleosomes, Chromosomes.
• B. Chromatin, Nucleosomes, Chromosomes, DNA.
• C. DNA, Nucleosomes, Chromatin, Chromosomes.
• D. Chromosomes, Chromatin, Nucleosomes, DNA.

Answer: (C)

DNA Replication

In this video, we're going to briefly discuss how DNA replication produces replicated chromosomes. Later in our course, we will talk in more detail about the process of DNA replication. But in this video, we're only going to lightly introduce DNA replication just enough to help you understand how it produces replicated chromosomes. First, we need to recall from some of our previous lesson videos that before a cell can divide, it's critical that the DNA must be replicated.

The term 'replicated' has a few synonyms that can be commonly used by your professors or in your textbook. The term replicated is synonymous with 'synthesized' and 'duplicated'. Thus, DNA replication, DNA synthesis, and DNA duplication all mean the same thing. We can say that this process of DNA replication is a process that produces an exact copy of all the DNA inside of a cell. This is critical for a cell to do before it divides because when a single cell divides into two separate cells, each of those cells needs a copy of the DNA, and that can only happen if DNA replication occurs first.

This process of DNA replication will convert unreplicated chromosomes, or chromosomes that have not yet been replicated, into replicated chromosomes, or chromosomes that have gone through DNA replication. These replicated chromosomes that have gone through DNA replication are going to have two identical sister chromatids. The term 'sister' is used to imply that these chromatids are identical to one another in terms of their DNA sequence. The term 'chromatid' can be defined as half of a replicated chromosome, and the chromatids are going to be joined to one another at a region called the centromere, which can be thought of as the waist position of the chromosome.

If we look at our image, notice on the left-hand side what we're showing you is a single unreplicated chromosome, which kind of looks like a straight line in this image. We can also think of a single unreplicated chromosome as just one chromatid. After DNA replication, notice on the right-hand side what we have is a single replicated chromosome. This replicated chromosome has gone through the process of DNA replication, and what you'll notice is that our unreplicated chromosome is highlighted, and with this replicated chromosome, we still have that same unreplicated chromosome present. However, you'll notice that the unreplicated chromosome has been replicated. So here, what we have is another identical copy of the pink highlighted region.

We call these regions chromatids, and these two can be said to be two identical chromatids, or in other words, they are sister chromatids. What you'll notice is that this sister chromatid and this sister chromatid are attached to each other at this one position right here that we call the centromere. This is critical information because we can tell if a chromosome has gone through DNA replication because if the chromosome somewhat looks like an X, as you see here, then we know that it's gone through DNA replication. But if the chromosome does not look like an X and looks more like a straight line, then we can say that it is an unreplicated chromosome that has not yet gone through DNA replication.

This can be very helpful information to note as we move forward and talk more about cell division. But for now, this here concludes our brief lesson on how DNA replication produces these replicated chromosomes, and we'll be able to get some practice as we move forward. So, I'll see you all in our next video.

The structures labeled Y in the image below are called:

• A. Centromeres.
• B. Centrioles.
• C. Sister chromatids.
• D. Spindles.

Answer: (C)

Here’s what students ask on this topic:

What is the genome and why is it important in cell division?

The genome refers to the complete set of all the DNA in a cell. It is crucial for cell division because it contains all the genetic information necessary for the cell's functions and inheritance. Before a cell divides, it must replicate its genome to ensure that each daughter cell receives an exact copy of the DNA. This replication is essential for maintaining genetic consistency across generations of cells, which is vital for the organism's development, growth, and repair.

How are nucleosomes formed and what role do they play in DNA organization?

Nucleosomes are formed when DNA wraps around histone proteins. Specifically, eight histone proteins form a core, and DNA wraps around this core, much like yarn around a spool. Nucleosomes play a critical role in DNA organization by compacting the DNA into a more manageable structure. This compaction is essential for fitting the long DNA molecules into the cell nucleus and for regulating gene expression and DNA replication.

What is the difference between chromatin and chromosomes?

Chromatin and chromosomes are both forms of DNA-protein complexes, but they differ in their degree of compaction and their role in the cell cycle. Chromatin is a loosely packed form of DNA found in non-dividing cells, allowing for easier access to the genetic material for processes like transcription. In contrast, chromosomes are tightly packed forms of DNA that appear during cell division, ensuring the DNA is accurately segregated into daughter cells. This tight packing is crucial for the efficient and error-free distribution of genetic material during mitosis and meiosis.

What are sister chromatids and how are they connected?

Sister chromatids are two identical copies of a single replicated chromosome, formed during DNA replication. They are connected at a region called the centromere, which can be thought of as the 'waist' of the chromosome. The centromere holds the sister chromatids together until they are separated during cell division, ensuring that each daughter cell receives an identical set of chromosomes.

How does DNA replication produce replicated chromosomes?

DNA replication produces replicated chromosomes by creating an exact copy of each chromosome in the cell. During this process, the DNA double helix unwinds, and each strand serves as a template for the synthesis of a new complementary strand. This results in two identical sister chromatids joined at the centromere, forming a replicated chromosome. This replication is essential for cell division, ensuring that each daughter cell receives a complete set of genetic information.