Chromosomes, chromatids and chromatin (video) | Khan Academy
Chromosomes are structures within the nuclei of eukaryotic cells that contain deoxyribonucleic acid (DNA) combined with proteins. Acknowledgments. I would like to thank my supervisor David Staněk for his guiding and tips on scientific . The relationship between chromatin and splicing. Although the numbers and sizes of chromosomes vary considerably between The DNA of eukaryotic cells is tightly bound to small basic proteins (histones) that form loops at the ends of chromosomes, thereby protecting the chromosome.
Before the cell can divide, DNA takes this very well-defined shape.
Most of the cell's life, when the DNA is actually doing its work, when it's actually creating proteins or proteins are being essentially transcribed and translated from the DNA, the DNA isn't all bundled up like this. Because if it was bundled up like, it would be very hard for the replication and the transcription machinery to get onto the DNA and make the proteins and do whatever else.
Normally, DNA-- let me draw that same nucleus. Normally, you can't even see it with a normal light microscope. It's so thin that the DNA strand is just completely separated around the cell. I'm drawing it here so you can try to-- maybe the other one is like this, right?
And then you have that shorter strand that's like this.
And so you can't even see it. It's not in this well-defined structure. This is the way it normally is. And they have the other short strand that's like that. So you would just see this kind of big mess of a combination of DNA and proteins, and this is what people essentially refer to as chromatin. So the words can be very ambiguous and very confusing, but the general usage is when you're talking about the well-defined one chain of DNA in this kind of well-defined structure, that is a chromosome.
Chromatin can either refer to kind of the structure of the chromosome, the combination of the DNA and the proteins that give the structure, or it can refer to this whole mess of multiple chromosomes of which you have all of this DNA from multiple chromosomes and all the proteins all jumbled together. So I just want to make that clear. Now, then the next word is, well, what is this chromatid thing? What is this chromatid thing? Actually, just in case I didn't, I don't remember if I labeled these.
These proteins that give structure to the chromatin or that make up the chromatin or that give structure to the chromosome, they're called histones.
Chromosomes and Chromatin - The Cell - NCBI Bookshelf
And there are multiple types that give structure at different levels, and we'll do that in more detail. So what's a chromatid? When it's just in its normal state, I have one version from my dad, one version from my mom.
Now, let's say it replicates. So my version from my dad, at first it's like this. It's a big strand of DNA. It creates another version of itself that is identical, if the machinery worked properly, and so that identical piece will look like this.
Chromosomes, chromatids and chromatin
And they actually are initially attached to each other. They're attached to each other at a point called the centromere. Now, even though I have two strands here, they're now attached. When I have these two strands that contain the exact-- so I have this strand right here, and then I have-- well, let me actually draw it a different way.
I could draw it multiple different ways. I could say this is one strand here and then I have another strand here. Now, I have two copies. They're coding for the exact same DNA. I still call this a chromosome. This whole thing is still called a chromosome, but now each individual copy is called a chromatid.
So that's one chromatid and this is another chromatid. Sometimes they'll call them sister chromatids. Maybe they should call them twin chromatids because they have the same genetic information. So this chromosome has two chromatids. Now, before the replication occurred or the DNA duplicated itself, you could say that this chromosome right here, this chromosome like a father, has one chromatid.
You could call it a chromatid, although that tends to not be the convention. People start talking about chromatids once you have two of them in a chromosome. And we'll learn in mitosis and meiosis, these two chromatids separate, and once they separate, that same strand of DNA that you once called a chromatid, you now call them individually chromosomes.
So that's one of them, and then you have another one that maybe gets separated in this direction. Let me circle that one with the green. So this one might move away like that, and the one that I circled in the orange might move away like this. Now, once they separate and they're no longer connected by the centromere, now what we originally called as one chromosome with two chromatids, you will now refer to as two separate chromosomes.
Or you could say now you have two separate chromosomes, each made up of one chromatid. So hopefully, that clears up a little bit some of this jargon around DNA.Chromatin and Chromosomes
I always found it quite confusing. But it'll be a useful tool when we start going into mitosis and meiosis, and I start saying, oh, the chromosomes become chromatids. And you'll say, like, wait, how did one chromosome become two chromosomes? And how did a chromatid become a chromosome? And it all just revolves around the vocabulary. Thus, both the nuclease digestion and the electron microscopic studies suggested that chromatin is composed of repeating base-pair units, which were called nucleosomes.
Nonhistone proteins bind to the linker DNA between nucleosome core particles. B Gel electrophoresis of DNA fragments more More extensive digestion of chromatin with micrococcal nuclease was found to yield particles called nucleosome core particles that correspond to the beads visible by electron microscopy. Detailed analysis of these particles has shown that they contain base pairs of DNA wrapped 1.
One molecule of the fifth histone, H1, is bound to the DNA as it enters each nucleosome core particle. This forms a chromatin subunit known as a chromatosomewhich consists of base pairs of DNA wrapped around the histone core and held in place by H1 a linker histone. A The nucleosome core particle consists of base pairs of DNA wrapped 1. A chromatosome contains two full turns of DNA base more The packaging of DNA with histones yields a chromatin fiber approximately 10 nm in diameter that is composed of chromatosomes separated by linker DNA segments averaging about 80 base pairs in length Figure 4.
In the electron microscope, this nm fiber has the beaded appearance that suggested the nucleosome model. Packaging of DNA into such a nm chromatin fiber shortens its length approximately sixfold. The chromatin can then be further condensed by coiling into nm fibers, the structure of which still remains to be determined.
Interactions between histone H1 molecules appear to play an important role in this stage of chromatin condensation. The packaging of DNA into nucleosomes yields a chromatin fiber approximately 10 nm in diameter. The chromatin is further condensed by coiling into a nm fiber, containing about six nucleosomes per turn.
Photographs courtesy of Ada more The extent of chromatin condensation varies during the life cycle of the cell. Physically speaking, DNA is a long string of paired chemical units nucleotides that come in four different types, abbreviated A, T, C, and G, and it carries information organized into units called genes.
Genes typically provide instructions for making proteins, which give cells and organisms their functional characteristics. Image of a eukaryotic cell, showing the nuclear DNA in the nucleusthe mitochondrial DNA in the mitochondrial matrixand the chloroplast DNA in the stroma of the chloroplast.
Mitochondria, organelles that harvest energy for the cell, contain their own mitochondrial DNA, and chloroplasts, organelles that carry out photosynthesis in plant cells, also have chloroplast DNA.
The amounts of DNA found in mitochondria and chloroplasts are much smaller than the amount found in the nucleus. In bacteria, most of the DNA is found in a central region of the cell called the nucleoid, which functions similarly to a nucleus but is not surrounded by a membrane. Since all of the cells in an organism with a few exceptions contain the same DNA, you can also say that an organism has its own genome, and since the members of a species typically have similar genomes, you can also describe the genome of a species.
In general, when people refer to the human genome, or any other eukaryotic genome, they mean the set of DNA found in the nucleus. Mitochondria and chloroplasts are considered to have their own separate genomes. Chromatin In a cell, DNA does not usually exist by itself, but instead associates with specialized proteins that organize it and give it structure.
In addition to organizing DNA and making it more compact, histones play an important role in determining which genes are active. The complex of DNA plus histones and other structural proteins is called chromatin. Image of a long, double-stranded DNA polymer, which wraps around clusters of histone proteins.
The DNA wrapped around histones is further organized into higher-order structures that give a chromosome its shape. For most of the life of the cell, chromatin is decondensed, meaning that it exists in long, thin strings that look like squiggles under the microscope.
In this state, the DNA can be accessed relatively easily by cellular machinery such as proteins that read and copy DNAwhich is important in allowing the cell to grow and function. Condensation takes place when the cell is about to divide.
When chromatin condenses, you can see that eukaryotic DNA is not just one long string. Bacteria also have chromosomes, but their chromosomes are typically circular. Chromosomes Each species has its own characteristic number of chromosomes. Like many species of animals and plants, humans are diploid 2nmeaning that most of their chromosomes come in matched sets known as homologous pairs.
The 46 chromosomes of a human cell are organized into 23 pairs, and the two members of each pair are said to be homologues of one another with the slight exception of the X and Y chromosomes; see below. Human sperm and eggs, which have only one homologous chromosome from each pair, are said to be haploid 1n.
When a sperm and egg fuse, their genetic material combines to form one complete, diploid set of chromosomes. So, for each homologous pair of chromosomes in your genome, one of the homologues comes from your mom and the other from your dad. Image of the karyotype of a human male, with chromosomes from the mother and father false-colored purple and green, respectively.