Stated Clearly presents: What is a chromosome? In previous animations, we learned that DNA is a spiraling chain-like molecule made up of 4 different types of nucleotides. Scientists have labeled them A, C, T and G. We also learned that a gene is a long stretch of DNA a specific sequence of As, Cs, Ts, and Gs that code for something. What does a gene code for? Usually a protein or a group of proteins though there are a few exceptions. In multi-celled creatures like ourselves Proteins, which are coded for by genes, interact with other proteins and molecules to make up living cells. Cells make up tissues, tissues make up organs, and organs ultimately make up entire living creatures. Individuals have different traits - in this case, differences in beak shape - largely because of differences, or mutations, in their genetic code. These differences can either cause changes in a specific protein shape (and therefore its function) or mutations can cause when or how much of a particular protein will be reproduced. Now that you clearly understand these three terms...and, by the way, feel free to pause the video to study them if need be... ...it is now time to talk about chromosomes. A chromosome is an entire chain of DNA along with a group of stabilizing proteins. You may have seen images showing DNA trapped inside a chromosome sort of like this. This is NOT how chromosomes hold their DNA. If you were to unravel a chromosome you would see that its made of a thread sort of like a spool of yarn. Zoom in again and you'll see that that fiber is a collection of protein orbs called histones which are wrapped with a string-like structure. Zoom in on that string and you'll find that this is the chain of DNA. It is extremely long - millions of nucleotides long and often contains hundreds of genes along its length. In this image, we're looking at a chromosome in its condensed yarn-like form but chromosomes are only packaged tightly like this when a cell is reproducing. This structure helps with durability as the cell is splitting in 2. For most of a cell's life, however, chromosomes exist in a loose, noodle-like structure, along with other chromosomes safe inside the nucleus (or centerpiece) of a cell. Here is a photograph of an entire set of human chromosomes taken from a cell right as it was getting ready to reproduce. Your genome, the entire collection of genes that help make you who you are, are written across 23 pairs of chromosomes. That's 46 all together. With very few exceptions, each cell in your body contains its own full copy of your chromosomes your entire genetic code. Eye ball cells only use the eye ball cell genes the rest are turned off. Liver cells only use liver cell genes the rest are turned off. and so on for each cell type. One member of each pair of your chromosomes came from your mother, the other came from your father If you were to examine the 2 individuals of a pair you'd find that for the most part both contain the exact same genes in the exact same locations If you were to zoom in to the actual genetic code, however you would find slight sequence variations between them. These are the results of mutations. Most of these mutations happened long ago and have been passed down from parent to child for many generations. Others, however, are completely unique to you. These mutations occurred either in the sperm or egg which fused to make you or they occurred early on in your development making it into all or many of your cells as you matured. These unique mutations represent brand new genetic information - sequences of DNA that may have never been seen before in the history of the cosmos. So, to sum things up, what exactly is a chromosome? A chromosome is an entire chain of DNA along with a group of stabilizing proteins. Humans typically have 23 pairs of chromosomes in each of their cells - that's 46 all together. In your body, 1 member of each pair came from your father, the other from your mother. For most of a cell's life, chromosomes exist in a loose, noodle-like structure. Chromosomes are copied and then condensed like spools of yarn for durability when a cell prepares to reproduce by splitting in 2. I'm Jon Perry, and that is a chromosome Stated Clearly. This animation was paid for by our viewers. If you'd like to see more, consider supporting us at patreon.com/statedclearly. So long for now. Stay curious.