Chromosomes: Structure & Physical Features

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

You've probably heard about chromosomes before, but how much do you truly know about them? This lesson will discuss their structure and features and will go over all of the vocabulary you need to know to be a chromosome expert!

What is a Chromosome?

There's a lot of DNA, or deoxyribonucleic acid, in your cells. But how much? Although there is some debate on the length, scientists believe that if you took all of the DNA out of your cells and stretched it out, you would have a whole lot of DNA. For example, some say you could make it to the sun and back four times, others say you could make it to Pluto and back, and some even say the stretched out DNA would be about two times the diameter of the solar system. Maybe they can't agree, but scientists do know that there's a huge amount of DNA inside of your body. So how does all of that DNA fit into your teeny, tiny cells?

Excellent question. To answer it, I bring you the chromosome! Chromosomes are made up of DNA and proteins and are found in the nucleus of eukaryotes. Eukaryotes are more complex cells found in animals, plants, fungi, and protists. In prokaryotes, or organisms with simple cells like bacteria, the DNA is found floating in the cytoplasm of the cell.

A simple cell, or prokaryote, versus a more complex cell, or eukaryote. Notice the simple cell does not have a nucleus.

If you were to zoom in on a chromosome, you would see genes, which are sections of your DNA that act as instructions to build certain proteins. You inherit your genes from your parents and genes are the reason you look the way you do! For example, you have several genes that control the production of a pigment called melanin in your eyes. So you may have blue eyes because your genes tell your body to produce less melanin, or brown eyes because your genes tell your body to make more melanin!

You inherit a set of chromosomes, and therefore genes, from your mother and a set from your father. Each set of chromosomes is referred to as homologous chromosomes. I remember homologous because homo means the same, so homologous chromosomes are the same chromosomes. Homologous chromosomes are considered the same because they are the same size and carry the same genes.

Note the homologous chromosomes for humans. You can see that the chromosomes are paired.

Now things start to get tricky. Each homologous chromosome may have different forms of a gene which may result in variations of how the gene is expressed. These ''forms'' are referred to as alleles. For example, let's say you receive the allele that tells your body to produce a lot of melanin (thus resulting in brown eyes) from your mother and the allele that tells your body to produce less melanin (thus resulting in blue eyes) from your father. Although both of your parents passed down the gene for eye color, each gave you a different form of that gene (one form that results in brown eyes and one form that results in blue eyes).

Because you inherit a set of chromosomes from each parent, most of your cells have 23 pairs of chromosomes, or 46 individual chromosomes. This is referred to as diploid. I always remember diploid because di means two and diploid means you have two sets of chromosomes. Different species have different numbers of chromosomes. For example, your dog has 39 pairs and your pet rat has 21 pairs.

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