Learning Objectives

Learn what a chromosome is,

Why we have 23 pairs,

Why one pair is special,

Where chromosomes live, and

What they are made out of!

All humans are special. This is because we each have a unique set of DNA. This DNA is found in our chromosomes. Today, we are going to learn more about these cool chromosomes!

Every species has a certain number of chromosomes: female jack jumper ants have only 2, a fruit fly has 8, humans have 46, and atlas blue butterflies have around 450!

Notice how all of these numbers are even? That’s because many humans and plants are diploid meaning that all of their chromosomes come in pairs. So as humans, we have 23 pairs of chromosomes, making 46 in total. 22 of these pairs are said to be

Micrograph of condensed chromosomes
Figure 1: Chromosomes can be pictures while they are dividing then they are moved around to form this image known as a karyotype.

homologous of one another. This means that they are very similar to each other in size, shape, and genetic information. One set of our chromosomes are haploid. This chromosome pair has unique genetic information in each chromosome. These chromosomes are our sex chromosomes. Our X and Y chromosomes determine what biological sex we are. If you have XX chromosomes then you are female, and if you have XY chromosomes it means you are male.


As mentioned, 22 of your chromosome pairs are very similar. We call these pairs of chromosomes the autosomes. Each “side” of your chromosome pair comes from each of your biological parents. So when a human egg and human sperm come together, each cell brings one side of a chromosome pair together. This is why meiosis results in half of the number of chromosomes in each daughter cell. When sperm and egg come together, chromosomes find their pair resulting in 44 autosomes and 2 sex chromosomes.


Let’s take a look at where chromosomes are located. Chromosomes are found in the nucleus. Chromosomes are made up of tightly wound strands of DNA and proteins. Each chromosome carries specific genetic information. The genetic information part of a

Decomposition from chromosomes to DNA structure
Figure 2: Chromosomes “unraveled” to see from their home in the nucleus to their DNA structure.


chromosome comes from the DNA. Each human has their own set of DNA. A cell’s set of DNA is known as a genome. Each human has their own genome, and sometimes more broadly, a genome describes the similar genetic makeup of an entire species.


In the next unit you will learn more about the structure of DNA. To conclude, humans have 46 chromosomes that are stored in the nucleus that are made up of proteins and DNA. 22 pairs of chromosomes are very similar and one pair is not. This special pair is our sex chromosomes which have specific genetic information and determine what biological sex we are. Our chromosomes are all unique and make up our own human genome!


Figure 1 Image Credit: (licensed under CC BY NC)
Figure 2 Image Credit: (licensed under CC BY NC)
Second Paragraph: Source: Wikipedia ( License of original source: CC-BY 4.0. Modifications: rewording

Fifth Paragraph: Source: Khan Academy ( License of original source: CC-BY 4.0. Modifications: rewording




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