1. Biology 1-2a
Chromosome Structure

2. 1-2a: Chromosome Structure
Now that we know how DNA replicates and how it is used as the instructions to make proteins, let’s explore how it is found within our cells….

3. 1-2a: Chromosome Structure
Each human cell contains approximately 2m of DNA, if it was stretched end to end.
The nucleus of an average human cell is only 6µm in diameter
Obviously, the DNA must somehow be condensed in order to fit into the nucleus.

4. The condensing process can be compared to packing 40 km of very fine thread into a tennis ball!!

5. 1-2a: Chromosome Structure
Let’s look at the process starting from the DNA strand:
The double helix DNA strand begins to wrap around small proteins called histones, creating a structure that looks like beads on a string.
Each of the “beads” (the combination of histones and looped DNA) is called a nucleosome.

6. 1-2a: Chromosome Structure
Next, these nucleosomes begin to coil together forming a ropelike strand of condensed DNA and proteins called chromatin.

7. For the majority of their life, your cells exist with the DNA in their nucleus in the form of chromatin
This allows the specific sections of DNA that code for protein production to be easily accessible when needed.

8. However, when your cells are replicating themselves, the chromatin strands within the nucleus need to be easily separated from each other.
If they stayed in the loose coils of chromatin they would get all tangled up!

9. 1-2a: Chromosome Structure
For cell division, the chromatin fibers begin to loop and coil upon themselves (imagine super twisting an elastic) until they form the shape of a chromosome.

10. Remember that in cell division, all the DNA in your cells must first be duplicated and then separated into the new cells.
Each condensed chromosome is actually two exact copies of the same chromatin strand connected together by a protein called a centromere.

11. 1-2a: Chromosome Structure
Final question:
Where do “genes” fit into all of this?

12. 1-2a: Chromosome Structure
A gene is a segment of DNA that contains the instructions for making a particular protein.
Genes can range in length from a few hundred to more than 2 million nucleotides long.

13. To Summarize:
Chromosomes are made up of tightly coiled chromatin fibers
Chromatin is made up of a strand of DNA that is wrapped around histones to form “beads” called nucleosomes that then wrap around each other.
A gene is a section of a DNA that contains the instructions for making a protein.