Chromosomes

Chromosomes are genetic structures found in cells during cell division. A chromosome is made from tightly packed strands of DNA and a huge number of proteins called histones.

True chromosomes are found only in eukaryotic cells. Prokaryotic cells contain much less DNA which is packaged with proteins but not into ‘true’ chromosomes.

Human cells contain 46 chromosomes. The full length of the DNA in one chromosome is around 4 cm (1.5 inches) long.

The width of human cells range between 10-120 µm. A bone marrow cell is around 40 µm wide. The length of the DNA packed into the 46 chromosomes of a bone marrow cell is around 46 000 times longer than the width of a cell. In order for all of this DNA to fit inside the nucleus of the cell it has to be packaged incredibly well.

Eukaryotic chromosomes

Chromosomes form during cell division from strands of DNA and histone proteins. They are condensed structures that can easily be distinguished from one another and are found within the nucleus of eukaryotic cells. In order for DNA to be packaged into chromosomes, several levels of coiling and wrapping is required. Eukaryotic cells typically have between 10-50 chromosomes.

Chromatin

DNA is not always packaged into chromosomes. In between cell divisions, DNA remains less densely packed in strands known as chromatin.

Chromatin is formed by the first two levels of coiling of DNA. Firstly, a strand of DNA is wrapped around proteins called histones. As the DNA wraps around histone proteins small beads are formed along the DNA strand called nucleosomes. One nucleosome is around 10 nm wide.

DNA + histone = nucleosome

To form chromatin, a strand of DNA with nucleosomes interacts with additional histone proteins. The interactions lead to further folding and coiling of the DNA strand into a chromatin fiber. A fiber of chromatin is around 30 nm wide.

Chromatin fibers are packed into the nucleus of eukaryotic cells. Each chromatin fills a specific space in the nucleus of the cell. Sections of chromatin that contain active genes are positioned closer to the edge of the nucleus. The looser packing of chromatin in between cell divisions makes it easier to access DNA for transcription.

DNA is most commonly found packaged as chromatin. The exceptions being when DNA is being replicated and transcribed.

Formation of chromosomes

During cell division, chromatin is packed tighter into chromosomes. The next stage of packaging sees chromatin loop around a scaffold of proteins to form what are known as looped domains. Loop domains are around 300 nm wide, 10 times wider than chromatin.

Loop domains continue to fold together to form a distinct structure known as a chromatid. A chromatid is one half of a chromosome. Chromatids are replicated to form a chromosome.

DNA + histones = nucleosomes (10 nm wide)
↓
folded nucleosomes = chromatin (30 nm)
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looped chromatin = loop domains (300 nm)
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folded loop domains = chromatid
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2 x chromatid = chromosome

Prokaryotic ‘chromosomes’

Prokaryotic cells contain far less DNA than eukaryotic cells. A prokaryotic cell has a single ring of DNA which stretches to around 1 cm long. In comparison, the eukaryotic cells of humans have 46 chromosomes which each stretch out to around 4 cm long.

Instead of forming ‘true’ chromosomes the DNA of prokaryotic cells coils up tightly into a condensed ball. Instead of being wrapped around histone proteins the DNA of prokaryotic cells twists so much that the twists fold over onto themselves and eventually form a ball. This process is known as supercoiling.

Last edited: 14 March 2016