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Chromosomes and Plasmids

DNA comes in a variety of different forms. On this page we will look at two important types of DNA molecules: chromosomes and plasmids. Chromosomes are very large DNA molecules containing many genes. Plasmids are much smaller DNA molecules containing only a few genes.

Chromosomes are large DNA molecules

A chromosome is a very large DNA molecule. In some organisms the chromosomes are linear (a straight line with two ends). In other organisms they are circular (a loop).

Diagram showing a linear chromosome and a circular chromosome. The linear chromosome looks like a piece of rope with two ends. It is labelled, "Linear Chromosome". The circular chromosome looks like a look of rope. It is labelled, "Circular chromosome".

Some organisms have linear chromosomes. Other organisms have circular chromosomes.

A chromosome is made up of a single very long DNA molecule, which is very highly coiled.

This DNA molecule usually contains hundreds or thousands of genes, each coding for a different protein.

Diagram of a chromosome. The chromosome is shown on the left of the diagram. It is shown as a thick, curvy, vertical line, almost like a piece of rope. It is labelled, "Chromosome. A very large DNA molecule". A circle to the right shows part of the chromosome zoomed in. It shows that it is made up of highly coiled DNA, which looks like a tangle of string. It is labelled, "Highly coiled DNA". Another circle to the right shows a section of this DNA zoomed it. It appears as a vertical line which blue boxes around several sections of it. These are labelled, "Genes". Another circle to the right shows one of the genes zoomed in. It shows a DNA double-helix, labelled "DNA". Text at the bottom of the image says, "A chromosome is one very long DNA molecule which is highly coiled. It contains hundreds or thousands of genes, each of which codes for a different protein. Each gene is a section of the DNA molecule - usually hundreds or thousands of base pairs long."

A chromosome is a very large DNA molecule containing many genes. Each gene codes for a different protein.

A single gene can be thousands of base pairs long. Since a single chromosome can contain thousands of genes, and there are sections of chromosome in between the genes, a single chromosome can be many millions of base pairs long.

A chromosome is like a recipe book for proteins

Because a chromosome contains many genes, it allows the cell to make many different proteins.

Diagram with the title, "Each gene codes for a different protein". Part of a DNA molecule is shown - represented by a horizontal line. There are blue rectangles around five different parts of the line to represent 5 genes. These are labelled "Gene 1" to "Gene 5". Each gene has an arrow come out from the bottom of it pointing to a protein. The five proteins are all different. The protein from gene 1 is labelled "Protein 1" and so on.

Each gene in a chromosome codes for a different protein.

A chromosome is like a recipe book for proteins.

A real recipe book contains lots of recipes, which are instructions for making different meals.

A chromosome contains lots of genes, which are instructions for making different proteins.

In this recipe book analogy, the paper that the recipe book is made out of represents the DNA. This is because DNA is the material that the chromosome is made out of.

Recipe bookChromosome
Contains many recipes.Contains many genes.
Each recipe is the instructions for making a particular meal.Each gene is the instructions for making a particular protein.
The recipe book is made out of paper.The chromosome is made out of DNA.

After DNA replication, a linear chromosome is X-shaped

Before a cell divides, all of its DNA is copied. This is called DNA replication.

During DNA replication an identical copy of every chromosome is made. This identical copy remains attached to the original copy. If the original chromosome was linear then the structure that is formed looks like a letter X.

Even though this structure is made up of two DNA molecules, it is still called a chromosome. Therefore, during DNA replication the number of chromosomes does not change, but each chromosome becomes an X shape made up of two identical DNA molecules.

Diagram showing a linear chromosome before and after DNA replication. On the left is a drawing of the chromosome before DNA replication, labelled as such. It looks like a piece of rope with two ends, stood up vertically (it is not completely straight - it is somewhat curvy). On the right is a drawing of the same chromosome after DNA replication, also labelled as such. It looks like two copies of the original chromosome, back to back, touching in one place to form an X-shape. There is a label pointing to two of the arms of the X which reads, "Two identical copies of original chromosome".

After DNA replication, each chromosome is an X shape, made up of two identical copies of the original chromosome.

When you see microscope images of chromosomes, they are usually images of chromosomes after DNA replication and therefore the chromosomes are usually X-shaped.

Microscope image of all of the chromosomes from a human body cell. Each chromosome appears as a dark purple X shape. They vary greatly in size. They are randomly arranged.

Microscope image of human chromosomes. They have been stained dark purple. Each chromosome is roughly X-shaped.

When the cell divides, the two copies are pulled apart and become separate chromosomes which go into different cells.

A plasmid is a small, circular DNA molecule

Plasmids are circular DNA molecules which are much smaller than chromosomes.

A plasmid usually only has a small number of genes (usually fewer than a hundred).

Diagram of a plasmid. The title at the top says "Plasmid". Under this, there is a drawing of a plasmid. It is shown as a black circle. There are nine blue boxes around different parts of the plasmid. These shows the locations of genes. A label pointing to two of them says, "Genes". A zoomed in section of part of the plasmid shows that it is made up of DNA in a double-helix shape.

A plasmid is a small, circular DNA molecule containing a small number of genes.

(Note: Although most plasmids are circular, there are actually some that are linear. However you do not need to know this for the GCSE course. In your GCSE exams always say that plasmids are circular).

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.

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What is a chromosome?

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What is a plasmid?

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