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We are our DNA, since it is this chemical that makes all of our genetic information!
Contained in our cell nuclei, DNA is a long chain of tightly wound-up coded instructions which cells unravel, read and use to make proteins.
When cells make a certain protein, they read DNA in chunks called genes that code for a specific sequence of amino acids.
[Reminder: A protein is a folded polymer with a specific sequence of amino acids.]
Like proteins, DNA is also a polymer, but of repeating nucleotides - these are made of a phosphate group on a sugar attached to a nitrogenous base, linking together to make the double helix.
There are four different bases, A, T, C, G, and they’re very specific about what they pair with:
Adenine with Thymine and Cytosine with Guanine
These groups are complementary base pairs and they are joined by weak hydrogen bonds, linking the two double helix strands in the middle.
So far, we have only said that DNA codes for proteins. This is not entirely true because there are two types!
Coding DNA are the genes that the cells read and make proteins from.
Non-coding DNA are like on-off switches for coding DNA - they control gene expression by telling the cell to use or ignore certain genes, making sure we only get the proteins we need.
So, to make a protein there are two stages: transcription of DNA (in the nucleus) and translation (in the cytoplasm):
Transcription:
The cell reads the genes to find the amino acid order - but DNA is too big to get out of the nucleus to build the protein, so it uses a messenger instead!
An enzyme temporarily breaks the weak hydrogen bonds and unzips the double helix. It then uses one side as a template to make a copy of the genes there, with this new copy being called mRNA.
mRNA travels out into the cytoplasm and carries the code to the site of protein synthesis - an organelle called a ribosome.
Translation:
As the ribosome reads the mRNA, 100s to 1000s of amino acids are brought in and joined in the sequence the genes state, making the initial protein chain.
After protein synthesis, the polypeptide then folds into a specific 3D structure that makes the protein suitable for its function.
So clearly, we’re super dependent on DNA, and any change to it can have huge consequences!
But change happens, and when our DNA changes, we call this a mutation.
Mutations can change proteins by altering our coding and non-coding DNA, but usually their function is preserved, and little difference is seen in our appearance (our unique phenotypes).
But occasionally, if enough of the DNA is changed, the protein’s shape can be altered so much that it can’t do its job, like an enzyme without a working active site.
Now it's time for some questions.
