DNA damage and repair

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DNA damage and repair

USMLE® Step 1 questions

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An investigator is studying a young boy's case with recurrent pulmonary infections, unsteady gait, delayed language development, and uncoordinated eye and head movements. Photographs are attached to the case file that shows numerous superficial blanching nests of distended capillaries on sun-exposed areas. The condition described above is most likely due to an inherited mutation in a gene normally responsible for which of the following?  


Our DNA is like a library - found in the nucleus of our cells - with thousands of books.

Some of these books - called genes - are extremely important, because they carry the recipes for every single protein found in the cell.

Now, on a molecular level, DNA is made up of two strands of nucleotides, so each gene is just a segment of this nucleotide sequence.

Nucleotides of DNA are made out of a sugar - deoxyribose, a phosphate, and one of the four nucleobases - adenine, cytosine, guanine, and thymine - or, A, C, G, T for short.

The nucleotides on one strand pair up using hydrogen bonds with nucleotides on the opposing strand, to create the double-stranded DNA: specifically, A bonds with T, and C bonds with G, so they’re called complementary bases.

Now, the goal of DNA is to store information and pass it onto their daughter cells, and to use this information to create proteins.

To do this, there are two critical processes - DNA replication and gene expression.

DNA replication occurs during the cell cycle - more specifically, during the S phase of interphase.

So, the cell cycle is made up of interphase - when the cell prepares for division - and mitosis - or the actual splitting of the cell in two daughter cell.

Interphase has 3 subphases - G1, S and G2, and during the S subphase, the cell replicates its DNA, so that the two daughter cells get the exact same DNA during mitosis.

If we zoom onto the double- stranded DNA, we can see that during DNA replication, the two DNA strands are separated by an enzyme called DNA helicase.

Then another enzyme, DNA polymerase, uses each of the single strands as a template and adds complementary nucleotides to it.

Gene expression, on the other hand, is the process of decoding the information stored in the DNA in order for the cell to make proteins, and it includes transcription and translation.

Transcription is where RNA polymerase copies the nucleotide sequence of the gene and creates a messenger RNA molecule, or mRNA that has the same sequence, with one tweak: it has uracil nucleotides - or U - instead of thymine.


DNA damage is any abnormal change in the DNA sequence that may occur due to environmental factors, such as UV radiation or chemicals. The body's cells have mechanisms to repair this damage, which helps to ensure that damaged DNA doesn't accumulate and results in uncontrolled cell division and tumor formation. DNA repair mechanisms include mismatch repair, base excision repair, nucleotide excision repair, non-homologous end joining, and homologous recombination.


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