Question

Explain the current model for how expansion of nucleotide repeats happens. Draw a picture to illustrate the process. The following nucleotide sequence is found in a short stretch of DNA: 5’-ATGT-3’ 3’-TACA-5’ ​ If this sequence is treated with the mutagen hydroxylamine what will the sequences be after replication? Does treatment with hydroxylamine cause transitions or transversions?

Answer 1

The current model for the expansion of nucleotide repeats involves DNA slippage during replication. This phenomenon occurs when a DNA polymerase encounters a repetitive sequence, such as a nucleotide repeat, and temporarily slips or stutters during replication. It can result in the expansion of the repeat region.

Here's a simplified diagram to illustrate the process:

Original DNA sequence: 5'-ATGT-3'

3'-TACA-5'

During replication, the DNA polymerase synthesizes a complementary strand based on the template. However, in the repetitive region, slippage can occur. This slippage leads to a misalignment between the template strand and the newly synthesized strand.

In the case of nucleotide repeat expansion, the slippage may result in the addition of extra repeat units. In this example, let's assume the slippage leads to an expansion of the repeat.

Expanded DNA sequence after replication with slippage:

Template strand: 5'-ATGT-3'

3'-TACA-5'

Synthesized strand: 5'-ATGTGTGT-3'

3'-TACACACA-5'

After replication with slippage, the expanded DNA sequence has additional repeat units (GT) compared to the original sequence.

Regarding the second part of your question, treatment with hydroxylamine can cause transitions in the DNA sequence. Hydroxylamine is a mutagen that can chemically modify DNA bases, particularly cytosine (C). It converts cytosine to uracil, which pairs with adenine (A) during replication, leading to a C to T transition.

Therefore, if the sequence "5'-ATGT-3'" is treated with hydroxylamine and then replicated, the resulting sequences after replication could be:

Original sequence: 5'-ATGT-3'

Replicated sequence: 3'-TACA-5'

After hydroxylamine treatment, the cytosine (C) in the sequence is converted to uracil (U). During replication, uracil pairs with adenine (A), leading to a C to T transition. Thus, the resulting replicated sequence would be "3'-TACA-5'".

In summary, the expansion of nucleotide repeats can occur through DNA slippage during replication, leading to the addition of extra repeat units. Treatment with hydroxylamine can cause transitions in the DNA sequence, converting cytosine (C) to uracil (U), which pairs with adenine (A) during replication, resulting in a C to T transition.

Step-by-step explanation: