![]() As a result, eukaryotic chromosomes are not nearly as supercoiled as prokaryotic chromosomes. In contrast, eukaryotic chromosomes are linear and packaged using histone proteins, which are not present in most prokaryotic cells. Due to the circular nature of most prokaryotic chromosomes, they are often highly supercoiled under normal growth conditions. Prokaryotic and eukaryotic chromosomal DNA is organized in different ways. One way to visualize supercoiling is to think about what happens when you twist a rubber band and then hold onto one end of it while trying to open it in the middle - the original coils will twist on top of each other to form a condensed, twisted ball. ![]() ![]() Supercoiling is a term used to describe what happens when the two strands of a double-stranded, double helical DNA molecule are separated from each other, which occurs during DNA replication and transcription. This simple, yet powerful, technique allows researchers to isolate DNA fragments of different sizes. Because DNA is negatively charged, molecular biologists often use agarose gel electrophoresis to separate different sized DNA fragments when DNA samples are subjected to an electric field - due to their negative charge, all of the DNA fragments will migrate toward the positively charged electrode, but smaller DNA fragments will migrate at a faster pace than larger DNA fragments. You might be interested to read that molecular biologists capitalize on this property of DNA to isolate DNA fragments of differing sizes. So, where does DNA’s negative charge come from? The phosphate groups that make up the sugar-phosphate backbone are responsible. In a nutshell, the structure of DNA can be thought of as a twisted ladder with its complementary base pairs making up the rungs of the ladder and the sugar-phosphate backbone of each strand making up each side of the ladder. The formation of phosphodiester bonds between adjacent nucleotides forms alternating sugar and phosphate groups, called the “sugar-phosphate backbone” of a DNA molecule. Nucleotides are covalently linked to one another via the formation of phosphodiester bonds between the sugar group of one nucleotide and the phosphate group of a second nucleotide.Īs you likely know, most DNA is found in a double-stranded form with complementary base pairing between the two DNA strands: A pairs with T, and C pairs with G. Each nucleotide consists of a phosphate group, a deoxyribose sugar group, and a nitrogenous base. Before we discuss where its negative charge comes from, let’s take a close-up view of the nucleotide monomers that make up DNA.įour different nucleotides are covalently linked to build DNA molecules: adenine (A), guanine (G), cytosine (C), and thymine (T). Since DNA strands of the helix run anti-parallel, the direction of the top strand will be 3’ to 5’.Hello Jery, DNA is a negatively charged polymer that is made up of nucleotide building blocks. The sugar-phosphate backbone of DNA has the phosphate on the 5’ carbon linked to the 3’ carbon of the next sugar. If the direction of the sugar-phosphate backbone of the bottom strand is 5’ to 3’ from left to right, what is the direction of the top strand from left to right?ĥ’ to 3’ (No, the DNA strands in a helix are anti-parallel.ģ’ to 3’ (No, the sugar-phosphate backbone of DNA is not 3’ to 3’.)ĥ’ to 5’ (No, the sugar-phosphate backbone of DNA is not 5’ to 5’.) Strand are the complement of the nucleotides on the other strand. Nucleotides base pair through hydrogen bonds A pairs with T, and G pairs with C. ![]() TACATATGCC (No, this is the reverse sequence, not the complement.)ĪATGCGCACG (No, A does not pair with C and T does not pair with G.) Reading left to right, what is the nucleotide sequence on the other strand of DNA in this section?ĬCGTATACAT (No, this is not the complementary sequence.) This section of DNA has a sequence of nucleotides like this: CCGTATACAT If you curl the fingers of your right hand in the direction of the helical twists, your thumb (No, The DNA helix does not have evenly spaced coils.)ĭNA is a right-handed double helix. (No, This is a left handed double helix.) The discovery of the structure of DNA was a landmark in molecular biology research. ![]()
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