The structure of nucleic acid chains (or DNA).

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W.Feltham

THE STRUCTURE OF NUCLEIC ACID CHAINS (or DNA).

        Nucleotides are joined together in DNA and RNA by phosphate ester bonds between the phosphate component of one nucleotide and the sugar component of the next nucleotide. An ester bond is a bond which occurs between a Carbon atom and an Oxygen atom.

More and more nucleotides can be added on by the same process of forming ester bonds until an immense chain is formed. But no matter how long a polynucleotide chain is, one end of the nucleic acid molecule always has a free -OH group on the sugar at the Carbon known as C3' (called the 3' end) and the other end of the molecule always has a phosphoric acid group at C5' (the 5' end). The Carbons get this name from a counting system illustrated in the next diagram. (Fig 1)

Beginning from the "right-hand" side of the sugar, count the Carbons....1', 2', 3' (where the phosphate group of the next nucleotide in a series can be linked via a chemical bond), 4', 5' (where the phosphate group of the previous nucleotide is linked via a chemical bond).

                                This "counting system" allows the                                         strand of nucleic                                                 acid to be oriented: the 5' end of                         the molecule always ends                                 with a phosphate and the 3' end of                         the strand always ends         with a                         sugar. You may be wondering why                 we don't just call the 5' end the                                 "top" of the DNA or RNA molecule                                                 and the 3' end the "bottom" of the                                                 molecule. But in order to name                                                         something the "top", we're assuming                                                 that that end of the molecule is "up".

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        Remember that cells don't have specific orientations and that the nucleic acid within the cells is tightly wrapped and coiled around special proteins in the nucleus. So the terms "top" and "bottom" or "left" and "right" are pretty useless in this situation. Any nucleotides in between the 3' and 5' nucleotides would be involved in phosphodiester bonds. These nucleotides on the ends of each strand have a "free" end which is not involved in such a bond.

Replication.

        DNA replication begins with a partial unwinding of the double helix at an area known as the replication fork. ...

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This essay is written well, however it is slightly worrying that the essay spends more time discussing replication than structure, which is the task set. Spelling, punctuation and grammar are used well and scientific terms are used correctly when appropriate.

The explanation of DNA strands is strong, using higher level concepts such as 3' and 5' ends. Just to note, I would always refer to them as 3-prime and 5-prime in an exam, just to prevent and ambiguity. I liked how they referred to the ester bonds in a DNA nucleotide, however their understanding could be proven if they'd included a diagram which had labelled this bond. Similarly, I would've liked to have a seen a discussion of the deoxyribose sugar base and the phosphate group. Including concepts such as this will ensure higher marks are within reach. When discussing DNA strands, I think it's key to show a diagram of the orientation of DNA nucleotides. This naturally leads onto discussion of complementary base pairing, which isn't discussed in the structure section. Yes, it is mentioned during replication, however I think this is something which needs to be addressed early on. Although discussing replication is not relevant to the task, plenty of higher level concepts are used. I particularly liked the discussion of DNA polymerase to separate the strands - a concept explored at A-Level.

This essay outlines the structure of DNA chains to a strong level for GCSE, even adding information about replication. I would've like to have seen an exploration of the structure of singular DNA nucleotides. Diagrams are used well to support the scientific explanations of replication, but there are plenty more that could've been included to directly answer the question.