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The structure of nucleic acid chains (or DNA).

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Introduction

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). ...read more.

Middle

The DNA polymerase (once it has reached its starting point as indicated by the primer) then adds nucleotides one by one in an exactly complementary manner, A to T and G to C. DNA polymerase is described as being "template dependent" in that it will "read" the sequence of bases on the template strand and then "synthesize" the complementary strand. The template strand is ALWAYS read in the 3' to 5' direction (that is, starting from the 3' end of the template and reading the nucleotides in order toward the 5' end of the template). The new DNA strand (since it is complementary) MUST BE SYNTHESIZED in the 5' to 3' direction (remember that both strands of a DNA molecule are described as being antiparallel). DNA polymerase catalyses the formation of the hydrogen bonds between each arriving nucleotide and the nucleotides on the template strand. In addition to catalysing the formation of Hydrogen bonds between complementary bases on the template and newly synthesized strands, DNA polymerase also catalyses the reaction between the 5' phosphate on an incoming nucleotide and the free 3' OH on the growing polynucleotide (what we know is called a phosphodiester bond!). ...read more.

Conclusion

This process is shown schematically below. Crick described the DNA replication process and the fitting together of two DNA strands as being like a hand in a glove. The hand and glove separate, a new hand forms inside the old glove, and a new glove forms around the old hand. As a result, two identical copies now exist. The process of DNA replication in all organisms is amazing, but in humans it seems particularly difficult to conceive. The sum of all genes in a human cell-the human genome-is estimated to be approximately 3 billion base pairs, and a single DNA chain might contain up to 250 million pairs of bases. What's even more incredible is how few mistakes are made in this process despite the immense size of human DNA! An error occurs only about once in each 10-100 billion bases. As you would probably expect, the complete process of DNA replication in human cells takes several hours. To replicate such huge molecules as human DNA at this speed requires not one, but many replication forks, forming replication bubbles and producing many segments of DNA strands that eventually meet up together and are joined to form the newly synthesized double helix. ...read more.

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Response to the question

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 ...

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Response to the question

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.

Level of analysis

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.

Quality of writing

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.


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Reviewed by groat 17/02/2012

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