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DNA Structure

Biology (Year 12) - DNA Structure and Function

Ben Whitten


Image: Nucleotide structure within a polynucleotide chain image, Image by Kep17, Sourced Under a Creative Commons 4.0 License from Wiki Commons Nucleotides are the building blocks of DNA that make up the nucleic acid that is DNA! Each nucleotide has 3 main components; a five-carbon (pentose) sugar molecule called deoxyribose, a phosphate group (negatively charged polyatomic ion) and a nitrogenous base (A, T, C, G). Every phosphate group is attached to two different sugar molecules by ester bonds and is then called a phosphodiester bond. The five carbon atoms in each sugar form a ring which is numbered 1' to 5' (' means prime!). One of the ester bonds is formed with the 3' carbon of one sugar ring, whilst the other is formed with the 5' carbon of the next sugar ring. The chain of alternating sugar molecules and phosphate groups is called the sugar-phosphate backbone, which is negatively charged. A strand of nucleotides has a particular direction using to phrase 5' to 3', where the 5' end starts with a phosphate group and the 3' end finishes with a sugar. It is important to note that DNA (and RNA) synthesis (creation) occurs in the 5' to 3' direction.

DNA's Shape

DNA molecules are double-helical (in a double helix), where the two strands are joined by the weak hydrogen bonds between complementary nitrogenous base pairs.

'Helix' simply refers to the spiral shape of the molecule. The two strands run in opposite directions to one another, meaning they can be called anti-parallel.

DNA's Structure Enables Function

DNA carries information in its different segments known as genes and enables certain traits (an inheritable characteristic) to be passed on to subsequent generations. DNA stores the genetic code for making proteins (a process called protein synthesis) and the inheritance of different genes cause an individual to have a very specific combination of proteins in its makeup. A section of DNA that actively codes for a specific protein is referred to as a gene. Genes have the ability to code for more than one kind of protein, and they interact with one another which causes changes to their expression. DNA can therefore be said to control the growth and development of an organism. The structural properties within DNA molecules (nucleotide composition, pairing and hydrogen bonding) is essentially what allows for DNA replication to occur!

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