Biology (UP) - DNA Structure


What is DNA and what is it made up of?

  • DNA holds genetic information and structure
  • It is made of the building block, a nucleotide.
  • Nucleotides are made of a phosphate group, a deoxyribose/pentose sugar and a nitrogenous base
  • DNA is a double helix, made of lots of nucleotides (polynucleotide)
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What are bases and their structures?

  • In DNA, nucleotides have a phosphate group, which is connected to the fifth or third carbon on the deoxyribose sugar. On the second carbon of the sugar, there is a hydrogen group (in RNA, this is a hydroxyl group). The nitrogenous base, joined to carbon one, is A,T, C or G.
  • The phosphate and sugar remains the same in each nucleotide
  • The base can be double ring structures (Adenine and Guanine), or single ring structures (thymine, uracil and cytosine)
  • It always attatches phosphate, sugar and base, in that order
  • Adenine and Guanine are purines. Thymine, Uracil and Cytosine are pyrmidines. It always joins purine to pyrmidine e.g. A-T. This makes the bases complementary - one of each
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What is the structure of DNA?

  • DNA is a double helix, made of two strands called the sugar-phosphate backbone
  • Strands are antiparallel, meaning they don't run the same way. This means that no matter what end the DNA is read from, it always codes for the same thing
  • Hydrogen bonds form between complementary bases. They give accumulative strength, although they can be easily broken. Between C-G, three H bonds form and between A-T, two H bonds form
  • The coil formed by DNA has evolved to be this way to fit it all in. Needs a specific structure for role, and for it to fit in nucleus
  • DNA base sequence is made up of genes, which code for a particular protein. This is simply a section of the DNA
  • Some of the sequence codes for useful things. Some of it doesn't. This is junk DNA. It isn't really known why we have it - might code for something we used to have and now don't need. Everyone's junk DNA is different - coding areas are the same
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What is the 3 and 5 prime end in DNA?

  • The sugar joins to the phosphate of another nucleotide at the third carbon.
  • The phosphate then joins to the fifth carbon of the sugar in the same nucleotide.
  • This forms the sugar-phosphate backbone.
  • Nucleotides are joined together by phosphodiester (phosphate, two ester bonds; PO43-). Hydroxyl joins with phosphate group by ester bond; covalent bond and strong
  • The other strand will run 5, 3, 5, 3 etc because the strands are antiparallel
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Why does DNA replicate and what is required?

  • During interphase of the cell cycle, DNA must be copied so that the daughter cells have the same genetic material. The semi-conservative model is widely accepted.

There are four requirements for s-c replication:

  • 4 types of nucleotide, each with their bases, must be present
  • Both strands of DNA molecule act as a template for attatchment of those nucleotides
  • The enzyme DNA polymerase
  • A source of chemical energy to drive process
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How does DNA replicate?

  • The enzyme, DNA helicase, separates DNA strands by breaking the hydrogen bonds holding them together. DNA unwinds
  • DNA helicase completes strand splitting. Each exposed polynucleotide strand acts as a template so that complementary free nucleotides bind by specific base pairing
  • Once active nucleotides are bound, they are joined by condensation reaction by DNA polymerase. The DNA polymerase forms phosphodiester bonds between the nucleotides, so that they form the 'missing' polynucleotide strand
  • Once this has finished, the result is two identical DNA strands. One of the strands comes from the parent strand, and retains half of the original material. The other is the daughter strand, which has been built based on the parent strand. This is called semi-conservative replication
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What evidence is there for semi-conservative repli

  • It was proven by microbiologists Messelson and Stahl, who were working on E.Coli
  • They centrifuged DNA, placed it in a caesium chloride gradient. Depending on the mass, DNA floats at different levels

Here's how it works:

  • Two bacteria samples grown, one in N15 and other in N14. Sample of DNA taken from bacteria and centrifuged. Heavy bacteria settled lower than the other. (DNA in starting culture is grown in N15. DNA therefore contains all heavy N15, which is a heavy istope of nitrogen, taken into the strands)
  • Bacteria with N15 DNA placed in a solution with N14, a lighter isotope. The DNA will now float higher because it is less dense. This is because it is taking in the less dense nitrogen. The DNA formed will be 50% N15, and 50% N14 because it has one parent strand containing N15, and one daughter strand containing N14. (If conservative, heavy strand to bottom and light strand to top).
  • Second generation is grown in N14. This will form one mixed molecule and one light molecule. The light will float higher still - only N14 is available.
  • Third generation is grown in N14. 75% of the DNA will be made up from lighter N14
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