Topic 8.1

Structure of DNA

Nucleotide structure

Individual nucleotides of DNA are made up of:

• Sugar – Deoxyribose
• Phosphate group
• Organic base

Single-ring bases – Cytosine (C) and Thymine (T)

          OR

Double-ring bases – Adenine (A) and Guanine (G)

T=A

C=G

=Hydrogen bonds

 

                             

*These three components are combined as a result of condensation reactions to give a mononucleotide.

-Two mononucleotides may combine as a result of a condensation reaction between the deoxyribose sugar of one mononucleotide and the phosphate group of another. This new structure is a dinucleotide.

-A long chain of mononucleotides is a polynucleotide.

-It is not just DNA that contains nucleotides.

DNA structure

-In 1953, James Watson and Francis Crick worked out the structure of DNA following Rosalind Franklin’s work on X-ray diffraction patterns of DNA.

-DNA is made up of two strands of nucleotides (polynucleotides).

-Each starnd is extremely long and joined together by hydrogen bonds formed between certain bases.

-Simply: Ladder – phosphate and deoxyribose molecules alternate to form the uprights and organic bases pair together to form the rungs.

 

The double helix

-The ladder-like arrangement of the two polynucleotide chains is twisted.

-The uprights of phosphate and deoxyribose wind around each other to form a double helix.

-They form the structural backbone of the DNA molecule.

-For each complete turn of the helix, there are ten base pairs.

Function of DNA

-DNA is the hereditary material responsible for passing genetic information from cell to cell and generation to generation.

-There are around 3.2 billion base pairs in DNA of a typical mammalian cell.

  This means there is an almost infinite variety of sequences of bases on the length of a DNA molecule, providing immense variety in living organisms.

How DNA molecule is adapted to its functions:

• Very stable and can pass between generations without change.
• Two separate strands, joined with only hydrogen bonds, allowing the to separate during DNA replication and protein synthesis.
• Extremely large molecule, therefore carries an immense amount of genetic information.
• Having base pairs within the helical cylinder of deoxyribose-phosphate backbone, genetic information is protected from being corrupted by outside chemical and physical forces.
• The function of DNA depends on the sequence of the base pairs it posseses.