ka1: the structure of DNA

• Structure of DNA — nucleotides (deoxyribose sugar, phosphate and base), sugar– phosphate backbone, base pairing (adenine–thymine and guanine–cytosine) by hydrogen bonds and double stranded antiparallel structure, with deoxyribose and phosphate at 3’ and 5’ ends of each strand respectively, forming a double helix.
• The base sequence of DNA forms the genetic code
• Organisation of DNA — prokaryotes have a single circular chromosome and smaller circular plasmids.
• Eukaryotes all have linear chromosomes, in the nucleus, which are tightly coiled and packaged with associated proteins. They also contain circular chromosomes in their mitochondria and chloroplasts. Yeast is a special example of a eukaryote as it also has plasmids.
• The associated proteins are called histones.
• All cells store their genetic information in the base sequence of DNA.  The genetic code is determined by this sequence of bases.
• The sequence of amino acids in a protein is determined by this code.
• DNA is:

  • Deoxyribonucleic acid.
  • A type of nucleic acid.
  • Consists of a double helix.
  • Each strand is made of repeating units called nucleotides.
• DNA nucleotides contain deoxyribose sugar.

  It has 5 carbons (numbered in the diagram).+

  
• There are four different organic bases in DNA:

  • A    Adenine
  • T    Thymine
  • G    Guanine
  • C    Cytosine.

   

  This means that there are four different nucleotides in DNA+

  
• The sugar of one nucleotide joins with the phosphate of the next nucleotide.  This forms a chain of nucleotides which make up a strand of DNA.  The chain has a sugar phosphate backbone.  The sequence of bases make up the genetic code.  The strand runs from the 5’ carbon to the 3’carbon. 

  
• Two strands join together to form a molecule of DNA.

  Only certain bases can pair together:

  • Adenine pairs with thymine
  • Guanine pairs with cytosine

   

  This is known as complementary base pairing. Weak hydrogen bonds between the bases hold the two strands together. The two DNA strands twist to form a double helix.  Note that one strand runs in the 5’ to 3’ direction and the other is in the 3’ to 5’ direction.  This arrangement is described as being an antiparallel structure, with deoxyribose sugar and phosphate at 3’ and 5’ end of each strand respectively forming a double helix.

  
• The DNA on one chromosome is millions of base pairs long.  A section of DNA which is thousands of base pairs long is called a gene.  Each gene carries a code for one protein.

  
• Prokaryotes

  These are incredibly ancient cells.  Bacteria are prokaryotes.  They have the following features:

   

  • No nucleus.
  • No membrane bound organelles e.g. chloroplasts or mitochondria.
  • DNA is ‘naked’ in the cell.
  • Single chromosome is in a circular form.
  • Ribosomes are present.
  • Plasmids are present.
• Eukaryotes

  Much more complex cells characteristic of 'higher order' life
• Eukaryotic cell structures

  • Membrane bound nucleus
  • Linear chromosomes supported by proteins
  • Plasmids present in some yeasts but absent in animal and plant cells
  • Membrane bound organelles such as chloroplast and mitochondria
  • Mitochondria and chloroplasts contain their own small circular chromosomes
  • Ribosomes present
• DNA in the linear chromosomes in the nucleus of eukaryotes is tightly coiled and packaged with associated proteins called histones