3.1.5.1 Structure of DNA and RNA

• DNA has

  a double helix structure

  
• pentose sugars


  sugar with 5 carbon atoms

  
• Nucleotides


  are the monomers that make up DNA and RNA

  
• Nucleotide


  is a type of biological molecule made from three different components : ● a pentose sugar● a nitrogen-containing organic base● a phosphate group

  
• Base Pairs

  ● Adenine always pairs with thymine (A - T)● Guanine always pairs with cytosine (G - C)

  
• The nitrogenous base molecules occur in two structural forms


  ● Purines ● Pyrimidines

  
• Purines


  have a double ring structure e.g adenine,guanine

  
• pyrimidines


  have a single ring structuree.g cytosine,thymine,uracil

  
• Like DNA, RNA is made of

  nucleotides that contain :● a sugar● a phosphate group ● one of four different bases

  
• The two polynucleotide strands in a DNA molecule are


  antiparallel

  
• antiparallel

  they run in opposite directions
• The nucleotides in RNA form a


  polynucleotide strand with a sugar-phosphate backbone

  
• A DNA molecule is formed from


  two separate strands which wind around each other to form a spiral

  
• Two DNA polynucleotide strands join together by


  hydrogen bonds between the bases

  
• There are always equal amounts of


  ● adenine and thymine● cytosine and guanine— in a DNA molecule
• Two antiparallel strands twist to form


  a DNA double helix

  
• The sugar in RNA nucleotides is


  a ribose sugar

  
• There are four possible bases in DNA


  ● adenine (A)● thymine (T) ● cytosine (C)● guanine (G)

  
• The nucleotides in RNA form

  a single polynucleotide strand

  
• Uracil (U)


  ● a pyrimidine ● replaces thymine as a base in RNA

  
• A DNA nucleotide is made from

  ● a phosphate group● the pentose sugar deoxyribose ● a nitrogen-containing organic base

  
• RNA strands are

  much shorter than most DNA polynucleotides

  
• DNA molecules are very long and are coiled up very tightly


  so a lot of genetic information can fit into a small space in the cell nucleus

  
• Each DNA nucleotide has the same sugar and phosphate, but


  the base on each nucleotide can vary

  
• DNA Polynucleotide Strand

  
• The strands are polynucleotides,


  made up of lots of nucleotides joined together in a long chain

  
• Hydrogen bonds that form

  ● Two form between A and T● Three form between C and G

  
• The two ends of a polynucleotide strand are different

  ● one end has a phosphate group● other has a hydroxyl (OH) group — attached to the sugar

  
• complementary base pairing // specific base pairing


  Each base in a DNA molecule can only join with one particular partner

  
• The structure of a DNA nucleotide

  
• Molecules of DNA and RNA are

  polymers of nucleotides

  
• sugar-phosphate backbone


  The chain of phosphates and sugars

  
• Many nucleotides join together to form polynucleotide strands//chains via

  a condensation reaction between● the phosphate group of one nucleotide● the sugar of another— forming a phosphodiester bond

  
• The structure of a nucleotide and the arrangement of the DNA double helix is

  the same in all living organisms

  
• An RNA nucleotide consists of


  ● a base (A, U, C or G)● ribose sugar● a phosphate group

  
• DNA was first observed in the 1800s, but many scientists at the time doubted that it could carry the genetic code because


  it has a relatively simple chemical composition
• Scientists analysed a section of double stranded DNA with

  68 bases in total (34 base pairs) and 22 of the bases were adenine
• The overall structure of a DNA molecule consists of

  ● a polynucleotide strand with a sugar-phosphate backbone● bases hydrogen bonding between them — keeping the strands coiled tightly together● antiparallel strands

  
• DNA and RNA have different shapes

  ● DNA is double-stranded — twisted into a double helix — held together by hydrogen bonds● RNA is single-stranded — has a (Ribose) pentose sugar
• Rosalind Franklin took an X-ray diffraction picture of DNA, showing


  that the molecule is a helix