Topic 1 - B: More Biological Molecules

• DNA and RNA are both types of nucleic acid. They're found in all living cells and they both carry information
• DNA - deoxyribonucleic acid is used to store genetic information
• RNA - ribonucleic acid is similar in structure to DNA. One of its main functions is to transfer genetic information from the DNA to the ribosomes
• Ribosomes are the body's 'protein factories' - they read RNA to make polypeptides (proteins) in a process called translation
• Ribosomes are made from RNA and proteins and are found in the cytoplasm
• Nucleotide structure: Molecules of DNA and RNA are polymers of nucleotides.
• A nucleotide is a type of biological molecule which is made up of three different components:

  1. a pentose sugar (5 carbons)
  2. a nitrogen-containing organic base
  3. a phosphate group
• Nucleotides are the monomers that make up DNA and RNA
• Polynucleotide structure: many nucleotides join together to form polynucleotide strands (chains). The nucleotides join up via a condensation reaction between the phosphate group of one nucleotide and the sugar of another - this forms a phosphodiester bond
• a phosphodiester bond consists of the phosphate group and two ester bonds
• The chain of phosphates and sugars is known as the sugar-phosphate backbone
• DNA has a double helix structure. This means that a DNA molecule is formed from two separate strands which wind around each other to form a spiral. The strands are polynucleotides - made up of lots of nucleotides joined together in a long chain
• DNA molecules are very long ad are coiled up very tightly, so a lot of genetic information can fit into a small space in the cell nucleus
• DNA nucleotide structure: a DNA nucleotide is made from a phosphate group, the pentose sugar deoxyribose and a nitrogen-containing organic base
• Each DNA nucleotide has the same sugar and phosphate, the base on each nucleotide can vary though
• There are 4 possible bases:

  1. adenine
  2. thymine
  3. cytosine
  4. guanine
• Complementary base pairing: two DNA polynucleotide strands join together by hydrogen bonds between the bases. Each base can only join with one particular partner - complementary base pairing/ specific base pairing
• Adenine - Thymine
• Cytosine - Guanine
• This means that there are always equal amounts of adenine and thymine in a DNA molecule and equal amounts of cytosine and guanine
• 2 hydrogen bonds form between A and T
• three hydrogen bonds form between C and G
• The two polynucleotide strands are antiparallel - they run in opposite directions
• Two antiparallel strands twist to form a DNA double helix
• RNA structure: made up of nucleotides that contain a sugar, a phosphate group and one of four different bases
• The nucleotides of RNA also form a polynucleotide strand with a sugar-phosphate backbone
• Structure of RNA differs to DNA in 4 main ways

  1. the sugar in RNA nucleotides is a ribose sugar (not deoxyribose) - it is still pentose sugar though
  2. uracil (a pyrimidine) replaces thymine as a base - always pairs with adenine in RNA
  3. the nucleotides form a single polynucleotide trand
  4. RNA strands are much shorter than most DNA polynucleotides