Nucleic acids

• DNA


  Deoxyribonucleic acid - a type of nucleic acid
• RNA


  Ribonucleic acid - a type of nucleic acid

  
• DNA and RNA


  • Found in all living cells
  • Needed to build proteins, which are essential for the proper functioning of cells
  • Important information-carrying molecules
• Function of DNA


  To hold or store genetic information
• Function of RNA


  To transfer the genetic code found in DNA out of the nucleus and carry it to the ribosomes in the cytoplasm
• Nucleotide


  The repeating unit that DNA and RNA are made of, consisting of a pentose sugar, a phosphate group, and a nitrogenous base

  
• DNA nucleotide


  • Contains deoxyribose sugar, phosphate group, and one of four nitrogenous bases: adenine (A), cytosine (C), guanine (G), or thymine (T)

  
• RNA nucleotide


  • Contains ribose sugar, phosphate group, and one of four nitrogenous bases: adenine (A), cytosine (C), guanine (G), or uracil (U)

  
• Purines


  Nitrogenous bases with a double ring structure (adenine and guanine)

  
• Pyrimidines


  Nitrogenous bases with a single ring structure (cytosine, thymine, uracil)

  
• Phosphodiester bond formation


  1. Condensation reaction between phosphate group of one nucleotide and pentose sugar of next nucleotide

  2. Forms the sugar-phosphate backbone of DNA and RNA

  
• DNA structure


  • Two antiparallel polynucleotide strands
  • Strands held together by hydrogen bonds between complementary base pairs (A-T, C-G)
  • Forms a double helix

  
• RNA structure


  • Single polynucleotide strand
  • Contains ribose sugar and uracil instead of thymine

  
• Ribosome


  Organelle where protein synthesis occurs, composed of ribosomal RNA (rRNA) and proteins
• Ribosomes


  • Site of translation - 'read' RNA to make polypeptides
  • Eukaryotic ribosomes (80S) larger than prokaryotic ribosomes (70S)
  • Large subunit is site of translation
• DNA was first observed in the 1800s by Friedrich Miescher, who named it 'nuclein'
• Many early scientists doubted DNA could carry the genetic code due to its relatively simple chemical composition compared to proteins
• DNA was actually first observed in the 1800s by a Swiss scientist called Friedrich Miescher
• Miescher is credited with being the first person to discover DNA (although he named it 'nuclein') and define it as a distinct molecule in 1869
• Many scientific researchers at that time doubted that this newly discovered DNA molecule could carry the genetic code
• They doubted this because of the relatively simple chemical composition of DNA (because DNA was only made up of simple repeating nucleotides, which themselves were only composed of three parts: a phosphate group, deoxyribose, a nitrogen-containing organic base)

  
• Some scientists hypothesised that genetic information must be carried by proteins, which show much higher levels of chemical complexity
• Proteins are made up of 20 different amino acids whereas DNA is made up of only 4 different nucleotides
• It wasn't until the 1940s that the role of DNA in genetic inheritance began to be more fully researched and understood
• By 1953, experiments had confirmed that DNA carried the genetic code
• It was understood that, despite there being only 4 nucleotides, the use of the triplet code enabled much variation (the code is universal and degenerate)
• The location of DNA, protected in the nucleus, enabled the security of the genetic material rather than proteins that are found in the cytoplasm and susceptible to hydrolysis

  
• DNA is easily copied and therefore conserved throughout generations of cells and inherited between generations within families
• 1953 was also the year in which Watson and Crick confirmed the double-helix structure of DNA using Rosalind Franklin's X-ray data
• The relative simplicity of DNA led many scientists to doubt that it carried the genetic code and that this is perhaps why the function of DNA wasn't confirmed until a relatively long time after its initial discovery
• Semi-conservative replication


  DNA is copied so that the two new (daughter) cells produced will both receive the full copies of the parental DNA, with one polynucleotide DNA strand from the original DNA molecule and one newly created strand
• Importance of retaining one original DNA strand

  • Ensures genetic continuity between generations of cells
  • Ensures new cells can do the same role as old ones
• Semi-conservative replication


  1. Helicase unwinds the DNA double helix

  2. Each single polynucleotide DNA strand acts as a template for the formation of a new strand

  3. DNA polymerase catalyses condensation reactions to form the new strand

  4. The original strand and the new strand join together through hydrogen bonding between base pairs to form the new DNA molecule

  
• Nucleoside triphosphates/Activated nucleotides


  Free nucleotides in the nucleus which contain three phosphate groups, enabling them to take part in DNA replication

  
• Leading strand


  The template strand that DNA polymerase attaches to and synthesises the new strand continuously
• Lagging strand


  The other template strand where DNA polymerase moves away from the replication fork and synthesises the new strand in short Okazaki fragments
• Synthesis of complementary strands

  1. DNA polymerase can only build the new strand in the 5' to 3' direction

  2. DNA ligase joins the Okazaki fragments on the lagging strand to form a continuous complementary DNA strand

  
• The bases on each DNA strand pair up with each other, holding the two strands in a double helix
• Adenine always pairs with Thymine (A-T) and Cytosine always pairs with Guanine (C-G)

  
• Complementary bases


  The bases that pair up, with the frequency or number of adenine equal to the frequency or number of thymine, and the frequency or number of cytosine equal to the frequency or number of guanine