1.5 Nucleic acids

• Nucleotides are made up of three components that combine by condensation reaction.

  These are:

   One or more phosphate groups

   A pentose sugar

   An organic base which contains nitrogen
• Adenosine triphosphate (ATP) is an example of a nucleotide. ATP is the major energy

  currency of the cell – It provides energy for most reactions in most cells.
• To release energy from ATP the enzyme ATPase breaks the bond between the middle and

  terminal phosphate group; this releases energy. Adenosine diphosphate (ADP) and a

  phosphate group (Pi) are formed too.
• ATP is formed by phosphorylation.
• ATP energy release: When the bond between the middle and terminal phosphate group is broken, 30.6 kJ of energy is released. This is and exergonic reaction
• ATP is produced in the cytoplasm, the mitochondria (matrix and inner

  membranes) and in chloroplasts (thylakoid membranes).
• ATP provides energy for -

  Metabolic processes : To build large, complex molecules from smaller,

  simpler molecules

  Active transport

  Movement : For muscle contraction.

  Nerve transmission

  Secretion The packaging and transport of secretory products into

  vesicles in cells.
• Only one enzyme (ATPase) is needed to release energy from ATP, while many are

  needed in the case of glucose.
• The hydrolysis of ATP to ADP involves a single reaction that releases immediate

  energy. The breakdown of glucose involves a number of intermediates and it

  takes much longer for the energy to be released.
• ATP releases energy in small amounts when and where needed, whereas glucose

  contains large amounts of energy that may not be needed immediately.
• ATP is soluble and easily transported e.g. from companion cell to sieve element in phloem.
• ATP provides a common source of energy for many different chemical

  reactions, increasing efficiency and control by the cell.
• There are two types of nucleic acid; both are built up of nucleotides.

   Deoxyribonucleic acid (DNA).

   Ribonucleic acid (RNA).
• DNA nucleotides have the pentose sugar deoxyribose and the bases adenine, thymine,

  cytosine or guanine.
• RNA nucleotides have the pentose sugar ribose and the bases adenine, uracil, cytosine

  and guanine.
• Adenine and guanine are purine bases with a double ring structure.
• Thymine, uracil and

  cytosine are pyrimidine bases with a single ring structure.
• A pyrimidine base must bond

  with a purine base. Bases are complimentary to each other, adenine bonds with thymine

  or uracil (2 hydrogen bonds) and cytosine bonds with guanine (three hydrogen bonds).
• DNA is a double stranded polymer of nucleotides or polynucleotide.

  Each polynucleotide may contain many million nucleotide units.
• The alternating phosphate groups and deoxyribose sugars form the DNA backbone
• Purine bases bond with pyrimidine bases by hydrogen bonding.
• Base pairing links two polynucleotide chains in DNA. The polynucleotide chains are antiparallel to each other.
• The DNA molecule is twisted to from a double helix. The shape of the twisted double

  helix is maintained by hydrogen bonding.
• DNA is found in the nucleus of eukaryotic cells and has two functions – replication and

  protein synthesis.
• RNA is a single stranded polynucleotide.

  • RNA contains the pentose sugar ribose.

  • RNA does not contain the base thymine.

  • RNA is much shorter than DNA.
• Messenger RNA - mRNA is a long single-stranded molecule. It is synthesised in

  the nucleus and carries the genetic code from the DNA to the

  ribosomes in the cytoplasm. Each strand of mRNA contains

  the genetic code for one gene. Each gene codes for a

  particular polypeptide.
• Ribosomal RNA - rRNA is found in the cytoplasm and is a component part of

  ribosomes. Ribosomes are made of rRNA and protein and are

  synthesised in the nucleolus of the nucleus (they leave the

  nucleus via the nuclear pores). Ribosomes are the site of

  protein synthesis by a process called translation.
• Transfer RNA - tRNA is a small single stranded molecule folded into the

  shape of a clover leaf. Each tRNA molecule has an amino acid

  attachment site CCA. At the opposite end of the tRNA

  molecule there is a triplet of bases called an anticodon. tRNA

  molecules transport amino acids to the ribosomes. The

  anticodon bases form a complex with complimentary bases

  on the mRNA molecule (codon). This allows translation to

  take place.
• DNA is copied during semi-conservative replication (takes place during interphase)
• DNA replication occurs as follows:

   Hydrogen bonds holding the base pairs together break and two halves of the DNA

  molecule separate.

   DNA unwinds.

   As the DNA strands separate the enzyme DNA polymerase catalyses the addition of

  free nucleotides to the exposed bases; each chain acts as a template so that free

  nucleotides can be joined to their complimentary bases.

   This process results in the formation of two identical DNA molecules; each made

  up of one newly synthesised chain and one chain from the original molecule.
• Meselson and Stahl proposed the semi-conservative hypothesis of DNA replication. This

  hypothesis suggests that each DNA strand act as a template for new DNA. Each new

  strand of DNA formed is composed of an original strand and a newly synthesised

  strand.
• Meselson and Stahl cultured the bacterium Escherichia coli, for several generations on a

  medium containing amino acids made with the heavy isotope 15N. The bacteria

  incorporated the 15N into their nucleotides; nucleotides contain an organic base which

  contains nitrogen. After several generations all the DNA contained 15N.
• Meselson and Stahl’s experiment:

   The scientists extracted the bacterial DNA and centrifuged it.

   The DNA settled at a low point in the tube because it contained the heavy 15N isotope

   The bacteria were transferred to lighter isotope 14N medium and were allowed to replicate once.

   When extracts of DNA from the first generation culture were centrifuged it was shown to have a mid-point density (hybrid)

   When extracts of DNA were taken from the second generation grown in a 14N medium the DNA settled at mid-points and high-points in the tube after centrifugation
• The sequence of bases which make up a gene carry the genetic information to build the

  primary structure of a single polypeptide.
• Three bases code for a single amino acid; this is

  called the triplet code or a codon.
• Protein synthesis requires the transcription of a gene

  into a mRNA molecule, from the original DNA template. The code within the mRNA

  molecule is then translated into a polypeptide by a ribosome.
• Transcription - DNA does not leave the nucleus; it acts as a template for the production

  of mRNA (messenger RNA). The mRNA is copied from a specific region of DNA called the

  cistron. The cistron is equivalent to a gene and codes for a specific polypeptide.
• Transcription

  • DNA helicase breaks the H-bonds between bases to unwind and unzip a specific region of DNA
  • RNA polymerase attaches to the DNA at the beginning of the sequence that will be copied
  • RNA polymerase moves along the DNA aligning nucleotides to the opposite complimentary nucleotides on the DNA strand (antisense)
  • This results in the synthesis of a molecule of mRNA alongside the unzipped DNA
  • Behind the RNA polymerase the DNA strands re-join to reform the double helix
• After transcription: The mRNA carries the DNA code out of the nucleus through a nuclear pore to

  the cytoplasm and attaches itself to a ribosome.
• The antisense strand is the template DNA strand which is transcribed.