2.1.3 : Nucleotides and nucleic acids

• What do nucleic acids contain?

  Carbon, hydrogen, oxygen, nitrogen and phosphorus.
• What is an individual nucleotide made up of?

  A pentose monosaccharide (sugar), containing 5 carbon atoms.

  A phosphate group.

  A nitrogenous base.
• How are nucleotides linked together?

  Through condensation reactions to form a polynucleotide.

  Phosphate group on the fifth carbon of the pentose sugar forms a covalent bond with the hydroxyl (OH) group of the third carbon of a pentose sugar on another nucleotide.

  These bonds are called phosphodiester bonds.
• What are phosphodiester bonds?

  Link adjacent nucleotides to form a poly nucleotide.

  Forms a strong sugar-phosphate backbone.

  Can be broken down by hydrolysis, releasing the individual nucleotides.
• What does DNA stand for?

  Deoxyribonucleic acid.
• What is the sugar in DNA?

  Deoxyribose sugar - has one fewer oxygen atoms than ribose.
• What are pyrimidines?

  The smaller bases, which contain one single carbon ring structure.

  Thymine (T) and Cytosine (C).
• What are purines?

  Larger bases, which contain double carbon ring structures.

  Adenine (A) and Guanine (G).
• What bases can a DNA molecule have?

  Thymine (T).

  Cytosine (C).

  Adenine (A).

  Guanine (G).
• What is a DNA double helix made of?

  Two strands of polynucleotides coiled into a helix.

  The two strands are held together by hydrogen bonds between the bases.

  Each strand has a phosphate group (5') and one end and a hydroxyl group (3') at the other end.

  The strands are antiparallel and they are arranged to run in opposite directions.

  The pairing between the bases allows the DNA to be copied and transcribed.
• What DNA bases can pair together?

  Adenine (A) and Thymine (T) form two hydrogen bonds.

  Cytosine (C) and Guanine (G) form three hydrogen bonds.

  This is complementary base pairing.
• What is the complementary base pairing rule?

  Small pyrimidine base always binds to a larger purine base.

  This arrangement maintains a constant distance between the DNA 'backbones', resulting is parallel chains.
• What does complementary base pairing mean?

  DNA always has equal amount of adenine, thymine, cytosine and guanine.
• What does DNA store?

  All of the genetic information needed by an organism, which is passed on from generation to generation.
• What does RNA do?

  Transfers genetic information from DNA to the proteins that make up the enzymes and tissues of the body.
• Why is RNA needed?

  DNA is a very long molecules and is unable to leave the nucleus in order to supply the information directly to the sites of protein synthesis.

  So, a short section corresponding to a single gene is transcribed into messenger RNA (mRNA).
• How is an RNA nucleotide different to a DNA nucleotide?

  Pentose sugar on RNA is ribose.

  The thymine base on DNA is replaced with Uracil (U) on RNA.

  But the base pairing rules still apply as Uracil is a pyrimidine that can form 2 hydrogen bonds with adenine.
• What happens to RNA after protein synthesis?

  The RNA molecules are degraded in the cytoplasm.

  The phosphodiester bonds are hydrolysed and the RNA nucleotides are release and reused.
• What happens when the cell prepares to divide?

  The two strands of DNA double helix separate and each strand becomes a template for the new double-stranded molecule.

  This is called DNA replication.
• What does semi-conservative replication mean?

  DNA replication where each new DNA molecule consists of one original strand and one newly synthesized strand.
• What must happen for DNA to replicate?

  The double helix has to unwind and two strands must be separated.

  This means the hydrogen bonds holding the complementary bases together must be broken.

  Free DNA nucleotides then pair with their complementary strands.

  Hydrogen bonds are then formed between them, and the new nucleotides join to their adjacent nucleotides with phosphodiester bonds.
• What is an enzyme and what is its role in DNA replication?

  DNA replication is controlled by enzymes.

  Enzymes are a class of proteins that act as biological catalysts for biochemical reactions.
• What does the enzyme DNA helicase do?

  Before replication, the unwinding and separating of the two DNA strands is carried out by DNA helicase.

  It travels along the DNA backbone, catalysing reactions that break the hydrogen bonds between complementary bases.

  This process is like the strand 'unzipping'.
• What does DNA polymerase do?

  Catalyses the formation of phosphodiester bonds between the newly exposed bases on the template strands during the 'unzipping' process.
• What is a replication error?

  When an incorrect sequence occurs in a newly-copied strand (bases not matched exactly).

  These errors occur randomly and lead to a change in the sequence (mutation).
• What is the genetic code?

  The genetic code is the set of rules by which information encoded within genetic material (DNA or RNA) is translated into proteins.
• What is the triplet code?

  The triplet code is a sequence of three nucleotides in DNA or RNA that codes for a specific amino acid during protein synthesis.

  The sequence of 3 bases is called a 'codon' and each codon codes for an amino acid.
• What is a section of DNA called?

  A section of DNA that contains the complete sequence of bases to code for an entire protein is called a gene.
• What is degenerate code?

  Degenerate code is when multiple codons code for the same amino acid.
• How many amino acids are there?

  20
• What is the process of transcription?

  Transcription is the process by which an RNA molecule is synthesized from a DNA template.
• What is the process of translation?

  Translation is the process in biology where the information encoded in mRNA is used to synthesize a protein.
• What three types of activity require energy (ATP)?

  Synthesis

  Transport

  Movement
• What is an ATP molecule made of?

  Nitrogenous base.

  A pentose sugar (ribose).

  Three phosphate groups.
• What is ATP used for?

  Used for energy transfer in all cells of living things.

  It is the universal energy currency.
• What are the properties of ATP?

  Small - moves in and out of cells easily.

  Water soluble - energy-requiring processes happen in aqueous environments.

  Releases energy in small quantities - energy is not wasted.

  Easily regenerated - can be recharged with energy.