Nucleic acids

• What does DNA stand for and what is the structure of a nucleotide?

  DNA - deoxyribonucleic acid

  Nucleotides contain a pentose sugar (deoxyribose), a phosphate group and a nitrogenous base. The base can be one of the following: adenine, guanine, thymine and cytosine.

  
• What are pyrimidines and purines?

  Pyrimidines: have a single ringed structure e.g. thymine and cytosine

  Purines: have a double ringed structure e.g. adenine and guanine
• What is base pairing?

  Base pairing is where complementary bases attach to each other by hydrogen bonds. Purines always pair with pyrimidines.

  A joins to T: 2 hydrogen bonds

  C joins to G: 3 hydrogen bonds
• What is the structure of DNA?

  A condensation reaction occurs between deoxyribose sugar and phosphate group bonding nucleotides together forming phosphodiester bonds. This forms the sugar-phosphate backbone. DNA is made up of 2 strands of nucleotides that are anti-parallel, they are joined together by hydrogen bonds between bases forming a double helix structure.
• What is the function of DNA?

  The sequence of bases is an example of information storage that forms the codes to build proteins. Molecules are long so store large amounts of information. Base pairing rules means complementary strands of nucleotides can be replicated. Double helix structure gives stability and also makes the molecule compact. Hydrogen bonds allow for easy unzipping for copying and reading information and many of them provide stability. As the base pairs are within the helical cylinder they are protected from being corrupted to some extent.
• Why did scientists initially doubt DNA was the genetic code?

  It was a simple chemical composition and in 1800s they thought genetic code must be carried by proteins.
• How did Crick and Watson prove DNA carried genetic information?

  1953- they carried out an experiment that showed DNA did carry genetic information and discovered the double helix structure which helps DNA carry out its function.
• What is the structure of RNA and what does it stand for?

  RNA - ribonucleic acid

  RNA is one nucleotide strand, it is a relatively small molecule. The sugar is ribose instead of deoxyribose and the base is of thymine is replaced with uracil.

  
• What is the general function of RNA?

  They transfer genetic information from DNA to ribosomes and are found in the cytoplasm.
• What is the structure and function of mRNA?

  mRNA: messenger RNA

  Structure - one nucleotide stand that is a copy of one strand from a double helix, every 3 bases form a codon, that codes for an amino acid, they are smaller then DNA so can leave the nucleus. They are short-lived molecules so if digested by enzymes there is no loss to the cell.

  Function - to code for proteins and carry the genetic code out of the nucleus into the cytoplasm.
• What is the structure and function of tRNA?

  tRNA - transfer RNA

  Structure: single-stranded but folded into the shape of a clover leaf, held in place by hydrogen bonds. They contain an amino acid binding site which is an anti-codon, it is complementary to a specific codon, therefore there are 20 different types of tRNA as there are 20 different amino acids.

  Function: carries amino acids to ribosomes for protein production, each tRNA molecule is specific to an amino acid due to there anti-codon.
• What is the structure and function of rRNA?

  rRNA - ribosomal RNA

  Structure: a large complex molecule made up of double and single helices found in the cytoplasm.

  Function: They join to proteins to form ribosomes meaning they are found in ribosomes. They form the site for protein synthesis.
• What are the differences between DNA and RNA?

  DNA: larger, thymine, deoxyribose, double-stranded, nucleus

  RNA: smaller, uracil, ribose, single strand, cytoplasm
• What is the significance of 5' and 3'?

  The enzyme DNA polymerase can only attach nucleotides to OH- ions on the 3' carbon molecule. This is why they run antiparallel so they form chains in opposite directions which prevents them from the 2 strands joining together.
• What are the 4 requirements for semi-conservative replication?

  1.4 types of nucleotides each of their bases A,T,C,G.

  2. both strands of DNA act as a template

  3. DNA polymerase enzyme

  4. a source of chemical energy required to drive the process
• What is the process of semi-conservative replication?

  . enzyme DNA helicase breaks hydrogen bonds linking base pairs of DNA

  . double helix splits into 2 strands and unwinds, each strand acts as a template

  . free nucleotides line up and bond to specific bases by hydrogen bonding

  . the enzyme DNA polymerase bonds nucleotides in a condensation reaction forming phosphodiester bonds

  . one strand is the original and one is new hence the name 'semi-conservative'.
• What experiment did Meselson and Stahl conduct to prove semi-conservative replication?

  1.grow bacteria in a N$^{15}$ medium for many generations

  2. transfer bacteria to a N$^{14}$ medium, allow to form new generations - DNA replicate every 20 minutes.

  3. extract DNA and place in a centrifuge 10 separate substances based on density

  4. Following results:

  
• What results would you expect if the conservative theory was proved correct?

  Parents all heavy.

  1st generation: 1 light and 1 heavy

  2nd generation: 1 heavy and the rest light