DNA as the genetic code

• What are DNA nucleotides composed of?

  A DNA nucleotide contains pentose sugar (deoxyribose), phosphate group and the inorganic nitrogenous bases adenine, guanine, cytosine, and thymine.

  
• What two bases are purine + what is their structure?

  The purine bases adenine (A) and guanine (G) are larger with a double ring structure.
• What two bases are pyrimidine + what is their structure?

  The pyrimidine bases thymine (T) and cytosine (C) are smaller with a single ring structure.
• What will each base pair with?

  A will only bond with T (2 hydrogen bonds) and C will only bond with G ( 3 hydrogen bonds).
• How do the amounts of each base compare with each other?

  Due to specific base pairing, the relative amount of A will always equal T, C will always equal G.
• How is a dinucleotide formed?

  If a condensation reaction occurs between the sugar of one nucleotide and the phosphate group of another (a phosphodiester bond) a dinucleotide is formed.
• What is formed when lots of nucleotides are joined together?

  If lots of nucleotides are joined a polynucleotide chain is formed.
• Describe the general structure of RNA. (2)

  RNA is built up from four types of bases.

  => The larger purine bases adenine and guanine.

  => The smaller pyrimidine bases uracil and cytosine.
• What produces single polynucleotide chains?

  RNA nucleotides are polymerised to produce single polynucleotide chains.

  
• What are the three types of RNA?

  There are three types of RNA: messenger RNA, transfer RNA and ribosomal RNA.

  
• What are the main features to ribosomal RNA (rRNA)? (2)

  - Large complex structure made in the nucleolus.

  - Forms over half the mass of ribosomes.
• What are the main features to messenger RNA (mRNA)? (5)


  - Has a single chain twisted into a helix whose length and base sequence vary.

  
  

  - Is a complimentary copy of the template DNA strand

  
  

  - Formed during transcription.

  
  

  - Carries the code for the gene concerned, from the nucleus to the ribosomes in the cytoplasm

  
  

  - It has a short life and it is involved in protein synthesis, as its base sequence codes for a polypeptide.
• What are the main features to transfer RNA (tRNA)? (5)

  - A small molecule

  
  

  - Has a single strand of RNA folded into a clover leaf shape.

  
  

  - There are at least 20 types of tRNA - each capable of carrying a different amino acids.

  
  

  - At the midpoint of the chain is an important sequence of three bases - the anticodon.

  - This determines which amino acid attaches by lining up against the appropriate codon on the mRNA in protein synthesis.

  
  

  - It is important in bringing amino acids to the correct position on the mRNA during the process of translation.
• What is the central dogma of molecular biology?

  
• How does DNA store information?

  DNA stores information as sequences of bases.
• What is the function of the information stored by DNA?

  This information acts as instructions to synthesise proteins.
• What spells out which amino acid will be used to make a protein (in terms of DNA)?

  The linear order of the nitrogenous bases (base sequence) acts as a language or code, which spells out which amino acids will be used to make a protein.
• What differences to each species have in terms of DNA + what is the ripple effect of this?

  Every species has different DNA and hence produces different proteins, in particular, enzymes, which in turn control cell metabolism.
• How do the DNA in other species vary?

  The DNA of different species varies in the sequence of base pairs along its length.
• How does DNA (in humans) carry information?

  DNA carries information in the form of a genetic code which is a code for the amino acid sequence (i.e. the primary structure) of a protein.
• What is a gene?

  A gene is a length of DNA which gives instructions for making one protein.
• Where is the code stored?

  The code is stored in the molecular structure of one strand of the DNA molecule (the sense or coding strand)
• What are the 5 features of the genetic code?

  - Triplet code

  - Degenerate code

  - Non-overlapping code

  - A universal code

  - Start + stop triplets
• How many acids are used to make proteins + what must the genetic code be able to do? (2)

  - Twenty different amino acids are used to make proteins.

  
  

  - The Genetic Code must be able to code for all twenty.
• How many combinations does a triplet code produce?

  A triplet code of three bases per amino acid produces sixty four possible combinations, i.e. more than enough. (43 = 64)
• What is each amino acid coded for by?

  Each amino acid is coded for by a three-letter 'word' consisting of a sequence of three DNA bases - known as a codon or a base triplet e.g. CGT contains the bases cytosine, guanine and thymine and codes for the amino acid alanine.
• How many triplets can code for the same amino acid? (2)

  - In many cases, more than one triplet of bases can code for the same amino acid.

  
  

  - For example, GCA, GCG, GCC, and GCT all code for alanine.
• What may a mutation not alter? (2)

  - One important implication of degeneracy is that a mutation which substitutes one DNA base for another may not alter the amino acid coded for.

  
  

  - The mutation may have no effect on the protein produced.
• Which 2 bases in a codon are most important in determining which amino acid is coded for?

  Note that the first 2 bases in a codon, are most important in determining which amino acid is coded for.
• Does the code overlap? (3)

  - Non-overlapping code i.e. the code is read in discrete three's.

  
  

  - There is no overlap in the coding sequences.

  
  

  - e.g. the sequence GGAGCACCTCAGTGCTCA is always read as GGA GCA CCT CAG TGC TCA.
• Explain what universal code means in terms of DNA.

  - The DNA base triplets code for the same amino acids in all organisms, with very few exceptions.

  
  

  - e.g. the base sequence GCA, for example codes for alanine in all organisms.
• What has universal code made possible + how?

  - This has made the development of gene technology possible.

  
  

  - It means that genes from one organism, e.g. human, can be inserted into a totally different organism, e.g. yeast, and are still able to function.
• What other type of codons are present + what is their function? (3)

  - There are three codons which do not code for amino acids (UGA, UAG, UAA).

  
  

  - Instead they are reading instructions for ribosomes ("full-stops" in the recipe).

  
  

  - These are known as stop sequences.
• What does TAC code for?

  TAC codes for methionine, but also acts as a start triplet, starting a coding sequence.
• What does bulk of our DNA code for?

  The bulk of our DNA does not code for proteins, as far as we know.
• What are the exons separated by?

  The coding sequences, or exons are separated by long stretches of non-coding DNA.
• What do eukaryotic chromosomes contain as well as exons?

  In eukaryotic chromosomes, genes contain short lengths of non-coding DNA, called introns, in addition to the exons.
• What happens to the introns during protein synthesis?

  These introns have to be edited out during the process of transcription in protein synthesis.
• Where does protein synthesis occur?

  Occurs in all cells at the ribosomes
• What does protein synthesis involve?

  Involves the assembly of amino acids in the correct order into polypeptide chains as directed by the genetic code on the DNA.