Bases and codes (A1.2.4, A1.2.8, A1.2.9, A1.2.10)

• What is a codon?

  Groups of three bases are known as codons and translate to specific amino acids. For example, AUG (start codon) signals the beginning of protein synthesis. This sequence directs the synthesis of proteins, which determine an organism's traits and functions.
• How many different codons are there?

  There are 64 different codons, because each base within it can be any one of four.
• What does it mean that the genetic code is universal?

  The genetic code is universal, so all living organisms, and all viruses, use it. Any exceptions are changes in the meaning of one of the 64 codons. This supports the LUCA theory.
• What are nucleic acids?

  Nucleic acids are polymers of nucleotides. They may be either DNA or RNA.
• What are the five different bases?

  Adenine (A)

  Thymine (T)

  Cytosine (C)

  Guanine (G)

  Uracil (U) (instead of Thymine in RNA)
• What is a purine base?

  Purines: double-ringed structures

  Guanine (DNA,RNA)

  Adenine (DNA,RNA)
• What is a pyrimidine base?

  Pyrimidines: single ring structure

  Cytosine (DNA,RNA)

  Thymine (DNA, RNA)

  Uracil (RNA)
• What is the genetic code?

  The order in which the different types of nucleotides are arranged in DNA or RNA serves as a code for storing genetic information in all living organisms.

  A code is a system in which one symbol signifies the meaning of another symbol.
• What is a gene?

  A gene is a specific sequence of nitrogenous bases in DNA nucleotides that codes for a functional protein or RNA. Genes can vary from hundreds to millions of nucleotides long. They act as units of hereditary, passing genetic traits from parents to offspring.

  The diversity of possible gene sequences and lengths means there is limitless capacity of DNA for storing genetic information.
• How is a gene decoded?

  The process of decoding a gene consists of two major steps:

  Transcription

  The synthesis of mRNA using a DNA template

  Translation

  The synthesis of a polypeptide from mRNA
• What is complimentary base pairing?

  The nitrogenous bases on each antiparallel strand of DNA are held together by hydrogen bonds.

  The hydrogen bonds form between complimentary nitrogenous bases- meaning that bases pair up with one another in a consistent way- known as complimentary base pairing.
• How do Adenine and Thymine pair?

  With two hydrogen bonds (DNA)
• How do Cytosine and Guanine pair?

  With three hydrogen bonds (DNA and RNA)
• How do Uracil and Adenine pair?

  With two hydrogen bonds (RNA)
• What is base pairing in RNA?

  In tRNA, complimentary sections within a single strand of RNA base-pair with each other, causing the molecule to fold in on itself and form a complex, three-dimensional shape. During translation, a mRNA codon will complimentary base pair with the tRNA anti-codon.
• How do bases pair during DNA replication?

  DNA replication occurs in interphase, and is essential for reproduction, growth, and tissue replacement in multicellular organisms.

  DNA replication is semi-conservative- meaning that each of the original DNA strands acts as a template for the creation of a new strand. The enzyme DNA polymerase III builds the new strand by reading the template and adding the complimentary DNA nucleotide. This results in the newly built strand having the same sequence of bases as the template strand. Therefore, replication builds two identical DNA molecules, each with one original and one new strand.
• What is transcription?

  Transcription is the synthesises of mRNA using a DNA template. The enzyme RNA polymerase builds an mRNA strand by reading the DNA template, adding the complimentary mRNA nucleotide.

  Transcription results in the mRNA strand having the same complimentary sequence of bases as the DNA, thereby maintaining the information stored in the sequence of nucleotides of the code.
• What is translation?

  Translation is the synthesis of a polypeptide from mRNA. Ribosomes can build polypeptides through reading mRNA templates and binding coded amino acids to the polypeptide chain.

  The amino acids are brought to a ribosome by tRNA. The tRNA forms a temporary bond to the mRNA using complimentary base pairing.
• Why is DNA diverse?

  DNA stores information (for future use) in the sequence of nitrogenous bases.

  Each nucleotide can have one of four nitrogenous bases. The bases can be in any order in a DNA strand of any length. So, in a DNA strand of n base pairs long, there are 4^n possible sequences. Furthermore, while A always pairs with T and C always pairs with G, these pairs can be arranged in any order along the DNA strand. Mutations also introduce new sequences.

  DNA can also store an incredibly large amount of information in an extremely small volume.

  Computer scientists are developing ways to use DNA to store digital information from devices that generate and use data in exponentially increasing amounts.