DNA, genes and protein synthesis

• What is a gene?

  A gene is a segment of DNA that contains the instructions for building a specific protein or performing a specific function in an organism.
• The gene is a base sequence of DNA that codes for:

  • the amino acid sequence of polypeptide
  • or a functional RNA, including ribosomal RNA and transfer RNAs
• How many bases code for one amino acid?

  Three
• How many amino acids are there?

  20
• Features of the genetic code

  • Degenerate code
  • Non-overlapping
  • Universal
• Chromosome structure

  Two threads joined at a single point (chromatid). DNA in chromosomes is held by histones.
• How is DNA packed into a chromosome?

  DNA double helix molecule, DNA combined with histones and a DNA-histone complex is coiled, coils fold to form loops, loops coil and pack together to form the chromosome
• Homologous chromosomes

  Two chromosomes that carry the same genes but not necessarily the same alleles of the gene.
• What is an allele?

  An allele is a variant form of a gene. It is one of the possible versions of a gene that can exist at a specific location on a chromosome. Alleles can differ in their DNA sequence, resulting in different traits or characteristics.
• Codon

  The sequence if three bases on mRNA that codes for a single amino acid.
• Genome 


  The complete set of genes in a cell, including those in mitochondria and chloroplasts.
• Proteome Refers to the proteins produced by a given type of cell under a certain set of chromosomes.
• Proteome 


  The proteins produced by a given type of cell under a certain set of conditions
• Complete proteome 


  The full range of proteins produced by the genome.
• Ribonucleic acid (RNA) structure

  • Pentose sugar (ribose)
  • Organic base (A,C,G,U)
  • Phosphate group
• What are the two types of RNA?

  • messenger RNA (mRNA)
  • transfer RNA (tRNA)
• Messenger RNA (mRNA) structure 


  Thousands of mononucleotides, long strand in a single helix. Contains codons and these determines the amino acid sequence of a specific polypeptide that will be made.
• Transfer RNA (tRNA) structure 


  80 nucleotides in a single stranded chain folded into a clover-leaf shape, with one end of the chain extending beyond the other. At the opposite end there is an anticodon which is a sequence of three other organic bases. Each tRNA molecule is specific to one amino acid and has an anticodon specific to that amino acid.
• Transcription 


  A complementary section of part of this sequence is made in the form of a molecule called pre-mRNA.
• Translation 


  The mRNA is used as a template to which complementary tRNA molecules attach and the amino acids they carry are linked to form a polypeptide.
• There are 64 possible codons (3 bases x 20 amino acids)
• Most amino acids have more than one codon - this means there's redundancy in the genetic code
• Splicing of pre-mRNA

  The base sequences corresponding to the introns are removed and the functional exons are joined together.