8 - DNA, genes, protein synthesis

• Gene


  A section of DNA that contains the coded information for making polypeptides and functional RNA
• Gene


  • The coded information is in the form of a specific sequence of bases along the DNA molecule
  • Polypeptides make up proteins and so genes determine the proteins of an organism
  • Enzymes are proteins and control chemical reactions, responsible for an organism's development and activities
  • A gene is a section of DNA located at a particular position, called a locus, on a DNA molecule
• Genetic code


  The code that determines the amino acid sequence of a polypeptide
• Discovering how DNA bases coded for amino acids

  1. Suggested a minimum of three bases that coded for each amino acid

  2. Reasoning: Only 20 different amino acids regularly occur in proteins, each amino acid must have its own code of bases on the DNA, only four different bases are present in DNA, if each base coded for a different amino acid only four different amino acids could be coded for, using a pair of bases 16 different codes are possible which is still inadequate, three bases produce 64 different codes, more than enough to satisfy the requirements of 20 amino acids
• Triplet


  Three bases that code for each amino acid
• Genetic code


  • A few amino acids are coded for by only a single triplet
  • The remaining amino acids are coded for by between two and six triplets each
  • The code is known as a 'degenerate code' because most amino acids are coded for by more than one triplet
  • A triplet is always read in one particular direction along the DNA strand
  • The start of a DNA sequence that codes for a polypeptide is always the same triplet, which codes for the amino acid methionine
  • Three triplets do not code for any amino acid, these are called 'stop codes' and mark the end of a polypeptide chain
  • The code is non-overlapping, each base in the sequence is read only once
  • The code is universal, with a few minor exceptions each triplet codes for the same amino acid in all organisms
• Much of the DNA in eukaryotes does not code for polypeptides
• Non-coding sequences


  Made up of multiple repeats of base sequences, found between genes
• Exons


  Coding sequences within genes that code for amino acids
• Introns


  Non-coding sequences within genes that separate the exons
• Some genes code for ribosomal RNA and transfer RNAs
• Prokaryotic cells


  DNA molecules are shorter, form a circle and are not associated with protein molecules. Prokaryotic cells therefore do not have chromosomes.
• Eukaryotic cells


  DNA molecules are longer, form a line (linear) rather than a circle and occur in association with proteins called histones to form structures called chromosomes.
• Mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotic cells, is short, circular and not associated with proteins.
• Chromosomes


  • Only visible as distinct structures when a cell is dividing
  • Widely dispersed throughout the nucleus for the rest of the time
  • When first become visible, appear as two threads, joined at a single point
  • Each thread is called a chromatid because DNA has already replicated to give two identical DNA molecules
  • DNA in chromosomes is held by histones
  • Considerable length of DNA found in each cell is highly coiled and folded
• DNA-histone complex


  1. DNA helix is wound around histones to fix it in position

  2. Coil is looped and further coiled before being packed into the chromosome

  3. In this way, a lot of DNA is condensed into a single chromosome
• Gene


  Occupies a specific position (locus) along the DNA molecule
• Homologous chromosomes


  • Sexually produced organisms have one set from mother and one set from father
  • Total number is referred to as the diploid number (in humans this is 46)
  • Carry the same genes but not necessarily the same alleles
• Allele


  • One of a number of alternative forms of a gene
  • Genes are sections of DNA that contain coded information in the form of specific sequences of bases
  • Each gene exists in two, occasionally more, different forms
  • Each individual inherits one allele from each of its parents
  • When alleles are different, each has a different base sequence, therefore a different amino acid sequence, so produces a different polypeptide
• Changes in the sequence of a gene

  Produces a new allele of that gene and results in a different sequence of amino acids being coded for
• The new amino acid sequence will lead to the production of a different polypeptide, and hence a different protein</b>
• When the protein produced is an enzyme, it may have a different shape that does not fit the enzyme's substrate, resulting in the enzyme not functioning properly or at all
• Ribonucleic acid (RNA)


  A polymer made up of repeating mononucleotide sub-units, forming a single strand in which each nucleotide is made up of the pentose sugar ribose, one of the organic bases adenine, guanine, cytosine and uracil, and a phosphate group
• Messenger RNA (mRNA)


  A long strand consisting of thousands of mononucleotides, arranged in a single helix, with a base sequence determined by the sequence of bases on a length of DNA in a process called transcription. It leaves the nucleus via pores in the nuclear envelope and enters the cytoplasm, where it associates with the ribosomes and acts as a template for protein synthesis.
• Transfer RNA (tRNA)


  A relatively small molecule made up of around 80 nucleotides, folded into a clover leaf shape with one end of the chain extending beyond the other, to which an amino acid can easily attach. There are many types of transfer RNA, each of which binds to a specific amino acid, and at the opposite end is a sequence of three other organic bases known as the anticodon.

  
• The sequence of nucleotide bases in DNA determines the sequence of amino acids in the proteins of an organism

  
• DNA is largely confined to the nucleus in eukaryotic cells, while the synthesis of proteins takes place in the cytoplasm
• Transferring the coded information

  1. Sections of the DNA code are transcribed onto a single-stranded molecule called ribonucleic acid (RNA)

  2. Messenger RNA (mRNA) is small enough to leave the nucleus through the nuclear pores and enter the cytoplasm, where the coded information it contains is used to determine the sequence of amino acids in the proteins which are synthesised there

  
• Codon


  The sequence of three bases on messenger RNA that codes for a single amino acid
• Genome


  The complete set of genes in a cell, including those in mitochondria and/or chloroplasts
• Proteome


  The full range of proteins produced by the genome, sometimes referred to as the complete proteome
• The complementary base pairings that RNA forms are guanine with cytosine, and adenine with uracil

  
• During protein synthesis, an anticodon pairs with the three complementary organic bases that make up the codon on mRNA