Genetics

• Eukaryotic DNA

  Long, linear, associated with proteins called histones, tightly coiled into chromosomes (DNA molecule + its associated proteins)

  
• Prokaryotic DNA

  Short, circular, not associated with proteins/histones
• Mitochondria and chloroplasts in eukaryotic cells have their own DNA
• Mitochondrial and chloroplast DNA

  Short, circular, not associated with proteins/histones

  
• Gene


  Sequence of DNA bases that codes for: the amino acid sequence of a polypeptide, a functional RNA e.g. ribosomal RNA and tRNAs

  
• A gene occupies a fixed position, called a locus, on a particular DNA molecule

  
• Genetic code


  Sequence of DNA triplets (or mRNA codons) codes for sequence of amino acids

  
• Genetic code

  • Universal - the same specific DNA base triplets code for the same amino acids in all living organisms
  • Non-overlapping - Discrete, each base can only be used once and in only one triplet
  • Degenerate - The same amino acid can be coded for by more than one base triplet

  
• In eukaryotes, much of the DNA doesn't code for polypeptides

  
• Non-coding regions in eukaryotic DNA


  • Between genes - Non-coding multiple repeats (or Variable Number Tandem Repeats (VNTRs) in second year)
  • Within genes - Only exons code for amino acid sequences, which are separated by one or more non-coding sequences, called introns

  
• Genome


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


  The full range of proteins that a cell/genome is able to produce

  
• Alleles


  Different version (sequence of bases / triplets) of the same gene
• Homologous pair of chromosomes

  Same size chromosomes with same genes, but different alleles
• Protein synthesis

  1. Transcription - Production of mRNA from DNA in the nucleus

  2. Translation - Production of polypeptides from the sequence of codons carried by mRNA in the cytoplasm on ribosomes

  
• Messenger RNA (mRNA)


  Made by transcription in the nucleus, acts as a template for translation in the cytoplasm, sequence of bases on RNA determines sequence of amino acids in polypeptide chain, straight chain molecule, sequence of bases on RNA determined by sequence of bases on DNA, chemically unstable so breaks down after a few days

  
• Transfer RNA (tRNA)


  Carries an amino acid, has an amino acid binding site, has an anticodon (3 bases) that is complementary to the mRNA codon, each tRNA specific to one amino acid, in relation to its anticodon, single polynucleotide strand folded into a three-leafed clover shape held together by hydrogen bonds

  
• Transcription in the nucleus

  DNA double helix unzipped / unwound by helicase, hydrogen bonds broken, RNA nucleotides align next to their complementary bases on the template strand forming temporary hydrogen bonds, uracil replaces thymine in RNA, RNA polymerase joins adjacent nucleotides forming phosphodiester bonds, when RNA polymerase reaches stop codon, mRNA (prokaryotes) or pre-mRNA (eukaryotes) detaches from DNA, mRNA leaves nucleus via nuclear pore

  
• Eukaryotic genes

  Contain exons (coding regions) and introns (non-coding regions), whole gene transcribed to pre-mRNA which contains introns & exons, splicing - introns removed and exons spliced together in different combos for different proteins

  
• Prokaryotic DNA doesn't contain introns, mRNA produced directly from DNA with no splicing

  
• Translation at the ribosome

  Sequence of mRNA codons determines sequence of amino acids, tRNAs carry specific amino acids in relation to their anticodon, at the ribosome tRNA codon binds to mRNA codon via complementary base pairing and hydrogen bonds, first codon is the start codon, two amino acids joined by condensation forming a peptide bond using energy from ATP, tRNA detaches (without its amino acid), ribosome moves along mRNA to next codon, continues until stop codon (polypeptide released)

  
• ATP

  Hydrolysis of ATP, to ADP + Pi, releases energy for the bond between the amino acid and its corresponding tRNA molecule, and for peptide bond formation between amino acids
• tRNA

  Attaches to and transports a specific amino acid, in relation to its anticodon, anticodon complementary base pairs to mRNA codon forming hydrogen bonds, brings two amino acids together for the formation of a peptide bond, about 60 types of tRNAs to carry 20 different amino acids

  
• Ribosomes

  Attaches to mRNA and houses tRNA, allowing codon-anticodon complementary base pairing, allows peptide bonds to form between amino acids

  
• Gene mutation


  A change in the base sequence of DNA (on chromosomes), can arise spontaneously during DNA replication

  
• Effect of gene mutation


  Changes sequence of codons on mRNA, changes sequence of amino acids in the primary structure of the polypeptide, changes position of bonds and tertiary structure of the protein (and active site if enzyme), substrate can't bind to active site and form an enzyme-substrate complex

  
• Base deletion


  One nucleotide / base removed from DNA sequence, changes triplet / codon sequence from the point of mutation (frameshift), changes sequence of codons on mRNA and amino acids in primary structure, changes tertiary structure / shape of protein

  
• Base substitution


  Nucleotide / base in DNA replaced with another, changes one mRNA codon and one amino acid, or due to genetic code degeneracy the new triplet may still code for the same amino acid so the sequence of amino acids remains unchanged

  
• Meiosis


  Before meiosis, DNA replicates so there are two copies of each chromosome (sister chromatids), meiosis I separates homologous pairs, crossing over creates genetic variation, independent segregation increases genetic variation, meiosis II separates chromatids, creates 4 genetically varied haploid cells
• Mechanisms creating genetic variation in meiosis


  • Crossing over between homologous chromosomes exchanges alleles, creating new combinations of maternal and paternal alleles
  • Independent segregation of homologous chromosomes results in random alignment at the equator, creating different combinations of maternal and paternal chromosomes and alleles in daughter cells
• Meiosis I (first division)


  Separates homologous pairs
• Chromosomes arrange into homologous pairs
• Crossing over (prophase I)


  Creates genetic variation in gametes
• Independent segregation (metaphase I)


  Increases genetic variation in gametes
• Meiosis II (second division)


  Separates chromatids
• Creates 4 haploid cells (from a single diploid parent cell) that are genetically varied
• How meiosis creates genetic variation


  • Crossing over between homologous chromosomes
  • Alleles exchanged between chromosomes
  • Creates new combinations of maternal and paternal alleles on chromosomes
  • Independent segregation of homologous chromosomes
  • Random alignment of homologous pairs at equator → random which chromosome from each pair goes to each daughter cell
  • Creates different combinations of maternal and paternal chromosomes and alleles in daughter cells
  • Random fertilisation when two gametes fuse to form a zygote
• Mutations in the number of chromosomes – chromosome non-disjunction


  1. Homologous chromosomes fail to separate during meiosis I OR sister chromatids fail to separate during meiosis II

  2. One gamete has an extra copy of this chromosome and the other has none

  3. Upon fertilisation, zygote has one fewer (dies) or one extra chromosome (survives)

  4. Arises spontaneously

  5. Causes genetic diseases e.g. down's syndrome in humans – extra copy of chromosome 21
• Independent segregation
• Crossing over