Topic 4

• DNA

  Carries genetic information
• RNA

  Transfers genetic information from DNA to ribosomes
• Components of a DNA nucleotide

  • Deoxyribose
  • Phosphate group
  • Adenine
  • Cytosine
  • Guanine
  • Thymine
• Components of an RNA nucleotide

  • Ribose
  • Phosphate group
  • Adenine
  • Cytosine
  • Guanine
  • Uracil
• Purine bases

  Adenine and guanine
• Pyrimidines

  Thymine, uracil and cytosine
• Phosphodiester bonds

  Formed in condensation reactions to join nucleotides
• DNA molecule

  Double helix composed of two polynucleotides joined by hydrogen bonds between complementary bases
• RNA

  Relatively short polynucleotide chain
• Genetic code

  Order of bases on DNA, consisting of triplets of bases coding for particular amino acids (codons)
• Gene

  Sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain
• Locus

  Location of a gene
• Introns

  Non-coding sections of DNA
• Exons

  Coding regions of DNA
• Genetic code

  • Non-overlapping
  • Degenerate
  • Contains start and stop codons
• Each amino acid is coded for by one or more combination of triplets, resulting in 64 possible triplets
• Genes are separated by non-coding repeats of bases
• A change in the base sequence of DNA alters the amino acid sequence and the protein, which can have various effects
• Harmful mutations

  • Cystic fibrosis
  • Sickle cell anaemia
• Eukaryotic cells

  DNA molecules are long and linear, found in the nucleus and associated with histones to form chromosomes
• Prokaryotic cells

  DNA is short and circular, not associated with proteins
• Humans have 46 chromosomes in every cell, arranged into homologous pairs
• Homologous pair

  Two chromosomes that carry the same genes, but can carry different alleles
• Allele

  Alternative form of the same gene
• Transcription

  Occurs in the nucleus, involves DNA and mRNA
• Translation

  Involves mRNA, tRNA and ribosomes
• Messenger RNA (mRNA)

  Long single strand created during transcription, complementary to DNA
• Transfer RNA (tRNA)

  Small molecule made up of around 80 nucleotides, folded into a clover leaf shape with an amino acid attachment site and an anti-codon
• Transcription

  DNA uncoils and strands separate

  2. One DNA strand used as template to make mRNA

  3. rna polymerase joins free floating nucleotides to exposed bases. Phosphodiester bonds form between backbone to create mRNA

  4. In eukaryotes, pre-mRNA is spliced to remove introns
• Translation

  mRNA attaches to ribosome, tRNA collects amino acids

  2. tRNA attaches to mRNA by complementary base pairing

  3. Amino acids join by peptide bonds, tRNA detaches

  4. Process repeats until stop codon reached
• Ribosomes can join up to 15 amino acids per second, and up to 50 ribosomes can move along the same mRNA strand
• Substitution mutation

  One nucleotide in the DNA sequence is replaced by another
• Deletion mutation

  A nucleotide in the DNA sequence is lost, leading to a frameshift
• Polyploidy

  Changes in the whole set of chromosomes, resulting in more than two sets
• Non-disjunction

  Chromosomes fail to separate correctly in meiosis, resulting in gametes and zygotes with an extra or missing chromosome
• Down's syndrome is the result of non-disjunction, where individuals have an extra chromosome 21
• Meiosis

  • Produces genetically different haploid daughter cells through independent assortment of chromosomes and crossing over of chromatids
• Meiosis 1

  Homologous chromosomes pair up, crossing over may occur, cell divides to produce daughter cells with one chromosome from each homologous pair
• Meiosis 2

  Chromatids of each chromosome are separated, producing 4 haploid daughter cells
• Genetic diversity

  The total number of different alleles in a population