Genetics, Variation and Interdependence

• Nucleotides


  Consist of a pentose sugar, a phosphate group, and an organic base
• DNA


  Made up of a deoxyribose sugar, a phosphate group, and one of four organic bases (A,C,G,T). It is double-stranded, and hydrogen bonds between the bases form a helix shape.
• Role of DNA


  Carries genetic information, determines our inherited characteristics
• RNA


  Made up of a ribose sugar, a phosphate group, and one of four organic bases (A,C,G,U). It is single stranded.
• Role of RNA


  Transfers genetic information from DNA to ribosomes for protein synthesis
• Bases


  • Purine (double ring) = adenine, guanine
  • Pyrimidine (single ring) = cytosine, thymine, uracil
• DNA in eukaryotic cells

  • Found in nucleus, long and linear, associated with histone proteins to form chromosomes. Mitochondria and chloroplasts contain prokaryotic-like DNA.
• DNA in prokaryotic cells


  • Short and circular, not associated with proteins
• Genetic code


  The order of bases on DNA. Consists of codons (triplets of bases that code for a particular amino acid).
• Features of the genetic code

  • Non-overlapping= each triplet is only read once
  • Degenerate= more than one triplet codes for the same amino acid (64 possible triplets for 20 amino acids)
  • Universal= same bases and sequences used by all species
• Gene


  A sequence of bases on a DNA molecule that codes for a specific sequence of amino acids to make a polypeptide. Can also code for functional RNA.
• Locus

  The fixed position on a DNA molecule occupied by a gene
• Allele


  Different versions of the same gene, found at the same locus on a chromosome
• Exons


  Regions of DNA that code for amino acid sequences. Separated by one or more introns.
• Introns


  Regions of DNA that do not code for anything.
• Introns are found between exons and within genes
• Genome

  The complete set of genetic information contained in the cells of an organism
• Proteome

  The complete set of proteins that can be produced by a cell
• Messenger RNA (mRNA)


  • A long, single strand
  • Its base sequence is complementary to the DNA it was transcribed from
• Transfer RNA (tRNA)


  • A single strand of around 80 nucleotides that is folded over into a clover leaf shape
  • On one end is an anti-codon, on the opposite end is an amino acid binding site
• Transcription


  The process that produces mRNA
• Transcription takes place in the nucleus
• Transcription


  1. DNA uncoils into two strands with exposed bases. One used as a template

  2. Free nucleotides line up next to their complementary bases, and are joined together by RNA polymerase
• In eukaryotic cells, pre-mRNA must be spliced to remove introns, leaving only the coding regions. Then it moves out of the nucleus and attaches to a ribosome
• Translation

  The process that produces proteins
• Translation takes place in the cytoplasm (on ribosomes)
• Translation


  1. The anti-codon of tRNA attaches to complementary bases on the mRNA

  2. Amino acids bonded to tRNA form peptide bonds, continuing to form a polypeptide chain until a stop codon is reached

  3. This process requires ATP
• Mutation


  An alteration to the DNA base sequence
• Mutations often arise spontaneously during DNA replication
• Substitution mutation


  When a nucleotide in the DNA sequence is replaced by another. This is more likely to be a quiet mutation, meaning no change occurs in the amino acid sequence.
• Deletion mutation


  When a nucleotide in the DNA sequence is lost. This is more likely to be harmful and significant, as it leads to a frame shift which means the entire amino acid sequence will be different.
• Mutagenic agent


  Factors that increase the rate of gene mutation. Examples: X-rays, UV light, gamma rays, certain chemicals e.g. in alcohol and tobacco.
• Polyploidy chromosome mutation


  Where an individual has three or more sets of chromosomes instead of two.
• Chromosome non-disjunction

  When chromosomes fail to separate correctly in meiosis, resulting in gametes with one more or less chromosome than normal.
• Meiosis


  A form of cell division that produces four genetically different haploid cells (cells with half the number of chromosomes found in the parent cell) known as gametes.
• Meiosis vs Mitosis


  • Meiosis produces four genetically different cells with half the number of chromosomes as the parent cells. Mitosis produces two genetically identical cells with the same number of chromosomes as the parent cells.
• Meiosis I


  1. Homologous chromosomes pair to form bivalents.

  2. Crossing over (exchange of sections of genetic material) occurs at chiasmata.

  3. Cell divides into two. Homologous chromosomes separate randomly. Each cell contains either maternal or paternal copy.
• Meiosis II


  1. Independent segregation of sister chromatids.

  2. Each cell divides again, producing 4 haploid cells.
• Meiosis produces genetic variation through crossing over during meiosis I and independent assortment (random segregation) of homologous chromosomes & sister chromatids
• Population


  All the organisms of a particular species that live in the same place