Unit 4 Flashcards

• DNA in eukaryotes


  Bound to proteins (histones) in linear chromosomes in nucleus
• DNA in prokaryotes


  Unbound circular DNA in cytosol, and in mitochondria and chloroplasts of eukaryotes
• DNA structure


  • Double stranded molecule
  • Nucleotide = phosphate + sugar + one nitrogenous base
  • Complementary base pairing: A-T, C-G
  • Weak hydrogen bonds between bases
  • Covalent bonds P-S stronger
  • Strands are antiparallel
• Types of RNA


  • Messenger RNA (mRNA)
  • Transfer RNA (tRNA)
  • Ribosomal RNA (rRNA)
• RNA


  • Single stranded
  • Has uracil instead of thymine
• DNA Replication


  1. Helicase unzips DNA

  2. DNA polymerase attaches to RNA primer and joins free nucleotides following complementary base pairing

  3. On leading strand, DNA pol moves towards replication fork and synthesises continuously

  4. On lagging strand, DNA pol moves away from replication fork and synthesises in Okazaki fragments
• Meiosis


  1. Occurs in gonads

  2. Allows sexual life cycle with fusion of gametes

  3. Promotes genetic variation

  4. Two phases - Meiosis I and II

  5. Reduction division: 1 diploid cell divides to form 4 haploid daughter cells

  6. Homologous chromosomes carry same sequence of genes, not necessarily same alleles

  
• Crossing Over


  Swapping of DNA segments between non-sister chromatids of homologous chromosomes at chiasmata in Prophase I
• Random Assortment


  Homologous chromosomes line up at the equator as bivalents in one of two random orientations in Metaphase I

  
• Spermatogenesis


  Uninterrupted process that begins in puberty
• Oogenesis


  Arrested in stages, begins in fetus
• Both spermatogenesis and oogenesis result in formation of haploid gametes from diploid cells, and occur in gonads

  
• Random Fertilisation


  1. Leads to variation in offspring genotypes

  2. Millions of genetically distinct sperm

  3. Each oocyte genetically distinct

  4. Random fusion of sperm and egg

  
• Genome


  Total complement of all genes in an individual
• Gene


  Specific DNA sequence that codes for a particular polypeptide; molecular unit of heredity
• Transcription


  1. RNA sequence produced using DNA template

  2. Initiation: RNA polymerase binds to promoter region

  3. Elongation: RNA polymerase binds nucleotides in 5' to 3' direction

  4. Termination: RNA polymerase crosses stop sequence, pre-mRNA detaches
• Translation


  1. Amino acid sequence (polypeptide) formed from mRNA template

  2. 3 bases (codon) on mRNA code for a specific amino acid

  3. Anticodons on tRNA align opposite codons

  4. Ribosomes aid peptide bond formation between adjacent amino acids

  5. Ribosome binds and moves along mRNA synthesising polypeptide chain until stop codon reached
• Non-coding nucleic acid sequences

  • Introns
  • tRNA
  • Centromeres
  • Telomeres
• Gene Expression


  • Synthesis of a functional gene product (protein OR functional RNA)
  • Is regulated
  • Used by all known life (genetic code is universal)
• Polypeptide chain undergoes pleating, coiling and folding to form final functional protein
• Twin Methodology Example


  Researchers compare effect of environmental changes between monozygotic (identical) twins to deduce if an environmental factor impacts gene expression and phenotype
• DNA sequences of monozygotic twins are identical, and age, gender, in-utero effects are largely constant. Differences in histones (tagging, packing) accumulate as individuals age, which can be attributed to environment and affect gene expression and phenotype.

  
• Structural genes


  Genes that code for any RNA or protein product other than a regulatory factor
• Regulatory genes


  Genes that code for transcription factors, which regulate structural genes
• Epigenetics


  Studies factors that regulate phenotypic expression of genes, without modifying DNA sequences
• Regulating phenotypic expression


  1. Transcription factors can attach to start of gene to block RNA polymerase or upregulate/downregulate transcription

  2. During translation, mRNA survival time, length of stabilizing polyA tail, supply of amino acids vary
• Differential gene expression, controlled by transcription factors, regulates cell differentiation for tissue formation and morphology. Cells in the body show specialisation in structure and function yet all contain the same genetic information.
• Hox genes


  Code for transcription factors to ensure relevant genes in particular parts of the body are switched on/off as required for proper development
• SRY gene


  Codes for a transcription factor responsible for switching on maleness genes, triggering cell differentiation and development of testes
• Mutation


  • Change in sequence of DNA bases
  • Ultimate source of genetic variation
  • Can change phenotypic makeup of population
• Disadvantageous mutations (lethal or sublethal) are removed from populations if affected organisms die before reproductive age, but can be hidden in the phenotype of heterozygotes

  
• Somatic mutations


  Occur in diploid body cells, only affect individual, not passed to offspring
• Germline mutations


  Occur in cells that give rise to gametes and can be passed to offspring

  
• How mutations in genes and chromosomes arise

  • Errors in DNA replication
  • Errors in cell division
  • Damage by mutagens (physical, including UV and ionising radiation, and chemical)

  
• Missense mutation


  Single base substitution results in incorrect amino acid in polypeptide
• Silent mutation


  Single base substitution has no impact as same amino acid still coded for (degeneracy)
• Nonsense mutation


  Single substitution codes for a stop codon, resulting in premature termination of polypeptide
• Frameshift mutation


  Deletion or insertion of one or more bases shifts the reading frame, resulting in incorrect amino acid sequence beyond the mutation
• Types of block mutations

  • Duplication - segment repeated
  • Inversion - DNA sequence reversed
  • Deletion - segment missing
  • Insertion - segment inserted from another chromosome
  • Translocation - segment exchanged with another chromosome
• Inheritance Patterns - Monohybrid

  • Autosomal dominant/recessive: 2 possible phenotypes, F1 phenotypic ratio 3:1