Chapter 4

• Prokaryotic DNA


  Short, circular and not associated with proteins. The DNA is not in the nucleus as prokaryotes don't have one. Prokaryotes can also contain plasmids (small loops of self-replicating DNA)
• Eukaryotic DNA

  Long, linear and associated with proteins called histones. Located in the nucleus. The DNA and the histone condense to form a chromosome
• Mitochondria and chloroplasts DNA

  Similar to prokaryotic DNA - short, circular and not associated with proteins
• Gene


  A base sequence of DNA that codes for: the amino acid sequence of a polypeptide, a functional RNA including ribosomal RNA and tRNAs. A gene occupies a fixed position on a DNA molecule called a locus

  
• Codon

  Triplets of bases that code for a specific amino acid
• Genetic code


  • Non-overlapping - each triplet is only read once and the triplets don't share any bases
  • Degenerate - more than one triplet codes for the same amino acid, reduces the number of mutations
  • Universal - the code is the same for all organisms
• Introns


  Non-coding parts of DNA
• Exons


  Coding parts of DNA
• Homologous chromosomes


  Contain the same genes at the same loci but may contain different forms of the gene called alleles
• Alleles


  Different forms of the same gene with small base changes resulting in slightly different versions of the same protein
• Genome


  Complete set of genes in a cell
• Proteome


  The full range of proteins that a cell is able to produce
• mRNA (messenger RNA)

  Short, single-stranded linear chain produced during transcription with a base sequence complementary to DNA

  
• tRNA (transfer RNA)


  A single polynucleotide with a clover shape, has an amino acid binding site at one end and an anti-codon binding site at the other end

  
• Transcription


  1. DNA helicase unwinds the two DNA strands

  2. Only 1 DNA strand acts as the template

  3. Free mRNA nucleotides align and pair with their complementary base

  4. RNA polymerase joins the mRNA together to form pre-mRNA

  
• Splicing


  Where introns are removed from the pre-mRNA and exons are attached back together via a condensation reaction
• Translation

  1. The ribosome attaches to the start codon of the mRNA

  2. tRNA with the complementary anticodon aligns with the mRNA with complementary base pairing

  3. Two tRNA molecules attach to mRNA at a time

  4. The ribosome moves along mRNA, allowing another tRNA to attach to the next codon along

  5. The two amino acids attached to the tRNA molecules are joined by peptide bonds - requires ATP

  6. This process keeps occurring until the ribosome reaches a stop codon

  
• Gene Mutation


  Involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include base deletion and base substitution
• Substitution Mutation


  When one base is replaced by another, resulting in either a change to a single amino acid or the amino acid might stay the same due to the degenerate nature of the genetic code
• Deletion Mutation


  When one or more bases are removed, changing the codon at the point of mutation and all following codons resulting in a frameshift
• Non-disjunction


  Occurs where chromosomes fail to separate correctly in meiosis, resulting in gametes and zygotes with one more or one less chromosome than they should

  
• Polyploidy

  An individual has three or more sets of chromosomes instead of two, common in plants

  
• Meiosis


  A form of cell division that gives rise to four daughter cells that are genetically different and have half the number of chromosomes found in the parent cell
• Meiosis I


  1. Prophase I: DNA condenses and becomes visible as chromosomes. Crossing over may occur.

  2. Metaphase I: The bivalents line up along the equator of the spindle.

  3. Anaphase I: The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends.

  4. Telophase I: The chromosomes arrive at opposite poles. Spindle fibres start to break down. Nuclear envelopes form around the two groups and nucleoli reform.

  
• Meiosis II


  1. Prophase II: The nuclear envelope breaks down and chromosomes condense. A spindle forms at a right angle to the old one.

  2. Metaphase II: Chromosomes line up in a single file along the equator of the spindle.

  3. Anaphase II: Centromeres divide and individual chromatids are pulled to opposite poles.

  4. Telophase II: Nuclear membranes form around each group of chromosomes.

  
• Independent Assortment


  The random arrangement of homologous chromosomes during meiosis I, leading to various combinations of chromosome arrangement in the daughter cells

  
• Crossing Over


  When pairs of chromosomes line up, they can exchange some of their genetic material, leading to a different combination of alleles on the gene
• Genetic Diversity


  The small differences in DNA base sequences between individual organisms within a species population

  
• Mutation results in the generation of new alleles and contributes to genetic diversity or the size of the gene pool
• There needs to be some level of genetic diversity within a population for natural selection to occur
• Differences in the alleles possessed by individuals in a population result in differences in phenotypes
• Environmental factors affect the chance of survival of an organism i.e. act as a selection pressure
• Selection pressures increase the chance of individuals with a specific phenotype surviving and reproducing over others
• A population with a large gene pool or high genetic diversity has a strong ability to adapt to change
• Natural Selection


  1. Random mutation can result in new alleles of a gene

  2. Many mutations are harmful or neutral but under certain environmental conditions, the new alleles could lead to an increased chance of survival and increased reproductive success

  3. The advantageous allele is passed on to the next generation

  4. As a result, over several generations, the new allele will increase in frequency in the population
• Anatomical Adaptations

  Physical adaptations that could be either external or internal
• Selection pressure


  Factors that increase the chance of survival of an organism
• Gene pool


  The total genetic diversity within a population
• Natural Selection


  1. Random mutation can result in new alleles

  2. Advantageous allele is passed on to the next generation

  3. New allele will increase in frequency in the population
• Natural selection


  • Results in species that are better adapted to their environment
  • Adaptations may be anatomical, physiological or behavioural