4. Genetics, Biodiversity and Classification

• Three components of nucleotides:

  • A pentose sugar
  • A phosphate group
  • An organic base
• Structure of DNA:

  • Made up of a deoxyribose sugar, a phosphate group, and one of four organic bases (A,C,G,T)
  • Double-stranded
  • Hydrogen bonds between the bases form a helix shape
• Role of DNA:

  • Carries genetic information
  • Determines our inherited characteristics
• Structure of RNA:

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

  • Transfers genetic information from DNA to ribosomes for protein synthesis
• Bases that are purine and pyrimidine:

  • Purine (double ring): adenine, guanine
  • Pyrimidine (single ring): cytosine, thymine, uracil
• Differences between DNA in eukaryotic and prokaryotic cells:

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

  • 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
• Where introns are found:

  • Between exons 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
• Structure of messenger RNA (mRNA):

  • A long, single strand with a base sequence complementary to the DNA it was transcribed from
• Advantages of using mRNA for translation:

  • Shorter & contains uracil, breaks down quickly so no excess polypeptide forms
  • Single-stranded & linear, ribosome moves along strand & tRNA binds to exposed bases
  • Contains no introns
• Structure of transfer RNA (tRNA):

  • A single strand of around 80 nucleotides folded into a clover leaf shape
  • One end has an anti-codon, the opposite end has an amino acid binding site
• Product of transcription:

  • mRNA
• Location of transcription:

  • In the nucleus
• Process of transcription:

  • DNA uncoils into two strands with exposed bases, one used as a template
  • Free nucleotides line up next to their complementary bases and are joined by RNA polymerase
• After transcription, mRNA in eukaryotic cells:

  • Pre-mRNA must be spliced to remove introns, leaving only the coding regions
  • Moves out of the nucleus and attaches to a ribosome
• Product of translation:

  • Proteins
• Location of translation:

  • In the cytoplasm (on ribosomes)
• Process of translation:

  • The anti-codon of tRNA attaches to complementary bases on the mRNA
  • Amino acids bonded to tRNA form peptide bonds, continuing to form a polypeptide chain until a stop codon is reached
  • This process requires ATP
• Mutation is an alteration to the DNA base sequence and often arises spontaneously during DNA replication
• A mutation may not lead to a change in the amino acid sequence because:

  • The genetic code is degenerate, so the mutation may end up coding for the same amino acid as the original triplet
  • The mutation may occur in an intron
• A substitution mutation is when a nucleotide in the DNA sequence is replaced by another, often resulting in no change in the amino acid sequence
• A deletion mutation is when a nucleotide in the DNA sequence is lost, leading to a frame shift and significant changes in the entire amino acid sequence
• A mutagenic agent is a factor that increases the rate of gene mutation, examples include X-rays, UV light, gamma rays, and certain chemicals found in alcohol and tobacco
• A polyploidy chromosome mutation is where an individual has three or more sets of chromosomes instead of two
• Chromosome non-disjunction occurs when chromosomes fail to separate correctly in meiosis, resulting in gametes with one more or less chromosome than normal
• Meiosis is a form of cell division that produces four genetically different haploid cells known as gametes, with half the number of chromosomes found in the parent cell
• Meiosis differs from mitosis in that:

  • 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
• During 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, and homologous chromosomes separate randomly, with each cell containing either a maternal or paternal copy
• During meiosis II:

  1. Independent segregation of sister chromatids

  2. Each cell divides again, producing 4 haploid cells
• Meiosis produces genetic variation through:

  1. Crossing over during meiosis I

  2. Independent assortment (random segregation) of homologous chromosomes and sister chromatids, resulting in new combinations of alleles
• Population: All the organisms of a particular species that live in the same place
• Allele: Different forms of a particular gene, found at the same locus (position) on a chromosome. A single gene could have many alleles