bio unit 3 🧫 🔬

• DNA


  Deoxyribonucleic acid
• DNA structure


  • Double helix
  • Composed of nucleotides
  • Very long molecule, 10,000's of base pairs
  • Each DNA strand runs anti-parallel
  • Ladder like structure: Sides made of alternating phosphate and sugar molecules
  • Rungs made of nitrogenous bases held together by weak hydrogen bonds that can be broken easily during replication
• Nucleotide


  Has 3 components: 5 carbon sugar, negatively charged phosphate group + a nitrogen base

  
• Complementary base pairing


  Adenine - Thymine, Guanine - Cytosine
• Purine


  Adenine and guanine
• Pyrimidine


  Thymine and cytosine
• DNA replication


  1. Helicase unwinds parent DNA

  2. Strands coated in single stranded binding proteins

  3. Nucleotides of template strand exposed and unpaired

  4. Primers synthesised by RNA primase

  5. DNA polymerase extends nucleic acid chain

  6. Free nucleotides attach to complementary bases

  7. Synthesis occurs in 5' to 3' direction, creating leading and lagging strand

  8. Lagging strand made of Okazaki fragments

  9. Ligase joins DNA strands together
• DNA is a helical double-stranded molecule that occurs bound to proteins in chromosomes in nucleus, and as unbound circular DNA in the cytosol of prokaryotes, and in the mitochondria and chloroplasts of eukaryotic cells
• Nucleosome


  Structural unit of a eukaryotic chromosome, consisting of a length of DNA coiled around a core of histones
• Chromosomes and genes


  Are made of DNA
• Chromosome


  A long DNA strand which is wrapped around histones
• Gene


  A short segment of DNA which encodes for specific characteristics, allele is a specific form of gene which is responsible for variation of how trait is expressed
• DNA location


  • Plasmids and nucleoid in prokaryotes
  • Mitochondria, chloroplasts and nucleus in eukaryotes
• Proteins, including enzymes and structural protein, are essential to cell structure and functioning
• Enzyme examples


  • Helicase (unwinds DNA)
  • Amylase (breaks down glucose into simple sugars)
  • Protein Carriers (required in active transport)
• Protein synthesis


  1. Transcription in nucleus

  2. Translation at ribosome
• Transcription


  • Only anti-sense (lagging) strand is transcribed
  • RNA polymerase attaches to promoter region and unzips DNA
  • Messenger RNA is complementary transcribed using DNA as template
  • Transcription stops when termination sequence is reached
• Post-transcription changes


  • Spliceosome removes non-coding introns
  • Methylated cap added to 5' end
  • Adenine tail added to 3' end
• Translation


  1. mRNA moves from nucleus to cytoplasm and binds to ribosome

  2. tRNA brings amino acids to ribosome

  3. Starts with start codon AUG/Methionine

  4. Codon has 3 bases, 1 codon codes for 1 amino acid

  5. tRNA reads mRNA codon and binds to site on ribosome

  6. Amino acids joined by polypeptide bonds to form polypeptide chain

  7. Polypeptide chain travels to endoplasmic reticulum in vesicle for final folding to become functioning protein
• DNA carries blueprint to assemble proteins, RNA plays role in making proteins
• Genetic code


  Triplet base code: coding and non-coding DNA, many genes contain information for protein production
• Codon


  A triplet base, 3 nitrogen bases
• Anti-codon (tRNA)


  Complementary to the mRNA section
• Differences between DNA and RNA

  • DNA is double stranded, RNA is single stranded
  • DNA uses thymine, RNA uses uracil
  • DNA is longer, RNA is shorter
• Differences between prokaryote and eukaryote ribosomes

  • Prokaryote ribosome has 2 unequal subunits, eukaryote ribosome has 2 equal subunits (40s and 60s)
  • Prokaryote ribosome is smaller, eukaryote ribosome is larger
  • Prokaryote ribosome is located in mitochondria and chloroplasts, eukaryote ribosome is bound to ER and free in cytoplasm
• Binary fission in prokaryotes

  1. Replication of DNA

  2. Each DNA copy attaches to opposite ends of the cell membrane

  3. Cell increases in size

  4. Each copy of duplicated chromosome attaches to a different part of the cell

  5. Cell begins to pull apart separating chromosomes (cytokinesis)

  6. Wall forms across the cell and divides into 2 identical cells
• Mitosis


  1. Interphase: DNA replication occurs, DNA not condensed into chromosomes

  2. Prophase: DNA condensed into visible chromosomes, spindle fibres appear

  3. Metaphase: Chromosomes line up at equator

  4. Anaphase: Chromosomes are pulled apart to either side of cell

  5. Telophase: Cytokinesis occurs, nuclear membrane reforms, 2 identical daughter cells produced (diploid 2n)
• Cell cycle


  • Cell spends majority of time in interphase
  • During S phase DNA replicated before cell division
  • Mitosis starts in M phase and ends in C phase
• Meiosis


  1. Prophase I: DNA condenses into chromosomes, homologous chromosomes pair up and crossing over occurs

  2. Metaphase I: Homologous chromosomes line up at equator

  3. Anaphase I: Microtubules pull homologous chromosomes apart to opposite ends of cell

  4. Telophase I: Spindle fibres broken up, new nuclear membrane forms, chromosomes uncoil and cell divides

  5. Prophase II: Chromosomes condense, nuclear membrane breaks down, new spindles form at right angles

  6. Metaphase II: Spindle fibres line attach to chromosomes and line at equator

  7. Anaphase II: Sister chromatids pulled to opposite ends, cell elongates

  8. Telophase II: Sister chromosomes uncoil, new nuclear membrane forms, 2 cells divide again to form new haploid cells (gametes (n))
• Chiasmata


  Crossing over between homologous chromosomes during meiosis I
• Phenotypic expression of genes depend on interaction of genes and environment
• Gene expression


  Cells do not express all genes in their genome at same time, some genes active (produced via protein synthesis) while others inactive or switched off
• Eukaryotic gene expression


  Only 2% of human genome contains genes that are transcribed, coding proteins: exons, no coding proteins: introns
• DNA methylation


  Attachment of methyl group to nucleotide/histone protein, prevents genes from being transcribed
• Epigenetics


  Study of chemical modifications to gene function that aren't due to DNA sequence change
• Environmental influence on gene expression

  • Himalayan Rabbits: temperature effects fur colour
  • Hydrangea: pH of soil effects flower colour
• Mutation


  Permanent change in DNA structure, can be caused by environmental factors
• Physical mutagens


  • Radiation (X-Ray): cause loss of adenine/guanine bases and creates gaps, incorrect bases inserted during DNA replication
  • UV: hydrogen bonds along DNA strand are broken causing adjacent thymine bases to bond
  • Nuclear Radiation: causes breaks in DNA strand
• Chemical mutagens


  • Alcohol: causes DNA breaks which permanently alters it
  • Mustard Gas: causes guanine to be replaced by other bases
• Biological mutagens


  • Virus
  • Bacteria: no actual DNA changes but causes genes to switch on/off
  • Micro-organisms