Processes

• Semi conservative replication

  • DNA helicase breaks hydrogen bonds between complementary bases, unwinding the double helix
  • Both strands act as templates
  • Free DNA nucleotides attracted to exposed bases + join by specific complementary base pairing
  • Hydrogen bonds form between adenine-thymine and guanine-cytosine
  • DNA polymerase joins adjacent nucleotides on new strand by condensation reactions
  • Forming phosphodiester bonds
• Cell fractionation + ultracentrifugaiton

  1. Homogenise tissue= Disrupt cell membrane, break cells+ release

  2. Put in a : Cold =reduce enzyme activity →so O not broken down , Isotonic = h2o doesn’t move by osmosis → so don’t burst , Buffered = keep pH constant → so enzymes don’t denature solution

  3. Filter homogenate = Remove large, unwanted debris

  4. Ultracentrifugation - separates organelles in order of mass

  ● Centrifuge homogenate in tube at a high speed

  ● Remove heaviest organelle + respin at a higher speed + repeat
• Cell cycle

  1 Interphase:

  ● (S phase) DNA replicates semi-conservatively

  ○ Leading to 2 chromatids (identical copies) joined at a centromere

  ● (G1/G2) number of organelles & volume of cytoplasm increases, protein synthesis

  2. Mitosis

  ● Nucleus divides

  ● To produce 2 nuclei with identical copies of DNA produced by parent cell

  3. Cytokinesis

  ● Cytoplasm and cell membrane (normally) divide

  ● To form 2 new genetically identical daughter cells
• Mitosis

  Prophase

  ● Chromosomes condense, becoming shorter / thicker

  ○ Appear as 2 sister chromatids joined by a centromere

  ● Nuclear envelope breaks down

  ● Centrioles move to opposite poles forming spindle network

  Metaphase

  ● Spindle fibres attach to chromosomes by their centromeres

  ● Chromosomes align along equator

  Anaphase

  ● Spindle fibres shorten / contract

  ● Centromere divides

  ● Pulling chromatids (from each pair) to opposite poles of cell

  Telophase

  ● Chromosomes uncoil, becoming longer / thinner

  ● Nuclear envelopes reform = 2 nuclei

  ● Spindle fibres / centrioles break down
• Binary fission

  1. Replication of circular DNA

  2. Replication of plasmids

  3. Division of cytoplasm to produce 2 daughter cells

  ● Single copy of circular DNA

  ● Variable number of copies of plasmids
• Virus replication

  1 Attachment proteins attach to complementary receptors on host cell

  2. Inject viral nucleic acid (DNA/RNA) into host cell

  3. Infected host cell replicates virus particles

  a Nucleic acid replicated

  b. Cell produces viral protein / capsid / enzymes

  c. Virus assembled then released
• Absorption of sodium ions and glucose (or amino acids) by cells lining the mammalian ileum:

  1 Na+ actively transported from epithelial cells to blood (by Na+/K+pump)

  ● Establishing a conc. gradient of Na+ (higher in lumen than epithelial cell)

  2 ● Na+ enters epithelial cell down its concentration

  gradient with glucose against its concentration gradient

  ● Via a co-transporter protein

  3 ● Glucose moves down a conc. gradient into blood via

  facilitated diffusion
• Phagocytosis

  1 Phagocyte attracted by chemicals antigens on pathogen

  2 Phagocyte engulfs pathogen by surrounding it with its cell membrane

  3 Pathogen contained in phagosome in cytoplasm of phagocyte

  4 Lysosome fuses with phagosome and releases lysozymes

  5 Lysozymes hydrolyse pathogen
• Formation of tissue fluid

  1 At the arteriole end of capillaries:

  2. Higher hydrostatic pressure inside capillaries (due to contraction of ventricles) than tissue fluid

  3. Forcing water (and dissolved substances) out of capillaries

  4. Large plasma proteins remain in capillary
• Return of tissue fluid

  1 At the venule end of capillaries:

  2. Hydrostatic pressure reduces as fluid leaves capillary (also due to friction)

  3. Increasing concentration of plasma proteins lowers water potential in capillary

  4. Water enters capillaries from tissue fluid by osmosis down a water potential gradient

  5. Excess water taken up by lymph capillaries and returned to circulatory system through veins
• Cohesion tension theory - xylem

  Leaf:

  1 Water lost from leaf by transpiration- water evaporates from mesophyll cells into air spaces and water vapour diffuses through (open) stomata

  2. Reducing water potential of mesophyll cells

  3. So water drawn out of xylem down a water potential gradient

  Xylem

  4. Creating tension in xylem

  5. Hydrogen bonds result in cohesion between water molecules so water is pulled up as a continuous column

  6. Water also adheres to walls of xylem

  Root

  7. Water enters roots via osmosis
• Translocation - phloem

  1 At source, sucrose is actively transported into phloem sieve tubes / cells

  2. By companion cells

  3. This lowers water potential in sieve tubes so water enters (from xylem) by osmosis

  4. This increases hydrostatic pressure in sieve tubes - creates a hydrostatic pressure gradient

  5. So mass flow occurs - movement from source to sink

  6. At sink, sucrose is removed by active transport to be used by respiring cells or stored in storage organs
• Transcription

  1 Hydrogen bonds between DNA bases break

  2. Only one DNA strand acts as a template

  3. Free RNA nucleotides align next to their complementary bases on the template strand

  ● In RNA, uracil is used in place of thymine (pairing with adenine in DNA)

  4. RNA polymerase joins adjacent RNA nucleotides

  5. This forms phosphodiester bonds via condensation reactions

  6. Pre-mRNA is formed and this is spliced to remove introns, forming (mature) mRNA
• Translation

  1 mRNA attaches to a ribosome and the ribosome moves to a start codon (AUG)

  2. tRNA brings a specific amino acid

  3. tRNA anticodon binds to complementary mRNA codon

  4. Ribosome moves along to next codon and another

  tRNA binds so 2 amino acids can be joined by a

  condensation reaction forming a peptide bond

  ● Using energy from hydrolysis of ATP

  5. tRNA released after amino acid joined polypeptide

  6. Ribosome moves along mRNA to form the polypeptide, until a stop codon is reached