Unit Book

• Eukaryotic vs Prokaryotic DNA

  Eukaryotic:

  1. DNA has introns and exons
  2. DNA is associated with histones
  3. Linear
  4. DNA is in a membrane (nucleus)
  5. Longer

  Prokaryotic:

  1. No introns, only exons
  2. DNA is not associated with histones (naked)
  3. Non-linear, it is circular
  4. DNA is in cytoplasm
  5. Shorter
• Gene


  A base sequence of DNA that codes for a polypeptide or functional RNA
• Locus


  The position of a gene on a chromosome
• Intron


  A non-coding sequence that is found within a gene
• Exon


  A sequence of DNA that codes for a polypeptide
• Chromosome


  The DNA double helix combined with histones, coiled up, forming chromatin
• Homologous

  2 chromosomes that carry the same genes
• Allele

  An alternative form of a gene
• Histone

  A circular protein combined with DNA to make a chromosome
• Genome

  The complete set of genes in a cell
• Proteome


  The full range of proteins produced by the genome
• Triplet Code


  3 bases form a triplet code; each triplet codes for 1 amino acid
• Universal

  The triplet code codes for the same amino acid in organsims
• Non-overlapping


  The bases are read in triplets and then the next 3 triplets are read
• Degenerate


  When amino acids are coded for by more than one triplet code
• Chromosome structure

  
• mRNA Structure and Function

  Structure: Single strand of RNA nucleotides

  Function: Carries the triplet code cor a specific polypeptide

  
• rRNA Structure and Function

  Structure: 2 subunits (large&small) combined with protein -> ribosome

  Function: Site of protein synthesis

  
• tRNA Structure & Function

  Structure: Small single strand of RNA folded into a clover-leaf shape

  Function: Carry amino acids to ribosome

  
• Why is the Genetic Code Universal?

  • Every organisms DNA is made up of the same 4 bases
  • Every 3 bases forms a triplet code
  • Each triplet code determines 1 amino acid
  • Each triplet code codes for the SAME amino acid in all organisms
  • The sequence of bases and quantity of DNA determines the organisms characteristics
• Why is Genetic Code degenerate?

  • A number of different triplet codes/codons can code for the same amino acid
• Why is Genetic Code Non-overlapping?

  • Each base in the DNA molecule is only read once- as a series of 3/in triplets
• If all cells contain the same DNA in the nucleus, why are cells in the body different?

  • All cells have potential to produce all proteins as they have the DNA present in their nuclei
  • Each cell expresses (transcribes and translates) a limited number of those genes & in a different combo

  -> because some genes are "switched on"/ expresses and others are not
• Semi-Conservative Replication


  1. A representative portion of DNA, about to undergo Replication
  2. The 2 strands of DNA separate. The H bonds between the bases break
  3. Free nucleotides are attracted to their complementary bases
  4. Once lined up, they are joined by enzyme DNA polymerase
  5. Finally, all nucleotides have joined to form complete nucleotides chain. Two identical strands of DNA formed. Each strand retains ½ genetic material ∴ this replication is semi-conservative

  
• Meselson and Stahl


  Steps:

  1. They grew bacteria for many generations on a medium with ¹⁵N
  2. Then transferred bacteria to medium of ¹⁴N & allowed it to replicate once, for single gen
  3. The transferred bacteria again to ¹⁴N allowed & allowed it to replicate once, for a 2nd gen
  4. Repeated again for 3rd gen
  5. At each step, sample was centrifuged in special solution to different DNA could be separated
  6. Lighter DNA nearer to top

  
• Meselson and Stahl explanation


  1. All bases in DNA contain N
  2. There are two isotopes, normal ¹⁴N and heavy ¹⁵N
  3. Bacteria will incorporate N from growth media into any new DNA made
  4. Bacteria grown on ¹⁴N would be lighter/less dense that bacteria grown on medium containing ¹⁵N
  5. So when samples are centrifuged, they will collect as band in different positions according to density

  
• DNA Semi-Conservative


  1. Let bacteria divide and replicate their DNA
  2. Grow E-Coli with heavier ¹⁵N
  3. Move bacteria to medium with lighter ¹⁴N. All new DNA will now be lighter
  4. Check bacterial DNA & found all DNA molecules were same density (half ¹⁴N, half ¹⁵N) so not conservative
  5. To distinguish between semi-conservative and dispersive, DNA divided again. If same density, dispersive, if ½contained ¹⁵N + ¹⁴N and half ¹⁴N, semi
  6. -> Found it was semi-conservative

  
• Features of DNA + how they are important for semi-conservative replication

  • Complementary base pairings- allows accurate replication
  • Weak/easily broken H bonds between bases- allows 2 strands to separate/unzip
  • Two strands- both can act as template
• Roles of Mitosis

  • Growth of tissues
  • Repair of tissues (by replacing cells)
  • Asexual reproduction
  • Development of body plan
  • Proliferation of WBC (clonal expansion)
  • Production of new STEM cells
• Cell Cycle


  • Most cells do not divide continously
  • Instead undergo regular cycle of division, separated by periods of growth

  This is known as the cell cycle

  1. Interphase
  2. Mitosis
  3. Cytokinesis

  
• Steps


  Interphase(95%)

  • G1: Growth phase 1- protein synthesis/transcription/formation of mRNA, organelles replicate ATP required
  • S: Synthesis phase- DNA replication, ATP required
  • G2: Growth phase 2- cell growth, cell increases in size

  Mitosis:

  • PMAT

  
• Checking the cell

  • To prevent mistakes & mutations the cell can only progress through cell cycle after passing checkpoints
  • If there is a problem, cell will pause cell cycle until fixed
  • If it cannot be fixed, cell will kill itself, preventing a tumour developing. Called apoptosis
• Cell Checkpoints

  • Cell growth checkpoint: Occurs towards the end of G1- checks whether cell is big enough + has made proper proteins for S phase. If not, cell goes through resting (G0) period until ready
  • DNA synthesis checkpoint: occurs during S phase. Checks whether DNA has been replicated correctly. If so, carried on to M
  • Mitosis checkpoint: occurs during M phase. Checks whether chromosomes are attached correctly to the spindle

  
• Cancer & Mitosis

  • Cancer is uncontrolled cell division leading to formation of a tumour
  • Causes by mutations in the genes that control mitosis
  • Mutant cells formed are structurally & functionally different from normal cells and generally die
  • However, those that survive clone themselves (mitosis( and form tumours
  • Malignant tumours grow rapidly, less compact, more likely to be life-threatening
  • Benign grow more slowly, more compact, less likely to be life-threatening
• Methotrexate: stops replication of DNA. Also works in fast dividing cells (eg hair cells leading to hair loss)

  Vinblastine: Inhibits metaphase stage of mitosis by interfering with spindle fibre formationn
• P


  • Spindle fibres form
  • Nuclear membrane breaks down
  • Chromatin condenses (coils & shortens) to form chromosomes

  
• M(etaphase)

  • Controversial join the chromosomes onto each spindle fibre at the metaphase plate (equator)
• A

  • Centromere splits in two
  • The sister chromatids are pulled to opposite poles as spindle fibres shorten (they contract)

  
• T


  • Nuclear membranes reform at the two poles
  • Chromatids uncoil back into chromatin

  
• Prophase


  • Chromosomes shorten/thicken/condense and become visible; due to DNA replication, chromosomes appear as 2 identical sister chromatids joined at the centromere