Topic 3

• Key eukaryotic organelles


  • Nucleus
  • Rough endoplasmic reticulum (rER)
  • Smooth endoplasmic reticulum (sER)
  • Golgi apparatus
  • Mitochondria
  • Chloroplasts
• Nucleus


  • Double-membrane with nuclear pores
  • Nucleolus
• Rough endoplasmic reticulum (rER)

  • Decorated with 80S ribosomes (translation)
  • Protein folding
• Smooth endoplasmic reticulum (sER)

  • Devoid of ribosomes
• Golgi apparatus


  • Devoid of ribosomes
  • Flattened cisternae
  • Different size cisternae
  • Presence of transport vesicles (coming to the Golgi) and secretory vesicles (leaving the Golgi)
  • Protein modification – addition of carbohydrate groups – formation of glycoproteins
• Mitochondria


  • Double membrane - outer and inner membrane surround the mitochondrion matrix
  • Inner membrane forms cristae – contains the proteins involved in cellular respiration
  • Mitochondrial DNA, 70S ribosome
  • Site of cellular respiration and ATP production
• Chloroplasts


  • Site of photosynthesis
  • Double membrane - outer and inner envelope membranes surround the chloroplast stroma (S)
  • Stacked grana thylakoids (GT) and unstacked stroma thylakoids (ST) in the stroma
  • Grana thylakoids contains the photosystems of the light-dependent reactions
  • Stroma contains the enzymes of the light-independent reactions
  • DNA, 70S ribosomes
• How do proteins get from the ribosome to the plasma membrane?

  1. At the ribosome - protein translation

  2. Inside the rough endoplasmic reticulum – protein folding

  3. Transport vesicles

  4. Golgi apparatus – protein modification (example carbohydrate addition – glycoprotein)

  5. Proteins packaging in secretory vesicles

  6. Secretory vesicles fuse with the plasma membrane; some proteins remain in the membrane (membrane proteins), some are secreted through exocytosis (secretory proteins)
• Cell cycle


  1. Interphase consists of G1, S and G2

  2. G1 – protein synthesis, organelles multiplied, growth

  3. S – DNA replication

  4. G2 - further growth

  5. Mitosis (M) – produces two diploid cells for growth, repair and asexual reproduction
• Mitosis


  1. Prophase: Nuclear envelope breaks down, Chromosomes condense visible as two sister chromatids joined together by a centromere, Centrioles migrate to opposite poles of cell, Spindle fibres form

  2. Metaphase: Chromosomes align in the middle of the cell, Spindle fibres attach to the centromere of sister chromatids

  3. Anaphase: Spindle fibres pull the sister chromatids towards the opposite poles of the cell

  4. Telophase: Chromosomes decondense and a nuclear envelope reforms, Cytokinesis
• Mitotic index


  Cells undergoing mitosis/ total number of cells
• Meiosis


  1. Meiosis I – separation of homologous chromosomes (carrying alleles for the same gene)

  2. Meiosis II – separation of sister chromatids (carrying copies of the same allele)

  3. Meiosis I increases genetic variation as a result of: Crossing over – exchange of alleles between nonsister chromatids of homologous pairs of chromosomes, Independent assortment – random arrangement of homologous pairs of chromosomes
• Sex-linked inheritance


  • Sex chromosomes, in humans, X and Y (Y chromosome smaller than X chromosome)
  • In humans, XX - female, XY – male
  • Egg cell and sperm cell have haploid nucleus – allow restoration of diploid number of chromosomes at fertilisation
  • Sex-linked diseases – mutations in genes on sexual chromosomes
  • X-linked recessive diseases, eg colour blindness, haemophilia; males have only one X chromosome, if they inherit only one copy of a recessive allele, it will be expressed (cannot be heterozygous for the disease) whereas females have two X chromosomes; both recessive alleles needed to be present to be expressed (can be heterozygous for the disease (carriers))
• Acrosome reaction


  1. The acrosome fuses with sperm cell membrane

  2. Enzymes (acrosin) are released through exocytosis

  3. These enzymes digest the zona pellucida

  4. So that the sperm cell membrane can fuse with the egg cell membrane
• Cortical reaction


  1. Fusion of the sperm cell membrane with the egg cell membrane

  2. Cortical granule release contents (enzymes) into zona pellucida through exocytosis

  3. Contents (enzymes) of cortical granules react with zona pellucida - the zona pellucida thickens – ensures egg cell is penetrated by one sperm cell only
• Lac operon


  1. When lactose absent, a repressor binds to the operator, preventing the binding of RNA polymerase to the promoter

  2. When lactose present, lactose binds to the repressor and prevents it from binding to the operator – RNA polymerase binds to the promoter, allowing transcription and gene expression
• Tissue


  A group of similar cells working together to perform a common function, eg muscle tissue, elastic tissue
• Organ


  A group of different tissues, working together to perform one or more functions, eg heart
• Organ system


  A group of different organs, working together to perform one or more functions, eg circulatory systems
• Stem cells


  Undifferentiated cell that has ability to differentiate into specialised cells, capable of unlimited division
• All cells in an organism contain the same DNA. Why are our cells looking so different?
• How stem cells become specialised and develop into heart muscle cells


  1. Certain signals cause activation of specific genes

  2. The active genes are transcribed to produce specific mRNA

  3. The specific mRNA is translated to produce specific protein

  4. Proteins determine the structure and function of a cell
• Totipotent


  A cell that has the ability to differentiate into all cell types
• Pluripotent


  A cell that has the ability to differentiate into most cell types, but not extra-embryonic cells – some genes are already activated/ inactivated
• Multipotent


  A cell that has the ability to differentiate into some cell types
• When an egg cell becomes fertilised, it is called a ...

  zygote
• After the first cell division there are two cells and after the fourth division the number of cells is ...
• Epigenetics


  Changes to DNA/ histones that do not alter the sequence of DNA nucleotides but can affect gene expression; epigenetics can be affected by the environment; may be passed on to further generations
• DNA methylation


  DNA wrapped tightly around histones, gene inactive, transcription factors cannot bind, transcription off
• Histone acetylation


  DNA wrapped loosely around histones, gene active, transcription factors can bind, transcription on
• Polygenic inheritance


  Characteristic determined by multiple genes at multiple loci, This leads to continuous variation (eg height)