Edexcel B Biology - Topic 2

• The Nucleus is surrounded by a double membrane (nuclear envelope), which has pores that allow molecules to enter/leave. The nucleus also contains chromatin and the site of ribosome production (nucleolus).
• The Rough Endoplasmic Reticulum is a series of flattened sacs within a membrane with ribosomes on the surface. The RER folds and processes proteins made on the ribosomes.
• The Smooth Endoplasmic Reticulum is a series of membrane-bound sacs. The SER synthesis and processes lipids and steroids.
• The Golgi Apparatus is a series of cisternae (fluid-filled, curved sacs) with vesicles surrounding the edges. The Golgi Apparatus sorts, processes and packages proteins and lipids. It also produces lysosomes.
• The Mitochondria are bound by a double membrane (envelope). The inner membrane is folded to form cristae, with a matrix on the inside containing enzymes needed for cellular respiration.
• The Centrioles are hollow cylinders containing a ring of microtubials arranged at right angles to each other. They are involved in cell division.
• Eukaryotic Cells have 80S Ribosomes - composed of a 60S subunit and a 40S subunit. It is the site of protein synthesis.
• Lysosomes are vesicles containing digestive enzymes and are bound by a single membrane.
• Prokaryotic - Cell Walls are made up of peptidoglycan and provides support to the cell.
• Prokaryotic - Slime Capsule, protective slimy layer which helps the cell retain moisture and adhere to surfaces.
• Prokaryotic - Plasmid is circular DNA.
• Prokaryotic - Flagellum, a tail like structure which is used for movement.
• Prokaryotic - Pili, hair like structure which attach to other bacterial cells and allows plasmids to move from cell to cell.
• Prokaryotic - 70S Ribosomes, composed of a 50S subunit and a 30S subunit.
• Prokaryotic - Mesosomes, inholdings of the inner membrane which contain enzymes required for cellular respiration.
• Bacteria can be classified via reaction to Gram stain.

  • Gram Positive bacteria have a wall with a thick layer of peptidoglycan and an inner plasma membrane. They appear violet after staining.
  • Gram Negative bacteria have a thin layer of peptidoglycan and an outer lipopolysaccharide membrane. They appear red after staining.
• Magnification is how much bigger the image is compared to the original object.
• Resolution is how far apart two points can be before they are seen as one.
• An optical microscope uses a beam of light to form an image of the specimen.
• A scanning electron microscope uses a beam of electrons to scan over the surface of the sample to produce a 3D image.
• Viruses are non-living structures which consist of a nucleic acid within a capsid, sometimes covered with a lipid layer (envelope).
• Ethical Implications of using untested drugs:

  • Difficult to obtain consent.
  • Unknown side effects.
  • May not be as effective as the current treatment.
• Lysogenic viruses insert DNA (provirus) into the DNA of a host cell which enables the viral DNA to be replicated via cell division of the host cell. The provirus can stay dormant if the virus produces repressor proteins which inhibit transcription.
• Lytic viruses insert DNA/RNA into the cytoplasm of the host cell, the viral genome is separate of the host cell genome. This leads to lysis of the host cell when a large number of viruses are assembled within the host cell.
• If lysogenic host cells become damaged or the immune system becomes weak, dormant viruses can enter the lytic pathway.
• Mitosis produces two genetically identical daughter cells.
• Three stages of the cell cycle:

  • Interphase - G1, S and G2; growth, DNA replication and preparation for division. Chromosomes condense to form chromatin.
  • Mitosis - prophase, metaphase, anaphase and telophase.
  • Cytokinesis - the cytoplasm divides, thus producing two daughter cells.
• Stages of Mitosis:

  1. Prophase - Spindle fibres appear, chromosomes condense and the nuclear membrane breaks down.
  2. Prometaphase - Spindle fibres attach to chromosomes via centromere.
  3. Metaphase - Chromosomes line up at the centre of cell.
  4. Anaphase - Centromeres divide, sister chromatids move to opposite poles of the cell.
  5. Telophase - Nuclear membrane reforms, chromosomes decondense and the spindle fibres disappear.
• Meiosis gives rise to genetic variation by:

  • Crossing over - the exchange of sections of DNA between homologous chromosomes.
  • Independent assortment - there are various combinations of maternal and paternal chromosomes arrangement.
• Chromosome mutations include:

  • Translocations - a part of one chromosome breaks off and reattaches to another chromosome.
  • Duplication.
  • Deletion - a section of a chromosome is remove, large loss of genes.
  • Inversion - a part of a chromosome flips its orientation with respect to the rest of the chromosome.
  • Non-disjunction - homologous chromosomes or sister chromatids fail to separate.
• Non-disjuction can cause:

  • More than two chromosomes in a cell, polysomy (Down syndrome - trisomy 21).
  • Less than two chromosomes in a cell, monosomy (Turners syndrome - monosomy of sex chromosomes, only the X chromosome is present).
• Spermatogenesis:

  1. Primordial germ cells undergo mitosis several times to form spermatogonia.
  2. Spermatogonia grow without further division to form primary spermatocytes.
  3. Primary spermatocytes undergo meiosis to form secondary spermatocytes (diploid).
  4. Secondary spermatocytes undergo meiosis to form spermatids (haploid).
  5. Spermatids differentiate to form mature spermatozoa.
• Oogenesis:

  1. Primordial germ cells undergo mitosis several times to form oogonia.
  2. Only one oogonium continues to grow to form a primary oocyte.
  3. The first meiotic division forms one secondary oocyte and one polar body.
  4. The second meiotic division of the secondary oocyte forms one haploid ootid and one polar body.
  5. The second meiotic division of the polar body forms two more polar bodies - which die as the ootid develops. This division is halted at prophase and only occurs in response to fertilisation to form the mature ovum.
• The Ovum:

  • Contains the zona pellucida which is a protective covering that sperm have to penetrate in order for fertilisation to happen. It prevents polyspermy.
  • Contains a haploid nucleus, full chromosomes set restored at fertilisation.
  • Contains cortical granules, which release substances which causes the zona pellucida to harden.
  • Contains follicle cells, which form a protective covering around the egg.
• Spermatozoa:

  • Have many mitochondria which provide energy for rotation of the flagellum.
  • Acrosomes contain digestive enzymes which break down the zona pellucida.
  • Haploid nucleus allows the restoration of the full chromosome set at fertilisation.
• Fertilization:

  1. Sperm head contacts the zona pellucida and acrosome reaction occurs: enzymes digest the ZP as the acrosome fuses with the cell membrane of the sperm and enzymes are released.
  2. The sperm head fuses with the cell membrane, allowing sperm nucleus to enter the egg cell.
  3. The cortical reaction occurs which causes the zona pellucida to harden.
  4. The nuclei fuse and the full chromosome set is restored, resulting in a diploid zygote forming.
• Pollen Formation:

  1. Diploid microspore mother cells in the anther undergo meiosis. They form four haploid microspores.
  2. Haploid microspores undergo mitosis to mature into pollen grains. Pollen grains consist of two nucleus, the generative nucleus and the pollen tube nucleus.
• Ovum Formation (Plants):

  1. Diploid megaspore in the ovule undergoes mitosis, an ootid and three polar bodies (which degenerate) are formed.
  2. The ootid undergoes three mitotic divisions to form an embryo sac. The embryo sac contains two polar nuclei (form the endosperm), an egg cell (form the zygote), two synergies (help the generative nucleus reach the egg cell) and three antipodal cells with a protective coating.
• Fertilisation in Plants:

  1. Pollen grain adheres to the stigma where it germinates.
  2. The pollen tube grows down the style via secretion of digestive enzymes.
  3. The pollen tube grows through the micropyle into the embryo sac.
  4. The generative nucleus of the pollen divides by mitosis to produce two sperm cells which enter the sac.
  5. Double fertilisation occurs: one of the male gametes fuse with the female nucleus to form a diploid zygote and the other male gamete fuses with two polar nuclei to form a triploid endosperm nucleus.