Science reviewer

• Interphase: the resting phase between successive mitotic divisions of a cell, or between the first and second divisions of meiosis.

  
• Prophase: the first stage of cell division, before metaphase, during which the chromosomes become visible and shortening and thickening of it as paired chromatids and the nuclear envelope disappears. The first prophase of meiosis includes the reduction division.

  
• Metaphase: the second stage of cell division, between prophase and anaphase, during which the chromosomes become attached to the spindle fibres. Chromosomes alligned to the center of the cell

  
• Mitosis


  A process of cell division that makes possible regeneration of body parts
• Mitosis


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• Interphase


  The period of growth and rest before a dividing cell enters mitosis
• Prophase


  1. Chromatin condenses and becomes short and thick to form chromosomes

  2. Each chromosome has two sister chromatids joined together at the centromere

  3. Nucleolus disappears

  4. Nuclear membrane starts to break down

  5. Centrosomes move apart to opposite poles forming a spindle
• Metaphase


  1. Microtubules of the spindle interact with chromosomes

  2. Chromosomes move and align along the middle of the cell or the equatorial plate

  3. Sister chromatids are held at the centromere by protein structures called kinetochores
• Anaphase


  1. Sister chromatids separate at the kinetochore

  2. Daughter chromosomes along with the divided centromere move towards the opposite poles

  3. Microtubules shorten

  4. Poles of the spindle move apart to help separate the chromosomes
• Telophase


  1. Daughter chromosomes arrive at the pole

  2. Chromatin starts to decondense

  3. Nuclei reform along with nucleoli

  4. Nuclear membrane reforms
• Cytokinesis


  1. In animals, a cleavage furrow forms in the plasma membrane which deepens and divides the cytoplasm into two daughter cells

  2. In plants, a cell plate forms at the center of the cell and grows outwards until it meets the existing wall, resulting in two daughter cells
• Mitosis is the process of separation of nuclear chromosomes into two identical daughter nuclei
• Mitosis involves four key phases: prophase, metaphase, anaphase, and telophase
• Monohybrid cross


  Study of the inheritance of one characteristic
• Punnett square


  Diagram used to predict the outcome of a particular cross, used by biologists to determine the probability of an offspring having a particular genotype
• Genotype


  The genes of the organism
• Dominant allele


  Represented by a capital letter
• Recessive allele


  Represented by a lowercase letter
• Constructing a Punnett square

  1. Start with an empty grid

  2. Enter the first parent's genotype, split into two columns

  3. Enter the second parent's genotype, split into two rows

  4. Fill in the possible offspring genotypes
• Punnett square example 1

  • Cross a heterozygous yellow pea pod (Yy) and a recessive green pea pod (yy) parent
  • 50% of the offspring would have the dominant yellow pea pods, 50% would have the recessive green pea pods
• Punnett square example 2

  • Cross two cystic fibrosis carrier parents (Cc)
  • There is a 25% chance the offspring will be cystic fibrosis sufferers (cc)
• Punnett squares show probabilities, not guaranteed results
• Meiosis


  The cell division that produces sperm and egg cells, where chromosomes are separated into two halves
• Meiosis


  The process by which haploid cells are produced from a diploid cell
• Meiosis


  1. Chromosomes are correctly sorted and distributed to create genetically unique cells with half the number of chromosomes as the original cell

  2. Occurs in special cells called germ cells within the gonads of males and females

  3. Two rounds of divisions are necessary - meiosis 1 and meiosis 2
• Mitosis vs Meiosis


  Mitosis results in two diploid daughter cells, meiosis results in four haploid cells
• Meiosis 1

  1. Homologous pairs of sister chromatids lie side by side in synapsis, forming a tetrad or bivalent

  2. Crossing over occurs - physical exchange between non-sister chromatids, increasing genetic diversity

  3. Homologous chromosomes separate and move to opposite poles
• Meiosis 2

  1. Sister chromatids attach to spindle fibers from opposite poles

  2. Sister chromatids separate and individual chromosomes move to the poles

  3. Cytokinesis occurs, forming four haploid daughter cells
• The haploid daughter cells will specialize into gametes (sperm or egg)
• The child receives half its chromosomes from its mother and half from its father
• Sources of genetic diversity


  • Crossing over in prophase 1
  • Independent assortment of homologous chromosomes in metaphase 1
  • Random pairing of gametes during sexual reproduction
• As the total number of chromosomes increases in an organism, the number of genetically distinct gametes increases exponentially (2^n)
• For humans with 23 chromosome pairs, there are over a million possible unique gamete combinations
• Plants take in water


  1. Through roots by absorption

  2. Give off water vapor from leaves (stomata) through transpiration
• Animals


  1. Drink water

  2. Obtain water from food

  3. Give off water through exhalation, sweating or excretion
• Water means life: therefore, we must maintain the normal water cycle by protecting our topsoil, the water table in subsoil, plants and bodies of water from pollution
• Water cycle

   

       It is a continuous movement of water from the atmosphere to the earth back to the atmosphere. It involves the following vital process: precipitation, evaporation, transpiration and condensation.

   
• Carbon-Oxygen cycle

   

       Organisms use and produce gases in photosynthesis and respiration. These gases flow through organisms and the environment in a cyclic process
• Role of living things in the carbon-oxygen cycle


  1. Plants photosynthesize, use carbon dioxide and produce oxygen

  2. Oxygen produced by plants used by animals when they respire, animals produce carbon dioxide

  3. Plants also carry out respiration, use oxygen and produce carbon dioxide
• Carbon-oxygen cycle


  1. Animals take in oxygen from atmosphere and give off carbon dioxide during respiration (day and night)

  2. Plants give off oxygen and take in carbon dioxide when they photosynthesize during the day

  3. At night, plants take in oxygen and give off carbon dioxide