Topic 3

• Advantages of sexual reproduction


  • Produces variation in offspring
  • Decreases chance of whole species becoming extinct
• Advantages of asexual reproduction


  • Only one parent is needed
  • Uses less energy and is faster as organisms don't need to find a mate
  • In favorable conditions lots of identical offspring can be produced
• Fertilisation


  The fusion of a male and female gamete to produce a zygote that undergoes cell division and develops into an embryo
• Adaptations of insect-pollinated plants


  • Large and bright petals to attract insects
  • Scented with nectar to attract insects
  • Sticky and moderate amounts of pollen grains
  • Anthers inside flower, stiff and attached so that insects can brush past
  • Stigma inside flower, sticky so pollen grains stick to it when an insect brushes past
• Adaptations of wind-pollinated plants


  • Small and dull petals - usually green or brown
  • No scent or nectar
  • Smooth and light pollen grains so they can be easily carried in the wind and in large amounts
  • Anthers outside flower, loose on long filaments so that pollen can be released easily
  • Stigma outside flower, feather so forms network to catch pollen grains drifting in the wind
• Seed and fruit formation

  1. Pollen grains land on stigma

  2. Pollen tube grows out of pollen grain and down style into ovary and then to ovule

  3. Male nucleus travels down pollen tube to fuse with female egg nucleus in ovule, forming zygote

  4. Zygote undergoes mitosis to form seed

  5. Ovule becomes seed and ovule wall becomes seed coat

  6. Ovary becomes fruit
• Conditions needed for seed germination

  • Water to activate enzymes to break down starch food reserves
  • Oxygen for aerobic respiration to release energy for growth
  • Warmth to increase growth rate
• Structures of a germinating seed

  • Embryo (young root and shoot)
  • Food store (starch)
  • Seed coat (protective covering)
• Asexual reproduction


  Produces clones as it only involves one parent, unlike sexual reproduction
• Natural asexual reproduction


  • Runners (e.g. strawberry plants)
• Artificial asexual reproduction


  Cuttings (tissue samples scraped from parent plant and placed in agar growth medium)
• Male reproductive system structures


  • Sex gland (produces semen with sperm)
  • Sperm duct
  • Testis (produces sperm and testosterone)
  • Penis (passes urine and semen out of body)
  • Urethra (tube inside penis to carry urine or semen)
• Female reproductive system structures

  • Ovary (contains ova)
  • Oviduct (connects ovary to uterus)
  • Uterus (has thick lining for implantation)
  • Cervix (ring of muscle at lower end of uterus)
  • Vagina (muscular tube)
• Secondary sexual characteristics in females


  • Breast development
  • Menstrual cycle begins
  • Growth of body hair
  • Widening of hips
  • Increased height
• Secondary sexual characteristics in males

  • Growth of penis and testes
  • Production of sperm
  • Growth of facial and body hair
  • Muscle development
  • Voice lowering and breaking
• Oestrogen


  Causes thickening of uterus in preparation for implantation, levels peak on day 10 then begin to fall
• Progesterone

  Maintains thick lining of uterus, inhibits release of LH and FSH, increases after ovulation
• FSH and LH


  FSH causes maturation of egg, LH stimulates release of egg during ovulation
• Placenta


  Allows diffusion of nutrients and gases between mother and developing foetus, takes over production of progesterone
• Amniotic fluid


  Liquid in bag (amnion) that surrounds and protects foetus, comes out when labour begins
• Genetic definitions

  • Gamete (reproductive cell with half chromosomes)
  • Genome (entire DNA of organism)
  • Chromosome (structure made of DNA)
  • Gene (section of DNA coding for a protein)
  • Allele/variant (different forms of a gene)
  • Dominant allele (only one needed to be expressed)
  • Recessive allele (two needed to be expressed)
  • Homozygous (two same alleles)
  • Heterozygous (one dominant, one recessive allele)
  • Genotype (combination of alleles)
  • Phenotype (observed physical characteristics)
  • Codominance (neither allele is dominant)

  
• DNA structure


  Double helix made of two strands with complementary base pairing (A-T, C-G), groups of three bases code for amino acids that form proteins, chromosomes are made of DNA
• Protein synthesis


  1. DNA helix untwisted and unzipped

  2. mRNA nucleotides match to complementary bases on DNA strand

  3. mRNA nucleotides joined together to create new strand
• Nucleotides


  Molecules that join up to make a long strand in the case of DNA
• Components of a nucleotide


  • One sugar molecule
  • One phosphate group (which forms the backbone)
  • One of the four different organic bases
• The 4 bases


  • A
  • C
  • G
  • T
• Complementary base pairing


  Only certain bases can join together: C joins to G and A joins to T
• Triplet of bases

  Codes for an amino acid
• Chromosomes

  Structures made up of long molecules of DNA
• Protein synthesis


  1. DNA helix is untwisted and unzipped

  2. mRNA nucleotides match to their complementary base on the strand

  3. The mRNA nucleotides themselves are then joined together, creating a new strand called a template strand

  4. The template strand of mRNA then moves out of the nucleus to the cytoplasm and onto structures called ribosomes

  5. At the ribosomes, the bases on the mRNA are read in threes to code for an amino acid

  6. The corresponding amino acids are brought to the ribosomes by carrier molecules

  7. These amino acids connect together to form a protein

  8. When the chain is complete the protein folds to form a unique 3D structure
• Punnett square


  Used to describe monohybrid inheritance (looking at only one characteristic, e.g. eye colour)

  
• Uppercase letters


  Represent dominant characteristics
• Lowercase letters

  Represent recessive characteristics
• Human body cells


  • Have 23 pairs of chromosomes
  • 22 control characteristics, and the chromosomes in each pair look very similar
  • The 23rd pair carries sex determining genes, and the two chromosomes can look different to each other (Y chromosomes are much smaller than X chromosomes)

  
• X chromosomes and Y chromosomes

  The two possible chromosomes in the 23rd pair
• Females have two X chromosomes, so therefore only pass on X chromosomes in their eggs
• Males have one X chromosome and one Y chromosome, so therefore can pass on X or Y chromosomes in their sperm
• Mitosis


  1. Cell grows as organelles grow and increase in number

  2. Synthesis of proteins occurs

  3. All 46 chromosomes are replicated

  4. Energy stores are increased

  5. Chromosomes line up at the equator of the cell

  6. Spindle fibres pull each chromosome of the 'X' to either side (poles) of the cell

  7. Two identical daughter cells form when the cytoplasm and cell membranes divide, each containing the same 46 chromosomes as the original cell

  
• Cell division by mitosis


  Occurs during growth and development, replacing damaged cells and also cloning
• Meiosis


  1. The cell makes copies of its chromosomes, so it has double the amount of genetic information

  2. The cell divides into two cells, each with half the amount of chromosomes, giving the normal amount of 46 chromosomes (diploid)

  3. Each cell divides into two again to produce four cells, each with 23 chromosomes (haploid)