class notes

Cards (22)

  • genetic variation:
    • new combination of alleles
    • different combination of characteristics
    • different expressed genes - protein synthesis
  • crossing over:
    • occurs during prophase 1
    • 2 homologous chromosomes lie side by side
    • chromosomes form links, called chiasmata
    • parts of the chromatids can swap places
    • no genes are lost as they have the same loci on both chromosomes
    • alleles are different which gives a new combination of alleles
    • multiple chiasmata can form
  • independent assortment:
    • occurs during metaphase 1
    • homologous pairs of chromosomes line up along the equator of the nucleus
    • each pair behaves independently from other pairs
    • random mix of maternal and paternal chromosomes in each gamete
    • different combination of alleles
  • hemaphrodites:
    • reproduce sexually by themselves
    • genetically varied offspring
    • still do meiosis, so crossing over and independent assortment
  • autosomes - chromosomes that are not sex pairs
  • autosomal linkage
    tendency of 2 characteristics to be inherited together because the genes that cause them are close together on the same chromosome due to close loci
  • autosomal linkage facts:
    • alleles do not assort independently as the genes are on the same chromosome
    • occurs during meiosis
    • alleles on the same chromosome tend to be inherited together
    • gametes only receive 1 chromosome for each homologous pair
  • Autosomal linkage affects the outcome of crosses, as dihybrid crosses predict 4 outcomes, and with heterozygous parents, a ratio of 9:3:3:1, yet in practicality, there are only 2 outcomes, with a 1:1 ratio.
  • When there is known autosomal linkage, offspring that have variation are recombinants, as there must have been crossing over for variety.
  • haploid nucleus

    contains 1 copy of each chromosome, so half of the genetic material (n)
  • diploid nucleus
    contains 2 copies of each chromosome, 1 from each parent (2n)
  • Although a diploid nucleus has genetic material of 2n during interphase, it increases to 4n during replication, before mitosis and meiosis.
  • plant fertilisation:
    1. pollen tube nucleus makes the pollen tube grow down the style to the ovule, by releasing digestive enzymes
    2. generative nucleus divides by mitosis to form 2 haploid nuclei
    3. pollen tube nucleus dissolves when the pollen tube enters the ovule
    4. 1 of the haploid nuclei fertilises the ovum to form the zygote
    5. other haploid nucleus fertilises the 2 polar nuclei to form the endosperm
  • The endosperm surrounds the embryo, and supplies nutrients e.g. amylose, glucose, fats, amino acids. It is a 3n structure, as it has a triploid nucleus.
  • Stem cells:
    • cell replacement
    • tissue repair
  • Purpose of stem cells:
    • specialised cells divide rarely
    • the more specialised the cell, the less it divides
    • specialisation/differentiation
    • cancer in specialised tissue is rare, as there is less division
  • Totipotent - stem cells that are capable of dividing to form any type of specialised animal cell
  • Totipotency is present in the cells of a 16 cell embryo. as the embryo develops, totipotence decreases so the stem cells are pluripotent.
  • There are more than 200 different types of cells in a human adult.
  • In an adult, most tissues contain stem cells. Stem cells are found in bone marrow, so that they can make all types of blood cells.
  • Application of stem cells:
    • bone marrow transplant
    • stem cell therapy
  • cancer
    • if cells divide when they're not supposed to, a tumor forms
    • uncontrollable cell division
    • mutation in any gene that regulates the cell cycle or controls cell death