no genes are lost as they have the sameloci 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:
pollen tube nucleus makes the pollen tube grow down the style to the ovule, by releasing digestive enzymes
generative nucleus divides by mitosis to form 2 haploid nuclei
pollen tube nucleus dissolves when the pollen tube enters the ovule
1 of the haploid nuclei fertilises the ovum to form the zygote
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