Samenvatting week 6
Meiosis
Diversity based on sexual reproduction
ensures survival
Propagation of genetic information into
the next generation
Equal contribution of genetic material
Alleles are variant versions of genes
Continuous generation random mutations
Rapid elimination of deleterious alleles
(individuals will not reproduce)
Rapid adaptation to a changing
environment
Mitosis and meiosis is not 4N but 2x 2N
From which stage onwards are cells
undergoing meiosis haploid?
o End of meiosis I
Meiose I start; diploid: 2 parental alleles
Meiosis II; start = haploid: 1 parental
allele present (either maternal or
paternal)
Homologous recombination through crossing-over = driver of
genetic diversity
Chiasma hold the bivalent together until the first
meiotic division
Chiasmata position chromatids that are glued
together by cohesins
The more space between the chromosomes
(chiasma not as strongly bonded) the more change
of crossing-over
Cohesins keeps the homologs together
4 combinations for 1 chromosome. For 3
chromosomes 43 = 64 combinations
Human: >7x1013 distinct gametes
Errors in chromosome segregation (trisomy 21 down’s syndrome)
Tissue maintenance
Maintenance is achieved by organization within an organ
o Cell communication (extracellular signals)
if a cell doesn’t get this it will automatically have a
programmed cell death
some cell signals can lead to proliferation (doubeling)
or differentiation
o Selective cell adhesion (only with same cell type)
Columnar (rechthoekig)
, Squamous (plat en dun)
Cuboidal (vierkant)
Stratified (meerdere cell lagen)
Always a basal lamina
Important function of epithelial barriers: keep outside world
outside
Tight junction (zonula occludens): seals neighboring cells
together in an epithelial sheet to prevent leakage of
extracellular molecules between them; helps polarize cells
Adherens junctions (zonula adherens): joins an actin bundle in
one cell to a similar bundle in a neighboring cell
Desmosome (macula adherens): joins the intermediate
filaments in one cell to those in a neighbor
Jap junction (nexus): forms channels that allow small,
intracellular, water-soluble molecules, including inorganic ions
and metabolites, to pass from cell to cell
Hemidesmosome: anchors intermediate filaments in a cell to
the basal lamina
o Cell memory (terminal differentiation)
Molecular level
Extracellular signals are necessary for cell survical, otherwise
apoptosis:
o Extracellular growth factors
o Survival factors from target cells: 30 min depletion starts
irreversible death program leading to apoptosis
o Mitogens
Cellular level
\
Renewal by stem cells
Four types of stem cells:
o Multipotent
o Pluripotent
o Totipotent
o Unipotent
Meiosis
Diversity based on sexual reproduction
ensures survival
Propagation of genetic information into
the next generation
Equal contribution of genetic material
Alleles are variant versions of genes
Continuous generation random mutations
Rapid elimination of deleterious alleles
(individuals will not reproduce)
Rapid adaptation to a changing
environment
Mitosis and meiosis is not 4N but 2x 2N
From which stage onwards are cells
undergoing meiosis haploid?
o End of meiosis I
Meiose I start; diploid: 2 parental alleles
Meiosis II; start = haploid: 1 parental
allele present (either maternal or
paternal)
Homologous recombination through crossing-over = driver of
genetic diversity
Chiasma hold the bivalent together until the first
meiotic division
Chiasmata position chromatids that are glued
together by cohesins
The more space between the chromosomes
(chiasma not as strongly bonded) the more change
of crossing-over
Cohesins keeps the homologs together
4 combinations for 1 chromosome. For 3
chromosomes 43 = 64 combinations
Human: >7x1013 distinct gametes
Errors in chromosome segregation (trisomy 21 down’s syndrome)
Tissue maintenance
Maintenance is achieved by organization within an organ
o Cell communication (extracellular signals)
if a cell doesn’t get this it will automatically have a
programmed cell death
some cell signals can lead to proliferation (doubeling)
or differentiation
o Selective cell adhesion (only with same cell type)
Columnar (rechthoekig)
, Squamous (plat en dun)
Cuboidal (vierkant)
Stratified (meerdere cell lagen)
Always a basal lamina
Important function of epithelial barriers: keep outside world
outside
Tight junction (zonula occludens): seals neighboring cells
together in an epithelial sheet to prevent leakage of
extracellular molecules between them; helps polarize cells
Adherens junctions (zonula adherens): joins an actin bundle in
one cell to a similar bundle in a neighboring cell
Desmosome (macula adherens): joins the intermediate
filaments in one cell to those in a neighbor
Jap junction (nexus): forms channels that allow small,
intracellular, water-soluble molecules, including inorganic ions
and metabolites, to pass from cell to cell
Hemidesmosome: anchors intermediate filaments in a cell to
the basal lamina
o Cell memory (terminal differentiation)
Molecular level
Extracellular signals are necessary for cell survical, otherwise
apoptosis:
o Extracellular growth factors
o Survival factors from target cells: 30 min depletion starts
irreversible death program leading to apoptosis
o Mitogens
Cellular level
\
Renewal by stem cells
Four types of stem cells:
o Multipotent
o Pluripotent
o Totipotent
o Unipotent
