Lecture 6 Flashcards
Meiosis
- reduced amount of genetic material by half
- produces hapliod gametes or spores, each containing one member of a homologous pair of chromosomes
Meiotic event
Genetic exchange between members of homologous pairs of chromosomes
Meiosis I
reductional division
Meiosis II
Equational division
When does DNA synthesis occur during meiosis
- occurs during interphase, before the beginning of meiosis I
- doesn’t occur again before meiosis II
Meiosis I - Prophase I
5 substages each with specific events
What indicated the end of prophase I?
centromeres are present on the equatorial place
Leptonema
chromosomes appear as long, single threads unassociated with each other (chromomeres)
Zygonema
- synapsis
- each pair of homologous chromosomes is know as bivalent
- chromatin still present
Pachynema
- each bivalent becomes shorter, thicker, and splits into 2 sister chromatids called tetrads
- homologous pairs line up
crossing over
exchange of genetic material between nonsister chromatids, promotes genetic variation
Diplonema
- within tetrads, sister chromatids separate
- chiasma: where chromatids are still intertwined
Diakinesis
- nucleus and nuclear envelope break down
- centromeres attach to spindle fibers
metaphase I
chromosomes at maximum shortness
terminal chiasmata
-holding non-sister chromatids together
what moves chromatids to metaphase plate
centromeres binding to spindle fibers
Anaphase I
- cohesion is degraded between sister chromatids
- homologous chromosomes separate
- nondisjunction may occur
First division: Meiosis I-Telophase I
- cytokinesis
- 2 haploid cells result
- nuclear membranes forms
- nuclei enter interphase
What occurs during metaphase, anaphase, and telophase I?
- homologous chromosomes separate and move toward the poles
- sister chromatids remain attached at centromeres
- duplicated chromosomes reach poles, each pole now has haploid set of chromosomes
- cytokinesis: 2 haploid daughter cells are formed
- nuclear envelope forms around chromosomes in some species
Prophase II
each dyad is composed of one pair of sister chromatids attached by the common centromeric region
dyad
group of 2 things
Metaphase II
the centromeres are positioned on the equatorial plate
Anaphase II
- sister chromatids are separated to opposite poles
- each haploid daughter cell from meiosis II has one member of each pair of homologous chromosomes
Telophase II
- one member of each pair of homologous chromosome is now a monad
second round of cytokinesis…
results in 4 haploid cells
spermatogenesis
production of male gametes in testes
oogenesis
production of female gametes in ovary
- formation of ova in the ovaries
- 4 daughter cells do not receive equal cytoplasm
- only 1 daughter cell receives cytoplasm (primary polar body)
- undergoes meiosis I and II
- develops into ovum
Primary spermatocyte
- undergoes meiosis I
- produces 2 secondary spermatocytes
Secondary spermatocyte
- undergo meiosis II
- produces a total of 4 haploid spermatids
Fungi life cycles
haploid vegetative cells arise via meiosis-proliferate via mitotic cell division
Plants life cycles
life cycle alternates between diploid, sporophyte stage and haploid gametophyte stage
Chromosomes
- visible only during mitosis and meiosis
- chromatin fibers that make up chromosomes coil and condense
folded-fiber model
electron microscope of mitotic chromosomes in varying states of coiling led to this model
