Unit 4: Meiosis Flashcards
Meiosis (body cells must contain…):
-While autosomal/somatic/body cells must contain a complete set of chromosomes, gametes possess just half the genetic information of parent cells. This is accomplished through meiosis.
Meiosis:
•(2n—> n—> n ): produces gametes/sex cells.
•two things that make it different from mitosis:
1. Produces daughter cells with fewer chromosomes than parent cells.
2. Produces chromosomes with different combinations of genes compared to those in parent cells.
•involves the same stages of cell division as mitosis; however, in meiosis, each stage occurs twice.
Prophase I:
•like in mitosis, meiosis begins with interphase, or DNA replication. However, entering prophase I the cell contains pairs of homologous chromosomes…
•chromatin condenses to form chromosomes, spindle fibres form, nuclear membrane dissolves, etc.
•homologous chromosomes match up (“synapsis”) for crossing over.
-crossing over swaps genetic information and increases diversity.
-happens ONLY during prophase I of meiosis.
Metaphase I:
•instead of lining up in single file, chromosomes line up as homologous pairs, so that chromosomes of maternal and paternal origin will end up at opposing poles.
•the orientation of each pair of homologous chromosomes is random/independent of the orientation of the other pairs (independent assortment).
-increases gamete diversity due to number of potential chromosome combinations.
Anaphase I:
•spindle fibres shorten, pulling homologous chromosomes apart.
•sister chromatids remain together, so that each new pole has a full set of DNA.
Telophase I:
•chromosomes uncoil, spindle fibres disappear.
•cytoplasm is divided and a nuclear membrane forms around each new set of chromosomes.
•each new cell is haploid because they contain either the maternal or paternal set of homologous chromosomes.
Meiosis (interphase):
-interphase does NOT occur before the cells enter prophase II (i.e. DNA is not replicated).
Meiosis II:
-considered mitotic division because n—> n.
Prophase II:
•chromosomes visible.
•nuclear membrane disappears.
•mitotic spindle forms.
Metaphase II:
•chromosomes align single file on plate.
Anaphase II:
•sister chromatids pulled apart to opposite poles.
Telophase II:
•nuclear membranes reform.
•each haploid cell formed after telophase II will proceed through cytokinesis to form gametes.
•in males, this process is referred to as spermatogenesis, and results in the formation of four viable sperm cells.
•in females, the cytoplasm is not equally divided amongst gametes, resulting in a single viable ovum and three polar bodies:
-this process is referred to as oogenesis.
Meiosis nondisjunction:
•major cause of chromosomal abnormalities.
•most occur in oogenesis due to maternal age.
•meiosis I: Anaphase I:
-homologous chromosomes (tetrad) fail to separate.
-most chromosomal abnormalities occur here.
•meiosis II: Anaphase II:
-sister chromatids fail to separate.
Nondisjunction results in:
•monosomy: missing a chromosome (2n = 45).
-if missing any full chromosome other than X, it is lethal.
-disorders: turner syndrome (XO).
•trisomy: extra chromosome (2n = 47).
-disorders: Down syndrome (21), Edwards syndrome (18), Patau syndrome (13), Klinefelter syndrome (XXY), Supermales (XYY), Trisomy X (XXX).
Genetic testing:
-looks for chromosomal abnormalities.
Genetic testing (amniocentesis):
•prenatal test (16th-20th week).
•a sample of fluid surrounding baby.
Genetic testing (chorionic villi sampling (CVS)):
•tissue taken from chorion/placenta.
•11th-14th week.
Genetic testing (karyotype):
•picture of the chromosomes.
•matched up by:
-size.
-banding pattern.
-centromere location.
