Lecture 13 - Meiosis I And Meiosis II Flashcards
How many chromosomes do you have?
23 pairs. Total of 46
Diploid
2 complete sets of chromosomes , one from each parent
Haploid
GAMETES… single set of unpaired chromosomes
What are the two functions of meiosis?
- Reduce # of chromosomes in half (46 to 23)
- Introduce genetic diversity (all daughter cells are genetically different from mother cell)
what happens during prophase I?
Synapsis- homologous replicated chromosomes pair up forming tetrads (4 sister chromatids together)
Crossover- exchange of genetic material between homologous chromatids. Place of crossover rims called the chiasmata
What happens during metaphase I?
Tetrads line up randomly at metaphase plate
What happens during anaphase I?
Each tetrad is pulled apart into two replicated chromosomes, each chromosome is still made of two sister chromatids
What happens during telophase I?
Nuclear envelope may or may not reform, depending on the cell.
NO DNA REPLICATION OCCURS AT THIS TIME
Interkinesis
Brief rest period between meiosis I and meiosis II
What is the result of meiosis I?
Each cell contains two copies (sister chromatids) of one member of each homologous pair (either the maternal or paternal chromosome) and none of the other chromosome.
Each cell will contain the haploid chromosomal number (n rather than 2n) because still-united sister chromatids are considered to be one chromosome as long as they are connected by a centromere (twice amount DNA in each chromosome).
Meiosis II
Meiosis II is just like mitosis except that the cells are haploid.
what happens during prophase II?
Chromosomes condense, and a new set of spindle fibres form
What happens during metaphase II?
Chromosomes line themselves along the the metaphase plate
What happens during anaphase II?
Sister chromatids separate and are pulled towards opposite poles of the cell
What happens during telophase II?
Nuclear membranes form around each set of chromosomes, and the chromosomes decondense
Summary of meiosis
During meiosis I, crossing over between maternal and paternal chromosomes result in an exchange of genetic material between members of a tetrad (pairs of homologous chromosomes); each tetrad randomly lines up at the equator, resulting in the independent assortment of single maternal and paternal homologues into two daughter cells.
Meiosis II resembles mitosis except the cells begin as haploid
Mitosis vs meiosis
Mitosis has one round of division; meiosis has two.
Synapsis and crossing over occurs in meiosis but not in mitosis.
Mitosis produces two diploid (2n) daughter cells identical to the mother cell. Meiosis produces four haploid (n) daughter cells that are all different from each other and from the mother cell.
Mitosis produces somatic (body) cells. Meiosis produces gametes (eggs and sperm).
Apoptosis
AKA programmed cell death
Causes certain cells to neatly self destruct
The process begins when mitochondrial membranes leak chemicals that activate enzymes called classes. These enzymes degrade the cell’s DNA and cytoskeleton, which leads to cell death.
The dead cell shrinks and is phagocytized by macrophages (a type of immune cell).
Theories of cell aging
Wear and tear theory: a lifetime of chemical damage and free radicals have cumulative effects.
Mitochondrial theory of aging: free radicals in mitochondria diminish energy production in each cell.
Immune system disorders: autoimmune responses, as well as the progressive weakening of the immune response, lead to a loss of homeostasis.
Genetic theory: the cessation of mitosis and cell aging are programmed into genes.
telomeres are strings of nucleotides that protect ends of chromosomes (like caps on shoelaces).
Every time a cell divides, the telomere shortens, so telomeres may act like an hour-glass on how many times a cell can divide.
Telomerase is an enzyme that lengthens telomeres. It is found in germ cells of embryos but absent in adult cells, except for cancer cells.
