Meiosis is a kind of cell division that leads to the production of gametes.
gametes: egg cells and sperm cells
contain half the number of chromosomes of an adult body cell
Adult body cells (somatic cells) are diploid, containing 2 sets of chromosomes.
Gametes are haploid, containing only 1 set of chromosomes.
Sexual reproduction includes the fusion of gametes
(fertilization) to produce a diploid zygote.
Life cycles of sexually reproducing organisms involve the
alternation of haploid and diploid stages.
Some life cycles include longer diploid phases, some
include longer haploid phases.
6. Features of Meiosis
Meiosis includes two rounds of division – meiosis I and meiosis II.
During meiosis I, homologous chromosomes (homologues) become closely associated with
each other. This is synapsis.
Proteins between the homologues hold them in a synaptonemal complex.
8. Features of Meiosis
Crossing over: genetic recombination between non-sister chromatids
physical exchange of regions of the chromatids
chiasmata: sites of crossing over
The homologues are separated from each other in anaphase I.
9. Features of Meiosis
Meiosis involves two successive cell divisions with no replication of genetic material
This results in a reduction of the chromosome number from diploid to haploid.
11. Meiosis I (four phases)
Cell division that reduces the chromosome number by one-half.
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
12. Prophase I
Longest and most complex phase (90%).
DNA condenses to form chromosomes.
Duplicated chromatids are joint together at centromere
Synapsis occurs: homologous chromosomes come together to form a tetrad and exchange
chromosomal matter called recombination or crossing over
Nuclear membrane begins to breakdown.
Centrosomes migrate to opposite ends of the cell
Microtubules appear and attach to chromosomes
14. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) that are similar in shape and size.
Homologous pairs (tetrads) carry genes controlling the same inherited traits.
Each locus (position of a gene) is in the same position on homologues.
Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 01 pair of sex chromosomes
19. Metaphase I
Tetrads align on the metaphase plate.
INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
3. Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations
20. Metaphase I
terminal chiasmata hold homologues together following
microtubules from opposite poles attach to each
homologue, not each sister chromatid
homologues are aligned at the metaphase plate side-by-
the orientation of each pair of homologues on the spindle
24. Telophase I
Each pole now has haploid set of chromosomes.
nuclear envelopes form around each set of chromosomes
sister chromatids are no longer identical because of crossing over
Cytokinesis occurs and two haploid daughter cells are formed.
26. Meiosis II
No interphase II
(or very short - no more DNA replication)
Meiosis II is similar to mitosis
27. Prophase II
Again chromosomes condense
Nuclear envelope dissolves and spindle apparatus form
Difference between prophase 1 and 2 is that daughter cells have only one copy of each
28. Metaphase II
Chromosomes align on the metaphase plate or the equator of the cell.
Since chromatids are no longer identical, there will be many possible ways to align.
metaphase platemetaphase plate
30. Telophase II
Nuclear membrane reforms and cytoplasm is divided into two haploid daughter cells
Four haploid daughter cells are produced
gametes = sperm or egg
38. Meiosis vs Mitosis
Meiosis produces haploid cells that are not identical to each other.
Genetic differences in these cells arise from:
-random alignment of homologues in metaphase I (independent assortment)
Mitosis produces 2 cells identical to each other.