MEIOSIS

3. WHAT IS MEIOSIS?
MEIOSIS IS A TYPE OF CELL DIVISION WHICH
INVOLVES TWO SUCCESSIVE NUCLEAR DIVISIONS
THAT PRODUCE FOUR HAPLOID CELLS. THE FIRST
DIVISION (MEIOSIS I) IS THE REDUCTION
DIVISION; THE SECOND DIVISION (MEIOSIS II)
SEPARATES THE CHROMATIDS.

4. MEIOSIS
THE FORM OF CELL DIVISION BY WHICH GAMETES, WITH HALF THE NUMBER OF
CHROMOSOMES, ARE PRODUCED.
DIPLOID (2N)  HAPLOID (N)
MEIOSIS IS SEXUAL REPRODUCTION.
TWO DIVISIONS (MEIOSIS I AND MEIOSIS II).

5. MEIOSIS
SEX CELLS DIVIDE TO PRODUCE GAMETES (SPERM OR EGG).
GAMETES HAVE HALF THE NUMBER OF CHROMOSOMES.
OCCURS ONLY IN GONADS (TESTES OR OVARIES).
MALE: SPERMATOGENESIS
FEMALE: OOGENESIS
MEIOSIS IS SIMILAR TO MITOSIS WITH SOME CHROMOSOMAL DIFFERENCES.

6. INTERPHASE I
SIMILAR TO MITOSIS INTERPHASE.
CHROMOSOMES REPLICATE (S PHASE).
EACH DUPLICATED CHROMOSOME CONSIST OF TWO IDENTICAL SISTER
CHROMATIDS ATTACHED AT THEIR CENTROMERES.
CENTRIOLE PAIRS ALSO REPLICATE.

7. INTERPHASE I
NUCLEUS AND NUCLEOLUS VISIBLE.
nuclear
membrane
nucleolus
cell membrane
chromatin

8. PROPHASE I
LONGEST AND MOST COMPLEX PHASE (90%).
CHROMOSOMES CONDENSE.
SYNAPSIS OCCURS: HOMOLOGOUS CHROMOSOMES COME TOGETHER TO FORM A
TETRAD.
TETRAD IS TWO CHROMOSOMES OR FOUR CHROMATIDS (SISTER AND
NONSISTER CHROMATIDS).

9. PROPHASE I- SYNAPSIS
Homologous chromosomes
sister chromatids sister chromatids
Tetrad

10. 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. 1 PAIR OF SEX CHROMOSOMES

11. HOMOLOGOUS CHROMOSOMES
Paternal Maternal
eye color
locus
eye color
locus
hair color
locus
hair color
locus

12. CROSSING OVER
CROSSING OVER (VARIATION) MAY OCCUR BETWEEN
NONSISTER CHROMATIDS AT THE CHIASMATA.
CROSSING OVER: SEGMENTS OF NONSISTER
CHROMATIDS BREAK AND REATTACH TO THE OTHER
CHROMATID.
CHIASMATA (CHIASMA) ARE THE SITES OF CROSSING
OVER.

13. CROSSING OVER - VARIATION
nonsister chromatids
chiasmata: site
of crossing over
variation
Tetrad

14. SEX CHROMOSOMES
XX chromosome - female XY chromosome - male

15. PROPHASE I
centrioles
spindle fiber
aster
fibers

16. METAPHASE I
SHORTEST PHASE
TETRADS ALIGN ON THE METAPHASE PLATE.
CHROMOSOMES ALIGN AT THE CENTER OF THE CELL.
INDEPENDENT ASSORTMENT OCCURS:
1. ORIENTATION OF HOMOLOGOUS PAIR TO POLES IS RANDOM.
2. VARIATION
3. FORMULA: 2N
EXAMPLE: 2N = 4
THEN N = 2
THUS 22 = 4 COMBINATIONS

17. METAPHASE I
metaphase plate
OR
metaphase plate

18. ANAPHASE I
HOMOLOGOUS CHROMOSOMES SEPARATE AND MOVE
TOWARDS THE POLES.
SISTER CHROMATIDS REMAIN ATTACHED AT THEIR
CENTROMERES.
CHROMOSOME PAIRS SEPARATE WITH SISTER
CHROMATIDS REMAINING TOGETHER.

19. ANAPHASE I

20. TELOPHASE I & CYTOKINESIS
EACH POLE NOW HAS HAPLOID SET OF
CHROMOSOMES.
CYTOKINESIS OCCURS AND TWO HAPLOID DAUGHTER
CELLS ARE FORMED.
TWO DAUGHTER CELLS ARE FORMED WITH EACH
DAUGHTER CONTAINING ONLY ONE CHROMOSOME OF
THE CHROMOSOME PAIR.

21. TELOPHASE I & CYTOKINESIS

22. MEIOSIS II
NO INTERPHASE II
(OR VERY SHORT - NO MORE DNA REPLICATION)
REMEMBER: MEIOSIS II IS SIMILAR TO MITOSIS
FOUR PHASES:
A. PROPHASE I
B. METAPHASE I
C. ANAPHASE I
D. TELOPHASE I & CYTOKINESIS

23. PROPHASE II
SAME AS PROPHASE IN MITOSIS
DNA DOES NOT REPLICATE.

24. METAPHASE II
SAME AS METAPHASE IN MITOSIS
CHROMOSOMES LINE UP AT THE CENTER OF THE CELL
CHROMOSOMES LINE UP AT THE CENTER OF THE CELL
metaphase plate
metaphase plate

25. ANAPHASE II
SAME AS ANAPHASE IN MITOSIS
SISTER CHROMATIDS SEPARATE
CENTROMERES DIVIDE AND SISTER CHROMATIDS MOVE SEPARATELY TO
EACH POLE.

26. TELOPHASE II & CYTOKINESIS
SAME AS TELOPHASE IN MITOSIS.
NUCLEI FORM.
CYTOKINESIS OCCURS.
CELL DIVISION IS COMPLETE.
REMEMBER: FOUR HAPLOID DAUGHTER CELLS
PRODUCED.
GAMETES = SPERM OR EGG

27. TELOPHASE II & CYTOKINESIS

28. MEIOSIS
2n=4
sex cell
diploid (2n)
n=2
n=2
meiosis I
n=2
n=2
n=2
n=2
sperm
haploid (n)
meiosis II

29. ALL 4 HAPOID CELLS BECOME
SPERM
IN OOGENESIS, ONLY 1 OF THE
HAPLOID CELLS BECOMES AN
EGG, AND THE OTHER 3 ARE
REABSORBED BY THE BODY.

32. MEIOSIS I

33. MEIOSIS II