1. Meiosis Notes

2. 1st…. A Review of Chromosomes

3. Chromosome Numbers
In a human, a body cell contains 46 chromosomes.

4. Chromosomes come in homologous pairs, thus genes come in pairs.
Homologous pairs – matching genes – one from female parent and one from male parent
When homologous chromosomes are paired together, this is referred to as a tetrad.
Example: Humans have 46 chromosomes or 23 pairs.
One set from dad
23 in sperm
One set from mom
23 in egg
Chromosome Numbers

5. Chromosome Numbers
Cells with both sets of homologous chromosomes are called diploid. These cells are represented by 2N. In humans, the diploid number is 46, so 2N=46.
In sexually reproducing organisms, the gametes (sex cells – sperm and egg) contain one set of chromosomes. They are called haploid, and are represented by the symbol N. In humans, the haploid is 23, so N=23.

6. Match the Information! 23 CHROMOSOMES GAMETES N SEX CELLS 2N
HAPLOID
DIPLOID N
SEX CELLS 2N
SOMATIC CELLS
BODY CELLS
YOU NEED TO GET OUT OF PPT SLIDE SHOW TO MOVE THE WORDS. OR YOU CAN TRANSFER IT INTO SMARTBOARD.
46 CHROMOSOMES
SEXUAL REPRODUCTION
ASEXUAL REPRODUCTION

7. MEIOSIS

8. PHASES OF MEIOSIS
Meiosis is a process of reduction division where the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell to produce 4 haploid cells.
Meiosis is divided into two process
Meiosis I
Meiosis II

9. PHASES OF MEIOSIS I During interphase, DNA and organelles are copied.
PRE-MEIOSIS - INTERPHASE
During interphase, DNA and organelles are copied.
46 chromosomes  92 chromosomes (diploid)
Don’t forget….centrioles are also copied!

10. PHASES OF MEIOSIS I PROPHASE I
Spindle fibers appear; nuclear membrane disappears.
Homologous chromosomes are paired, forming a tetrad
Each tetrad contains 4 sister chromatids
Crossing over occurs. Portions of chromatids are exchanged, contributing to genetic variation in offspring

11. CROSSING OVER
As homologous chromosomes pair up and form tetrads in Meiosis I, they actually exchange portions of their chromatids.
Crossing over is the reason for genetic diversity! It produces new combinations of genes.

12. PHASES OF MEIOSIS I
METAPHASE I
Tetrads/homologous chromosomes line up in the middle of the cell
Spindle fibers attach

13. PHASES OF MEIOSIS I
ANAPHASE I
Spindle fibers pull the homologous chromosomes toward opposite ends of the cell

14. PHASES OF MEIOSIS I Cytokinesis occurs; two new nuclear envelopes form
TELOPHASE I
Cytokinesis occurs; two new nuclear envelopes form
End result is two daughter cells, each with 46 chromosomes

15. PHASES OF MEIOSIS II
PROPHASE II
Spindle fibers appear; nuclear envelope disappears.
Question of the day: why is there not another interphase?

16. PHASES OF MEIOSIS II Chromosomes line up in the middle of the cell
METAPHASE II
Chromosomes line up in the middle of the cell

17. PHASES OF MEIOSIS II
ANAPHASE II
Sister chromatids separate and move toward opposite ends of the cell

18. PHASES OF MEIOSIS II
TELOPHASE II
Cytokinesis occurs; four new nuclear envelopes form
End result is four daughter cells, each with 23 haploid chromosomes

19. PHASES OF MEIOSIS II - CYTOKINESIS
MALES
FEMALES
In males, cytokinesis makes four equal cells that develop heads and tails – sperm – called spermatogenesis
FEMALES
In females, cytokinesis is unequal; one cell gets most of the cytoplasm to become the mature egg, while the other three usually die and are known as polar bodies. Called oogenesis.
Egg and Sperm are called gametes.

21. Chromosome Separation Problems
Failure of the homologous chromosomes to
separate normally during meiosis -- nondisjunction
1. missing a chromosome – monosomy (45 chromosomes)
2. having an extra chromosome – trisomy ( chromosomes)
3. most embryos fail to survive, but some do

22. (47 chromosomes) – Down Syndrome
Short, round face, upper eyelids cover the inner part of the eye, mental retardation – trisomy three #21 chromosomes
(47 chromosomes) – Down Syndrome

23. Male with longer-than-average limbs, sterile – XXY
Male with longer-than-average limbs, sterile – XXY Klinefelter’s Syndrome

24. Female with short stature, webbed neck, sterile – XO Turner’s Syndrome

25. deletion / translocation / duplication / inversion
Parts of chromosomes break off and reattach in different ways, occurs in meiosis:
deletion / translocation / duplication / inversion
Deletion
Duplication
Inversion
Translocation

26. Detecting chromosome mutations:
1. Picture of individual’s chromosomes – karyotype
2. Amniotic fluid surrounding an embryo is removed for analysis (done 3½ to 4 months of pregnancy) – amniocentesis
3. Analysis of chorionic villi which grows between mother’s uterus and placenta (done 2
months of pregnancy – chorionic villi sampling (CVS)

29. COMPARISONS! MEIOSIS MITOSIS Occurs in sex cells ONLY
During first meiotic division, crossing over occurs
During second meiotic division, chromosome duplication does not occur
Produces four cells, each with half a set of complete chromosomes
Cells are haploid (N = 23)
Cells are GENETICALLY DIFFERENT from each other
Occurs in ALL body cells
Chromosomes are duplicated
Chromosomes are pulled apart by spindle fibers
An identical, complete set of chromosomes moves towards each side of cell
Cytokinesis
End product are two IDENTICAL cells
Daughter cells are diploid
2N = 46