1. Cell Division (Continued) – Meiosis
Chromosomes are DNA!
Chromosomes contain genetic information

2. Cell Division – Mitosis (Review)
– Division of a somatic cell that results in 2 genetically identical daughter cells
Cells must divide for growth, repair of tissues, and asexual reproduction
Cell division begins in Interphase when the chromosomes duplicate

3. Mitosis occurs in somatic or body cells
Chromosomes duplicate
Parent cell
2 new daughter cells identical to parent cell
Daughter cells are genetically identical to parent cell – same kind and number of chromosomes
Mitosis occurs in somatic or body cells
Ex: liver, heart, skin, stomach
Every organism has its own unique number of chromosomes. Humans have 46.
This is called its diploid number or the total number of chromosomes in a somatic cell.
Diploid means “2 sets” and is written as “2N”.

4. Body cells of adult organisms have 2 sets of homologous (matching) chromosomes – 1 set from female parent and 1 set from male parent

5. Cell Division –Meiosis
– the process in which the number of chromosomes in the original cell is reduced by HALF through the separation of homologous chromosomes
Meiosis occurs in sex organs only
Males (XY) – sex organs are the testes in humans
Females (XX) – sex organs are the ovaries in humans
Meiosis also occurs in the sex organs of other animals, plants, fungi, etc…

6. Meiosis
Mitosis

7. Meiosis Division 2 Division 1 Chromosomes duplicate
Daughter cells have half as many chromosomes as parent cell
Parent cell

8. Meiosis produces sex cells – cells with ½ the number of chromosomes as the original cell
Males – meiosis produces 4 sperm
Females – meiosis produces 1 (viable) egg
The other 3 cells are called polar bodies – they give up their cytoplasm to nourish the 1 good egg.
Egg and sperm (sex cells) are also called gametes

9. Gametes have ½ the number of chromosomes as somatic (body) cells
Gametes have ½ the number of chromosomes as somatic (body) cells. We call this the haploid number. Haploid means “1 set” and is written as “N”.
If human diploid number is 46, what is its haploid number? 23
Diploid # of a dog – 78 Haploid # of a dog – 39
Diploid # of a fly – 8 Haploid # of a fly – 4

10. When does meiosis occur in humans?
Males beginning at puberty
Females before birth – all eggs are produced before birth and at puberty eggs mature

11. Chromosome Number
Remember, chromosome number is unique to each kind of organism and all cells (except sex cells) in an organism have the same kind and number of chromosomes.
Ex: All humans have 46 chromosomes and all cells in the human body (except sperm and egg) have 46 chromosomes.
This is why the chromosome number in sex cells must be reduced in half by meiosis
Ex: Humans have 46 chromosomes in their somatic cells, but 23 chromosomes in their sex cells
(egg and sperm)

12. WRONG!!!

13. Zygote develops into embryo and finally adult organism by mitosis23 46
Fertilized egg – zygote
Fertilization – process by which an egg and sperm unite
Zygote – fertilized egg
Embryo – organism in early stage of development

14. Fertilized egg – zygote
Without meiosis …………
Fertilized egg – zygote 46 92

15. Unique events in Meiosis
Homologous (matching) chromosomes pair up before 1st cell division
Homologous chromosomes:
-look alike
-code for same traits
-receive one from each parent

16. During 1st division, homologous chromosomes exchange genes during process called “crossing over”
These homologous chromosomes separate during 2nd division of meiosis – so chromosomes in gametes are different from each other due to crossing over
Crossing over increases genetic variation and is the reason why siblings look different

17. No crossing over – daughter cells are identical to parent cells
Crossing over occurs –causes genetic variation
(Daughter cells are NOT identical to parent cell)

18. Comparing Mitosis and Meiosis

19. Unique features of Meiosis
Comparison of Meiosis and Mitosis

21. Female (XX) = before birth # of Divisions (Draw picture) 1 2
Mitosis
Meiosis
What kind of cells?
Somatic cells
Male (XY) = Sperm
Female (XX) = Egg
When does this occur?
Any time
Male (XY) = puberty
Female (XX) = before birth
# of Divisions
(Draw picture) 1 2
# of Daughter cells
Male (XY) = 4 sperm
Female (XX) = 1 viable egg
# of Chromosomes
Same as parent cell
diploid or 2N
In humans 46
Half as many as parent cell
haploid or N
In humans 23
Type of Reproduction
Asexual
Sexual
Genetic Composition
Daughter cells
identical / not identical
to parent cell
Genetic variation
Pairing of Homologous Chromosomes
YES / NO
Crossing over of genes
Function/Importance
Growth, repair; development of adult from zygote
Production of gametes:
eggs and sperm
Sex Cells