1. Cell Replication
Mitosis & Meiosis

2. Cell Division Somatic cell production Male adult 2N Female adult
Many mitosis
divisions
Male adult 2N
Female adult
Female embryo 2N
Male embryo
Meiosis
A single set of chromosomes
Egg 1N
Sperm
Several mitotic
divisions
Somatic cell production
A double set of chromosomes
Embryo 2N
Many mitotic
divisions
Somatic cell production
Adult 2N
Gamete production
Fertilization
Zygote 2N

3. Mitosis in Onion Cells
If the cells of a growing root tip are examined, a proportion of them are in mitosis. Cells in different stages of division can be seen.
Mitosis is the process by which the cell produces two new daughter cells from the original parent cell.
Prophase
Anaphase
Late anaphase
Telophase

4. Cell division for somatic growth and repair.
Mitosis Micrographs
Cell division for somatic growth and repair.
1. Interphase
2. Prophase
3. Metaphase
5. Telophase
4. Anaphase

5. Interphase Interphase is the first step of the process
Cells prepare for Mitosis by accumulating materials required
At this point, individual chromosomes cannot be seen
This is the longest phase in which the DNA replicates
1. Interphase

6. Interphase
Chromosomes need to be duplicated into double stranded chromosomes, so each daughter cell can receive an identical copy

7. Prophase
During Prophase, the chromosomes begin to condense, and become visible under a microscope
Eventually they can be seen as individual chromosomes
The nuclear membrane breaks down
Centrioles appear at opposite ends of the cell
2. Prophase

8. Metaphase
During Metaphase, the chromosomes line up through the middle of the cell
Spindle fibres extend from the centrioles, and meet up with the chromosomes in the middle
The ends of the spindle fibres attach to the chromosomes
3. Metaphase

9. Anaphase
During Anaphase, the spindle fibres begin to contract, pulling the chromosomes. They split apart at the centromere, separating them into 2 chromatids
These chromatids are now single stranded chromosomes
The spindle fibres draw the chromosomes to either end of the cell, each side having the exact copies
4. Anaphase

10. Telophase
During Telophase, the opposite occurs to prophase. The chromosomes begin to expand.
The chromosomes can no longer be seen individually
A nuclear membrane begins to form around each set of chromosomes
The spindle dismantles and disappears
5. Telophase

11. Cytokinisis
Cytokinesis is not part of Mitosis, although it starts occurring during Anaphase or Telophase
It is the separation of the two daughter cells
The membrane pinches inwards and a cleavage furrow appears
The membrane is formed and 2 daughter cells are born!!

12. Cell Division haploid (1N) diploid (2N) Somatic cell production
Many mitosis
divisions
Male adult 2N
Female adult
Female embryo 2N
Male embryo
Meiosis
A single set of chromosomes
Egg 1N
Sperm
Several mitotic
divisions
Somatic cell production
A double set of chromosomes
Embryo 2N
Many mitotic
divisions
Somatic cell production
Adult 2N
Gamete production
Fertilization
Zygote 2N
haploid (1N)
diploid (2N) 12

13. Crossing over may occur at this stage in meiosis
Intermediate cell 1N
First Division
(Reduction division)
The purpose of meiosis is to produce haploid sex cells. Haploid sex cells have only one copy of each homologous pair of autosomes plus one sex chromosome.
Second Division
('Mitotic' division) 1N
Gametes
(eggs or sperm) 1N

14. 2N
Interphase
DNA replication
Meiosis I 2N
Prophase 1
Synapsis and crossing over
The first division of meiosis is called a ‘reduction’ division because it reduces (halves) the number of chromosomes. One chromosome from each homologous pair is donated to each intermediate cell. 2N
Metaphase 1
Bivalents line up on the equator
Anaphase 1
Telophase 1 1N
Intermediate cell

15. NOTE: Half the full chromosome complement shown
Intermediate cell
NOTE: Half the full chromosome complement shown
Meiosis II 1N
Prophase 2
The second division of meiosis is called a ‘mitotic’ division, because it is similar to mitosis. Sister chomatids of each chromosome are pulled apart and are donated to each gamete cell.
Metaphase 2
Anaphase 2 1N
Gamete (egg or sperm)
Telophase 2

16. Mitosis and Meiosis Compared
Homologous chromosomes pair up at the equatorial plate
Homologous chromosomes do not pair up at the equatorial plate
Reduction division
Meiosis I
Cell division
‘Mitotic’ division
Meiosis II
Cell division 1N 2N

17. Non-disjunction in Meiosis I
The meiotic spindle normally distributes chromosomes to daughter cells without error. Sometimes, homologous chromosomes or sister chromatids fail to separate. In these cases, one gamete receives two of the same type of gamete and the other gamete receives no copy. This mishap, called non-disjunction, results in abnormal numbers of chromosomes in the gametes. This example shows non-disjunction in meiosis I; homologous chromosomes fail to separate properly at anaphase during meiosis I.
Meiosis II
Non-disjunction in Meiosis I
n+1
n–1

18. Non-disjunction in Meiosis II
Non-disjunction can also occur in meiosis II, when sister chromatids fail to separate during anaphase of meiosis II.
In cases of non-disjunction, if either of the aberrant gametes unites with a normal one at fertilization, the offspring will have an abnormal chromosome number (e.g. 2N+1 or 2N–1).
This is known as aneuploidy and it accounts for chromosomal defects such as Down syndrome (trisomy 21) and Edward syndrome (trisomy 18).
Meiosis II
Non-disjunction in Meiosis II
n+1
n–1 n

19. Overview Mitosis Meiosis
Cells undergo mitotic division during which time the genetic material is doubled and divided into two cells.
Meiosis
Meiosis is a reduction division that results in the formation of haploid (N) cells from diploid (2N) ones.
Its purpose is to produce gametes for sexual reproduction.
During meiosis, genetic material is exchanged between chromosomes; this introduces genetic variation into the offspring.

20. Helpful Hints to remember
Cell division takes place for the growth, development, repair and replacement of tissues
Mitosis replaces cells and meiosis produces gametes for reproduction
Males gamete = sperm formed in the testes and female gamete = ovum formed in the ovaries
Haploid the daughter cells have half the number of chromosomes of the parent cell.
Diploid have double, in other words the daughter and parent cells have the same number of chromosomes
Each human body cell has 23 pairs, or 46 chromosomes. Sperm and ova have only 23 chromosomes

21. Helpful Hints to remember MITOSIS
I Inter = Prepare/replicate DNA
P Pro = Condense
M Meta = Line up
A Ana = Split
T Telo = expand/2 new cells/diploid