1. 4.2 Mitosis and Meiosis Big Ideas
Mitosis is a process of asexual reproduction that produces genetically identical cells
It distributes the diploid number of chromosomes to daughter cells
Meiosis occurs in sexual reproduction
It distributes an assortment of chromosomes to the gametes

2. The timing and regulation of cell division are critical for normal growth and development
A control system that depends on specific proteins within the cell regulates the sequence of events in the cell cycle
If a cell reproduces at the wrong time or in the wrong place it can form a mass of cells (tumour)
Cancer is a disease caused by the severe disruption of the mechanisms that regulate the cell cycle
Disruption leads to uncontrolled cell division
Cancer is so dangerous because of its ability to spread. A tumour displaces normal tissue as it grows. If the tumour is malignant and is not removed it can metastasize meaning it spreads to other tissues or organs in the body. Many different biochemical changes can affect the cell cycle and result in cancer and that’s why there is not single cure but rather there are multiple approaches to controlling the progress of the disease.

3. Mitosis & Asexual Reproduction
Cell division is essential for living things and is the basis of reproduction for every organism
Single-celled organisms often reproduce by simple cell division in which a single cell or group of cells splits into two new genetically identical cells
The two new cells rapidly grow and develop into organisms that are genetic clones of one another
Called asexual reproduction: produces offspring that inherit all their genetic material from one parent

4. Cells of the body are known as somatic cells
Asexual reproduction also enables multicellular organisms to grow and to repair and replace damaged cells
Cells of the body are known as somatic cells
How often they divide depends on the type of cell and its role in the body
When a single-celled organism or somatic cell divides it must go through a precise sequence of steps in order to create daughter cells that are genetically identical to the original parent cell
Some specialized cells never divide, while others divide daily

5. Phases of Mitosis – pages 106/107
After interphase, the cell's DNA has been replicated and the M phase begin
Mitosis distributes the duplicated sets of chromosomes to two daughter nuclei and cytokinesis divides the nucleus producing two daughter cells
Mitosis involves 4 phases and how long mitosis takes depends on the cell (minutes to days)

6. During interphase the cell replicates its DNA and makes other new molecules and organelles
Chromosomes are not yet visible because they are still loosely packed fibres of chromatin
The presence of a nucleolus (a dark-staining region on the nucleus) indicates that a cell is actively making proteins

7. 1. Prophase:
The sister chromatids become visible and in animal cells, the centrioles separate and move to opposite sides of the nucleus
The sister chromatids attach to the long fibres of the spindle
By the end of the stage, the nucleolus disappears and the nuclear envelope breaks down

8. 2. Metaphase:
The sister chromatids are fully attached to the spindle fibres
The spindle fibres move the chromatids and line them up across the middle of the cell
3. Anaphase:
The sister chromatids separate from their partners to become individual chromosomes
The spindle fibres pull the chromosomes to opposite poles/ends of the cell

9. Chromosomes begin to uncoil and take the form of chromatin
4. Telophase:
Chromosomes begin to uncoil and take the form of chromatin
Spindle fibres disappear and nuclear envelopes re-form around each set of daughter chromosomes
At the end of this stage mitosis is complete
Cytokinesis completes cell division by dividing the cytoplasm into two daughter cells each with its own nucleus
Occurs at the same time as telophase
Mitosis video: 6 min

10. Meiosis & Sexual Reproduction
Sexual reproduction: the process in which two parents provide genetic material in order to produce offspring
With sexual reproduction some of the genetic material from each parent combines producing offspring that differ genetically from both parents
Each of the offspring inherits a unique combination of traits

11. In animals, meiosis occurs in the sex organs
Meiosis: is a form of cell division that produces four cells, each containing half the number of chromosomes as the parent cell
Sexual reproduction depends on this specialized type of cell division
In animals, meiosis occurs in the sex organs
Testes in males and ovaries in females
Almost all cells in an individual organism have the same number and types of chromosomes
Human somatic cells have _______________ of chromosomes for a total of _______ chromosomes
23 pairs, 46

12. Karyotype: a display of all the chromosomes in a cell or individual
Each chromosome has a counterpart that resembles its size and shape
One chromosome of each pair is inherited from each of your parents
The two chromosomes of each matching pair are called homologous chromosomes
Each member of a pair carries the same series of genes controlling the same inherited traits

13. The total number of chromosomes is called the diploid number
Most human cells are diploid meaning they contain two sets of chromosomes
The total number of chromosomes is called the diploid number
Represented as 2n, in humans 2n=____
An exception to diploid cells are the sex cells (sperm and eggs = gametes) which contain only a single set of chromosomes, one from each homologous pair
A cell with a single set of chromosomes is called a haploid number
Represented as n, in humans n= _____

14. The haploid gametes are produced through the process of meiosis
In the life cycle of sexually reproducing organisms, the nucleus of a haploid sperm from the father fuses with the nucleus of a haploid egg cell from the mother
The fusion of the two gametes is called fertilization and results in a zygote which is diploid because it contains two pairs of homologous chromosomes, one from each parent (2 x n = 2n)

15. Autosomes: all other chromosomes
Therefore, meiosis enables the zygote to have the correct number of chromosomes
Eventually the diploid zygote will develop into a sexually mature adult with trillions of cells produced by mitosis!
Sex chromosomes: one set of chromosomes that determine an individuals sex
Autosomes: all other chromosomes
In humans, the 23rd pair of chromosomes are the sex chromosomes
Go back to karyotype on slide 13
Females have two X chromosomes, males have one X and one Y chromosome, an egg cell contains one X chromosomes and a sperm cell contains either an X or a Y chromosome

16. Phases of Meiosis – pages 110/111
Many phases of meiosis are similar to those of mitosis
As with mitosis, DNA replicates before meiosis begins
Unlike mitosis, this single round of DNA replication is followed by two distinct cell division stages: meiosis I and meiosis II
By the end of meiosis II, the original diploid cell has become four haploid cells

17. Meiosis I – Homologous Chromosomes Separate 1. Prophase I:
Begins with two complete sets of chromosomes in the form of sister chromatids
Each duplicated chromosome pairs with its corresponding homologous chromosome forming a tetrad
The homologous chromosomes then exchange portions of DNA
Genetic Recombination: the exchange of DNA segments

18. 2. Metaphase I:
The tetrads move to the middle of the cell and line up
3. Anaphase I:
Homologous chromosomes separate from the tetrad and migrate to opposite poles of the spindle
Sister chromatids remain attached at their centromeres

19. The nuclear membrane re-forms around each cluster of chromosomes
4. Telophase I:
The nuclear membrane re-forms around each cluster of chromosomes
Chromosomes of each daughter cell are present in duplicates as sister chromatids
Each cell is considered haploid even though they contain the diploid number of chromosomes, because they only contain half the chromosomes but two of each
The cells must go through a second division in order to have the correct number of chromosomes
Draw on board

20. Meiosis II – Sister Chromatids Separate (in each cell)
1. Prophase II:
Not preceded by a round of DNA replication***
In each haploid daughter cell a spindle forms and attaches to the centromeres of the sister chromatids
2. Metaphase II:
The spindle attached to the centromeres moves the sister chromatids so that they line up in the middle of the cell

21. 3. Anaphase II:
The sister chromatids separate and move to opposite poles of the cell
Once separated they are considered individual chromosomes
4. Telophase II:
Chromosomes begin to uncoil and take the form of chromatin
Spindle fibres disappear and nuclear envelopes re-form around each set of daughter chromosomes
Cytokinesis splits the cells one more time, resulting in four haploid cells

22. New Gene Combinations
Offspring that result from sexual reproduction are genetically different from their parents and from one another
The genetic variety in offspring arises through the process of meiosis and fertilization
Each parent contributes a unique combination of genes to the offspring and this set of genes codes for a unique combination of traits
Mitosis and Meiosis video: 12 min
Mitosis 0-5:50 min

23. The way in which chromosomes assort or distribute during meiosis contributes to variation in the gametes
How the chromosomes in each homologous pair (tetrad) line up and separate in metaphase I occurs by chance
Therefore, the assortment of chromosomes in the resulting gametes is random
Called random assortment

24. In addition to the random assortment of chromosomes, in prophase I there is an exchange of genetic material between homologous chromosomes
Called crossing over
When crossing over takes place, homologous chromosomes are closely paired along their lengths
Each gene on one pair of sister chromatids is precisely lined up with the corresponding gene on the adjacent pair of sister chromatids

25. This close association between homologous chromosomes in early meiosis is called synapsis
At regions called chiasmata, chromatids from different pairs are attached and segments of the chromatids are exchanged
Genetic recombination results in chromosomes with new combination of genetic information different from those carried by the original chromosomes

26. One single cross over event can affect many genes on a chromosome
Without this process, every chromosome produced by meiosis would be purely of maternal origin or purely of paternal origin
One single cross over event can affect many genes on a chromosome
Since more than one cross over event can occur in each tetrad, it is not surprising that the resulting gametes and offspring can be so varied
Homework: page 115 Mitosis #1,2,14
Meiosis #3-7,9-13,15,16 STUDY PAGE 113