1. Lecture 7 Mitosis & Meiosis

2. Cell Division Essential for body growth and tissue repair Interphase
G1 phase
Primary cell growth phase
S phase
DNA replication
G2 phase
Microtubule synthesis
Mitosis
Nuclear division
Cytokinesis
Cytoplasmic division

3. Chromosomes: Carriers of Inherited Traits
Chromosomes were first observed by the German embryologist Walther Fleming in 1882
Chromosomes exist in somatic cells as pairs
Homologous chromosomes or homologues
The number of chromosomes varies enormously from species to species
The Australian ant Myrmecia spp. has only 1 pair
Some ferns have more than 500 pairs

4. Human Chromosomes Humans have 46 chromosomes
The 23 pairs of homologous chromosomes can be organized by size
This display is termed a karyotype

5. Basic Terminology Diploid cells have two copies of each chromosome
Replicated chromosomes consist of two sister chromatids
These are held together at the centromere

6. Chromosomes Chromosomes are composed of chromatin
Complex of DNA (~ 40%) and proteins (~ 60%)
A typical human chromosome contains about 140 million nucleotides in its DNA
This is equivalent to
About 5 cm in stretched length
2,000 printed books of 1,000 pages each!
In the cell, however, the DNA is coiled

7. Chromosomes: Packaged DNA
The DNA helix is wrapped around positively-charged proteins, called histones
200 nucleotides of DNA coil around a core of eight histones, forming a nucleosome
The nucleosomes coil into solenoids
Solenoids are then organized into looped domains
The looped domains appear to form rosettes on scaffolds

8. Mitosis The phases of mitosis are: Cytokinesis
Prophase Xm Xf
Condense
Metaphase Xm Xf
Line Up
Anaphase Im If
Separate
Telophase Im If
Divide
Cytokinesis
Cleavage furrow formed in late anaphase by contractile ring
Cytoplasm is pinched into two parts after mitosis ends

9. Early Prophase Asters are seen as chromatin condenses into chromosomes
Early mitotic spindle
Pair of centrioles
Centromere
Aster
Chromosome, consisting of two sister chromatids
Asters are seen as chromatin condenses into chromosomes
PLAY
Prophase
Figure

10. Late Prophase Nucleoli disappear
Fragments of nuclear envelope
Polar microtubules
Kinetochore
Kinetochore microtubule
Spindle pole
Nucleoli disappear
Centriole pairs separate and the mitotic spindle is formed
PLAY
Prometaphase
Figure

11. Metaphase
Chromosomes cluster at the middle of the cell with their centromeres aligned at the exact center, or equator, of the cell
This arrangement of chromosomes along a plane midway between the poles is called the metaphase plate
Metaphase
Metaphase plate
Spindle
PLAY
Metaphase
Figure

12. Anaphase Centromeres of the chromosomes split
Daughter chromosomes
Anaphase
Centromeres of the chromosomes split
Motor proteins in kinetochores pull chromosomes toward poles
PLAY
Anaphase
Figure

13. Telophase and Cytokinesis
New sets of chromosomes extend into chromatin
New nuclear membrane is formed from the rough ER
Nucleoli reappear
Generally cytokinesis completes cell division
Telophase and cytokinesis
Nucleolus forming
Contractile ring at cleavage furrow
Nuclear envelope forming
PLAY
Telophase

14. Controlling the Cell Cycle
G0 is an extended rest period
The eukaryotic cell cycle is controlled by feedback at three checkpoints
Cell growth is assessed at the G1 checkpoint
DNA replication is assessed at the G2 checkpoint
Mitosis is assessed at the M checkpoint
PLAY
Control of the cell cycle

15. Fingers and toes form from these paddle-like hands and feet
Cell Death
Fingers and toes form from these paddle-like hands and feet
During fetal development, many cells are programmed to die
Programmed cell death
Human cells appear to be programmed to undergo only so many cell divisions
About 50 in cell cultures
Only cancer cells can divide endlessly

16. What is Cancer? Cancer is unrestrained cell growth and division
The result is a cluster of cells termed a tumor
Benign tumors
Encapsulated and noninvasive
Malignant tumors
Not encapsulated and invasive
Can undergo metastasis
Leave the tumor and spread throughout the body

17. What is Cancer?
Most cancers result from mutations in growth-regulating genes
There are two general classes of these genes
1. Proto-oncogenes
Encode proteins that stimulate cell division
If mutated, they become oncogenes
2. Tumor-suppressor genes
Encode proteins that inhibit cell division
Cancer can be caused by chemicals, radiation or even some viruses

18. Cancer, cell division, and p53 protein
PLAY
How tumor suppression genes work

19. New molecular therapies for cancer
Receiving the signal to divide
Stopping tumor growth
Stepping on the gas
Passing the signal via a relay switch
Checking that everything is ready
Amplifying the signal
Releasing the “brake”
Potential cancer therapies are being developed to target seven different stages in the cancer process
Stages 1-6
Prevent the start of cancer within cells
Focus on the decision-making process to divide
Stage 7
Act outside cancer cells
Prevents tumors from growing and spreading

20. Importance of Generating Diversity
Diversity is what allows gene populations to survive changes in their environment
The greater the diversity, the more likely some individuals will have the traits to survive a major change
Genetic diversity is the raw material that fuels evolution
No genetic process generates diversity more quickly than sexual reproduction

21. Two Types of Reproduction
Asexual reproduction
Creates a faithful copy
Does not involve fertilization
Contain one set of chromosomes
Sexual reproduction
Maximizes diversity
Involves the alternation of meiosis and fertilization
Meiosis halves the number of chromosomes from each parent cell
Fertilization restores the number of chromosomes
Contains two sets of chromosomes

22. The Sexual Life Cycle
The life cycles of all sexually-reproducing organisms follows the same basic pattern
Haploid cells or organisms alternate with diploid cells or organisms
There are three basic types of sexual life cycles
Spend most of life in haploid form
Spend most of life in diploid form
Spend half of life in haploid form and half in diploid form

23. The Sexual Life Cycle in Humans

24. Discovery of Meiosis
Meiosis was first observed by the Belgian cytologist Pierre-Joseph van Beneden in 1887
Gametes (eggs and sperm) contain half the complement of chromosomes found in other cells
The fusion of gametes is called fertilization or syngamy
It creates the zygote, which contains two copies of each chromosome

25. Meiosis PLAY Unique features of meiosis Mitosis Meiosis I Meiosis II
Prophase Xm Xf
Condense
Metaphase Xm Xf
Line Up
Anaphase Im If
Separate
Telophase Im If
Divide
Mitosis
Meiosis I mX Xf Xm fX Xm fX mX Xf Xm Xf Xf Xm Xf Xm Im If If Im
Meiosis II Xf Xm
PLAY
Unique features of meiosis

26. Meiotic Cell Division: Meiosis I
Figure 27.7
Tetrads line up at the spindle equator during metaphase I
In anaphase I, homologous chromosomes still composed of joined sister chromatids are distributed to opposite ends of the cell
At the end of meiosis I each daughter cell has:
Two copies of either a maternal or paternal chromosome
A 2n amount of DNA and haploid number of chromosomes
In telophase I:
The nuclear membranes re-form around the chromosomal masses
The spindle breaks down
The chromatin reappears, forming two daughter cells
PLAY
Prophase I, Metaphase I, Anaphase I, Telophase I

27. Meiotic Cell Division: Meiosis II
Mirrors mitosis except that chromosomes are not replicated before it begins
Meiosis accomplishes two tasks:
It reduces the chromosome number by half (2n to n)
It introduces genetic variability
PLAY
Meiosis II

28. Differences Between Mitosis & Meiosis
Purpose
Create new cells
(faithful copies)
Create new individuals
(diversity)
Where it occurs
Body cells
Special reproductive organs
Number of divisions after one chromatin replication event
One
Two
How chromosomes
Line up at metaphase
Individually
Homologous pairs in
Meiosis I,
Individually in Meiosis II
Number of daughter cells
Two
Four
Homologous chromosomes in daughter cells
Two (diploid)
One (haploid)
Makeup of daughter cells vs. parent cell
Same as parent cell
Different from parent cell
PLAY
Differences between meiosis &. mitosis

29. Evolutionary Consequences of Sex
Sexual reproduction increases genetic diversity through three key mechanisms
Independent assortment
Crossing over
Random fertilization

30. Independent assortment
Three chromosome pairs
23 combinations
In humans, a gamete receives one homologue of each of the 23 chromosomes
Humans have 23 pairs of chromosomes
223 combinations in an egg or sperm
8,388,608 possible kinds of gametes

31. Crossing over
DNA exchanges between maternal and paternal chromatid pairs
This adds even more recombination to independent assortment that occurs later

32. Random fertilization
The zygote is formed by the union of two independently-produced gametes
Therefore, the possible combinations in an offspring
8,388,608 X 8,388,608 =
70,368,744,177,664
More than 70 trillion!
And this number does not count crossing-over