1. Cell Cycle and Cell Division

2. Cell Division
Creates two identical “daughter” cells roughly ½ the size of the parent cell
Before the cell divides:
DNA of the cell (chromosomes) must be copied in interphase
chromosomes must be separated in mitosis
Actual cell division is cytokinesis.

3. Why do cells have to divide
Surface area/volume ratio
Too much demand placed on DNA
Larger the cell, more proteins need to be made!
Only one copy of each DNA molecule in a cell!
Signals from outside the cell

4. Cell Division
In multicellular organisms, the somatic (body cells) cells undergo mitosis.
Germ cells (cells destined to become sperm eggs) divide by a related process called meiosis.
Prokaryotic cells (no nucleus) divide by a process called binary fission.

5. Forms of DNA DNA + Proteins = Chromosomes
Most of the time, DNA is “unspooled” into loose strands – called chromatin
can be used to provide instructions in this form.
Before a cell divides, the DNA winds around histone proteins and becomes visible as “chromosomes”
The chromosomes can be counted in this form.

6. Structure of DNA Nucleosome Chromosome DNA double helix Coils
Supercoils
Histones

7. Chromosome number
Eukaryotic organisms have a specific number of different types of chromosomes.
They have two of each type – so chromosomes come in pairs.
Cells with chromosomes present in pairs are said to be diploid (2N)
one of each is haploid

8. Chromosome identity
Each chromosome type can be identified by shape and size.
If stained, characteristic patterns of bands can be seen as well.
A karyotype is a display of all of an individual’s chromosomes arranged by type.
Can be used to identify major genetic disorders.

10. Chromosome number Each organism has a different number of chromosomes:
Camel Chicken 78
Opossum Housefly 22
Bat Corn 24
Lentil 14 Rice 24
Goat Barley 14
Apple 34 Lettuce 12
HUMANS HAVE 46! 23 different pairs

11. How many chromosomes?
In a diploid cell, one set of chromosomes comes from the mother and another from the father.
Each human gets 23 from each parent (46 total)
44 are autosomes (general body characteristics) and two are sex chromosomes (determine sex and carry general characteristics)

12. Being Diploid
The two corresponding chromosomes are called homologous chromosomes. Homologous chromosomes need not be genetically identical.
For example, a gene for eye color at one locus (location) on the father chromosome may code for green eyes, while the same locus on the mother chromosome may code for brown.

13. Before Cell Division, each chromosome must replicate!
Individual chromosomes replicate and form sister chromatids.
Each sister chromatid is destined for one of the two resulting daughter cells

14. Sister chromatids together are considered one chromosome.
After separation, the two independent copies are sister chromosomes.
Each sister chromosome goes to a different cell.

15. After Replication  Chromosomes

16. The Cell Cycle
Sequence of events a cell goes through as it grows and divides
(G1 Phase) Cell grows and synthesizes proteins and new organelles
(S Phase) Chromosomes replicate
(G2 Phase) Organelles and molecules used in cell division are produced
(M Phase) mitosis (chromosomes) and cytokinesis (cytoplasm) are divided.

17. Cell Division M-Phase Consists of TWO steps (Mitosis and Cytokinesis)
Mitosis  process by which a cell separates its duplicated genome into two identical halves. Mitosis only separates the newly replicated chromosomes; DNA replication does not occur during mitosis.
broken down into five phases: (PMAT)
Prophase, Metaphase, Anaphase, Telophase.
Cytokinesis which divides the cytoplasm and cell membrane.

18. Mitosis  Prophase Longest phase of mitosis
Chromosomes condense (become visible)
Centrioles (in cytoplasm) separate and move to opposite sides of cell
Nuclear membrane breaks-down
Microtubule structure called the spindle develops (attaches from centrioles to chromosomes

19. Chromosome Structure
Prior to separation, the two sister chromatids are attached together in a specialized region of the chromosome known as the centromere.

20. Mitosis  Metaphase
Chromosomes line-up along center of cell (metaphase plate)
Each chromosome is connected to its centromere by a spindle fiber

21. Mitosis  Anaphase
Sister chromatids separate into separate chromosomes
Separated chromosomes pulled to opposite sides

22. Mitosis  Telophase
Chromosomes move together at opposite ends of the cell and become less condensed
Spindle breaks apart
Two new nuclear membrane form
Result is one cell with 2 nuclei!

23. Cytokinesis Remember, NOT part of mitosis Animals Plants
Cell membrane pinches off cytoplasm into two equal parts at a region called the cleavage furrow
Plants
Cell Plate develops between two new nuclei which grows into a separating membrane and ultimately a separating cell wall

24. Mitosis Animation Cell Cycle Animation

25. Limits to Division?
Problem with eukaryotes is that they have to replicate linear chromosomes. The polymerase enzyme can’t work all the way to the end, so the chromosome gets shorter with each round of replication.
Solution: use special ends called telomeres that don’t contain genes. “Expendable” DNA.
When cell runs out of telomere, can’t divide any more.
May act as “replication counter” for cell.

26. Regulation of Cell Cycle
Not all cells move through cell cycle at same rate
Bone marrow cells/skin cells  continuous division
Nerve and muscle cells  seldom or never

27. Cycle Regulators
The cell cycle is regulated by special proteins called cyclins and cyclin-dependent kinases.
High concentrations of cyclin influences a cell to divide.
Internal Regulators  proteins that respond to internal stimuli: cell cycle checkpoints!
Ex. Cell will not enter mitosis until all chromosomes are replicated
External Regulators  proteins that respond to external stimuli
Ex. Cell will begin to divide rapidly after injury
Ex. When dividing cells come in contact with adjacent cells, division will slow

28. WHEN CELLS GO BAD!
Cell loses the ability to control growth; cancer is the result.
Cells do not respond to chemical signals that tell them to stop growing.
Do not differentiate.
Form masses of abnormal cells called tumors that damage surrounding tissues.
May rebuild telomeres and become “immortal”

29. Meiosis
We know that regular somatic (body) cells contain TWO sets of chromosomes (diploid/ 2N)
When a sexually reproducing organism produces gametes (sex cells) they must somehow separate these pairs of chromosomes so gametes only get one set.
WHY?

30. Ex. Humans
Normal Diploid (2N) somatic cell contains _____ chromosomes (_____ pairs)
Gametes (sperm and egg cells) need to contain _________ chromosomes.
We generate these HAPLOID (N) cells through the process of meiosis!

31. Steps of Meiosis Divided into two distinct stages Meiosis I Meiosis II
Starts with one diploid cell and ends with 4 haploid daughter cells
Before meiosis begins, DNA undergoes replication just like in mitosis!

32. Meiosis I: Prophase I
Appearance of the chromosomes, the development of the spindle, and the breakdown of the nuclear membrane (envelope).
Each replicated chromosome pairs up with its corresponding homologous chromosome
Paired chromosomes (4 chromatids) form a tetrad

33. Tetrads and crossing over
It is during this alignment that chromatid arms may overlap and temporarily fuse (chiasmata, or synapsis), resulting in crossovers
Segments of homologous chromosomes may switch places where overlap occurs.

34. What is Crossing Over?
Paired-up homologous chromosomes, may exchange portions of their chromatids
Advantage?

35. Meiosis I: Metaphase I
Here is where the critical difference occurs between Metaphase I in meiosis and metaphase in mitosis. In the latter, all the chromosomes line up on the metaphase plate in no particular order. In Metaphase I, the chromosome pairs are aligned on either side of the metaphase plate.

36. Meiosis I: Anaphase I
During Anaphase I the homologous pairs separate from each other and move along the spindle fibers to each pole of the cell.

37. End of Meiosis I
At the end, each daughter cell has a single set of chromosomes, half the total number in the original cell where the chromosomes were present in pairs.
While the original cell was diploid, the daughter cells are now haploid. This is why Meiosis I is often called reduction division.

38. Meiosis II
Meiosis II is quite simple in that it is simply a mitotic division of each of the haploid cells produced in Meiosis I.
There is no Interphase between Meiosis I and Meiosis II

39. Meiosis II: Prophase II
A new set of spindle fibers forms and the chromosomes begin to move toward the equator of the cell.

40. Meiosis II: Metaphase II
All the chromosomes in the two cells align with the metaphase plate.

41. Meiosis II: Anaphase II
Sister chromatids separate as they are pulled by spindle fibers

42. Meiosis II: Telophase II
A cleavage furrow develops, followed by cytokinesis and the formation of the nuclear membrane (envelope).
When Meiosis II is complete, there will be a total of four daughter cells, each with half the total number of chromosomes as the original cell.

44. Meiosis in Males and Females
In male animals, meiosis results in the formation of 4 ___________ cells
In female animals, meiosis results in the formation of one _______ cell and three small polar bodies which die.

45. Advantages/Disadvantages of sexual reproduction?
Recombination of maternal and paternal chromosomes in the gamete results in genetic variation among the offspring. In an environment which changes, this allows the process of natural selection to occur.