1. MITOSIS

2. The Cell Nucleus
Eukaryotic cells (plant, animal, fungal, and protistan cells) are characterized by having a membrane-bound cell nucleus.
The nucleus is a large organelle and contains most of a cell’s genetic material.
Eukaryotic cells, such as the animal cell (above), contain membrane-bound organelles, including a membrane-bound nucleus.
The nucleus (right), contains the cell’s genetic material.
The genetic material of a cell in interphase appears as dark threads

3. Structure of the Nucleus
The nucleus comprises:
The nucleoplasm: a highly viscous liquid, similar to cytoplasm, which surrounds the chromosomes and nucleolus.
Almost all the cell’s genetic material.
A double layered nuclear membrane, which isolates and protects the DNA from molecules that could damage its structure or interfere with function.
The nucleolus
Almost all DNA replication and RNA synthesis occur in the nucleus.
Genetic material (chromatin)
Nucleolus
Double layered nuclear membrane perforated with nuclear pores

4. Introduction to DNA
The majority of genetic material in eukaryotic cells is housed within the nucleus.
Some is also found in the mitochondria, and the chloroplasts of plant cells.
In eukaryotic cells, the genetic material is deoxyribonucleic acid (DNA).
DNA is packaged as long strands called chromosomes within the nucleus.
DNA is often referred to as the unit of inheritance.
It provides all the information required for a cell to reproduce and construct a new organism.

5. Chromatin: DNA molecules wrapped around histone protein.
The Genetic Material
When cells are not dividing, the chromosomal DNA is dispersed within the nucleus as fiber-like chromatin.
Chromatin is made up of:
a cell’s DNA
proteins (mainly histone)
Chromatin is packed in a way that allows a large amount of genetic material to be organized in a compact way in the nucleus.
If the DNA from one human cell was stretched out it would stretch to approximately 1 m long.
Chromatin: DNA molecules wrapped around histone protein.
DNA
Histone protein

6. Chromosomes
A visible change in nuclear structure, called chromosome condensation occurs in preparation for cell divsion.
The interphase chromatin, condenses to form compact chromosomes, which are easily seen using microscopy.
Diagrammatic representation of a chromosome.
Scanning electron microscope (SEM) view of sex chromosomes in the condensed state during a cell division. The smaller chromosome is the ‘Y’ while the larger one is the ‘X’ chromosome. X Y

7. Cell Division
Cell division is the process where a parent cell divides into two daughter cells.
There are two types of cell division:
Mitosis occurs in somatic cells.
Meiosis occurs in the sex organs and produces sex cells (gametes).
The examination of a root tip of an onion plant (left) shows a proportion of the cells are undergoing mitosis (some indicated with arrows).
Meiosis (meiotic division) produces sex cells or gametes, sperm and ovum (above).
Sperm
Ovum (egg)

8. The Centrosome
All eukaryotic cells contain a centrosome, also called the microtubular organizing center. It has a central role in cell division.
Within an centrosome of animal and algal cells, there is a pair of centrioles.
During cell division, the centrosome divides and the centrioles replicate, producing two centrosomes, each with its own pair of centrioles.
The two centrosomes move to opposite ends of the nucleus.
Each centrosome produces microtubules. These form the spindle responsible for separating the replicated chromosomes into two daughter cells.
Plant cells have centrosomes, with a similar role to those in animal cells, but they lack centrioles.
Each centriole (cross section above) is made up of a ring of nine groups of microtubules.

9. Introduction to Mitosis
During mitosis, an existing parent cell divides into two new daughter cells (right).
The cells are genetically identical.
There is no change in chromosomal number.
Cells are diploid, containing two sets of chromosomes.
In humans the diploid number is 46
Mitosis is associated with the growth and repair of somatic cells in the body.
Normal male karyotype
Humans have 23 pairs of chromosomes, 22 pairs of autosomes and 1 pair of sex chromosomes.
The karyotype on the right is for a normal male. The sex chromosomes (XY in this example) are highlighted.

10. Mitosis and the Cell Cycle
Mitosis is just one phase of the cell cycle.
There are three main phases in the cell cycle:
Interphase (itself comprising three stages)
Mitosis (nuclear division)
Cytokinesis (division of the cytoplasm)
The cells in this section are in various stages of the cell cycle. In a dividing cell, the mitotic phase phase alternates with an interphase, or growth period.
The cell cycle
Interphase
Mitosis C
Cytokinesis

11. Interphase G2
Interphase accounts for 90% of the cell cycle. (longest phase of cell cycle.
Interphase consists of three stages:
First gap phase (G1) The cell grows and develops
Synthesis (S) The cell duplicates its genetic material (chromosomes).
Second gap phase (G2)
The nucleus is well defined
The chromosomes condense into chromatids in preparation for division
The centrosome is replicated
Replication of organelles S M
The cell cycle C G1
Nucleolus
Centrosome is replicated
Chromatid
Nuclear membrane

12. Mitosis The mitotic cycle is broken down into six phases.
The example below is for a plant cell.
Early Prophase
Late Prophase
Metaphase
Anaphase
Telophase
Late Anaphase

13. Mitosis: Early Prophase
Nuclear membrane
Nuclear membrane disintegrates
Prophase is the first stage of mitosis. In early prophase:
the nuclear membrane disintegrates
the nucleolus disappears
the chromatin condenses into visible chromosomes.
Replicated centrosomes
Nucleolus disappears
The chromatids condense into chromosomes

14. Mitosis: Prophase In late prophase:
Centromere and kinetochore
Centrosome
In late prophase:
the chromosomes continue to coil and appear as double chromatids.
Each chromosome is a pair of sister chromatids, one a copy of the other (copying of DNA occurred in S phase)
the sister chromatids are joined by a centromere.
the centrosomes move to opposite poles (ends) of the cell. As they do so, they form the mitotic spindle between the poles.
the kinetochores mature and attach to the spindle.
Chromatids
A newt lung cell in late prophase (stained with fluorescent dyes). The mitotic spindles appear green and the nucleus appears blue.

15. Mitosis: Metaphase
During metaphase the chromosomes become aligned at the equator of the cell.
Kinetochores attach the chromosomes to the spindle and align them along the metaphase plate at the equator of the cell.
The metaphase plate is an imaginary plane equidistant between the two poles.
Kinetochores are disc like structures to which the spindle fibers attach.
The spindle fibers are made up of microtubules and associated proteins, joined at the ends (the spindle poles).
Mitotic spindle
Chromosomes

16. Mitosis: Early Anaphase
In anaphase, the chromosomes are pulled to opposite poles of the cell.
the centromeres divide, freeing the two sister chromatids from each other.
Each chromatid is now considered to be a chromosome.
The spindle fibers begin moving the once-joined sisters to opposite poles of the cell.
Chromosomes
Spindle
Anaphase is the shortest mitotic phase

17. Mitosis: Late Anaphase
By late anaphase, the chromosomes have moved to opposite poles.
The kinetochore microtubules shorten as the chromosomes approach the poles.
The cell starts to elongate
By the end of anaphase, the two poles of the cell have a complete collection of chromosomes.
Centrosome
Mitotic spindle
Chromosomes

18. Mitosis: Telophase
Telophase is characterized by the formation of two new nuclei.
The cell continues to elongate.
The daughter nuclei begin to form at the two poles of the cell where the chromosomes have gathered.
The nucleoli reappear and the chromatin becomes less tightly coiled (less condensed).
In plant cells, the cell plate forms where the new cell wall will form.

19. Cytokinesis The division of the cytoplasm is termed cytokinesis.
Mitosis & Cell Cycle
Cytokinesis
Cell wall
The division of the cytoplasm is termed cytokinesis.
Cytokinesis is usually well underway by the end of telophase, so that the appearance of two new daughter cells follows shortly after the end of mitosis.
In plant cells, the cell plate forms where the new cell wall will form.
In animal cells, a cleavage furrow pinches the cell in two.
The two daughter cells are now separate cells in their own right.
Two cells are formed
Nucleus

20. Mitosis: Review Interphase Early Prophase Late Prophase
Cell enters mitosis
Metaphase
DNA replicated.
Centrosome replicated.
Nucleus still well defined.
DNA continues condensing.
Nuclear membrane disintegrates.
Nucleolus disintegrates.
Chromosomes appear as chromatids.
Mitotic spindle forms.
Centrosomes move to opposite poles.
Chromosomes line up on the metaphase plate.
Anaphase
Cytokinesis
Telophase
Late Anaphase
Nuclei reform.
Cell plate forms (plants)
Elongation of the cell.
Chromosomes separate to opposite poles.
Two independent cells.

21. Mitosis in the Root Tip
Zone of specialization
Mitosis in plant cells occurs only in regions of meristematic tissue.
The meristematic tissue is located at the tip of every stem and every root.
In contrast, mitosis can occur throughout the body of a growing animal.
Root tip growing in this direction
Zone of elongation
Zone of cell division
Meristematic tissue (area of cell division)
Root cap