1. Cellular Reproduction I
GENE 330

2. The Cell Cycle and DNA replication in Eukaryotes
Although cell division occurs in all organisms, it takes place differently in prokaryotes and eukaryotes.
In a population of dividing cells , each cell passes through a series of defined stages, which constitutes the cell cycle.
The cell cycle can be divided into two major phases based on cellular activities: M phase and Interphase.
M phase includes 1. process of mitosis (duplicated chromosomes are separated into two nuclei), and 2. cytokinesis (the cell divides into two daughter cells).
Interphase is a period between cell divisions (cell grows and is engages in metabolic activities).

3. The Cell Cycle
In order for an organism to grow, 3 events must take place:
The cell mass must increase
There must be a duplication of the genetic material
A division process must occur so that each daughter cell receives an equal and identical complement of the genetic material
These occurrences take place in an ordered progression of events during the cell’s life span, or cell cycle

5. Cell Cycles in Vivo
One of the properties that distinguishes various types of cells within a multicellular animal or plant is their capacity to grow and divide.
We can accept three broad categories of cells:
1. Cell, such as nerve cells, muscle cells, or red blood cells, that are highly specialized and lack the ability to divide.
2. cells, that normally do not divide but can be induced to begin DNA synthesis and divide when given an appropriate stimulus (liver cells, lymphocyte).
3. Cells that normally possess a relatively high level of mitotic activity (spermatogonia, hematopoietic stem cells, cells of apical meristems).

6. Cell cycles can range in length from as short as 30 minutes in a cleaving frog embryo, whose cell cycles lack both G1 and G2 phases, to several month in slowly growing tissues (mammalian liver).
With a few exception, cell that have stopped dividing (temporarily or permanently) in the culture or in the body, are present in a stage preceding the initiation of DNA synthesis. Cell are in the Go state.
The cell must generate an internal signal to proceed from Go or G1 into S phase.
Once the signal to begin DNA replication has been generated, the cell invariably completes that round of DNA synthesis and continues through mitosis.

7. The Cell cycle
Definition – The sum total of the division related events that occur between the time a cell completes one cell division, and the time it completes the next one.
2 basic elements of a cell cycle :
Genetic material in both the nucleus and the organelles must replicate completely, and 1 copy must end up in each of the daughter cells : nuclear division
Cytoplasmic material and membranes must arrange so that there will be two/more complete cells to receive this information : cytokinesis

8. The Cell Cycle
A typical eukaryotic cell cycle can arbitrarily begin in what is called the G1 phase . At this point, the cell contain two copies of each chromosome (C2 -the normal diploid state of eukaryotic cell).
Sometimes late in G1 phase synthesis of histones is one of the first indications of incipient DNA replication.
The cell then enters synthesis, or S phase. During this stage, the DNA is replicated and the histones and nonhistone proteins are deposit on the daughter DNA molecules to reproduce the chromatin structure.
When replication is complete, the cell enters what is called the second gap phase or G2 phase. It has a DNA content four times the haploid amount (C4). In most eukaryotic cells, the total time required for G1, S, and G2 phases is many hours.
In the interphase, the chromatin is dispersed throughout the nucleus and is actively engaged in transcription.
At the end of G2, the cell is ready to enter the process called mitosis, during which it divides.

9. The eukaryotic cell cycle
The eukaryotic cell cycle. Changes in the amount of DNA (blue line) and rate of histone synthesis (red line) with time during two cell cycles. The DNA content is measured in units of the haploid genome (C). The time scale is typical of many eukaryotic cells.

10. The Cell Cycle
The M phase, or the division phase, is the final part of the cell cycle.
This phase contains the different stages of mitosis e.g. prophase, metaphase, anaphase and telophase.
In prophase , the replicated chromosomes condense into the typical methaphase chromosome structures. The nuclear membrane disintegrates, and the mitotic spindle forms. The spindle consists of contractile microtubules that pull pairs of chromatids apart so that the daughter cells each receive identical sets of chromosomes. In telophase the nuclear membrane then re-forms about each daughter nucleus, and the cell itself divides.
This cellular division is called cytokinesis.
After division , the chromosomes of the daughter cells decondense , and a new G1 phase begins.
The 2 daughter cells are identical to each other and to the parent cell.

11. Mitosis

12. The Cell Cycle
Cells can leave the mitotic cell cycle at the G1 to the following two condition
In many tissues of higher organisms, the G1 phase becomes very prolonged after growth and tissue differentiation is complete. The most extreme examples are nerve cells. Such nondividing cells are in permanently arrested G1 phase, which is often called G0 phase.
cells can further develop to reproduction cells (gametes and spores) by entering meiosis.

14. The cell cycle trigers
The key process, that triggers the successive stages in the cell cycle appears to be phosphorylation of a number of nuclear proteins, and the key enzymes are a set of kinases. Its activation was found to require assotian with specific small proteins called cyclins.
There exist several cyclin-dependent kinases and number of cyclins to associate with them.
Each transition in the cell cycle appears to have unique cyclin/kinase complex as trigger.
A somewhat simplified view of the roles of these proteins in mammalian cells is shown in next Figure.

15. The cyclin-dependent kinase CDK2 is involved in the entrance to S phase, and cdc2, with cyclins A and B, regulate mitosis. Cdc2 is phosphorylated at entry to G2, but must be dephosphorylated for mitosis to commence. In this somewhat oversimplified figure, is indication of CDK2 and cyclins E and A (CE,CA) in triggering S phase, the role of cdc2 and its phosphorylation at the S/G2 boundary, and the participation of cyclins A(CA) and B(CB) in the entry to mitosis.

16. Replication of the DNA
The basic mechanisms of DNA replication are quite similar in eukaryotes and prokaryotes. The enzymology is not fundamentally different.
In both cases, replication is semiconservative and is continuous on one strand and discontinuous on the other.
As in prokaryotes, eukaryotic replication on the retrograde arm entails assembly of short RNA primer molecules, elongation from the primers by a DNA polymerase, and ligation of Okazaki fragments.
A significant difference in eukaryotic and prokaryotic DNA replication is the size of the Okazaki fragments. They are much smaller in eukaryotes (only 135 bases long, or about the size of the DNA on a nucleosome).
Eukaryotic cells contain five DNA polymerases.
Three of these – α,δ, and ε – are used during S phase replication.