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What happens in each phase?

Cl. During this phase, also called the first gap phase, the cell grows larger and makes more of its ribosomes, organelles, and proteins. This phase ensurc that division will produce functional daughter cells, ones that are the right size and have all the parts they need.
S phase. In this phase, the cell synthesizes a complete copy of the DNA in its nucleus, which it needs in order to divide allowing it to give one full set each of its two daughter cells. During S phase, the cell also duplicates a microtubule-organizing structure called the centrosome.
G2. Once DNA synthesis is complete, the cell enters a second gap phase.
What happens in each phase?
During M phase, the cell divides its copied nuclear DNA and cytoplasm to form two new cells. M phase is further divided into twc phases: mitosis and cytokinesis.
In mitosis, the nuclear DNA of the cell condenses into visible chromosomes and is pulled apart by the mitotic spindle, a specializec structure made out of microtubules.
In cytokinesis, the cytoplasm of the cell is split in two, making two new cells. Cytokinesis usually begins just as mitosis is ending, with a
Mitosis takes place in four stages: prophase (sometimes divided into early prophase and prometaphase), metaphase, anaphase, and telophase.


Mitosis phase by phase
Early prophase:
•Centrosomes replicate just before prophase.
•Chromosomes start to appear as the chromatin coils up, becoming shorter and thicker:
Late prophase:
•Nuclear envelope ‘disappears’ (breaks up into small PROUETAPHASE vesicles which can’t be seen with a microscope).
‘Nucleolus “disappears .
Mitosis phase by phase
Metaphase: METAPHASEMetaphase
•Each centrosome reaches a pole.
•Sprindle made from protein microtubules is completed.
•Chromosomes line up across the equator of the sprindle; they are attached by their centromeres to the sprindle.
Spindle one spir•vdk
Mitosis phase by phase
•Each chromosome splits at the centromere.
•The chromatids start to be pulled apart by microtubules.ANAPHASE •Chromatids move to opposite poles, centromeres
Mitosis phase by phase
•Nucleolus and nuclear envelope reform.
•Chromatids have reached the poles of the sprindle; they will now uncoil again.
•Each new cell will have only one chromatid of each chromosome, but the complete genetic set, HOW COME?
•Once the telophase is reaching the end,
Figure 12.10 Cytokinesis in animal and plant cells, 
Centromeres, centrosomes, and centrioles
•Centromere is needed for the separation of the chromosomes during mitosis.
•Each metaphase chromosome has two kinetochores at its centromere, one on each chromatid,
•Kinetochores are made of proteins that bind specifically to the DNA in the centromere and also to the microtubules.
•Shortening of the microtubules during anaphase: allows the
Chromosomes and the mitotic spindle during mitosis
174 Kinetochore Kinetochc
2/11 Centrornt
spindle pole spindle pole
Biological significance of mitosis
•Growth: the two daughter cells are identical; same number of chromosomes and genetically identical.
Replacement of cells and repair of tissues
•Asexual reproduction: Mitosis is the basis of
Spore Formation: Reproduces by forming spores. Under favourable conditions spores develop into new individuals, E.g. Fern, fungi, bacteria.

Fragmentation: Organism with filamentous body, break into two or more fragments. Each fragment grows into a
Binary fission: A single parent cell divides into two daughter cells .
e.g. Amoeba, paramecium, bacteria.
Budding: Parent cell produces bud, it gets detached and develops into new individual e.g. Yeast,
Regeneration: Organism’s body breaks up into one or several parts. Each part develops into a new individual Ex-planaria, hydra

Vegetative Reproduction: Organism produces new individuals by a vegetative part of the plant.
Ex- potato, onion, ginger,
Types of Asexual Reproductio 
Telomeres are repetitive stretches of DNA located at the ends of linear chromosomes, rich in guanine and cytosine. They protect the ends of chromosomes in a manner similar to the way the tips of shoelaces keep them from unraveling.
Every time a cell carries out DNA replication the chromosomes are shortened by about 25-200 bases (A, C, G, or T) per replication.
However, because the ends are protected by telomeres, the only par of the chromosome that is lost, is the telomere, and the DNA is left undamaged.
What happens to telomeres as we age?
Each time a cell divides, 25-200 bases are lost from the ends of the telomeres on each chromosome. Two main factors contribute to telomere shortening during cell division:
The “end replication problem” during DNA replication: the copyin enzyme cannot run to the end of the DNA and complete the replication. This accounts for the loss of about 20 base pairs* per cel division.
2.Oxidative stress: The amount of this stress in the body is thought t be affected by lifestyle factors such as diet, smoking and stress.
How is telomere length maintained?
Telomerase is an enzyme that adds the TTAGGG telomere sequence to the ends of chromosomes.
Telomerase is only found in very low concentrations in our somati cells. Because these cells do not regularly use telomerase they age leading to a reduction in normal function. The result of ageing cells, an ageing body.
Telomerase is found in high levels in germline-cells (egg and sperm) and stem cells. In these cells telomere length is maintained after DN replication and the cells do not show signs of ageing.
Stem cells
•Stem cells are unspecialized (undifferentiated) cells that are characteristically of the same family type (lineage).
•They retain the ability to divide throughout life.
•VVhen a stem cell divides, each new cell has the potential to remain stem cell or to develop and give rise to cells that can become highly specialized.
•Stem cells contribute to the body’s ability to renew and repair its tissues. Unlike mature cells, which are permanently committed to their fate, stem cells can both renew themselves and create new cell
Potency: extent of the power of a stem cell to produce different cell types.
•Totipotent: cells that can produce any type of cell
•Pluripotent: embryonic cstem cells.
•Multipotent: cells that can only produce a few types of cells. For example, stem cells in the bone marrow.
Cancer is an abnormal growth of cells caused by multiple changes in gene expression leading to dysregulated balance of cell proliferation and cell death and ultimately evolving into a population of cells that can invade tissues and metastasize to distant sites, causing significant
•Cancer shows us the importance of controlling cel division properly, because cancers are the result of uncontrolled mitosis.
‘Cancer starts when certain cells undergo changes ir the genes that control cell division (mutation). •Oncogenes: particular term for a mutated gene that causes cancer.
•Benign tumours: do not spread! Not cancerous, but may become cancerous.
•Malignant tumours: interfere with the normal functioning of the area where they are located.
Metastasis: Cells of malignant tumours can break of and spread through the blood and lymph to other parts of the body.
Steps in the development of cancer
Oncogenes transformed by carcinogens.Normal

  1. Cancerous cell does not respond to signals: continues to divide out of control’.
  2. Tumour is formed. It gets bigger. Tumour cells look “different’ under the microscope.
  3. Tumour is supplied with blood


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