Topic 1.6 Cell Division

Chapter 12Campbell Reece

5 W’sWho does cell division? All cells (except most brain cells)

What is cell division? Dividing of the DNA and cytoplasm/cell contents into two separate identical cells

Where does it occur? In most places of all organisms

When does it happen? During the mitotic stage of the cell cycle (depends on the type of cell) ie. Red blood cells every day, heart cells 50 times in our lives, embyro cells are the most rapid

Why does it happen? For growth/development, repair damaged cells, replace old cells or infected ones, and asexual reproduction (bacteria)

Cell division is more complex in eukaryotes than prokaryotes

The process of cell division in eukaryotes may be divided into two steps

MitosisThe allocation of two complete sets of chromosomes to each of the

daughter cells

CytokinesisThe division of the parent cell into two daughter cells with an

equivalent complement of cytoplasm and organelles from the parent cell.

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter10/animation_-_cytokinesis.html

Cell Division

• As you will recall from Topic 1.5(U1) all cells can only be formed by division of pre-existing cells

• All eukaryotic cells go through a cycle over and over again known as the cell division cycle. It involves interphase, mitosis, and cytokinesis.

1.6 (U1) Mitosis is division of the nucleus into two genetically identical daughter cells.

• The cell cycle outlines and describes how cells behave during their growth and division stages.

• Once cells reach a certain size they will divide into two genetically identical daughter cells in a process known as mitosis.

• The cell cycle involves two major phases; a growth phase and a division phase. The largest portion of the cell cycle is spent in interphase (growth phase).

1.6 (U4) Interphase is a very active phase of the cell cycle with many processes occurring in the nucleus and cytoplasm.

• Interphase is the longest phase in the cell cycle and it includes three phases; G1 , S, and G2 .

Phases of Interphase Major Events

G1 Growth of the cell and increase in number of organelles.

S Replication of chromosomes with copies remaining attached to one another

G2 Further growth occurs, organelles increase in number, DNA condenses to form visible chromosomes, microtubules begin to form.

Overall Cell is performing the tasks appropriate to its type along with many metabolic reactions such as protein synthesis . For example, a cell of the pancreas may be actively secreting insulin to lower high glucose levels in the body.

Interphase under the microscope!

1.6 (U2) Chromosomes condense by supercoiling during mitosis.

• You will learn that DNA condenses into chromosomes during mitosis and this occurs during a process known as condensation of chromosomes.

• The DNA will wrap itself around proteins (histones) which will then group together and followed by more wrapping and grouping together. This process is called supercoiling.

• In the end a chromosome is produced which consists of two identical molecules of DNA known as sister chromatids held together in the middle by a centromere.

1.6 (U2) Chromosomes condense by supercoiling during mitosis.

• Chromosomes must be supercoiled or they would not fit inside the nucleus (5µm) of human cells (10µm.

• Human chromosomes are between 15 000µm and 85 000 µm in length! http://www.youtube.com/

watch?v=N5zFOScowqo

Four Phases of Mitosis

• The process of mitosis in the cell cycle is when the nucleus of the cell divides and forms two new genetically identical nuclei.

• This takes place over the four phases of mitosis and towards the end there is a division of the cytoplasm and all its contents.

• Then the cell begins to form two separate new daughter cells, each containing one nucleus and they are an exact copy of each other.

Prophase• During prophase the

chromatin (DNA) begins to condense by super-coiling.

• Spindle fibres begin to form from the pair of centrioles in animal cells that begin to move to opposite poles of the cell.

• The nuclear envelope surrounding the nucleus begins to dissolve.

Metaphase• During this stage of mitosis,

the spindle fibres attach to the sister chromatids at the centromere.

• The chromosomes are guided by the spindle fibres to line up along the equatorial plate in the cell. One spindle fibre microtubule from each pole is attached to each of the sister chromatids.

• This is the longest phase in mitosis lasting about 20 minutes.

Anaphase• During this phase of

mitosis the spindle fibres begin pulling the sister chromatids to opposite poles.

• The sister chromatids separate at the centromere and are pulled toward opposite ends of the cell as the spindle fibres shorten.

• This phase is very short lasting only a few minutes.

Telophase• During this phase in mitosis,

the chromosomes have arrived at each end and a nuclear membrane begins to form around them.

• The chromosomes uncoil and become less dense which makes them no longer visible.

• The spindle fibres begin to break down and the process of cytokinesis begins.

http://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.html

1.6 (U3) Cytokinesis occurs after mitosis and is different in plant and animal cells

• This involves the division of the cytoplasm and all its contents.

• In animal cells the process of cytokinesis is known as cleavage.

• In plant cells cytokinesis is quite different due to the presence of the cell wall.

Cytokinesis in Animal Cells

• The first sign of cytokinesis in animal cells is the appearance of a cleavage furrow near the location where the metaphase plate was located.

• There is a contractile ring of actin microfilaments and myosin molecules that interact together to cause the ring to contract which deepens the furrow and separates the cell contents. The process is like pulling on the drawstrings of your hoodie!

• The parent cell is pinched into two separate, identical daughter cells. http://highered.mheducation.com/sites/

9834092339/student_view0/chapter10/animation_-_cytokinesis.html

Cytokinesis in Plant Cells• During telophase vessicles from the Golgi apparatus follow

microtubules to the middle of the cell. Once they reach the middle they coalesce and form a cell plate which will develop into the cell membranes of the daughter cells.

• The vessicles carry the materials such as pectin and other substances needed for the new cell wall and they collect in the cell plate as it grows. This will develop into the middle lamella that will attach the new cell walls to one another.

• The cell plate grows until it joins with the membrane surrounding the cell. This separates the cell into two identical daughter cells and a new cell wall forms from the contents of the cell plate and cellulose that has been deposited by each daughter cell.

1.6 (U5) Cyclins are involved in the control of the cell cycle.

• Cyclin is a regulatory protein that assists in the control of the progression of cells through the cell cycle.

• Cyclins ensure that each task in the cell cycle is not only performed on time but in the proper order. The next task will not begin until the previous tack has been completed.

1.6 (U5) Cyclins are involved in the control of the cell cycle.

• The cell contains enzymes known as cyclin-dependent kinases. When the cyclin binds to these enzymes they are activated and will attach phosphate groups to other proteins contained in the cell.

• The attachment of the phosphates activates these proteins and they begin to carry out the specific tasks of the cell cycle.

Four main Cyclins• Cyclin D triggers cells to move fromG0 to G1 and G1 into S phase• Cyclin E prepares the cell for DNA replication in S phase• Cyclin A activates DNA replication inside the nucleus in S phase• Cyclin B promotes the assembly of the mitotic spindle and other

tasks in the cytoplasm to prepare for mitosis.

1.6 (U6) Mutagens, oncogenes and metastasis are involved in the development of primary and secondary tumors.

Benign tumors• Tumors are abnormal

groups of cells that develop in the tissues of the body. In benign tumors, the cells adhere to one another and do not invade other tissues or move to other body parts. Benign tumors do not usually cause much harm.

Malignant Tumors• When cells in a tumor

become detached and move to other parts of the body and develop into secondary tumors these are classified as malignant. Malignant tumors, known as carcinomas are more often than not life-threatening. These tumors cause a disease known as cancer.

Causes of Cancer

• Carcinogens are chemicals and agents that are known to cause cancer.

• Some known carcinogens include some viruses, chemical mutagens, high energy radiation such as x-rays and short-wave UV light.

• Mutagens are agents that cause gene mutations and these mutations can cause cancer.

Mutagens & Mutations

• When random changes occur that alter the base sequence of genes we call them mutations. Most of the time these mutations in genes do not cause cancer unless they occur in genes that are known to become cancer causing following mutation known as oncogenes.

• Oncogenes are involved in controlling the cell cycle and cell division. If there is a mutation in these genes it can cause uncontrolled cell division which can lead to the formation of tumors.

Mutations

• The chance of a cell becoming a tumor cell is small due to the fact that several mutations would have to occur.

• However, due to the significant number and types of cells in the human body, the chance of tumor formation during our lifetime is significant.

• A primary tumor forms when a tumor cell begin dividing by mitosis into a large group of cells.

• If cells from the primary tumor move in a process known as metastasis and begin to develop in another part of the body it is known as a secondary tumor.

1.6 (A1) The correlation between smoking and incidence of cancers

• See handout given in class.