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UNIT 2:  GENETIC CONTINUITY

 

The ability of a single cell to multiply and develop into a complex multicellular organism, with specific physiological traits, a specific body form, and specific physical and molecular adaptations, is called genetic continuity.  Genetic engineering is the science that studies and deals with all aspects of DNA, from mapping the chromosomes, and genes of individual organisms, to cloning and developing designer organisms.  This branch of science has improved the quality of life for humans.  Genetic engineering research has benefited humans in many areas, such as in the detection of birth defects in the developing fetus, and in the use of gene therapy to cure inherited disorders like certain cancers, hemophilia, heart disease, and cystic fibrosis.  Did you know that scientists at the University of Wales have located the gene thought to be linked to late-onset Alzheimer’s disease, and that a Swedish professor, by the name of Bengt Saltin, has discovered that a gene therapy used in flies, can be used for humans to produce faster, stronger, more powerful athletes?  The ability to understand and manipulate DNA does have both its pros and cons.  For example, you could be denied a job, or be discriminated against on the basis of your genetic makeup.  A genetically modified organism could potentially multiply out of control and disrupt the balance of the entire ecosystem.  By the end of this unit, you will have gained the necessary knowledge to develop an informed opinion about the proper use of genetic information.

 

Mitosis and Meiosis

 

The process of cell reproduction where only one parent cell is required is called asexual reproduction.  This process, which involves four distinct phases, is called mitosis.  The offspring that results is identical to the parent and to all other offspring.  Another process, called sexual reproduction, is where a new individual cell arises from two separate parent cells.  Here, a few extra phases are required, and the offspring possess half of its traits from one parent cell, and the other half from the other parent cell.  Both strategies have evolved throughout life’s history.  Each has its own benefits and downfalls.

 

A.  Cell Division:  Mitosis and Cytokinesis

 

• the process of cell division is necessary for growth and development of multicellular organisms, for tissue repair, and for the replacement of ageing cells
• the rate of cell division varies, depending on the stage of the organism’s lifecycle, and on the tissues in question – it occurs much faster in developing tissue, and in tissues that are exposed to a lot of wear and tear, and slower in tissues that are more permanent, like muscle and nerve tissue
• thee inability to regenerate certain kinds of cells may explain the ageing process
• mitosis occurs when a parent cell divides to produce two daughter cells, where the daughter cells are identical to each other
• in order for this to happen, the DNA of the parent cell must be duplicated and passed on to both daughter cells
• with every round of mitosis, the total number of cells doubles
• Figure 5.2, p. 120, shows how the genetic information that is passed onto the daughter cells is packaged
• the hereditary information in the nucleus of the parent cell is packaged as long strands of DNA that are condensed, folded and, in association with proteins, formed into chromosomes

 

1.  When does a cell say, “O.K….divide!”

 

• one of the postulates of the cell theory is that all cells come from pre-existing cells
• there are two key factors that restrict cell growth and initiate cell division:

• as a cell grows, its surface to volume ratio decreases because the volume of a cell increases much faster than its surface area as it’s growing – which causes the cell to inefficiently distribute materials into, out of, and throughout its body at some point during its growth
• as a cell grows larger, the nucleus has difficulty controlling the activities of the increased volume of the cytoplasm and organelles, which results in an inefficiency to self-regulate

·         however, it is still not known why some cells need to divide while others do not and what factors trigger cells divisions in different cell types

 

 

2.  The Cell Cycle

 

·         Figure 5.4, p. 122, illustrates the cell cycle of a typical eukaryotic cell

·         copy the figure into your notebook

·         the period between cell divisions is called interphase – a period where the cell undergoes growth, duplicates its genetic material, and prepares for mitosis

·         the length of this particular phase depends on the organism and cell type

·         cell division takes place in two parts – mitosis and cytokinesis

·         mitosis ensures the equal distribution of a complete set of DNA to each daughter cell

·         cytokinesis ensures that each daughter cell receives cytoplasm and the necessary organelles to begin life

·         most of the cell’s lifecycle is spent in interphase, during which many specific functions are performed such as respiration, photosynthesis, synthesizing products (i.e. anabolic processes), repairing damage, and fighting disease

·         during interphase, a very important process takes place that duplicates the DNA called replication – this phase is called the S phase, which stands for the “synthesis” phase

·         before this important phase, the cell undergoes its initial growth spurt called the G₁ phase – which stands for the “first gap” phase

·         after the S phase, the cell enters the G₂ phase – or “second gap” phase, where it prepares itself for cell division

 

 

3.  Mitosis -- “Pick Many Apples Today?”

 

·         although mitosis is a continuous process, it is divided into four distinct stages:  prophase, metaphase, anaphase, and telophase

·         the major events of each stage of mitosis are the same for all eukaryotic cells, however, the equipment involved may vary

·         for example, plant and yeast cells do not have centrioles – organelles that anchor the spindle fibres, along which the chromosomes move

·         as well, plant cells and yeast cells do not have asters and astral rays – organelles clearly seen in animal cells

·         the differences between plant and animal mitosis are illustrated in Figures 5.6 through 5.10, pp. 124-127.

 

PROPHASE

·         the DNA becomes visible during this phase since it is here that the coiling, condensing, and protein “wrapping” occurs

·         tiny fibres of protein called astral rays form around each pair of centrioles – they look like stars

·         each duplicated section of DNA is now condensed and visible as a strand called a chromatid

·         each chromatid is connected to its “sister” at a single point called a centromere (see Figure 5.5, p. 122)

·         the nuclear envelope breaks down

·         the nucleolus decreases in size until it disappears

·         more protein fibres are made to connect each centriole pair at opposite poles called mitotic spindles

·         the chromatids attach themselves onto the mitotic spindles, and begin to migrate to the center of the cell (see Figures 5.6 and 5.7, pp. 124-125)

 

METAPHASE

 

·         chromosomes line up at the equator (middle) of the cell or otherwise known as the metaphase plate

·         the chromosomes are held together by their centromeres midway between the poles and perpendicular to the spindle fibres

·         since the chromosomes at the metaphase stage are the thickest and most condensed they will ever get, they are often photographed during this stage for studying purposes – the entire cell’s chromosome set is photographed and they are all organized in a particular fashion to create what is known as a karyotype

ANAPHASE

·         the chromosomes separate into their two chromatid strands, at the centromere, and each sister chromatid moves along the spindle fibre, to opposite “poles” of the cell

 

TELOPHASE

·         the cell returns to the interphase condition

·         the opposite events that occurred in prophase, occur in telophase

·         at the end of this phase, two distinct nuclei are visible within the single cell

 

4.  Cytokinesis:  The Conclusion to Cell Division

 

·         during this last stage of cell division, a few things happen:

·         the cytoplasm divides and is equally distributed to each daughter cell

·         all necessary organelles are made and equally distributed to each daughter cell

·         in animal cells, the cell membrane pinches inward at the equator of the cell, producing a furrow – see Figure 5.8, p. 126 – that continues to deepen until two separate cells are formed, each with its own nucleus

·         in plant cells, the formation of a cell plate forms across the equator of the cell during the end of anaphase, not during cytokinesis, like in animal cells

 

5.  The Aftermath

 

·         the products of mitosis are two daughter cells that have the exact same genetic information as the parent cell, as well as the exact same number of chromosomes as the parent cell

·         if any of the daughter cells produced by mitosis were not identical to the parent cell in any way, due to some malfunction in cell division, they would be abnormal and might not survive

·         a very important process takes place after cell division called differentiation

·         differentiation is responsible for differences that occur among cells

·         all the cells of a multicellular organism possess the exact same nucleus, thus the exact same genetic information

·         however, each specialized cell (i.e. nerve cell, muscle cell, skin cell, bone cell, etc.) has its own specific function, dictated by a specific section or sections of DNA

·         for example, a bone cell is “directed” to be a bone cell by the specific section or sections of DNA that code for bone cell function – a nerve cell is “directed” to function as a nerve cell by the specific parts of DNA that code for nerve cell operation

·         specific proteins “turn on” the necessary sections of DNA that need to be “turned on” and “listened to” by each specific cell

·         this means that even though a specialized cell, of a multicellular organism, contains the complete set of genetic data, not all of the data is utilized by that cell – a muscle cell only “pays attention to” the DNA part that codes for that muscle cell’s function

·         mitosis only happens in body cells called somatic cells – not in reproductive cells (gametes)

·         each type of organism has a characteristic number of chromosomes present in each of its somatic cells

·         for example, all human somatic cells contain 46 chromosomes – 23 from the mother, and 23 from the father – all fruit flies have 8, pea plants have 14, goldfish have 94

·         this number that is found in the somatic cells of organisms is called the diploid number

 

6.  Cancer and Mitosis

 

·         cancer is defined as uncontrollable, or abnormal, cell division

·         cancer cells are different than normal cells in two ways:

·                                                          they divide in an uncontrollable manner, producing “wild” cells with unusual characteristics such as being very large, very small, having huge nuclei, or having an abnormal number of chromosomes

·                                                          they continue to divide and pile up on one another, forming “clumpy” tissue known as tumors

·         not all tumors are harmful -- those that do not show a tendency to spread, are called benign

·         those that do show a tendency to spread, are called malignant – this type is capable of moving throughout the body to invade new tissues – a process called metastasis

·         various therapies are used to treat rapid cell division called radiation therapy – therapy that is directed at specific sites in the body that upsets the mitotic process, thereby disrupting cell division – either the chromosomes do not line up on the metaphase plate properly, or migration of chromosomes to opposite poles of the cell does not take place

·         unfortunately, radiation therapy, known as chemotherapy, affects all dividing cells – healthy and cancerous ones

·         another form of therapy, called immunotherapy, uses the body’s own immune defences to treat cancer

 

Homework:         1-6, p. 130

 

• for a detailed view and animation of the cell cycle and cell reproduction, click on http://ecology.about.com/library/blmitosisanim.htm
• for more animations click on http://www.cellsalive.com/cell_cycle.htm for cell cycle information, http://www.cellsalive.com/cell_cycle.htm for an animal cell reproduction animation, http://web.grcc.cc.mi.us/biosci/pictdata/mitosis/planmito.htm for a plant cell reproduction animation, http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html for a tutorial on the cell cycle and mitosis, and http://www.bio.unc.edu/faculty/salmon/lab/mitosis/mitosislinks.html for movies, pictures, lessons, and other mitosis links