Cell Cycle
CH12 Reading Guide CH13 Reading Guide another study guide for chapter 12 and chapter 13 Slides 12 slides 13
Interphase: Interphase generally lasts at least 12 to 24 hours in mammalian tissue. During this period, the cell is constantly synthesizing RNA, producing protein and growing in size. By studying molecular events in cells, scientists have determined that interphase can be divided into 4 steps: Gap 0 (G0), Gap 1 (G1), S (synthesis) phase, Gap 2 (G2).
Gap 0 (G0): There are times when a cell will leave the cycle and quit dividing. This may be a temporary resting period or more permanent. An example of the latter is a cell that has reached an end stage of development and will no longer divide (e.g. neuron).
Gap 1 (G1): Cells increase in size in Gap 1, produce RNA and synthesize protein. An important cell cycle control mechanism activated during this period (G1 Checkpoint) ensures that everything is ready for DNA synthesis. (Click on the Checkpoints animation, above.)
S Phase: To produce two similar daughter cells, the complete DNA instructions in the cell must be duplicated. DNA replication occurs during this S (synthesis) phase.
Gap 2 (G2): During the gap between DNA synthesis and mitosis, the cell will continue to grow and produce new proteins. At the end of this gap is another control checkpoint (G2 Checkpoint) to determine if the cell can now proceed to enter M (mitosis) and divide.
Mitosis or M Phase: Cell growth and protein production stop at this stage in the cell cycle. All of the cell's energy is focused on the complex and orderly division into two similar daughter cells. Mitosis is much shorter than interphase, lasting perhaps only one to two hours. As in both G1 and G2, there is a Checkpoint in the middle of mitosis (Metaphase Checkpoint) that ensures the cell is ready to complete cell division.
I tried that AI to explain the similarities and differences between mitosis and meiosis.
Q. can you explain the difference between mitosis and meiosis?
A. Mitosis and meiosis are both types of cell division, but they serve different purposes. Mitosis is the process by which a single cell divides into two identical daughter cells. This is the process by which cells reproduce, and it is how an organism grows and develops. Meiosis, on the other hand, is the process by which a single cell divides into four non-identical daughter cells. This process is important in sexual reproduction, as it creates the genetic diversity necessary for the survival of a species.
Q. ok but what are the steps of mitosis and meiosis, and how are they similar, and how are they different?
A. Both mitosis and meiosis are complex processes that involve several steps. In general, both processes involve the replication of a cell's DNA and the subsequent separation of the copied DNA into new cells. However, the specific steps involved in each process are slightly different, and the end result of each process is also different.
The steps of mitosis are as follows:
Interphase: In this phase, the cell's DNA is replicated, and the cell grows and prepares for division.
Prophase: In this phase, the cell's nucleus begins to form, and the chromosomes that carry the cell's genetic information become visible.
Metaphase: In this phase, the chromosomes line up along the center of the cell.
Anaphase: In this phase, the chromosomes are separated and begin to move to opposite ends of the cell.
Telophase: In this phase, the cell's nucleus re-forms at the opposite ends of the cell, and a new cell wall begins to form around each set of chromosomes.
The steps of meiosis are similar to those of mitosis, but there are some key differences. The steps of meiosis are as follows:
Interphase: In this phase, the cell's DNA is replicated, and the cell grows and prepares for division.
Prophase I: In this phase, the cell's nucleus begins to form, and the chromosomes that carry the cell's genetic information become visible. Unlike in mitosis, the chromosomes in meiosis are not identical to one another.
Metaphase I: In this phase, the chromosomes line up along the center of the cell.
Anaphase I: In this phase, the chromosomes are separated and begin to move to opposite ends of the cell. Unlike in mitosis, the chromosomes in meiosis are not separated into identical sets.
Telophase I: In this phase, the cell's nucleus re-forms at the opposite ends of the cell, and a new cell wall begins to form around each set of chromosomes.
Prophase II: In this phase, the cell's nucleus begins to form again, and the chromosomes that were separated in Anaphase I are now visible.
Metaphase II: In this phase, the chromosomes line up along the center of the cell.
Anaphase II: In this phase, the chromosomes are separated and begin to move to opposite ends of the cell. This time, the chromosomes are separated into identical sets.
Telophase II: In this phase, the cell's nucleus re-forms at the opposite ends of the cell, and a new cell wall begins to form around each set of chromosomes.
So, to summarize, the main differences between mitosis and meiosis are:
Mitosis produces two identical daughter cells, while meiosis produces four non-identical daughter cells.
The chromosomes in mitosis are identical to one another, while the chromosomes in meiosis are not.
In meiosis, the chromosomes are separated into non-identical sets in Anaphase I, and then into identical sets in Anaphase II. In mitosis, the chromosomes are separated into identical sets in Anaphase.
Meiosis is important in sexual reproduction, while mitosis is important in growth and development.
Prophase
Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. The nucleolus disappears. Centrioles begin moving to opposite ends of the cell and fibers extend from the centromeres. Some fibers cross the cell to form the mitotic spindle.
Prometaphase
The nuclear membrane dissolves, marking the beginning of prometaphase. Proteins attach to the centromeres creating the kinetochores. Microtubules attach at the kinetochores and the chromosomes begin moving.
Metaphase
Spindle fibers align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome.
Anaphase
The paired chromosomes separate at the kinetochores and move to opposite sides of the cell. Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules.
Telophase
Chromatids arrive at opposite poles of cell, and new membranes form around the daughter nuclei. The chromosomes disperse and are no longer visible under the light microscope. The spindle fibers disperse, and cytokinesis or the partitioning of the cell may also begin during this stage.
Cytokinesis
In animal cells, cytokinesis results when a fiber ring composed of a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus. In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells.
MeiosisIn sexual reproduction, two parents give rise to an offspring with an unique gene combination from either of them — each parent gives 1/2 of his/her genes to the offspring. A gene is a discrete unit of information on the DNA that codes for one protein, perhaps one of the many enzymes needed by our bodies.Somatic cells have two sets of chromosomes; one set from each parent. For example, in humans one set = 23 chromosomes, so our somatic cells have 46 chromosomes arranged in 23 pairs. The two chromosomes in each pair are referred to as being homologous chromosomes, so we could say that humans have 23 pairs of homologous chromosomes. The two chromosomes of each pair carry genes for the same trait (for example, eye color) at the same location, but not necessarily the same form of that gene (for example, brown vs. blue eyes).An important exception to this is the sex chromosomes, the X and Y chromosomes. Although these chromosomes pair with each other, they are not the same size. The X-chromosome is longer and has genes for many traits with no match on the Y-chromosome. A person with XX would be female and someone with XY would be male (although, that’s not true of all other organisms). All the other chromosomes are calledautosomes.Somatic cells have two sets of autosomes (however many pairs that is) and one pair of sex chromosomes so are called diploid or 2n cells. Thus, humans would have 44 + XX or 44 + XY chromosomes, and fruit flies would have 6 + XX or 6 + XY. Gametes or sex cells (eggs from female and sperm from male) have one chromosome from each autosome pair and one sex chromosome (one set of chromosomes), thus are calledhaploid or 1n. Human eggs would have 22 + X chromosomes, and human sperm would have 22 + X or 22 + Y chromosomes. Similarly, fruit fly eggs would have 4 + X chromsomes and their sperm would have 3 + X or 3 + Y chromosomes.
Meiosis is a special type of cell division that produces gametes with half as many chromosomes. The opposite process would be syngamy or fertilization, which is the union of the egg and sperm to restore the 2n number. This results in a zygote, the first cell formed by fertilization, a completely new and different organism with unique genetic information different from either parent. The zygote divides and grows to form an embryo which developes into a young organism, then an adult.
Life cycles of all sexually-reproducing organisms follow this pattern of alternation of generations. The 2n adult produces 1n gametes by the process of meiosis. These unite in the process of syngamy to produce a new 2n generation. Thus, the life cycles alternate between 1n and 2n stages, and between the processes of meiosis and syngamy. It is because of the way in which genes recombine in meiosis and syngamy that we have the whole study of genetics.
The steps in meiosis are similar to mitosis and even have the same names. However, there is a significant difference in how the chromosomes line up initially. In mitosis, chromosomes line up individually, while in meiosis, the two chromosomes in each homologous pair line up next to each other. This pairing process is called synapsis, and the resulting homologous pair is called a bivalent in reference to the two chromosomes or a tetrad in reference to the four sister chromatids involved.
Interphase is the same in both mitosis and meiosis, but in meiosis, it is followed by two cell divisions. These two division processes are referred to as Meiosis I and Meiosis II, and result in a total of four daughter cells, each with a 1n chromosome number.
In prophase I, notice the difference in how the homologous chromosomes behave. They come together and match up (synapsis) in pairs (tetrads or bivalents). In human females, this stage happens prior to birth when the ovaries are forming, and then stops. A baby girl is born with all the precursor egg cells she will ever have in a sort-of "suspended animation" until puberty (hence abdominal x-rays are dangerous for any young to middle-aged human female, not just pregnant women, and hence there is a greater likelihood that a 40-yr-old mother will have a baby with Down Syndrome – due to incorrect meiosis — than a 20-yr-old mother).
In metaphase I, the bivalents line up, not individual chromosomes, so there’s a 50:50 chance of which chromosome of each pair faces which pole of the cell. Human “eggs” go about this far through meiosis before they are shed from the ovaries at ovulation.
In anaphase I, the homologous chromosomes separate, and one of each pair travels to each of the two poles of the cell, thereby reducing the chromosome number from 2n to 1n. Note that the sister chromatids stay together.
Two daughter cells are formed during telophase I. These usually go immediately into the second cell division (meiosis II) to separate the sister chromatids.
Meiosis II is pretty much like mitosis, in that the sister chromatids are separated. This results in four daughter cells, each with an 1n chromosome number. In human females, meiosis II in the precursor egg cells never happens until/if a sperm first enters the egg to fertilize it. Fertilization triggers Meiosis II, and then the sperm nucleus unites with the resulting egg nucleus. Thus, the unfertilized “eggs” that a woman sheds each month are not true eggs. Also in human females, division of the cytoplasm is not even. This provides a way of keeping as much cytoplasm as possible with the future egg/zygote. Rather than equal-sized gametes, one big egg and three smaller polar bodies with minimal cytoplasm are formed.
Interestingly, because the homologous pairs line up during Metaphase I, there is a 50:50 chance of which one of each pair will go to each of the poles of the cell (like flipping a coin, where you can get either heads or tails). Therefore, in humans with 23 pairs of chromosomes, a gamete (egg or sperm) could have 223 or 8,388,604 possible combinations of chromosomes from that parent. Any couple could have 223 × 223 or 70,368,744,177,644 (70 trillion) different possible children, based just on the number of chromosomes, not considering the actual genes on those chromosomes. Thus, the chance of two siblings being exactly identical would be 1 in 70 trillion. In addition, something called crossingover, in which the two homologous chromosomes of a pair exchange equal segments during synapsis in Meiosis I, can add further variation to an individual’s genetic make-up.