unit 9: cellular replication - inetteacher.com
TRANSCRIPT
Unit 9: Cellular Replication
Chapter 8 – Read P. 121-137
Chapter 15 – P. 301
Chapter 26 – P. 552-553
The Cell Cycle
4 Types of Asexual Reproduction : Binary Fission, Budding, Fragmentation and Sporulation
Introduction
Asexual reproduction may take place in an unicellular organism or a
multicellular organism.
In an unicellular organism, it is very similar to cell replication or division
because the products are increase number of cells that are identical to the
original parent cell.
Likewise, with multicellular organisms, the organism produced will be identical
to the original parent however more the one cell is involved in being reproduced.
1. Binary FissionBinary fission is the most common type of asexual reproduction and occurs in a wide variety of organisms. It is perhaps most commonly known as the reproductive method of unicellular prokaryotes (bacteria).
These organisms are unicellular and will follow the generic cell cycle. They will grow as an individual, replicate its DNA and then continue growing. When time to divide, the cytoplasm, cell membrane and cell wall will split in half producing two cells, each will be half the initial size and identical in terms of genetics to the original cell.
The second growth helps to separate the copied DNA by increasing distance between the two DNA molecules. This ensures that each new cell will receive a copy of the DNA.
2. Budding
Budding is really a type of binary fission.
It is commonly found in yeast cells which are also unicellular organisms that belong to Kingdom Fungi.
Growth occurs and DNA is replicated and more growth occurs, however when division occurs the splitting of the cytoplasm is uneven. It appears that a much smaller cell ('bud') is growing from the original cell and the division occurs when this bud breaks off.
The two cells after are identical however they are not of equal size more half of the original size.
https://www.youtube.com/watch?v=DuvWFim2ltU
3. Fragmentation
Fragmentation is a type of asexual reproduction that occurs in multicellular Fungi.
The cells of some of these organisms are long thread like structures known as hyphae. They are multinucleate meaning that they contain many nuclei.
When a piece of hyphae breaks off, as long as a nucleus is present and there may be many in the broken piece, it is able to grow an entire new organism.
https://www.youtube.com/watch?v=J4OIv2P_BDg
4. SporulationSporulation is another type of asexual reproduction commonly found in fungus.
It involves the production of spores which is basically reproductive cells that can grow into new cells.
The spores, once produced, will undergo the generic cell cycle, where its method of division is mitosis. This is another type of asexual reproduction that we will discuss shortly.
Rather than only doing mitosis, spores help with the success of fungi because they are able to be stored in special spore cases until they are ready to be released. Only if conditions are favorable, they will then grow and undergo mitosis to produce new individual cells.
https://www.youtube.com/watch?v=jzprmPlijQo
Mitosis: Asexual Reproduction in Eukaryotic Cells
• Mitosis is the asexual division of a eukaryotic cell to produce two identical cells.
Increased Complexity
• The complexity of mitosis is a result of the complexity of eukaryotic cells. There is more than one single DNA molecule but rather numerous DNA molecules all with very specific codes for all duties within the cell like protein building as well as genes. This DNA must be packaged efficiently in order to fit in a well-defined nucleus.
• It is packaged into chromosomes with the help of proteins known as histones. During G1, S and G2 (also known as Interphase), the DNA is not recognizable.
• However during Mitosis, the chromosomes become visible and the orchestration of dividing this now doubled DNA without the presence of the nuclear envelope can be very confusing. The cell containing other organelles must ensure that after division each new cell has a copy of the DNA molecules therefore organization during division in of key importance.
• It is microtubule spindles that will hold the chromosomes to ensure that each new cell gets what it requires.
Stages of Mitosis
https://www.youtube.com/watch?v=1cVZBV9tD-A
Stages of Mitosis: Interphase
Any reference material that diagrams a cell cycle does so for mitosis. G1, S and G2 of a eukaryotic cell is known as interphase in mitosis.
It is commonly referred to as the resting stage yet the cell is not resting but actively metabolizing, growing and replicating DNA.
This occurs 90% of the time in a eukaryotic cell cycle.
The other 10% of the time the cell is dividing or performing mitosis, of which there are various stages.
Stages of Mitosis: Prophase
Prophase is the first stage in which the nuclear envelope disintegrates and the chromosomes become visible.
The DNA is doubled (after S phase) and therefore the chromosomes appear as 'X’.
It is important to understand that chromosomes are not doubled but only DNA and the doubled DNA remains together in the chromosome as two chromatids joined together at the center known as the centromere.
Stages of Mitosis: Metaphase
During metaphase, the spindles from the centrioles have attached to the centromeres of the chromosomes which have lined up at the middle 'equator' of the cell.
Stages of Mitosis: Anaphase
It is then during anaphase that the spindles will pull the identical chromatids apart to opposites end of the cell. The chromosomes now appear as straight lines because they represent only one copy of the DNA at each end of the cell.
Stages of Mitosis: Telophase
The last phase, known as telophase is when cytokinesis or splitting of the cytoplasm occurs. This is the actually division that produces two identical cells from the original cell with the chromosomes now linear and only one copy of the cell's DNA back within the confines of the nucleus.
Special Notes about Mitosis
Meiosis: Sexual Reproduction in Eukaryotic Cells
• Meiosis is the sexual division of a eukaryotic cell to produce four cells that have a reduction in chromosome number.
Why is it Required?
• Meiosis produces variation in eukaryotic organisms. This variation is essential, not only for interest and diversity in a population but also for adaptation and evolution within a population.
• If the environment changes, variation allows for individuals that are best fit for that environment to be selected for, survive and reproduce. If all individuals were clones of each other, which would be the result of asexual reproduction, then all individuals would not only look alike but could face mass extinction when the environment changes.
• The same complexity and structures observed in mitosis is also seen in meiosis.
Stages of Meiosis : Interphase• Meiosis also follows the cell cycle where G1, S and G2 of a eukaryotic cell is
known as interphase.
• The difference is that the stages occur twice, in sequence, and is therefore known as meiosis I and meiosis II.
• Chromosomes occur in homologous pairs, which mean one set of chromosomes (maternal) are identical to the second set of chromosomes (paternal).
• For example, your cells have 46 chromosomes: 23 from your mother are homologous to the 23 from your father. They are the same size and the same structure. The only difference is the instructions that the genes carry.
• Meiosis I separate these homologous pairs whereas
meiosis II separates the chromatids (as seen in mitosis).
Stages of Meiosis I: Prophase I
• Prophase I is the first stage in which the nuclear envelope disintegrates and the chromosomes become visible. The DNA is doubled (after S phase) and therefore the chromosomes appear as 'X'. It is the same as prophase in mitosis.
Stages of Meiosis I: Metaphase I
• During metaphase I, the spindles from the centrioles have attached to the centromeres of the chromosomes.
• It is NOT the individual chromosomes that line up along the equator (as in metaphase of mitosis) but rather the homologous pairs line up at the equator of the cell.
Stages of Meiosis I: Anaphase I
• It is then during anaphase I that the spindles will pull the identical chromosomes apart to opposite ends of the cell.
Stages of Meiosis I: Telophase I
• During telophase I, cytokinesis occurs to produce two cells that have half the chromosome number that the original cell had. For example, each cell will have 23 chromosomes that contain double the amount of DNA. The chromosomes still at this point appear as 'X's because the doubled DNA produced during S and seen as chromatids joined at the centromere has not yet separated. This will be the function of Meiosis II.
Stages of Meiosis II: Prophase II
• Prophase II now occurs and the nuclear envelope in each of the two cells disintegrate.
Stages of Meiosis II: Metaphase II
• Metaphase II allows individual, but reduced, chromosomes to line up at the equator.
Stages of Meiosis II: Anaphase II
• Anaphase II the sister chromatids pull apart and move to opposite ends of the cell. The chromosomes are now linear (but reduced by half from meiosis I).
Stages of Meiosis II: Telophase II
• Telophase II and final cytokinesis produces four cells that each have half the number of chromosomes than the original parent cell each chromosome only with the 'regular' amount of DNA
Meiosis I – homologous chromosomes
Special Notes about Meiosis
Question 1Define the following terms (cite your source): ploidy, diploid, haploid, gametes, DNA, chromosomes, chromatin, centromere, chromatid, spindle, histones, and genes.
Question 2Write a paragraph, using the terms above, to show your understanding of their relationship. Do NOT simply redefine but rather compare and contrast to show that you understand the definitions that you just wrote.