Cellular Reproduction PDF

6 CELLULAR REPRODUCTION LEARNING OBJECTIVES At the end of this module the students will be able to: 1. Compare and contrast the processes of binary fission, mitosis, and meiosis, highlighting the key differences in their mechanisms and outcomes, including the...

6 CELLULAR REPRODUCTION LEARNING OBJECTIVES At the end of this module the students will be able to: 1. Compare and contrast the processes of binary fission, mitosis, and meiosis, highlighting the key differences in their mechanisms and outcomes, including the number of daughter cells produced and their genetic content. 2. Describe the stages of the cell cycle, including interphase and the mitotic phases, and explain the role of checkpoints in regulating cell division and preventing errors. 3. Analyze the significance of meiosis in sexual reproduction, explaining how it contributes to genetic diversity through crossing over and independent assortment, and how it produces haploid gametes necessary for fertilization. LEARNING CONTENT CELL CYCLES - The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells. o Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produces two identical (clone) cells. o The cell cycle has two major phases: interphase and the mitotic phase. During interphase, the cell grows and DNA is replicated. o During the mitotic phase, the replicated DNA and cytoplasmic contents are separated, and the cell divides. Interphase - During interphase, the cell undergoes normal growth processes while also preparing for cell division. In order for a cell to move from interphase into the mitotic phase, many internal and external conditions must be met. The three stages of interphase are called G1, S, and G2. G1 Phase (First Gap) - The first stage of interphase is called the G1 phase (first gap) because, from a microscopic aspect, little change is visible. However, during the G 1 stage, the cell is quite active at the biochemical level. The cell is accumulating the building blocks of chromosomal DNA and the associated proteins as well as accumulating sufficient energy reserves to complete the task of replicating each chromosome in the nucleus. S Phase (Synthesis of DNA) - Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration. In the S phase, DNA replication can proceed through the mechanisms that result in the formation of identical pairs of DNA molecules—sister chromatids—that are firmly attached at the centromeric region.  - The centrosome is duplicated during the S phase. The two centrosomes will give rise to the mitotic spindle, the apparatus that orchestrates the movement of chromosomes during mitosis. - At the center of each animal cell, the centrosomes of animal cells are associated with a pair of rod-like objects, the centrioles, which are at right angles to each other. Centrioles help organize cell division. - Centrioles are not present in the centrosomes of other eukaryotic species, such as plants and most fungi. G2 Phase (Second Gap) - In the G2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation. - Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase. - There may be additional cell growth during G2. - The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis. The Mitotic Phase - The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated, and move into two new, identical daughter cells. - The first portion of the mitotic phase is called karyokinesis, or nuclear division. The second portion of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic components into the two daughter cells. Coordination of Cell Division Multicellular organisms need to coordinate cell division across different tissues & organs – critical for normal growth, development & maintenance coordinate timing of cell division coordinate rates of cell division  not all cells can have the same cell cycle. Frequency of Cell division Frequency of cell division varies by cell type – embryo cell cycle < 20 minute – skin cells divide frequently throughout life 12-24 hours cycle – mature nerve cells & muscle cells do not divide at all after maturity permanently in G0 M metaphase anaphase prophase telophase C G2 interphase (G1, S, G2 phases) mitosis (M) cytokinesis (C) G S 1 Why are some cells regulate division? There are several factors that regulate the cell cycle and assure a cell divides correctly. 1. Checkpoints – DNA checked to make sure it is copying properly – Improper replication = mutation 2. Chemical signals - Internal and external Regulating the Cell Cycle - Experiments show that normal cells will continue to grow until they come into contact with other cells. - When cells come into contact with other cells, they stop growing. This is called contact inhibition. - This demonstrates that cell growth and division can be turned on and off. - Proteins called cyclins regulate the timing of the cell cycle. - Internal regulators: allow the cell to proceed to the next phase of the cell cycle only when certain processes have occurred inside the cell. Example: These proteins will not allow a cell to continue into G 2until all chromosomes have been duplicated during S phase. - External regulators: speed up or slow down the cell cycle depending on events outside of the cell. Example: Contact inhibition BINARY FISSION - Binary fission is a type of asexual reproduction typically observed in prokaryotes and a few single-celled eukaryotes. - In this method of asexual reproduction, there is a separation of the parent cell into two new daughter cells. - This process happens with the division and duplication of the parent’s genetic matter into two parts. Here, each daughter cell receives one copy of its parent DNA. - It is a primary method of reproduction in prokaryotic organisms. - Binary Fission occurs without any spindle apparatus formation in the cell. o In this process, the single DNA molecule begins replication and then attaches each copy to various parts of the cell membrane. o When the cell starts to get drawn apart, the original (actual) and replicated chromosomes get apart. - However, asexual mode of reproduction has a significant drawback. o All resultant cells are genetically identical, mirror copies of each other and the parent cell. o Most antibiotics work on this principle. o If a parent cell is vulnerable to an antibiotic, then all resultant daughter cells are vulnerable too. o If a mutation occurs in their genes, then it can render a particular strain resistant to antibiotics.  - Prokaryotes such as E. coli, Archaea as well as eukaryotes such as euglena reproduce through binary fission. The steps involved in the binary fission in bacteria are: 1. Replication of DNA - The bacterium uncoils and replicates its chromosome, essentially doubling its content. 2. Growth of a Cell - After copying the chromosome, the bacterium starts to grow larger in preparation for binary fissions. It is followed by an increase in cytoplasmic content. Another prominent trait of this stage is that the two strands migrate to opposite poles of the cell. 3. Segregation of DNA - The cell elongates with a septum forming at the middle. The two chromosomes are also separated in this phase. 4. Splitting of Cells - A new cell wall is formed at this phase, and the cell splits at the center, dividing the parent cell into two new daughter cells. Each of the daughter cells contains a copy of the nuclear materials as necessary organelles. MEIOSIS AND SEX CELLS - A process called meiosis produces haploid sex cells - During meiosis, two divisions of the nucleus occur. - These divisions are called meiosis I and meiosis II. MEIOSIS I - Before meiosis begins, each chromosome is duplicated, just as in mitosis. Prophase I - Occupies the longest divisions of meiosis Subdivided into five stages: 1. Leptonema – replicated chromosomes (leptotene) appear as long slender threads. 2. Zygonema – pairing of homologous chromosomes (synapsis); the pair is referred to as bivalent or tetrad (zygotene) 3. Pachynema – chromosomes continue to become shorter and thicker (pachytene), a series of exchange of genetic material can occur (crossing over) between specific regions of the homologous chromosomes. 4. Diplonema – the tetrad tends to repel each other (diplotene); crossing- over have taken place; chiasma, the area of contact between two chromatids, becomes distinct. 5. Diakinesis – coiling and contraction of the chromosomes continue; the bivalent migrate close to the nuclear membrane; the nucleolus disappear and the nuclear membrane begins to break down; spindle apparatus begins to form. Metaphase I - Synapsed homologous chromosomes are aligned at the equator of the cell - Spindle apparatus is completely formed. Anaphase I - The whole chromosomes from each tetrad separate and migrate toward the opposite poles of the cell. - The centromeres of each bivalent do not divide. - The chromatids (dyads) remain attached at their respective centromere. Telophase I - The dyads reach the poles of the cell - New nuclear membranes may form - New nucleolus may form - Cytokinesis occurs resulting into 2 daughter cells with haploid number of chromosomes.  MEOSIS II - Similar to the events of mitosis - Follows interkinesis - No chromosomes replication between meiosis I and meiosis II Prophase II - The dyads becomes thicker and shorter. The duplicated chromosomes and spindle fibers reappear in each new cell. Metaphase II - The centromeres of each dyad are directed to the equator of the cell - Then the centromere divide. Anaphase II - Single stranded chromosomes (monads) separate and migrate towards the opposite pole of the cell. Telophase II - The monads reach the pole of the cell - New nuclear membranes may form - Cytokinesis occurs resulting daughter cells with the same haploid number of chromosomes - The chromosomes uncoil and become thinner and invisible again. MITOSIS AND THE CELL CYCLE - All living organism has a life cycle - A life cycle begins with the organism’s formation, is followed by growth and development, and finally ends in death. - Individual cells have life cycles. THE CELL CYCLE - The cylindrical process of crowth and mitosis - Consists of four phases: three stages of interphase (G1, S, and G2) and mitotic phase. INTERPHASE - The preparatory stage for mitosis not the resting for the cell does not rest. - The nucleus is clearly visible with one or more distinct nucleoli - Chromosomes appear as irregular- granular form, thus cannot be recognized.  - Consists of three subdivisions: first growth period (G1), synthesis period (S), and second growth period (G2). G1 period (pre-synthesis interphase) - Growth of the cell - RNA and protein synthesis take place - Building of new protoplasm and cytoplasmic organelles - Enzymes necessary for DNA synthesis are synthesized S period (synthesis phase) - Most critical period - DNA synthesis and replication take place - RNA and protein synthesis continue - Synthesis of histone and other proteins necessary to maintain DNA synthesis occur - Centriole reproduction (in animal cell) begins G2 period (post-synthesis interphase) - Completion of DNA synthesis and replication - Continuation of RNA and protein synthesis - Prepares the cell to undergo mitosis. MITOSIS - Somatic cell division - A process that produces two daughter cells with the same quantity and quality of chromosomes as the parent cell. - Also called duplication division - Refers to the division of the nucleus (karyokinesis)  - Quickly followed by the division of the cytoplasm (cytokinesis).Prophase - The phase of preparation - Occupies almost 1/3 of mitosis - Chromosomes at first appear as thin threads and becoming shorter and thicker - Each chromosome is visible two chromatids held together by the centromere - Chromosomes move toward the equator of the cell - Centrioles move to opposite poles of the cell - Nucleolus no longer visible - Nuclear membrane start to disappear - Mitotic apparatus (asters and spindle fibers in animal cell) are nearly formed. Metaphase - Double- stranded chromosomes are aligned at the equator of the cell - Centromeres of each chromosomes are attached to the spindle fibers - Mitotic apparatus completely formed - Nuclear membrane completely disappeared Anaphase - The phase of migration - Centromeres of each chromosome divide - Two sets of single-stranded chromatids (daughter chromosomes) separate and move towards opposite poles of the cell - Cytokinesis begins (formation of cleavage furrow in animal cell) Telophase - The phase of reconstruction - Daughter chromosomes finally reached the opposite poles of the cell. - Chromosomes begin to become longer, thinner, and less distinct - Centrioles are replicated - Nucleolus reappear - Mew nuclear membranes form - Mitotic apparatus diasappears - Cytokinesis completed resulting into two daughter cells with the same quantity and quality of chromosomes as the parent cell. MITOSIS VS MEIOSIS Mitosis Meiosis Results in 2 Diploid Cells (2n) 4 Haploid Cells (n) Cells are Genetically Identical Genetically Different Occurs in Somatic (Body) Cells Sex Cells (Gamete) SYNTHESIS Cellular reproduction is the process by which cells create new cells, essential for growth, repair, and reproduction. There are several different types of cell division, each with its own unique purpose and mechanism. In prokaryotic cells, binary fission is the primary method of reproduction, where a single cell duplicates its DNA and divides into two identical daughter cells. Eukaryotic cells, on the other hand, utilize a more complex process called the cell cycle, which involves a series of stages leading to cell division. The cell cycle consists of interphase, where the cell grows and replicates its DNA, followed by the mitotic phase, which includes mitosis and cytokinesis. Mitosis is the process of nuclear division, where the duplicated chromosomes are separated into two identical nuclei, ensuring that each daughter cell receives a complete set of genetic information. While mitosis is crucial for growth and repair, sexual reproduction relies on a specialized form of cell division called meiosis. Meiosis involves two rounds of division, resulting in four haploid daughter cells, each containing half the number of chromosomes as the original parent cell. During meiosis, genetic diversity is introduced through crossing over and independent assortment, ensuring that each gamete (sperm or egg) carries a unique combination of genes. This genetic diversity is essential for the evolution and adaptation of species. Understanding the different types of cellular reproduction is crucial for comprehending the fundamental processes of life, from the growth of organisms to the inheritance of traits across generations. WORKSHEET DIRECTION: On a separate paper answer the following questions: A) The diagram below shows six cells in various phases of the cell cycle. Note the cells are not arranged in the order in which the cell cycle occurs. Use the diagram to answer questions 1-11. 1. Cells A & F show an early and a late stage of the same phase of the cell cycle. What phase is it? ______________________________________________________________ 2. Which cell is in metaphase? ______________________________________________________________ 3. Which cell is in the first phase of M phase (mitosis)? ______________________________________________________________ 4. List the diagrams in order from first to last in the cell cycle. ______________________________________________________________ 5. In cell A, what structure is labeled X? ______________________________________________________________ 6. Are the cells depicted plant or animal cells? Explain your answer. ______________________________________________________________ a. If it were the other type of cell what would be different in the diagrams? ________________________________________________________ 7. What is the longest phase of the cell cycle? ______________________________________________________________ 8. Why is mitosis important? ______________________________________________________________ 9. Predict what would happen if an individual had faulty spindle fibers ______________________________________________________________ 10. Predict what would happen if cytokinesis was skipped. ______________________________________________________________ REFERENCES 1. Alberts, B. et al. (2014). Molecular Biology of the Cell. 6th ed. Garland Science. 2. Cooper, G.M. (2000). The Cell: A Molecular Approach. 2nd ed. Sinauer Associates. 3. Lodish, H. et al. (2016). Molecular Cell Biology. 8th ed. W.H. Freeman. 4. Reece, J.B. et al. (2013). Campbell Biology. 10th ed. Pearson. 5. Sadava, D. et al. (2019). Life: The Science of Biology. 12th ed. Sinauer Associates.