Cell and NuclearDivision
Cell and Nuclear Division
This lesson develops Cell and Nuclear Division as part of Cellular Respiration. It uses the source curriculum to organize the important facts, relationships, examples, and vocabulary into a focused Stemosphere teaching sequence.
Keep the central model in view: Cell and Nuclear Division explains observable patterns by connecting structure, process, evidence, and quantitative relationships.
In Stemosphere terms, cell and Nuclear Division
Cell Proliferation: the process in which a cell grows and hereby divides to produce two daughter cells, leading to an increase in number of cells and a pathway for growth and repair. Examples include:
Embryological Development
early embryos grow through mitosis and cytokinesis from cell proliferation.
Tissue Replacement
Replacement of dead tissue.
Plant Meristems
a group of undifferentiated cells which reproduce and hence differentiate to produce plants, tissues and organs.
Figure 1: Cell Proliferation Breakdown
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In Stemosphere terms, the Cell Cycle—Interphase
Interphase: where the cell spends most of its cell cycle in, its stages include:
G1 Phase
The cell grows for mitosis and cytokinesis, involving a lot of protein synthesis allowing the cell to grow and build new organelles.
S Phase
DNA Replication begins, producing chromosomes WITH sister chromatids
G2 Phase
Growth and replication of Organelles.
Note
These processes are controlled by cyclins-a group of four proteins- which bind to and activate a group of enzymes known as cyclin-dependent kinases (CDK). Hence, this group attaches phosphates to specific proteins, which starts these reaction s. It is important to note that these cyclins vary in threshold level based on which reaction they are optimum for.
Figure 2: Cyclin Activity
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DNA Replication and Condensation of Chromosomes:
DNA Replication
DNA replication must occur before mitosis and meiosis and produces two identical copies held together by a chromosome.
In Stemosphere terms, sister chromatids are two identical copies of a duplicated chromosome and are separated during anaphase of mitosis and anaphase II of meiosis to form individual chromosomes.
Condensation of Chromosomes
Although chromosome mostly exists as chromatin through a cell’s life- which is not visible under a light microscope because chromosomes are spread out the nucleus- it later condenses to form visible chromosomes with sister chromatids during prophase of mitosis and prophase I and II of meiosis.
In Stemosphere terms, this is done in which DNA coils around histone proteins to form nucleosomes which coil around each other to form chromosomes with sister chromatids.
In Stemosphere terms, in spindle fibers, they are composed of protein microtubules which allow them to attach to the centromere of chromosomes by kinetochores. The microtubule motors on kinetochores move the chromosomes to each pole of th cell during anaphase.
Mitosis
Split into 4 key stages- PMAT- mitosis is a nuclear division which produces two genetically daughter nuclei, in which the cytoplasm of the parent cell undergoes cytokinesis after mitosis to produce two genetically identical daughter cells.
In Stemosphere terms, it is required for growth and development, tissue repair, and much more.
The process includes:
Prophase
Chromatin condenses to form chromosomes which are visible under a light microscope and are composed of IDENTICAL sister chromatids attached to the centromere. The nuclear membrane breaks down (barely visible), and centrioles move towards the poles leading to spindle fibers.
Metaphase
Spindle fibers attach to the centromeres of sister chromatids causing them to line up along the equator of the cell.
Anaphase
The spindle fibers separate the sister chromatids at the centromere, facilitated by microtubule motors on kinetochores moving each strand to the opposite poles on the cell.
Telophase
The chromosomes arrive at the poles of the cell, with a nuclear membrane forming at each set of chromosomes. Hence, the chromosomes uncoil to form chromatin, followed by the production of two identical nuclei and cytokinesis begins to form two identical daughter cells.
Figure 3: Mitosis in Animal and Plant Cells
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Haploid and Diploid Nuclei:
Haploid Cell
contains a haploid nucleus and one set of chromosomes (n).
Diploid Cell
contains a nucleus with two sets of chromosomes (2n).
In Stemosphere terms, in meiosis, it is often called reduction division where the parent cell is diploid and produces 4 haploid daughter cells.
In Stemosphere terms, in diploid cells, two sets of homologous chromosomes (same genes in same locations may have different alleles) are replicated before meiosis to give four sister chromatids.
In Stemosphere terms, when these homologous chromosomes pair up, they form bivalents during meiosis I, where the non-sister chromatids cross over to exchange their sections.
In Stemosphere terms, in tetrads, they are the actual four sister chromatids resulting from the pairing of homologous chromosomes.
Figure 4: Haploid and Diploid Cells
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Meiosis: a process of nuclear division which produces haploid cells and is split into two key divisions with each followed by cytokinesis, and they are:
Meiosis I: segregates the homologous chromosomes to produce two haploid cells and involved the following steps:
Prophase 1
homologous chromosomes pair up to form bivalents, in which crossing over occurs with the switch of alleles between non-sister chromatids through condensation of chromatin. The centrioles move towards the pole and start to produce spindle fibers, and the nuclear membrane breaks down.
Metaphase 1
The spindle fibers move the bivalents to the equator of the cell, and the sister chromatids are attached to the spindle fibers at the centromere. Random assortment occurs between maternal and paternal chromosomes as they line up.
Anaphase 1
Homologous chromosomes of the bivalents are separated and are pulled towards the poles of the cell by spindle fibers, through microtubule motors.
Telophase 1
The chromosomes uncoil as sister chromatids arrive at the pole. A nuclear membrane forms around the sister chromatids at each pole producing two haploid nuclei, followed by cytokinesis to produce two daughter cells.
Meiosis II: the second division of meiosis and produces four daughter nuclei, and involves the following steps:
Prophase II
Chromosomes as sister chromatids appear supercoiled and are in both haploid cells. The centrioles move towards the poles, producing the spindle fiber microtubules, followed by the breaking down of the nuclear membrane.
Metaphase II
Chromosomes with sister chromatids line up along the equator of the cells, attached to the spindle fiber by the centromere (no bivalents involved but singe chromosomes)
Anaphase II
Sister chromatids are pulled apart leading to single-stranded chromosomes and are moved by microtubule motors on kinetochores towards the poles.
Telophase II
The chromosomes reach the poles of each cell, and the chromosomes uncoil with the nuclear membrane forming around each set of chromosomes. After, cytokinesis occurs in both cells, forming 4 cells with haploid nuclei which are not identical.
Figure 5: Meiosis
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Non-Disjunction:
In Stemosphere terms, non-disjunction can occur during anaphase I or anaphase II and is the failure of one or more pairs of homologous chromosomes or sister chromatids to separate fully during nuclear division. This produces gametes with an extra or missing chromosome.
In Stemosphere terms, if a gamete with an extra chromosome is fertilized by a normal gamete, the zygote and offspring, and the offspring, three copies of chromosomes are observed. An example is down syndrome with chromosome 21, called trisomy.
In Stemosphere terms, with a missing chromosome fertilized by a normal gamete, the offspring and zygote will have one copy of the chromosome, called monosomy.
Figure 6: Trisomy and Monosomy
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The Law of Independent Assortment and The Law of Segregation:
In Stemosphere terms, this law states that when two or more characteristics are inherited, the combinations of offsprings do not always match the combination of traits in parent organisms.
In Stemosphere terms, it describes that the inheritance of a pair of genes is independent of the inheritance of another pair (metaphase I), with allele pairs separating independently of each other during gamete formation (meiosis). Therefore, the presence of one trait does not affect the presence of the other trait.
In Stemosphere terms, on the other hand, the law of segregation states that allele pairs randomly separate from one another during meiosis (anaphase I), where when organism inherit two copies of each gene from each parent, organism only donate one copy of each gene to their gamete. The two copies of each gene then segregate in meiosis.
Cytokinesis:
In Stemosphere terms, cytokinesis is the splitting of the cytoplasm of a parent cell to form two daughter cells.
In Stemosphere terms, in animal cells, a contractile ring of actin and myosin filaments attached to the plasma membrane and contract forming a cleavage furrow, forms around the equator of the cell. Later, the cleavage furrow deepens until the two daughter cells separate.
In Stemosphere terms, in plant cells, the Golgi apparatus produces vesicles of carbohydrates that move up and line up along the equator of the cell, and fuse to form a cell plate. Then, the membranes from the vesicles fuse with the plasma membrane to form two separate cells (fusion occurs on either side of the cell plate). Cellulose is secreted into the cell plate from the cell wall.
Note
During Cytokinesis, there are rare occasions where division of the cytoplasm is unequal between the two daughter cells.
Figure 7: Cytokinesis in Plant Cells and Animal Cells
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The Mitotic Index:
In Stemosphere terms, a high mitotic index suggests cancer is present in the tissue
Mutation of Cell Cycle Genes:
Tumor suppression genes
regulate cell division, and when mutated, uncontrolled cell division occurs.
Proto-oncogenes
regulate normal cell growth, and when mutated, become oncogenes which change cells into tumor cells and can lead to cancer.