Cellular Division: Cell Cycle Phases

Cellular Division Cell Cycle Phases: The cell cycle consists of two main sections known as interphase (primarily cell growth occurs here and there isn’t much division/proliferation here) and the other is mitosis (active cell division and have the four parts of it) Cancer cells do not undergo interphase since they are always constantly dividing Interphase consists of: 1. G1: longest phase of cell cycle, make extra organelles, and proteins 2. S: DNA replication occurs here (duplicates) 3. G2: directly preparing for mitosis (such as making microtubules) 4. G0: is an option that cell’s can undertake following G1 where no cell division occurs (such as neurons) and won’t typically come back into the cell cycle Cell Cycle Control: Two main checkpoints in the cell cycle: 1. Between G1 and the S phases (right before DNA replication) → Done by RB 2. Between G2 and mitosis (right before cell division) Cyclin Dependent Kinases (CDKs) add phosphate groups onto other proteins to activate/inactivate them (work together with cyclins) Are always present in the cell but their default function is inactive (needs to be activated and bound by cyclins) Specific cyclins are only generated at specific times For example, RB typically inhibits DNA replication but when it is phosphorylated by CDKs, it inactivates and allows the cell to enter the S phase Loss of Cell Cycle Control in Cancer: p53 is also known as the “guardian of the genome” and binds DNA directly in order to produce proteins which block the progression of the cell cycle (made from a tumor suppressor gene) One of these proteins is known as p21 which functions to inhibit CDK RB is another tumor suppressor gene When tumor suppressor genes become defective/inactivated, this can lead to cancer Fertilization Terminology: Gametes, Zygotes, Haploid, Diploid: Fertilization is the process of the sperm and ovum (egg) fusing together Gametes possess half of the number of chromosomes that somatic cells possess (haploid cells) Zygotes are fertilized eggs (once the sperm fuses with the egg) Homologous chromosomes are a pair of chromosomes that have the same genes (although their specific makeup could be different) Zygote Differentiating Into Somatic & Germ Cells: After mitosis, you still have the same number of chromosomes (23 pairs of homologous chromosomes) Mitosis keeps occurring in order to start generating more cells for the organism Some cells differentiate into the different parts of the body Somatic Cells: body cells with 23 pairs of homologous chromosomes Germ Cells: sex cells that have a haploid number of chromosomes (23 in humans) and through meiosis you will produce sperm/ova Mutations in somatic cells will not be passed onto offspring, but mutations in the germ cells has a chance of being passed onto your offspring Mitosis: Chromatin is the combination of DNA and special structural proteins (histones) Bivalent chromosomes consist of two sister chromatids 1. Prophase: bivalent chromosomes condense into tight packages, the mitotic spindle forms, centrosomes migrate to opposite sides of the cell, and the nuclear envelope dissolves 2. Metaphase: each chromosome lines up in a single file line in the center of the cell (mitotic spindles have already attached to the chromosomes) 3. Anaphase: the mitotic spindles pry each chromatid apart from its copy and drag them to the opposite side of the cell 4. Telophase: membrane forms around the two new groups of chromosomes and the mitotic spindle is disassembled (DNA begins unwinding) The cell splits via cytokinesis creating two clones of the original cell Interphase: Remember, during interphase the DNA isn’t tightly bound, it just kind of looks like spaghetti Sister chromatids are considered one chromosome Centromeres are where the two sister chromatids are connected to one another Centrosomes are where the mitotic spindles originate from (microtubules) and duplicate during the S phase of the cell cycle (each have two centrioles) → microtubules originating here connect to the kinetochores of the chromosomes (for pulling during anaphase) → ONLY FOUND IN EUKS since its is a membrane bound organelle Mitosis: Four steps: prophase, metaphase, anaphase, and telophase (definitions located above) Cytokinesis follows (cell splits into two separate cells) Comparing Mitosis & Meiosis: Mitosis starts with a diploid cell and ends with two identical diploid cells Meiosis starts with a diploid cell and ends with four different haploid cells Mitosis can keep on going and going while meiosis ends with the four different haploid cells Mitosis occurs in somatic cells Meiosis occurs in gamete cells Meiosis: Meiosis is used for basically only one purpose in the human body, the production of gametes (sex cells → sperm or egg cells) with a goal of making daughter cells with half as many chromosomes As in mitosis, in meiosis cells still must undergo interphase (G1, S, and G2) 2 Phases of Meiosis: 1. Meiosis I a. Prophase I: chromosomes begin to condense and homologous chromosomes pair up (think of 4, 2 pairs) i.Crossing Over can occur here (where homologous chromosomes switch genetic material) ii.Synaptonemal Complex holds together homologous chromosomes and aids with the crossing over event (chromosomes are actually positioned one on top of the other, they are just shown side by side to make it easier to visualize) and are made up of specialized proteins iii.Chiasmata are cross shaped structures that keep the homologous chromosomes connected to each other once the synaptonemal complex breaks down (each homologous pair needs at least one) b. Metaphase I: each chromosome from a homologous pair attaches to microtubules from just one pole (the chromosomes line up in a random order, mixing the chromosomes from mom or dad → Law of Segregation or Mendel’s First Law) c. Anaphase I: homologous pairs are pulled apart and moved to opposing poles (sister chromatids, however, remain intact) d. Telophase I: chromosomes arrive at opposite poles (some organisms the chromosomes decondense and the nuclear membrane reforms while in others this is skipped since meiosis II follows) and cytokinesis follows, forming two haploid daughter cells 2. Meiosis II is overall shorter and simpler process (a short rest period known as Interphase II may occur between meiosis I and II) a. Prophase II: chromosomes condense and the nuclear envelope dissolves (IF NEED BE), centrosomes move apart (replicated at some point) and the spindle forms between then b. Metaphase II: chromosomes line up individually along the metaphase plate c. Anaphase II: sister chromatids are separated and pulled towards opposite poles of the cell d. Telophase II: nuclear membrane forms around each set of chromosomes and the chromosomes decondense, followed by cytokinesis to finally split the cells into 4 haploid cells (NO SISTER CHROMATIDS) Gametes produced during meiosis are not genetically identical, since they are from mom, dad, and also exhibited crossing over during prophase I Embryonic Stem Cells: Morula: a small ball of cells resulting from division of a fertilized ovum (kind of turns into a sphere) and is made up of cells known as Blastomeres At around 16 cells, the morula begins to differentiate where the outer layer of cells is known as the Trophoblasts (eventually turns into the placenta) and the inner cells are known as the Embryoblasts (inner cell mass that ends up becoming the organism itself) Fluid known as the Blastocoel fills the space inside of the morula over time and once this occurs, the morula is now called a Blastocyst (occurs in humans) Embryonic Stem Cells have the capability to turn into any type of cell and therefore any part of the human body The idea here is that you could take the embryonic stem cells and place them anywhere there was damage and they’d differentiate to fit that specific environment (Ethically you’d have to kill the embryo that could go on to become a human in order to extract these embryonic stem cells) Somatic (Adult) Stem Cells are located in bone marrow and other parts of our body that can also aid in specific differentiation but they simply aren’t as plastic as embryonic stem cells are (meaning they can’t differentiate into any type of cells in the human body like embryonic stem cells can) Cancer: Contact Inhibition: when cells feel to crowded, they know to stop proliferating Also, unhealthy cells will undergo apoptosis in order to make way for other healthy cells in its vicinity Remember, mutations in somatic cells will not be passed onto offspring Neoplasm is a new and abnormal growth of tissue in some part of the body, especially as a characteristic of cancer Tumors are when neoplasms turn into noticeable lumps Benign (harmless) tumors are when the cells are growing a little out of control but not at a rate to cause alarm to its surroundings Malignant tumors are when cells are dividing so fast that they start becoming invasive to other tissues/parts of the body which is known as metastasis (mutations begin to occur more frequently) Additional Note: a karyotype is an image taken in order to visualize separate chromosomes, meaning it’d be best to do it during metaphase when the chromosomes are arranged as perfect X’s