Chapters 6 The Cell Cycle
How do organisms grow? or (2) cells divide to form new cells? Generally, do – (1) individual cells keep getting larger and larger? or (2) cells divide to form new cells?
Why don’t individual cells keep growing larger and larger? Cells are limited in size because they get their nutrients by diffusion. If cells become too large, nutrients such as oxygen and glucose (which are necessary for aerobic cell respiration) will not be able to diffuse quickly enough into the cell in order for it to survive.
Let’s start with some definitions….
Chromatin and Chromosome Remember: Chromatin is long and thin. It is uncoiled DNA. (DNA still keeps it’s double helix shape, however. A chromosome is coiled DNA. The DNA coils around proteins.
Chromosomes Compact Before chromosomes are created, DNA is copied The two exact copies are called chromatids Two chromatids are held together by a centromere
Chromatin and Chromosome
Gene A section of DNA that contains the code to make a protein.
Somatic Cells versus Gametes Gamete = reproductive (sex) cell. Each gamete contains 23 chromosomes. It is haploid (having 1 set of chromosome in the cell). Often written as n Somatic cell = body cell. Each somatic cell contains 46 chromosomes It contains the diploid number of chromosomes (having 2 sets of chromosomes in the cell) Often written as 2n
Diploid and Haploid Cells Two haploid cells merge to form one diploid cell. This process is called fertilization. Male gametes are called sperm; female gametes are called eggs Two sex cells (sperm and egg) merge to form a zygote.
Homologous Chromosomes Homologous Chromosomes are like a pair of shoes You get one “shoe” from your mother and one “shoe” from your father n + n = 2n
Karyotype ( a chart of chromosomes) showing homologous chromosomes - female
Homologous chromosomes are similar in size, shape and genetic content. Autosomes are pairs 1-22. The chromosomes of pair 23 are sex chromosomes and determine a person’s gender.
Zygotes and Specialization Once a zygote is formed, it undergoes cell division many, many times to form a multicellular organism. The cells specialize to perform specific functions for the organism.
Mitosis Mitosis: the process in cell division in which the nucleus of a cell is divided into two nuclei. Each nucleus has the same number of chromosomes as the original cell.
Prokaryotic Cell Division Prokaryotes (bacteria) have only one chromosome. It is circular in shape and is attached to the cell membrane. When a prokaryote divides, the cell makes a copy of its DNA, then the cell divides. Each cell has one copy of the chromosome. Prokaryotic cell division is called binary fission. Prokaryotes do not go through mitosis, nor do they have gametes or zygotes.
Control of Cell Cycle
The Cell Cycle Cell Cycle: a repeating sequence of growth and division during the life of a cell. The cell cycle has three parts: 1. Interphase: the part of the cell cycle in which an organism grows and carries out its metabolism Before it is about to divide, the cell will copy its DNA.
The Cell Cycle, Parts 2 and 3 2. Mitosis: Division of the nucleus. There are 4 phases of mitosis 3. Cytokinesis: the splitting of the cytoplasm of the cell.
Biology is the only subject in which multiplication is the same thing as division…
The Cell Cycle: Cell Growth, Cell Division
Getting from there to here… Function of cell division making new cells continuity of life asexual reproduction unicellular organisms growth repair & renew Cell cycle life of a cell from origin to division into 2 new daughter cells amoeba Unicellular organisms Cell division = reproduction Reproduces entire organism& increase population Multicellular organisms Cell division provides for growth & development in a multicellular organism that begins as a fertilized egg Also use cell division to repair & renew cells that die from normal wear & tear or accidents
Centrioles Cell division in animal cells, pair of centrioles organize microtubules spindle fibers guide chromosomes in mitosis
Getting the right stuff What is passed on to daughter cells? exact copy of genetic material = DNA mitosis division of organelles & cytoplasm cytokinesis chromosomes (stained orange) in kangaroo rat epithelial cell notice cytoskeleton fibers
Overview of mitosis I.P.M.A.T. interphase prophase (pro-metaphase) cytokinesis metaphase anaphase telophase
Interphase 90% of cell life cycle Normal day in and day out cell doing its “everyday job” produce RNA, synthesize proteins/enzymes prepares for duplication if triggered
Cell cycle Cell has a “life cycle” cell is formed from a mitotic division cell grows & matures to divide again cell grows & matures to never divide again G1, S, G2, M liver cells G1G0 epithelial cells, blood cells, stem cells brain / nerve cells muscle cells
Interphase Divided into 3 phases: G0 G1 = 1st Gap S = DNA Synthesis cell doing its “everyday job” cell grows S = DNA Synthesis copies chromosomes G2 = 2nd Gap prepares for division cell grows (more) produces organelles, proteins, membranes G0 signal to divide
Interphase Nucleus well-defined Prepares for mitosis DNA loosely packed in long chromatin fibers Prepares for mitosis replicates chromosome DNA & proteins produces proteins & organelles
Copying / Replicating DNA Synthesis phase of Interphase dividing cell replicates DNA must separate DNA copies correctly to 2 daughter cells human cell duplicates ~3 meters DNA each daughter cell gets complete identical copy error rate = ~1 per 100 million bases 3 billion base pairs in mammalian genome ~30 errors per cell cycle mutations (to somatic cells)
Organizing DNA DNA is organized in chromosomes ACTGGTCAGGCAATGTC DNA Organizing DNA DNA is organized in chromosomes double helix DNA molecule wrapped around histone proteins like thread on spools DNA-protein complex = chromatin organized into long thin fiber condensed further during mitosis histones chromatin double stranded chromosome duplicated mitotic chromosome
Copying DNA & packaging it… After DNA duplication, chromatin condenses coiling & folding to make a smaller package DNA mitotic chromosome chromatin
Mitotic Chromosome Duplicated chromosome 2 sister chromatids narrow at centromeres contain identical copies of original DNA homologous chromosomes homologous chromosomes Centromeres are segments of DNA which have long series of tandem repeats = 100,000s of bases long. The sequence of the repeated bases is quite variable. It has proven difficult to sequence. sister chromatids single-stranded homologous = “same information” double-stranded
Mitosis Dividing cell’s DNA between 2 daughter nuclei 4 phases “dance of the chromosomes” 4 phases prophase metaphase anaphase telophase
Prophase Chromatin condenses visible chromosomes chromatids Centrioles move to opposite poles of cell animal cell Protein fibers cross cell to form mitotic spindle microtubules actin, myosin coordinates movement of chromosomes Nucleolus disappears Nuclear membrane breaks down
Transition to Metaphase Prometaphase spindle fibers attach to centromeres creating kinetochores microtubules attach at kinetochores connect centromeres to centrioles chromosomes begin moving
Metaphase Chromosomes align along middle of cell metaphase plate meta = middle spindle fibers coordinate movement helps to ensure chromosomes separate properly so each new nucleus receives only 1 copy of each chromosome
Anaphase Sister chromatids separate at kinetochores move to opposite poles pulled at centromeres pulled by motor proteins “walking”along microtubules actin, myosin increased production of ATP by mitochondria Poles move farther apart polar microtubules lengthen
Separation of chromatids In anaphase, proteins holding together sister chromatids are inactivated separate to become individual chromosomes 1 chromosome 2 chromatids 2 chromosomes single-stranded double-stranded
Chromosome movement Kinetochores use motor proteins that “walk” chromosome along attached microtubule microtubule shortens by dismantling at kinetochore (chromosome) end Microtubules are NOT reeled in to centrioles like line on a fishing rod. The motor proteins walk along the microtubule like little hanging robots on a clothes line. In dividing animal cells, non-kinetochore microtubules are responsible for elongating the whole cell during anaphase, readying fro cytokinesis
Telophase Chromosomes arrive at opposite poles Spindle fibers disperse daughter nuclei form nucleoli form chromosomes disperse no longer visible under light microscope Spindle fibers disperse Cytokinesis begins cell division
Cytokinesis – Cell Division Animals constriction belt of actin microfilaments around equator of cell cleavage furrow forms splits cell in two like tightening a draw string Division of cytoplasm happens quickly.
Cytokinesis in Animals
Mitosis in whitefish blastula
Mitosis in animal cells
Cytokinesis in Plants Plants cell plate forms vesicles line up at equator derived from Golgi vesicles fuse to form 2 cell membranes new cell wall laid down between membranes new cell wall fuses with existing cell wall
Mitosis in plant cell
onion root tip
Origin of replication chromosome: double-stranded DNA replication of DNA elongation of cell cell pinches in two ring of proteins Evolution of mitosis Mitosis in eukaryotes likely evolved from binary fission in bacteria single circular chromosome no membrane-bound organelles
prokaryotes (bacteria) Evolution of mitosis protists dinoflagellates A possible progression of mechanisms intermediate between binary fission & mitosis seen in modern organisms protists diatoms eukaryotes yeast Prokaryotes (bacteria) No nucleus; single circular chromosome. After DNA is replicated, it is partitioned in the cell. After cell elongation, FtsZ protein assembles into a ring and facilitates septation and cell division. Protists (dinoflagellates) Nucleus present and nuclear envelope remains intact during cell division. Chromosomes linear. Fibers called microtubules, composed of the protein tubulin, pass through tunnels in the nuclear membrane and set up an axis for separation of replicated chromosomes, and cell division. Protists (diatoms) A spindle of microtubules forms between two pairs of centrioles at opposite ends of the cell. The spindle passes through one tunnel in the intact nuclear envelope. Kinetochore microtubules form between kinetochores on the chromosomes and the spindle poles and pull the chromosomes to each pole. Eukaryotes (yeast) Nuclear envelope remains intact; spindle microtubules form inside the nucleus between spindle pole bodies. A single kinetochore microtubule attaches to each chromosome and pulls each to a pole. Eukaryotes (animals) Spindle microtubules begin to form between centrioles outside of nucleus. As these centrioles move to the poles, the nuclear envelope breaks down, and kinetochore microtubules attach kinetochores of chromosomes to spindle poles. Polar microtubules extend toward the center of the cell and overlap. eukaryotes animals
Video animation: Cancer Cancer – the enemy! Cancer – the uncontrolled dividing of cells Video animation: Cancer
Cell Cycle - Normal vs. Cancerous Cells Normal Chicken Stomach Cells: Interphase – 120 min Prophase – 60 min Metaphase – 10 min Anaphase – 3 min Telophase – 12 min Cancerous Chicken Stomach Cells: Interphase – 16 min Prophase – 15 min Metaphase – 2 min Anaphase – 1 min Telophase – 3 min
Animated Introduction to Cancer Biology Types of Tumors Benign tumor – slow-growing or has stopped growing and does not spread to other tissues Malignant tumor– is actively growing and tends to spread to other tissues Metastasis – the spreading of cancer cells to other parts of the body Animated Introduction to Cancer Biology