Chapter 9: The Cell Cycle and Cellular Reproduction (Outline) Interphase Mitotic Stage Control of the Cell Cycle Apoptosis Mitosis and Cytokinesis Phases of Mitosis Cytokinesis in Animal and Plant Cells Prokaryotic Cell Division

The Cell Cycle An orderly set of stages and substages between one division and the next Just prior to the next division: The cell grows larger The number of organelles doubles The amount of DNA is doubled as DNA is replicated The two major stages of the cell cycle: Interphase Mitotic stage

Interphase Most of the cell cycle is spent in interphase Cells perform normal functions, depending on body location Interphase time varies widely Nerve and muscle cells are permanently arrested (i.e. cell reaches end stage of development and no longer divide); G0 stage Embryonic cells complete entire cycle in few hours In adult mammalian cells, interphase lasts for 20 hours (i.e. 90% of cell cycle) Consists of three phases; G1, S and G2

Interphase (cont.) G1 Phase: S (Synthesis) Phase: G2 Phase: Between the end of mitosis and beginning of S phase Cell increases in size and doubles its organelles Accumulates raw materials for DNA synthesis S (Synthesis) Phase: Growth and DNA synthesis or replication occurs Chromosomes are duplicated with 2 identical chromatids G2 Phase: Between DNA replication and the onset of mitosis Protein synthesis increases in preparation for division

M (Mitotic) Stage Involves two main processes: Mitosis (karyokinesis) Nuclear division Daughter chromosomes are identical to parental nuclei and distributed to two daughter nuclei Cytokinesis Division of the cell cytoplasm Results in two genetically identical daughter cells

The Cell Cycle

Control of the Cell Cycle Signal – an agent that influences the activities of a cell Growth factors Signaling proteins received at the plasma membrane Ensure that the stages follow one another in the normal sequence Cause completion of cell cycle (even cells arrested in G0)

Cell Cycle Checkpoints Internal signals Family of proteins called cyclins Cell cycle stops at the G2 checkpoint if DNA replication not completed Prevents initiation of the M stage before completion of the S stage Allows time for any DNA damage (i.e. exposure to solar radiation or X-rays) to be repaired

Cell Cycle Checkpoints (cont.) Signal protein p53 in mammalian cells Stops cycle at G1 when DNA is damaged Initiates DNA attempt at repair If successful, cycle continues to mitosis If not, apoptosis is initiated RB (retinoblastoma) protein responsible for interpreting growth signals and nutrient availability signals

Cell Cycle Checkpoints (cont.) There is also a cell cycle checkpoint that occurs during the mitotic stage The cell cycle stops if the chromosomes are not distributed accurately to the daughter cells

Apoptosis The sequence of events during apoptosis Often defined as programmed cell death Cells harbor apoptosis enzymes (caspases) Ordinarily held in check by inhibitors Can be unleashed by internal or external signals The sequence of events during apoptosis Cell rounds up and loses contact with its neighbors Nucleus fragments and plasma membrane blisters Cell fragments are engulfed by white blood cells

Apoptosis (cont.)

Apoptosis and Cell Division Mitosis and apoptosis are opposing forces Cell division (mitosis) increases the number of somatic (body) cells Apoptosis decreases the number of somatic cells Example: Tadpole tail disappears (apoptosis) to become a frog Webbed fingers and toes of human embryos disappear through apoptosis Both mitosis and apoptosis are normal parts of growth and development (homeostasis)

Eukaryotic Chromosomes DNA in the chromosomes of eukaryotes is Associated with histone proteins In very long threads and collectively called chromatin Before mitosis begins: Chromatin condenses (coils) into distinctly visible chromosomes Each species has a characteristic chromosome number Example: humans 46, corn 20, goldfish 94

Chromosome Number Most familiar organisms are diploid Have two chromosomes of each type Humans have 23 different types of chromosomes Each type is represented twice in each body cell (diploid) Only sperm and eggs have one of each type (haploid) The n number for humans is n=23 Two representatives of each type Makes a total of 2n=46 in each nucleus

Chromosome Duplication Dividing cell is called the parent cell; the resulting cells are called the daughter cells At the end of S phase: Each chromosome is internally duplicated Consists of two identical DNA chains Sister chromatids Attached together at a single point (centromere) Protein complex (kinetochores) form on both sides of the centromere

Division of the Centrosome Centrosome – the main microtubule-organizing center of the cell Consists of a pair of barrel-shaped organelles – centrioles Present in animal cells only Organizes the mitotic spindle (composed of microtubules) Microtubules of the cytoskeleton disassemble when spindle fibers form

Phases of Mitosis Mitosis is a continuous process that is divided into five stages Prophase Prometaphase Metaphase Anaphase Telophase

Phases of Mitosis: Prophase Chromatin condensed and chromosomes are visible Nucleolus disappear Nuclear envelope fragments Spindle begins to assemble The 2 centrioles migrate in opposite poles Asters -arrays of microtubules radiate from centrioles (in animal cells) Chromosomes have no orientation, why?

Phases of Mitosis: Prometaphase (Late Prophase) Preparation for separation of sister chromatids Kinetochores appear on each side of the centromere Kinetochores attach sister chromatids to the kinetochore spindle fibers → chromatids are pulled toward opposite poles Chromosomes are still not in allignment

Phases of Mitosis: Metaphase Centromeres of chromosomes are now in alignment at the center of the cell This center is called metaphase plate Polar spindle – non attached spindle fibers Polar fibers reach beyond metaphase plate and overlap

Phases of Mitosis: Anaphase Kinetochore spindle fibers pull sister chromatids as they disassemble at the kinetochore Two sister chromatids separate at centromere to the opposite poles Poles move further apart due to spindle fibers sliding past one another Microtuble proteins (kinesin and dynein) are involved Anaphase is the shortest phase

Phases of Mitosis: Telophase Spindle disappears New nuclear envelopes form around the daughter chromosomes Nucleolus appears in each daughter nucleus Ruminants of polar spindle fibers are still visible between the 2 nuclei Chromosomes become chromatin fibers

Cytokinesis Division of the cytoplasm to yield two daughter cells Allocates mother cell’s cytoplasm equally to daughter nucleus Encloses each in it’s own plasma membrane Often begins in anaphase, continues during telophase

Cytokinesis: Animal Cells During animal cytokinesis: A cleavage furrow appears between daughter nuclei Formed by a contractile ring of actin filaments Like pulling on a draw string Narrow bridge between 2 cells during telophase Eventually pinches mother cell in two

Cytokinesis in Animal Cells

Cytokinesis: Plant Cells In plant cells, cytokinesis begins by forming a cell plate Many small membrane-bounded vesicles originating in the Golgi complex Eventually fuse into one thin vesicle extending across the mother cell The membranes of the cell plate become the plasma membrane between the daughter cells Contents of vesicles become the middle lamella between the two daughter cells Daughter cells later secrete primary cell walls on opposite sides of middle lamella

Cytokinesis in Plant Cells

The Functions of Mitosis Maintenance of tissue Cut finger or skin Broken bone Replacement of damaged cells (i.e. accidents) Growth of multicellular organisms We all start out as a single cell…? Fertilized egg (zygote) Meristematic tissue (shoot tips) in plants

Stem Cells Stem cells – adult mammalian cells that retain the ability to divide Red bone marrow stem cells used for therapeutic cloning; used to produce human tissue Embryonic stem cells in reproductive cloning; used to produce new individual

Prokaryotic Cell Division Asexual reproduction is the formation of new individuals identical to the original parent cell Prokaryotic chromosome a ring of DNA Folded up in an area called the nucleoid 1,000 X length of cell Replicated into two rings prior to division Replicate rings attach to plasma membrane Binary fission Splitting in two between the two replicate chromosomes No spindle apparatus is formed Example: E. coli living in the intestines have a generation time of about 20 minutes!

Binary Fission of Prokaryotes

Comparing Prokaryotes and Eukaryotes Both binary fission & mitosis result in daughter cells that are identical to the parent cell Cellular division in unicellular organisms produces 2 new individuals – form of asexual reproduction Prokaryotes (bacteria & archea) Protists (algae & protozoans) Yeasts Cell division in multicellular organisms is part of the growth process and important for renewal & repair (e.g. plants & animals)

Cell Division and Function