Cellular Division Lecture 4 Genetics Dr. Heba Al-Fares

Cell Reproduction For any cell to reproduce successfully, three fundamental events must take place: its genetic information must be copied, the copies of genetic information must be separated from one another, and the cell must divide. All cellular reproduction includes these three events, but the processes that lead to these events differ in prokaryotic (bacteria) and eukaryotic cells (protists, fungi, plants, & animals).

Cell Division All cells are derived from pre-existing cells New cells are produced for growth and to replace damaged or old cells.

Types of Cell Reproduction Asexual reproduction involves a single cell dividing to make 2 new, identical daughter cells Mitosis & binary fission are examples of asexual reproduction Sexual reproduction involves two cells (egg & sperm) joining to make a new cell (zygote) that is NOT identical to the original cells Meiosis is an example

Prokaryotic Cell Reproduction When prokaryotic cells reproduce, the circular chromosome of the bacterium is replicated. The two resulting identical copies are attached to the plasma membrane, which grows and gradually separates the two chromosomes. Finally, a new cell wall forms between the two chromosomes, producing two cells, each with an identical copy of the chromosome. Under optimal conditions, some bacterial cells divide every 20 minutes. At this rate, a single bacterial cell could produce a billion descendants in a mere 10 hours.

Prokaryotic Chromosome The DNA of prokaryotes (bacteria) is one, circular chromosome attached to the inside of the cell membrane

Cell Division in Prokaryotes Prokaryotes such as bacteria divide into 2 identical cells by the process of binary fission Binary fission: the physical process whereby a bacterial cell divides into two daughter cells Single chromosome makes a copy of itself Cell wall forms between the chromosomes dividing the cell by the growth of a transverse septum Parent cell Chromosome doubles Cell splits 2 identical daughter cells

Eukaryotic Chromosomes All eukaryotic cells store genetic information in chromosomes Most eukaryotes have between 10 and 50 chromosomes in their body cells, potatoes have 48 chromosomes, fruit flies have 8, and humans have 46. There appears to be no special significance between the complexity of an organism and its number of chromosomes per cell. Each chromosome in one set has a corresponding chromosome in the other set, together constituting a homologous pair. The two chromosomes of a homologous pair are usually alike in structure and size, and each carries genetic information for the same set of hereditary characteristics. (An exception is the sex chromosomes,

cell division The process of cell division is necessary for life to function. In this process, one cell, the parent cell, divides into two daughter cells. This is called the cell cycle and has four distinct steps: a G1 (or gap-1) phase, an S phase, a G2 (or gap-2) phase, and mitosis.

Five Phases of the Cell Cycle G1 - a cell grows in size and prepares to replicate (or copy) the chromosomes. S – is the synthesizing phase, when the DNA molecule is replicated, doubling it. G2 - secondary growth phase collectively these 3 stages are called interphase M – mitosis; is the stage in which the cell actually divides into two daughter cells, each with its own copy of the DNA molecule C - cytokinesis

Cell Cycle

Interphase - G1 Stage 1st growth stage after cell division Cells mature by making more cytoplasm & organelles Cell carries on its normal metabolic activities There is a critical point in the cell cycle, termed the G1/S checkpoint, in G1; after this checkpoint has been passed, the cell is committed to divide. Before reaching the G1/S checkpoint, cells may exit from the active cell cycle in response to regulatory signals and pass into a nondividing phase called G0, which is a stable state during which cells usually maintain a constant size. They can remain in G0 for an extended period of time, even indefinitely, or they can re-enter G1 and the active cell cycle.

Interphase – S Stage Replication is the process of making replicas or copies (DNA synthesis) Synthesis stage DNA is copied or replicated If DNA synthesis is blocked (with drugs or by a mutation), the cell will not be able to undergo mitosis. Before S phase, each chromosome is composed of one chromatid; following S phase, each chromosome is composed of two chromatids Two identical copies of DNA Original DNA

Interphase – G2 Stage 2nd Growth Stage Occurs after DNA has been copied All cell structures needed for division are made (e.g. centrioles) Both organelles & proteins are synthesized

Centrioles is cytoplasmic bodies contain in special region in the centrosome. Function: The centrioles, small organelles that will produce spindle fibers needed to allow the cell to divide, also move to the opposite ends of the cell during this phase. Spindle fiber occurs during early stage of mitosis and meiosis composed of arrays of microtubules these fibers plays an important role in the movement of chromosome during cell division microtubules polymers of alpha and beta subunits of the protein tubulin. Centrosome: cellular structure from which microtubules originate.

What’s Happening in Interphase? What the cell looks like Animal Cell What’s occurring

Sketch the Cell Cycle DNA Copied Cells prepare for Division Cells Mature Daughter Cells Cell Divides into Identical cells

Mitosis Division of the nucleus Also called karyokinesis Only occurs in eukaryotes Has four stages Doesn’t occur in some cells such as brain cells

Four Mitotic Stages Prophase Metaphase Anaphase Telophase

Early Prophase Chromatin in nucleus condenses to form visible chromosomes through a process of DNA condensation and its associated histone protein in to a series of coiling. Mitotic spindle forms from fibers in cytoskeleton or centrioles (animal) Cytoplasm Nucleolus Nuclear Membrane Chromosomes

Late Prophase Nuclear membrane & nucleolus are broken down Chromosomes continue condensing & are clearly visible Centromere replicated during the formation of sister chromatids Multiproten complex called kinetochores attach to each sister centromere of each chromosome Spindle fiber consists of microtubules which are a chain of protein called tubulin finishes forming between the poles of the cell and attach to one sister chromatids from one side and to the other sister chromatids in the opposite side. The kinetochores act as the sites for attachment of microtubules

Late Prophase Chromosomes Nucleus & Nucleolus have disintegrated

Review of Prophase What the cell looks like What’s happening

Three Types of Spindle Fibers The mitotic spindle forms from the microtubules in plants and centrioles in animal cells Polar fibers extend from one pole of the cell to the opposite pole Kinetochore fibers extend from the pole to the centromere of the chromosome to which they attach Asters are short fibers radiating from centrioles important for positioning spindle apparatus within cell.

Sketch The Spindle

Metaphase Equator of Cell Pole of the Cell Chromosomes, attached to the kinetochore fibers, move to the center of the cell Spindle fibers apply tension to the chromosomes, causing them to line up in the center (the equator) of the cell. Equator of Cell Aster Pole of the Cell Chromosomes at Equator

Review of Metaphase What the cell looks like What’s occurring

Anaphase The chromatids are now two separate daughter chromosomes. Occurs rapidly The spindle fibers begin to shorten, pulling the Sister chromatids apart and toward opposite ends of the cell by kinetochore fibers As each sister chromatid moves; its two arms appear to trail its centromeres; a set of V-shaped structures with the points of the Vs directed to the poles The chromatids are now two separate daughter chromosomes.

Anaphase Review What the cell looks like What’s occurring

Removal of the tublin subunits from microtubules at the kinetochore and perhaps molecular motors, are responsible for the pole-ward movement of chromosomes during anaphase. Microtubules pull sister chromatids to opposite poles; the microtubules depolymerize (release free tubulin) at kinetochores resulting in shortening of microtubules

Telophase The daughter chromosomes (Sister chromatids) arrive at the opposite poles of the cell The spindle fibers disappear (disassembles). A new nuclear envelope also forms around the two sets of daughter chromosomes. Chromosomes are no longer visible under a light microscope during telophase. Nucleolus reappears CYTOKINESIS occurs Chromosomes reappear as chromatin

Comparison of Anaphase & Telophase

Cytokinesis The cytoplasm of the parent cell is divided into two daughter cells, each of which contains one set of identical chromosomes inside their new nucleus Means division of the cytoplasm Division of cell into two, identical halves called daughter cells In plant cells, cell plate forms at the equator to divide cell In animal cells, cleavage furrow forms to split cell

Cytokinesis Cleavage furrow in animal cell Cell plate in plant cell

Daughter Cells of Mitosis Have the same number of chromosomes as each other and as the parent cell from which they were formed But smaller than parent cell Must grow in size to become mature cells (G1 of Interphase)

Eukaryotic Cell Division Used for growth and repair Produce two new cells identical to the original cell Cells are diploid (2n) Chromosomes during Metaphase of mitosis Cytokinesis Anaphase Prophase Metaphase Telophase

Uncontrolled Mitosis Cancer cells If mitosis is not controlled, unlimited cell division occurs causing cancerous tumors Oncogenes are special proteins that increase the chance that a normal cell develops into a tumor cell Cancer cells