Cells Arise from Pre-existing Cells Cell division is at the heart of reproduction of both cells and organisms Cell division of unicellular organisms creates a new organism through binary fission (a type of reproduction) Cell division in multicellular organisms through mitosis either increases the number of cells in the organism (growth) or replaces worn-out or damaged cells, keeping the total number of cells relatively constant Multicellular organisms reproduce through meiosis whereby 2 different types of cells (gametes) fuse together to create a new organism

Asexual Reproduction Asexual reproduction involves the creation of genetically-identical offspring from a single cell of an organism Bacteria, yeast, protists, and certain plants and animals Asexual reproduction is a very efficient means of reproduction Faster than sexual reproduction Increases numbers of organisms quickly Ability to reproduce in absence of mate (male doesn’t need female and vice versa) Genetic diversity, however, is sacrificed

Binary fission Prokaryotes (Bacteria and Archaea) reproduce by a type of cell division called binary fission “dividing in half” These cells possess a single molecule of DNA, which is replicated (copied) prior to the cell dividing into 2

Sexual Reproduction The ability for an organism to form gametes, or sex cells (eggs and sperm in humans), defines sexual reproduction The fusion of 2 different gametes results in offspring that are genetically similar, but not identical to either parent; offspring inherits a combination of traits from each parent

Cell Cycle The sequence of events in the life of a cell is referred to as the cell cycle Cell cycles can be long (decades) nerve cells Cell cycles can be short (few days) skin cells Cells with short cycles can renew themselves through mitosis (cell division)

Cell Cycle

Interphase Most of the cell cycle in spent in interphase where the cell performs its various functions within the organism Interphase is divided into 3 stages The G1 phase: cell grows The S phase: DNA replication occurs each molecule DNA organized into a structure called a chromosome is copied before the cell divides so that each new cell contains the number molecules of DNA necessary for normal cellular functioning The G2 phase: cell grows G stands for “gap” (first and second gap) S stands for “synthesis” (DNA)

Chromosome duplication Sister chromatids Centromere Chromosome distribution to daughter cells

Mitosis Process by which one cell divides into 2 identical daughter cells Functions of mitosis growth replacement of old and dead cells repair of injured cells Phases of mitosis Prophase Metaphase Anaphase Telophase

Mitosis

Prophase Nuclear envelope disintegrates releasing the identical chromatid pairs into the cytoplasm Organelles called centrioles move towards opposite sides (poles) of the cell and synthesize mitotic spindle fibers the mitotic spindle is a web of fibrous proteins which are responsible for the equal division of all cellular material between the 2 daughter cells

Prophase

Metaphase Metaphase = middle Spindle fibers from each centriole attach to the chromatid pairs from opposite sides of the cell Chromatid pairs aligned to the middle (equator) of the cell

Metaphase

Anaphase Each centriole retracts the microtubules which pull the sister chromatids away from each other and toward opposite poles of cells the centromeres split and the 2 chromatids separate This stage ensures that when the cell divides down the equator, each daughter cell will have 46 molecules of DNA

Anaphase

Telophase Chromatids extend (loosen) 2 nuclear envelopes are created around the chromatin Mitotic spindle breaks down

Telophase

Cytokinesis Cytokinesis = cytoplasm movement Cytokinesis is the division of the cytoplasm (organelles and intracellular fluid) between 2 newly forming cells Follows telophase Creates a crease around cell equator called cleavage furrow pinches the cell in two

Sexual Life Cycle Made up of meiosis and fertilization Diploid cells somatic cells of adults have 2 sets of chromosomes (one set from each parent) Haploid cells gametes have only 1 set of chromosomes Offspring inherit genetic material from 2 parents

Sexual Life Cycle Stages Life cycles of sexually reproducing organisms involve the alternation of haploid and diploid stages In most organisms, the diploid state dominates Zygote first undergoes mitosis to produce diploid cells Later in the life cycle, some of these diploid cells undergo meiosis to produce haploid gametes

Features of Meiosis Meiosis includes two rounds of division Meiosis I and Meiosis II each has prophase, metaphase, anaphase, and telophase stages First meiotic division is termed the “reduction division” Results in daughter cells that contain one pair of sister chromatids Crossing Over genetic recombination between nonsister chromatids Second meiotic division does not further reduce the number of chromosomes separates the sister chromatids from each other

Crossing Over Allows the chromosomes to exchange genetic material

Final Result Four cells containing haploid sets of chromosomes In animals, develop directly into gametes sperm and eggs In plants, fungi, and many protists, divide mitotically to produce greater number of gametes

Meiosis vs Mitosis

Meiosis vs Mitosis