1. 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

2. 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

3. 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

4. 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

5. 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)

6. Cell Cycle

7. 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)

8. Chromosome
duplication
Sister
chromatids
Centromere
Chromosome
distribution to
daughter
cells

9. 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

10. Mitosis

11. 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

12. Prophase

13. 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

14. Metaphase

15. 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

16. Anaphase

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

18. Telophase

19. 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

20. 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

22. 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

24. 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

26. Crossing Over
Allows the chromosomes to exchange genetic material

27. 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

28. Meiosis vs Mitosis

29. Meiosis vs Mitosis