Organismal Development Part 1 Prokaryotes and Protists

Important concepts from previous units: Cells that divide by mitosis produce genetically identical offspring. Cells divide when they become inefficient at transport across the cell membrane. Cells that reproduce sexually depend on cell-to-cell communication/contact. All matter and energy for growth and development comes from the environment.

Modes (ways of doing) of reproduction Asexually – Basically, the making of genetically identical clones. No exchange of DNA occurs. Sexually – The fusion of two different gametes (sperm and egg) to create a Zygote (fertilized egg) that has variation. – Remember, variation is the key to survival in changing environments. – To create sperm and eggs, certain diploid cells have to undergo meiosis, which involves two divisions of chromosomes to become haploid cells.

Sexual Reproduction

Types of Asexual Reproduction: Binary Fission – term for Bacterial reproduction Budding – organisms, such as polyps (sessile form of jellyfish), are generating genetically identical “buds” that break off to become a new organism. Fragmentation and regeneration - Seen in the Platyhelminthes (flatworms) and Echinoderms (starfish). – For example, a single arm of a starfish gets removed. The starfish will grow (regenerate) a new arm in that same place. The removed arm will (fragment) will become a new starfish.

Asexual - Budding

Platyhelminthes - Regeneration

Binary Fission This is the process of Reproduction/Replication in prokaryotes (bacteria). – DNA replication(S phase) starts at the “origin” and works around the entire single, circular chromosome, this results in two identical chromosomes in the nucleoid region. – This is followed by producing a cleavage furrow (cytokinesis) to produce 2 new cells that are referred to as clones. The cleavage furrow is produced using actin and myosin microfilaments. How is Binary Fission related to mitosis in terms of evolution? – Binary Fission would have evolved into Mitosis as the DNA content increased dramatically and also the endosymbiant hypothesis occurred to produce “organelles”. The two major steps are the same: synthesis and division. – 100% genetically identical clones are produced by this process.

Asexual – Binary Fission Origin of replication Cell wall Plasma membrane Bacterial chromosome E. coli cell Two copies of origin Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Replication continues. One copy of the origin is now at each end of the cell. Origin Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. Two daughter cells result.

Bacterial Variation Processes Remember, variation increases survival chances within a changing environment. Transformation (This is a simple change of DNA content.) – A bacteria took in DNA from an external source. Recombination of DNA occurred. Variation “created”. – Biotechnology? This is what we do, in laboratories, to make bacteria “learn” new tricks.

Transformation

Transduction (This is when new DNA has been carried in by a virus thus creating the “change”. – A phage (virus) introduced the new DNA into the bacterial. The two DNAs combined into one genome.

– Bacteria exchange plasmids (small circular pieces of DNA) through a conjugation tube from the “male” to the “female” (Bacteria DO NOT have sexes like humans do.) – F factor (If a bacteria possess this gene, they are considered “male”.) (Shown as F+); (F- are “female”.)(They do NOT possess the F factor gene.) – Pili – This structure is a “sex whip” for pulling the “female” close so that a conjugation tube can be made between the two bacteria. The pili is created by expressing the F factor gene.

Conjugation in Bacteria Sex pilus 5 µm

PROTISTS (These are mostly single celled organisms.) Reproductive Means – Some are sexually reproducing organisms. Remember, this method favors variety. The organism makes haploid gamete structures through the process of meiosis. The gametes are released from the parent organism. The male and female gametes combine to form a genetically new diploid zygote. The zygote develops over time into the “adult” organism.

PROTISTS Some are asexual reproducing organisms. – This is a faster process but produces no variation; they are all clones. – Essentially, it is a process just like Binary Fission, except they have a nucleus with linear chromosomes and organelles in the cytoplasm; so therefore, there is a G2 phase and it is followed by Mitosis and Cytokinesis. (We call this whole process Mitosis. Just like we discussed with the Cell cycle)

Cell Cycle with checkpoints (Is all going according to plan?)

G2 followed by Mitosis

Mitosis continued followed by cytokinesis

Important concepts from previous units: Haploid refers to having one half the DNA content of a normal Diploid cell. We represent it as “n”. Diploid refers to having two halves (a whole) of the normal DNA content. We represent it as “2n”. S phase replicates the DNA of a cell. Mitosis has one division of DNA. Meiosis has 2 divisions of DNA.

FUNGUS Fungal Life Cycles Majority of the life is spent as haploid asexual organisms. – This allows the organism to reproduce much faster and colonize a dead organism for food and reduce competition with other organisms. – Haploid spores can be produced by mitosis and then released to reproduce, in favorable environments, very quickly.

single hypha Mass of hyphae (mycelium) Germ tube (Growing Spore) (initial hypha)

Fungi use the diploid state to create variation. (Remember, variation helps with survival in a changing environment. – All sexual reproduction, for all types of fungus, involves three phases: – Plasmogamy - This is the fusion of cytoplasms. (+=male; - =female) The “female” is signaled by the release of pheromones from the “male”. This fusion together of hyphae results in heterokaryon (Means “different nuclei”) or Dikaryotic (Means “two nuclei”). – Karyogamy – This is the fusion of nuclei. This makes the hyphae now 2n (Diploid) in genetic content. (VARIATION) – Meiosis of the diploid (2n) zygote to return to a haploid (n) state. Variation has been “created”.

PLANTS Sexual Reproduction in plants using Alternation of Generations – Sporophyte (2n) This generation produces diploid spores that undergo meiosis to become haploid (n) spores. The haploid spores are released (in the case of the seedless plants) into the environment or retained in the case of gymnosperms and angiosperms (the seed producing plants). Released spores hopefully will find a suitable environment to grow and produce gametophyte. Retained spores will develop into a single celled gametophyte, the sperm or egg. – The haploid gametophyte (n) produces haploid gametes that are released if male and retained if female. The male gamete, sperm, travels to the female gamete, egg, to fertilize and form a diploid zygote. The diploid zygote will grow into the new sporophyte generaration.

Asexual reproduction methods in plants (A.K.A. Vegetative Reproduction or Vegetative propagation) – Fragmentation – A piece of the original plant breaks off and lands implants the cut edge in the dirt. This fragment of cells begin to develop missing parts so long as Xylem tissue runs in right direction… up. – Cuttings – “Man” removes a piece and puts in water or soil to grow. (Like fragmentation; but caused by “man”.) – Grafting – This is the combining of two different plants. Stock – This is the part with established roots. Scion – This is the cutting to be attached to the stock plant. – Tissue Cultures – This uses plant cells to make clones. These cells are said to be totipotential (means “they can make all types of cells”).

Apoptosis This is the “programmed” cell death of cells to create important anatomical structures. – It is controlled by DNA genes within the cells of developing organisms. – An example would be the death and hollowing out of cells to create the water moving xylem tissue of plants.