THE CELL - Smallest living component of biological systems.
Two basic types of cells:
Prokaryotic (no nucleus or other membrane-bounded organelles)
Eukaryotic (nucleus and many membrane-bounded organelles).
Some facts and structures of import in prokaryotes (organisms composed of prokaryotic cells):
* prokaryotes are unicellular or colonial--there's no division of labor among cells.
* prokaryotic organisms are commonly called "bacteria," although not all prokaryotes are true bacteria. Close enough for our purposes, though.
* prokaryote cell wall is composed of a protein/sugar molecule called peptidoglycan, unique to prokaryotes.
* prokaryotic flagellum (whiplike "tail") is used for locomotion. (It superficially resembles the flagella we'll see in eukaryotic cells, but it is NOT the same at all.)
-composed of a protein called flagellin
-moves with rotation from the base
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And while we're on the subject of flagella that *look* superficially similar...
e.g. - wing of a bat and wing of a bird
e.g. - middle fingernail of human, front hoof of a horse
bones of bat wing and bones of bird wing
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e.g. - the vertebral column, found in all vertebrates, is primitive to all vertebrates.
e.g. - the vertebral column, found ONLY in vertebrates, is derived with respect to skeletal structures in other organisms that do not have a vertebral column. (That is, it's an "evolutionary innovation" unique to the vertebrates. It set them apart from other organisms, yet unites them all together as one group descended from a common ancestor that also had a vertebral column.)
Note that the above four terms are RELATIVE. You can't call one structure or characteristic "analogous" or "primitive." You must be comparing at least two different things.
Cellular structures: review in your text! Be familiar with the major organelles and with the general structure of the plasma membrane. Here are some helpful illustrations...
Plant cells, unlike animal cells...
1. lack lysosomes (little bags of digestive enzymes)
2. lack centrioles (involved in mitosis)
3. have chloroplasts (site of photosynthesis)
4. have large, central vacuole
5. have cell wall composed of a "jelly sandwich" of cellulose/pectin/cellulose.
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Cell reproduction: Asexual or Sexual
Mitosis: one parent cell divides to form two daughter cells, genetically identical to the parent and to each other. (Asexual)
Meiosis: one parent cell (called a "germ cell) divides to form daughter cells that have only half the genetic material of the original cell. (Sexual).
- the daughter cells are called gametes
- these are often sperm or ova
- when two complementary gametes fuse, they form a new cell which again has the same amount of genetic information as the original parent cell, but it is genetically different from the parent cell.
The fertilized ovum (fused sperm and egg) is called a ZYGOTE.
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Note of interest: all mitochondria in a zygote come from the female parent; the sperm mitochondria are lost after fusion of the gametes.
This is why mitochondrial DNA is sometimes fondly referred to as
"Eve's DNA" (think about it).
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Before we talk about cell division, we must mention...
A chromosome is composed of a central strand of DNA, connected to a network of proteins and RNA.
DNA is the genetic "blueprint" that tells the cell how to build the organism out of proteins.
- structural proteins are those which form the actual, physical structure of the organism (e.g., muscle tissue, hair, etc.)
- enzymes are protein catalysts that construct proteins into their finished products. (enzymes are not usually structural)
DNA is composed of functional subunits called genes.
Each gene controls (all or part of) a physical trait in the organism!
For example, humans have a gene that controls such traits as...
- eye color
- Hitchhiker's Thumb
- widow's peak
- hair color
The gene for a particular trait can have different forms. For example, the gene for eye color in humans can code for either blue eyes or brown eyes.
If one form of the gene masks the expression of the other form of the gene, we say that the masking one is DOMINANT and the masked one is RECESSIVE.
Every nucleated cell in your body has TWO COPIES of every gene: one from your mom and one from your dad.
The different forms of a particular gene are called the ALLELES of that gene.
(Think of the "gene" as ice cream and the "alleles" as vanilla, chocolate, strawberry, peach, tutti frutti, etc.)
During meiosis, the alleles of each gene segregate into the new daughter cells, so that each new gamete has ONLY ONE ALLELE (not the two of the parent cell) for each trait.
Genotype is the actual genetic makeup of the individual. (Bb)
Phenotype is the physical appearance of the organism. (brown eyes)
An organism that has two of the same allele for a particular trait is said to be HOMOZYGOUS for that particular trait.
And organism with two different alleles for a particular trait is said to be HETEROZYGOUS for that particular trait.
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The Wonderful World of Cell Division...
Here, a COMPARISON OF MITOSIS AND MEIOSISMitosis:
Interphase: The general life cycle condition of the cell, when it's not actively dividing. Rather, it is growing and metabolizing and doing all the things a happy, normal cell does.
Near the end of Interphase, the cell undergoes three phases in preparation for cell division:
- Gap 1: protein enzymes used for DNA replication are manufactured.
- Synthesis Phase (S phase): DNA in each chromosome doubles, but doesn't separate.
Gap 2: similar to G1, but with increase protein synthesis.
The phases of active cell division are:
1. Prophase
- nuclear membrane breaks down
- nucleolus disappears (ribosome manufacture pauses!)
- chromatin condenses into visible chromosomes
- at the end of Prophase, the duplicated chromosomes (remember, they did this in S phase) have unwound from each other, and are attached only at a small, constricted region called the centromere.) (See picture!)
2. Metaphase
- duplicated chromosomes line up along the equator of the cell.
3. Anaphase
- microtubules attached to the centromere pull the duplicated chromosomes apart, pulling one member of each identical pair to opposite ends of the dividing cell.
4. Telophase
- in plant cells, this is characterized by the formation of the cell plate.
- the cell plate begins forming in the center of the cell (remember, there's now a complete set of chromosomes--original to the set found in the original cell) and grows outward, like a disk.
- the cell plate is initially composed of cellulose. As it grows, a layer of PECTIN ("jelly") and another layer of celluose are laid down along the inside of each cell.
- Finally, a plasma membrane forms along the inside of the finished cell plate.
- Nuclear membrane reforms around the chromosomes
- Chromosomes become diffuse chromatin again
- Nucleolus reappears
- Cell eventually returns to interphase conditions.
Cytokinesis: the actual division of the original cell into two separate entities. In animals, the two cells "pinch off" from each other, somewhat like two little water balloons. In plants, cytokinesis is accomplished via cell plate formation.
MEIOSIS - sexual cell division.
Consists of two phases:
1. Meiosis I - reduction (by half) of the total number of chromosomes
2. Meiosis II - cell division very similar to mitosis (no further reduction in chromosome number)
The critical difference between Mitosis and Meiosis occurs during Meiosis I.
(Note: During prophase I of meiosis, the "mated pairs" of chromosomes come to lie in close apposition. As they do so, they break and re-connect, so that by the end of this process called "crossing over"--each chromosome is completely different and unique. (imagine originally having solid blue chromosomes and solid pink chromosomes. After crossing over, all the chromosomes are striped pink and blue)
At metaphase I, the duplicated chromosomes don't divide. Instead, they line up in "mated pairs" and one entire member of each pair will migrate to opposite ends of the cell.
End result: Instead of having SIX individual chromosomes, the cell now has THREE doubled chromosomes.
The number of sets of chromosomes in the cell was TWO. It is now ONE.
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Definitions from heck:
diploid - a cell with two complete sets of chromosomes.
(e.g. - a diploid human cell has 46 chromosomes; each set of chromosomes numbers 23)
haploid - a cell with one complete set of chromosomes.
("ploidy" is a term used to refer to the number of chromosomes sets in a cell's nucleus)
Regular "body" cells are almost always diploid. Gametes are haploid.
IN OUR EXAMPLE OF THE CELL WITH FOUR CHROMOSOMES...
At the end of Meiosis I, each cell contains three doubled chromosomes
that are identical to each other, but different (genetically shuffled
during crossing over) from the chromosomes in the original cell.
At the end of Meiosis II (which is essentially like mitosis), the doubled chromosomes have separated and migrated to opposite cells. Each new daughter cell contains three single chromosomes.
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Convention: The number of chromosome sets can be represented as "n" (with n = # of chromosomes per set; in humans, n = 23)
A diploid cell can be represented as 2n.
A haploid cell can be represented as n.
After meiosis, the new haploid cells usually undergo a process of gametogenesis, during which each is processed to become either a sperm or an egg (or whatever type of gamete it's going to be.)
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