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CELL REPRODUCTION AND CELL CYCLE
I. CELL SIZE LIMITATIONS
-Cells come in a variety of shapes and sizes.
-Cells may be a few microns across or up to one meter in length.
-Cells have limits on their possible sizes.
A. Diffusion Limits Cell size
1. The plasma membrane controls the flow of nutrients into and waste out of the cell.
2. This movement by diffusion works fine in small distances, but larger distances strain the
movement.
B. DNA Limits Cell Size
1. DNA controls all cell functions by controlling the rate of protein production/direction.
2. Since all cell functions rely on specific proteins, the rate at which proteins are made and
delivered greatly affects a cell’s life.
3. As long as the DNA in a cell makes enough protein, all is well.
4. Large cells require more proteins to function correctly.
a. Large cells don’t always have enough DNA to sufficiently operate the cell.
b. Simpler organisms have developed multiple-nucleated cells to generate
enough DNA, which leads to increased amounts of protein.
C. Surface Area-to-Volume Ratio
1. As a cell size increases, the volume increases much faster than the surface area.
D. The cell has specific processes to regulate cell size.
II. CELL REPRODUCTION
-Cell theory states that all cells arise from pre-existing cells by cell division
-Cell division produces two new, identical cells with complete sets of chromosomes.
A. Chromosome Discovery
1. Early biologists noticed key events that occurred around cell division.
a. Dark objects appeared just before cell division and disappeared right after
cell division.
2. These objects were named chromosomes:
chroma = dark / soma=body
a. Chromosomes are the carriers of genetic material which controls cell
function and identity.
b. Chromosomes are copied and passed on to the next generation.
B. Eukaryotic Chromosomes
1. Chromatin: long strands of DNA surrounding histone proteins
a. Chromatin resembles tangled rope
b. This material requires an organization before cell division occurs.
c. Chromatin eventually becomes chromosomes.
2. Chromosomes: long, tightly coiled strands of DNA around histone
proteins.
a. Chromosomes have an x-shape, with each half being called a chromatid.
b. The two chromatids are joined by a centromere.
c. Two sex chromosomes determine the gender of an organisms.
XX= Female XY= Male
d. All other chromosomes are called autosomes.
e. The copies of autosomes are called homologous chromosomes.
3. Chromosome number
a. Diploid: cell has two sets of chromosomes (2n)
b. Haploid: cell has only one set of chromosomes (1n)
III. CELL CYCLE
A. Cell cycle is the sequence of growth and division of a cell.
1. Almost all living things share the same cycle.
2. Three phases of the cell cycle:
a. Interphase- the first and longest phase of the cycle
i. Growth of the cell is rampant here.
ii. Chromosomes are duplicated
iii. There are 3 stages of interphase.
b. Mitosis- 2nd phase
i. Nuclear division occurs during mitosis
ii. No cell growth occurs during mitosis
c. Division of cytoplasm follows mitosis.
IV. INTERPHASE
A. Interphase is the busiest phase of the cell cycle.
B. Three phases of Interphase:
1. G1- First growth phase
- Chromatin is still loose and uncoiled
- Protein synthesis is in high gear.
2. S phase- Synthesis phase of interphase
-Chromatin condenses to form chromosomes
-Chromosomes are copied to make double chromosomes
-This is the only time that DNA is copied.
-Chromosomes divide to form identical sister chromatids connected by a
centromere.
3. G2- Second growth phase
-Chromosomes shorten and coil.
-Centrioles duplicate, preparing for cell division.
V. MITOTIC PHASES
-Cell size reaches a maximum limit after Interphase
-Cell can’t grow larger than the plasma membrane can support.
-Four stages: Prophase, Metaphase, Anaphase, Telophase
A. Prophase
1. First and longest phase
2. Chromosomes coil very tightly
3. Sister chromatids are readily visible
a. Sister chromatids are identical halves of doubled chromosomes
b. They are joined by a centromere, which function for chromatid movement
4. Nucleus and nuclear envelope begin to disintegrate
5. Centrioles migrate to cell poles
6. Spindle fibers develop into the spindle cage and connect to the centrioles
7. Centrioles and spindle fibers are microtubules
8. Plant cells have no centrioles, just spindle fibers
B. Metaphase
1. This is the 2nd phase, being relatively short
2. Centrioles become connected to centromeres via spindle fibers
3. Chromosomes move to the midline of the cell
a. The chromosomes are pulled by the contraction of the spindle fibers
4. Each sister chromatid is attached to a centriole by a separate spindle fiber
a. Chromatids are allowed to move to opposite poles
b. This allows complete chromosome sets in the new cells
C. Anaphase
1. 3rd phase of mitosis, relatively short
2. Chromatids begin to separate, each having a centromere
3. Chromatids split apart and are pulled to the cell poles
D. Telophase
1. 4th and last stage of mitosis
2. Chromatids finally reach the poles of the cell
3. Actions of Prophase are reversed:
a. Chromosomes begin to uncoil, returning to chromatin state
b. Spindle fibers begin to break down
c. The nucleolus and nuclear envelope reappear
d. Double membranes develop around the two new nuclei
E. Cytoplasmic Division
1. Cytokinesis is the division of cytoplasm
2. Animal cells have a division called cell furrowing: A deep row begins to penetrate
the cell equator, creating two identical halves
3. Plant cells don’t furrow because of the rigid cell wall
a. A cell plate develops at the cell equator
b. Plasma membranes develop on both sides of the new cell plate
F. Controlling Division
1. Three main checkpoints exist to control cell division.
a. G1- ensures cell growth is sufficient to enter S phase.
b. G2- ensures that DNA replication occurred correctly.
c. Mitosis- if all stages occur correctly, cell is signaled to exit mitosis.
2. If any checkpoints should fail, cancer can result.
VI. MEIOSIS
A. Meiosis has two complete cycles, Meiosis I and Meiosis II.
B. Meiosis I operates just as Mitosis, with only a few exceptions.
Interphase:
G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in
mass in preparation for cell division. Note that the G in G1 represents gap and the 1
represents first, so the G1 phase is the first gap phase.
S phase: The period during which DNA is synthesized. In most cells, there is a narrow
window of time during which DNA is synthesized.
Note that the S represents synthesis.
G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase.
The cell synthesizes proteins and continues to increase in size. Note that the G in G2
represents gap and the 2 represents second, so the G2 phase is the second gap phase.
In the latter part of interphase, the cell still has nucleoli present.
o
The nucleus is bounded by a nuclear envelope and the cell's chromosomes
have duplicated but are in the form of chromatin.
o
In animal cells, two pair of centrioles formed from the replication of one
pair are located outside of the nucleus.
Prophase I
-Chromosomes condense and attach to the nuclear envelope.
-Synapsis occurs (a pair of homologous chromosomes lines up closely together) and a
tetrad is formed. Each tetrad is composed of four chromatids.
-Crossing over may occur.
-Chromosomes thicken and detach from the nuclear envelope.
-Similar to mitosis, the centrioles migrate away from one another and both the nuclear
envelope and nucleoli break down.
-Likewise, the chromosomes begin their migration to the metaphase plate.
Metaphase I
-Tetrads align at the metaphase plate.
-Note that the centromeres of homologous chromosomes are oriented toward the opposite
cell poles.
Anaphase I
-Chromosomes move to the opposite cell poles. Similar to mitosis, the microtubules and
the kinetochore fibers interact to cause the movement.
-Unlike in mitosis, the homologous chromosomes move to opposite poles yet the sister
chromatids remain together.
Telophase I
-The spindles continue to move the homologous chromosomes to the poles.
-Once movement is complete, each pole has a haploid number of chromosomes.
-In most cases, cytokinesis occurs at the same time as telophase I.
-At the end of telophase I and cytokinesis, two daughter cells are produced, each with one
half the number of chromosomes of the original parent cell.
-Depending on the kind of cell, various processes occur in preparation for meiosis II.
-There is however a constant: The genetic material does not replicate again.
Prophase II
-The nuclear membrane and nuclei break up while the spindle network appears.
-Chromosomes do not replicate any further in this phase of meiosis.
-The chromosomes begin migrating to the metaphase II plate (at the cell's equator).
Metaphase II
-The chromosomes line up at the metaphase II plate at the cell's center.
-The kinetochores of the sister chromatids point toward opposite poles.
Anaphase II
-The sister chromatids separate and move toward the opposite cell poles.
Telophase II
-Distinct nuclei form at the opposite poles and cytokinesis occurs.
-At the end of meiosis II, there are four daughter cells each with one half the number of
chromosomes of the original parent cell.