Reproducing the cell
- Cells produce copies of themselves to make new cells.
- There three reasons why cells make new cells:
- Growth: Many living organisms start as single cells made from sperm and egg fusion.
- Single cells then multiply to produce thousands of copies.
- Most of the growth is by cell division.
- Repair: if an organism is wounded, cells around the wound reproduce themselves to repair the damaged tissue.
- Reproduction: single-celled organisms multiply from single cells to large populations by making copies of their cells. Some organisms make new organisms by copying themselves. This is called asexual reproduction.
Binary fission
- Bacteria multiply through the process of binary fission.
- Binary fission involves the steps below:
- Bacterial cells produce copies of their chromosomes.
- The bacterial cells become large as they produce copies of ribosomes and molecules in the cytoplasm.
- New cell walls and plasma membranes are synthesized to divide the cell into two.
- Some bacteria can finish the binary fission process in 10 minutes and produce thousands of cells from one cell in two hours.
The cell cycle
- Eukaryotic cells multiply at different rates.
- Different human cells have different replication behaviors:
- Some types of cells divide all the time. Cells on skin surfaces and mucous membranes are shed constantly and have to be replaced by dividing the cells in the tissues.
- Some cells only divide when signaled to multiply: cells in the liver may be triggered to divide upon damage to the organ.
- Some cells do not easily divide. There are some cells in the human nervous tissue that don’t divide.
- Mitosis is the dividing phase in eukaryotic cells.
- Interphase is the nondividing phase in eukaryotes.
- The interphase stage has three subphases: G1, S, and G2
Pausing during Gap 1
- Cells spend most of their time in the interphase stage called gap 1.
- Cells in G1 are very active and functioning cells.
- G1 serves as a checkpoint before they exit the phase failure to which they go to apoptosis.
- Cells that pass the G1 checkpoint must meet the following conditions:
- Signals instruct cells to undergo division.
- The cell’s DNA should be in good condition.
- Cells must be rich in nutrients.
- The cells should be large to divide.
The S phase and Gap 2
- During the S phase, the cells make copies of their DNA by replication.
- After copying DNA, cells move to Gap 2 phase.
- During G2, cells analyze what they did in the S phase at the G2 checkpoint.
- Before cells move from G2 into mitosis, the following conditions must be met:
- The DNA should not be damaged.
- The cell should copy all chromosomes.
- The cells should receive signals to proceed to mitosis.
Mitosis
- The major role of mitosis is to ensure chromosomes divided up correctly.
- Human cells have 46 chromosomes or 23 pairs.
- There are four phases of mitosis:
- Prophase: it is the stage where chromosomes coil tightly in a process called condensation. The main prophase events include:
- Chromosomes condense and become visible.
- The nuclear membrane dissolves to allow mitotic spindles to reach the central part of the cell.
- Mitotic spindles join to chromosomes.
- The nucleoli are digested completely.
- Metaphase: during this phase, the cells arrange the chromosomes by lining them at the center of the mitotic spindle. Cells cannot proceed to mitosis before passing through the metaphase checkpoint.
- Anaphase: during this phase, chromosome copies are separated to allow sister chromatids to move to opposite sides of the cell. Similar sister chromatids are separated into one new cell.
- Telophase: This phase ends mitosis by reversing prophase events. The following activities occur in this phase.
- Prophase: it is the stage where chromosomes coil tightly in a process called condensation. The main prophase events include:
- The chromosomes uncoil.
- The nuclear membrane is regenerated.
- The mitotic spindle is degraded.
Cytokinesis
- After chromosome separation and the nuclear membrane reformed, the cytoplasm separates into two cells.
- Cytoplasm separation is called cytokinesis.
- Cytokinesis occurs differently in animal and plant cells.
- Cytokinesis in plants: The vesicles containing wall material line up in the middle. The vesicles fuse and join wall material and vesicle membranes together. The wall materials form a cell plate.
- Cytokinesis in animal cells: after chromosome migration, the cell pinches into two new cells. A microfilament band form in the middle and contract to form a cleavage furrow.
Revision
What is the cell cycle?
a cycle of growth, development and division in cells
What are the 2 main phases of Cell Cycle?
Interphase and Mitotic Phase
What is Interphase?
the period during the cell cycle of a cell’s growth and development; longest phase
What is Mitotic phase?
It is the phase that follows Interphase and during this phase the cell reproduces
What does the the length of the cell cycle depend on?
how long the cell cycle takes depends on the type of cell that is dividing
What are the 3 names of the stages of Interphase?
G1, S and G2
What is G1 stage?
the 1st stage in Interphase during which the cell grows and normal cell functions happen including organelle replication
What is S stage?
the 2nd stage in Interphase during which cells grow and DNA replicates and organelle replication also occurs
What are sister chromatids?
the 2 new strands a duplicated chromosome created in S stage of Interphase
What is the centromere?
this holds together the sister chromatids
What is G2 stage?
the final stage in Interphase and during this phase the cell grows and prepares for mitosis and organelle replication also occurs
During which stage(s) do organelles replicate?
all 3 stages
What are the 2 stages of Mitotic Phase?
Mitosis and Cytokinesis
What happens in mitosis?
the nucleus and its contents divide
What happens in cytokinesis?
the cytoplasm and its contents divide and forms 2 new daughter cells
What are daughter cells?
the 2 new cells that result from mitosis and cytokinesis
What are the 4 phases of mitosis?
Prophase, Metaphase, Anaphase and Telophase (PMAT)
What happens in Prophase?
1st phase of mitosis when the DNA condenses and spindle fibers begin to form
What happens in Metaphase?
2nd stage of mitosis during which the chromosomes line up in a single file at the middle of the cell
What happens in Anaphase?
3rd stage of mitosis during which the sister chromatids separate and pull to the opposite sides
What happens in Telophase?
4th stage of mitosis during which the nucleus membrane reforms and the chromosomes unwind
Describe cytokinesis in animals
the cell membrane contracts or squeezes the middle of the cell until the cell divides
describe cytokinesis in plants
new cell wall forms in the middle of the cell and then 2 new cells form
What are the 4 results of the cell cycle
- reproduction 2. growth 3. replacement 4. repair
what happens during reproduction?
new organisms are made
What is growth?
allows multicellular orgranisms to grow from 1 cell to many
what happens during replacement?
worn out or damaged cells are replaced with new ones
what happens during repair?
new cells grow to repair damage
