Biology Notes (Part 14) The Cell Cycle

Introduction to the Cell Cycle

Cells arise through cell division of pre-existing cells.
==Meiosis== produces reproductive cells called gametes.
==Mitosis== produces all other cell types called somatic cells.
Mitosis and meiosis are usually accompanied by cytokinesis.
The ==Cell Cycle== is the sequence of events from cell formation to cell division.
Division between the M-phase (cell division) and interphase.
Most of the Cell Cycle is spent in Interphase (at least 90%).
Chromosomes are not condensed.
Cells are growing and preparing, fully prepared, or are fulfilling their specialized functions.
Within Interphase, there are three stages called ==G1,== next is ==S,== and lastly, ==G2.==
In the ==G1 phase==, the cell recovers from precious division.
Cell growth and the number of organisms increase, materials for DNA synthesis are accumulated, and cells perform their functional roles.
In the ==S phase==, DNA replication and cell growth occurs.
In the ==G2 phase==, preparation for cell division occurs.
In the ==G0 phase==, cells that have exited the cell cycle are fully arrested.
The M-phase consists of mitosis and cytokinesis.
At the start of Mitosis, replicated chromosomes condense.
During Mitosis, sister chromatids separate. Two daughter cells are formed by Cytokinesis.
The stages within Mitosis are ==prophase, prometaphase, metaphase, anaphase, telophase.==
In ==prophase==, chromatin condenses to form chromosomes, nuclear envelope fragments more, spindle apparatus forms, and centrosomes migrate to opposite “poles.”
In ==prometaphase==, nuclear envelope disappears, and spindle fibers attach kinetochores and begin to move chromosomes.
In ==metaphase==, chromosomes line up along the metaphase plate in the center of the cell.
In ==anaphase==, sister chromatids separate and move towards spindle poles.
In ==telophase==, the nuclear envelope reappears, and chromosomes return to chromatin form.
==Cytokinesis== is the division of the cytoplasm. It results in cells called daughter cells. During cytokinesis, a ==cleavage furrow== (which is a constriction formed by an actin ring during cytokinesis in animal cells that leads to cytoplasmic division) forms. However, when cytokinesis occurs in a plant cell, a cell plate will form. A ==cell plate== forms by Golgi vesicles, forming a temporary structure (phragmoplast) and fusing at the metaphase plate; ultimately leads to the formation of cell walls that separate the daughter cells.

Checkpoints in Mitosis

In the G1 checkpoint, if the cell size is adequate, nutrients are sufficient, social signals are present, and if DNA checkpoint is undamaged, then the cell passes the checkpoint. Mature cells do not pass this checkpoint, they enter the G0 phase.
In the G2 checkpoint, if chromosomes can replicate successfully, DNA is undamaged, and if activated MPF is present, then the cell passes this checkpoint.
In the M-phase checkpoint, the cell can pass it if chromosomes have attached to the spindle apparatus, and if chromosomes have properly segregated and MPF is absent.

Differences Between Mitosis and Meiosis

In Mitosis, there is one division of the parent cell, two daughter cells are produced, daughter cells are genetically identical, and this process occurs in most cells all the time (somatic cells).
In Meiosis, there are two division of the parent cell, four daughter cells are produced, daughter cells are not genetically identical, and this process only occurs at certain times (produces gametes).

How Do Cells Replicate?

Cells must replicate for life to exist.
The first step of cellular respiration is copying the DNA, the second step is to separate the copies, and the third step is diving the cytoplasm to create two complete cells.

Eukaryotic Chromosomes

==Eukaryotic chromosomes== are composed of DNA wrapped around proteins called histones.
Eukaryotic chromosomes are located in the nucleus.
For most of the cell cycle, DNA is located in ==chromatin==.
At the beginning of mitosis, chromatin begins to condense, making chromosomes more apparent.