Cell cycle Flashcards
The cell cycle
The cell cycle describes the events in the life of the cell; from the moment it is born, until its death/division.
The cell cycle is divided into two:
1. Interphase.
2. π phase.
Cells that are constantly dividing are usually non-specialized like skin and intestinal cells; while cells that
do not divide are usually specialized like nerve and muscle cells β these cells are not considered to be in the life cycle and are arrested in πΊ0
Interphase
Interphase is the longest part of the cell cycle (up to 90% of its life), and it includes 3 phases:
- πΊ1 phase
- π phase
- πΊ2 phase
πΊ1 phase
πΊ1 β the cell is created, increase in size and becomes functional according to the tissue it belongs to. Following the right signal, the cell prepares itself to the next stage (π·ππ΄ synthesis). That
preparation includes:
β Storage of energy.
- Storage of nutrients.
β Synthesis of proteins needed for π·ππ΄ synthesis.
Before entering the π phase, the cell passes the πΊ1/π checkpoint where it will verify that all the conditions are suitable in order to continue in the cycle, including checking the π·ππ΄ (done by a protein called π53). Following a success, π phase promoting factor leads the cell to the π phase.
π phase
The cell multiplies the genetic material and creating more 46 chromosomes. Since each
chromosome has an exact copy, the two of them are bond together. Such bonded chromosomes
are named βSister Chromatidsβ. The cell has 92 chromatids that are made of 46 sister chromatids
πΊ2 stage
Short phase during which the cell prepares itself to the actual cell division that occur in π phase. The cell goes through πΊ2/π checkpoint, to verify that all the conditions are suitable, and that π·ππ΄ synthases was indeed successful. Following a success, the π phase promoting factor will lead the cell to the π phase.
Important to remember about interphase
+πΊ0 phase β follows to the mentioned above β an arrested phase for cells like neurons and mussels that have no intention or ability to divide. Some cells like liver cells (hepatocytes) also exists in πΊ0, but they can return to the cycle in order to multiply when other cells are damaged.
+ π πππ’πππ‘πππ β the regulation of the cell cycle is important for cell survival. It prevents the passage of damaged π·ππ΄ to daughter cells and uncontrolled divisions.
+ Checkpoints β regulates the progress in the cell cycle and can prevent its progression to the other
stages if not all the requirements are met.
There are:
- πΊ1/π checkpoint.
- πΊ2/π checkpoint.
- πππ‘ππβππ π.
Checkpoint regulations:
- Cyclins β proteins that activate πΆπ·πΎ allowing the cell to progress from πΊ1 β π or πΊ2 β π.
There are:
βο§ π-promoting factor (πππΉ) β promotes π·ππ΄ synthesis.
β π-promoting factor (πππΉ) β promotes mitosis or meiosis. - Cyclin dependent kinase (πΆπ·πΎ) β adds phosphate to a protein.
- π53 β tumour suppresses gene, a protein that blocks the cell cycle if the π·ππ΄ is damaged. If
the damage is severe it can cause to apoptosis.
β Increased levels of this protein block the cell cycle and allows time for π·ππ΄ repair.
β Mutation or dysfunction in this protein can lead to cancer.
π΄ phase
π phase in divided into two types:
- Mitosis.
- Meiosis.
Mitosis
Mitosis - Separation of Sister Chromatids β division of somatic cells into two identical diploid cells.
Somatic cell division resulted in the creation of two identical daughter cells, each with diploid number of chromosomes. The goal is to ensure that exact copies of π·ππ΄ are passed to offspring. Mitosis is divided into 4 steps: ---step 1- Prophase ---step 2- Metaphase ---step 3- Anaphase ---step 4- telophase
Prophase of Mitoses
During prophase, chromatin condense into chromosomes. Nuclear envelop is broken
down realising all the Sister Chromatids (created in interphase) to the cytoplasm.
The Sister Chromatids are connected with one another by centromere, and each sister includes microtubules attachment sites called kinetochore. In every pole of the cell we can find a centrosome, and in each of them there are two centrioles made of microtubules.
They are serving for the growth of the mitotic spindle. The microtubules forming the mitotic spindle are advancing and binding the
Sister Chromatids.
Metaphase of Mitoses
The Sister Chromatids are arranged in the midline metaphase plane (done by spindle
fibres).
Anaphase of Mitoses
In this phase, the centromeres splits, and thus the separation of Sister Chromatids occurs by shortening of microtubules. The others microtubules that hasnβt bond to the of Sister Chromatids, bind to other microtubules from the other side, and keep growing from their base.
This causing a push on the centrosome that starts the actual division itself.
Telophase of Mitoses
Reversing the events of prophase. Nuclear fragments are used to create nuclear
envelope around the chromosomes. When completed, the chromosomes will uncoil themselves back to chromatin, and the spindle fibre will disappear. The cleavage furrow is becoming deeper by the activity of actin ring. This is the end of mitosis
Cytokinesis
A separate process from mitosis, but still a part of π phase. During cytokinesis, the cytoplasmic division occur and two cells are created.
Meiosis
Separation of Homologous Chromosomes β division of germ cells into 4 non identical haploid
gametes.
Meiosis is the production of gametes (haploid cells). Its goal is to mix and change the genetic material for
sexual reproduction purposes. Meiosis consist of two cytoplasmic divisions that eventually create four
different haploid cells. Therefore, meiosis is divided into two:
1. Meiosis Ξ - divided into 4 phases.
2. Meiosis ΞΞ(identical to mitosis, and includes the separation of Sister Chromatins during anaphase ΞΞ.
Eventually four different haploid cells are created
[sperm/oocyte]).
Prophase Ξ of Meiosis
The same as mitosis, but during prophase Ξ of meiosis, homologous chromosomes are looking for each other in a process called synapse, and forming tetrad that made of two Sister Chromatid. Then, crossing occur, and the Homologous chromosomes exchange genetic material (all the chromosomes in the tetrad).
Each place of exchange is called chiasma. Following crossing
over, different chromosomes are created. This is the most important event for our genetic variability.
Metaphase Ξ of Meiosis
The same as mitosis, but during metaphase Ξ of meiosis, the arrangement of Homologous Chromosome tetrad in the metaphase plane occur (arrangement according to sets of homologs not sister chromatids!).
Anaphase Ξ of Meiosis
The same as mitosis, but during anaphase Ξ of meiosis, the separation of Homologous
Chromosomes tetrad to the different poles occur. Instead of dividing the Sister Chromatid like in
mitosis, the division occur to the tetrad β the Homologous Chromosomes are broken down, and
in each side we end up with one set of Sister Chromatids (one set of 23 chromosomes and its
βcopiesβ of other 23 chromosome) instead of Homologous Chromosomes (two sets of the same
characteristics).
**Important to remember - During anaphase Ξ, Homologous Chromosomes separate, while during anaphase ΞΞ Sister Chromatins are separated.
Telophase Ξ of Meiosis
The same as mitosis, but in each side we end up with 46 chromosomes of one set only! This phase is followed by cytokinesis, and two Haploid cells 46 chromosomes (with only one set) are created. After the cytokinesis, Meiosis ΞΞ starts.
Differences Between Male and Female (Meiosis in Humans)
Oogenesis β production of female gametes. The process takes place in the ovaries and eventually produces one ovum and 3 polar bodies. At birth, oocytes arrested in meiosis Ξ (prophase Ξ), and at puberty proceeds to meiosis ΞΞ once a month until menopause.
Spermatogenesis β production of the male sperm. The process takes place in the testicles and eventually produces 4 spermatozoa. Sperm begins to develop at puberty and continue throughout the entire lifespan of the male.
Important to remember about the female Oogenesis production
Even though the female does not produce anymore oocyte after menopause, the productive system is
still functional; a woman can still get pregnant (with medical help) even when she is older. But, it is
important to remember that the body is old. Meaning:
1. The body is weakened by old age, and so apregnancy may be more difficult.
2. The genetic material in the oocyte may deform and change in time β the older the woman, the inferior the oocytes may be.
