Cell cycle

Question 1

The cell cycle

Answer 1

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. 𝑀 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 – these cells are not considered to be in the life cycle and are arrested in 𝐺0

Question 2

Interphase

Answer 2

Interphase is the longest part of the cell cycle (up to 90% of its life), and it includes 3 phases:

• 𝐺1 phase
• 𝑆 phase
• 𝐺2 phase

Question 3

𝐺1 phase

Answer 3

𝐺1 – 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 (𝐷𝑁𝐴 synthesis). That
preparation includes:
– Storage of energy.
- Storage of nutrients.
– Synthesis of proteins needed for 𝐷𝑁𝐴 synthesis.

Before entering the 𝑆 phase, the cell passes the 𝐺1/𝑆 checkpoint where it will verify that all the conditions are suitable in order to continue in the cycle, including checking the 𝐷𝑁𝐴 (done by a protein called 𝑝53). Following a success, 𝑆 phase promoting factor leads the cell to the 𝑆 phase.

Question 4

𝑆 phase

Answer 4

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 “Sister Chromatids”. The cell has 92 chromatids that are made of 46 sister chromatids

Question 5

𝐺2 stage

Answer 5

Short phase during which the cell prepares itself to the actual cell division that occur in 𝑀 phase. The cell goes through 𝐺2/𝑀 checkpoint, to verify that all the conditions are suitable, and that 𝐷𝑁𝐴 synthases was indeed successful. Following a success, the 𝑀 phase promoting factor will lead the cell to the 𝑀 phase.

Question 6

Important to remember about interphase

Answer 6

+𝐺0 phase – follows to the mentioned above – 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 𝐺0, but they can return to the cycle in order to multiply when other cells are damaged.

+ 𝑅𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 – the regulation of the cell cycle is important for cell survival. It prevents the passage of damaged 𝐷𝑁𝐴 to daughter cells and uncontrolled divisions.

+ Checkpoints – 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:
- 𝐺1/𝑆 checkpoint.
- 𝐺2/𝑀 checkpoint.
- 𝑀𝑒𝑡𝑎𝑝ℎ𝑎𝑠𝑒.

Question 7

Checkpoint regulations:

Answer 7

• Cyclins – proteins that activate 𝐶𝐷𝐾 allowing the cell to progress from 𝐺1 → 𝑆 or 𝐺2 → 𝑀.
  There are:
  – 𝑆-promoting factor (𝑆𝑃𝐹) – promotes 𝐷𝑁𝐴 synthesis.
  – 𝑀-promoting factor (𝑀𝑃𝐹) – promotes mitosis or meiosis.
• Cyclin dependent kinase (𝐶𝐷𝐾) – adds phosphate to a protein.
• 𝑝53 – tumour suppresses gene, a protein that blocks the cell cycle if the 𝐷𝑁𝐴 is damaged. If
  the damage is severe it can cause to apoptosis.
  – Increased levels of this protein block the cell cycle and allows time for 𝐷𝑁𝐴 repair.
  – Mutation or dysfunction in this protein can lead to cancer.

Question 8

𝑴 phase

Answer 8

𝑀 phase in divided into two types:

• Mitosis.
• Meiosis.

Question 9

Mitosis

Answer 9

Mitosis - Separation of Sister Chromatids – 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 𝐷𝑁𝐴 are passed to offspring. Mitosis is divided 
into 4 steps:
---step 1- Prophase
---step 2- Metaphase
---step 3- Anaphase
---step 4- telophase

Question 10

Prophase of Mitoses

Answer 10

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.

Question 11

Metaphase of Mitoses

Answer 11

The Sister Chromatids are arranged in the midline metaphase plane (done by spindle
fibres).

Question 12

Anaphase of Mitoses

Answer 12

In this phase, the centromeres splits, and thus the separation of Sister Chromatids occurs by shortening of microtubules. The others microtubules that hasn’t 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.

Question 13

Telophase of Mitoses

Answer 13

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

Question 14

Cytokinesis

Answer 14

A separate process from mitosis, but still a part of 𝑀 phase. During cytokinesis, the cytoplasmic division occur and two cells are created.

Question 15

Meiosis

Answer 15

Separation of Homologous Chromosomes – 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 Ι - divided into 4 phases.
2. Meiosis ΙΙ(identical to mitosis, and includes the separation of Sister Chromatins during anaphase ΙΙ.
Eventually four different haploid cells are created
[sperm/oocyte]).

Question 16

Prophase Ι of Meiosis

Answer 16

The same as mitosis, but during prophase Ι 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.

Question 17

Metaphase Ι of Meiosis

Answer 17

The same as mitosis, but during metaphase Ι of meiosis, the arrangement of Homologous Chromosome tetrad in the metaphase plane occur (arrangement according to sets of homologs not sister chromatids!).

Question 18

Anaphase Ι of Meiosis

Answer 18

The same as mitosis, but during anaphase Ι 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 – 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
“copies” of other 23 chromosome) instead of Homologous Chromosomes (two sets of the same
characteristics).

**Important to remember - During anaphase Ι, Homologous Chromosomes separate, while during anaphase ΙΙ Sister Chromatins are separated.

Question 19

Telophase Ι of Meiosis

Answer 19

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 ΙΙ starts.

Question 20

Differences Between Male and Female (Meiosis in Humans)

Answer 20

Oogenesis – 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 Ι (prophase Ι), and at puberty proceeds to meiosis ΙΙ once a month until menopause.

Spermatogenesis – 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.

Question 21

Important to remember about the female Oogenesis production

Answer 21

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 – the older the woman, the inferior the oocytes may be.