#haploid

There are 2 kinds of cell division: Mitosis and Meiosis. Mitosis produces diploids, while Meiosis produces haploids. Specifically, it produces gametes (sperm and ova).

In humans, a diploid cell has 46 chromosomes, while a haploid cell has 23.

A chromosome is a condensed and coiled strand of DNA. Chromosomes that have been replicated have 2 sister chromatids and are identical to each other. The centromere is a specialised region holding the sister chromatids together. Spindle fibres are responsible for connecting the centromere to the centrosome during division.

The Cell Cycle

The cell cycle is characterized by a regular sequence of growth and division. Some cells divide more often than others, for example, human stem cells have the ability to divide infinitely and differentiate, making them prime candidates for research, and possible transplants.

The cell cycle is regulated by proteins including those known as cyclins. Some are internal regulators, ensuring that the cell does not go through mitosis until certain conditions are met. Others, like growth factors and external regulators, speed up, slow down, or stop the cycle.

There are 5 stages during the cell cycle. Interphase, which is made of the G1 phase, the S phase, and the G2 phase, mitosis, and cytokinesis. There are checkpoints between each stage where specialised proteins assure that all is clear to move on.

G-1 (The first gap phase): This stage begins right after mitosis is complete, and a new cell has been made. The cell increases in size and prepares for DNA replication

G-1 checkpoint: As long as there are adequate resources, and the DNA is undamaged, growth factors stimulate the cell to move into the S phase. If it does not move on and does not die it will enter the G0 phase or the resting state.

S-Phase (Synthesis phase): DNA is synthesised via replication

S-Checkpoint: DNA is monitored to assure that synthesis is occurring properly

G-2 (The second gap phase): The cell produces organelles and components the new daughter cell will need to survive

G-2 Checkpoint: All chromosomes are undamaged, and replicated.

M-Phase (Mitosis): This is cell division, and is made of 4 steps. I will go over these steps later. Errors during cell division can be extremely harmful. Cancer is a disease where mitosis continually occurs, and improperly, creating a lump of useless cells that spread and damage the surrounding area.

90% of a cell’s life is spent in Interphase. During this phase, chromosomes will replicate, and the nucleoli will be visible. The nuclear membrane will also be intact.

Mitosis

Remember the Pneumonic: PMAT? These are the 4 stages of Mitosis. 

Prophase

Strands of chromosomes condense and become visible

The nucleoli disappear

Spindle fibres form in the cytoplasm and extend from one centrosome to the other.

Centrioles migrate to the poles

The nuclear membrane starts to disintegrate

Metaphase

Chromosomes line up single file at the equator, also known as the metaphase plate.

Centrosomes have ended up at opposite poles of the cell.

Spindle fibres run from centrosomes to the centromeres of the chromosomes.

Anaphase

The centromeres of each chromosome separate, and as they move, the spindle fibres pull the chromosomes apart

This is the shortest phase of mitosis

Telophase

Chromosomes cluster at opposite ends of the cell

The nuclear membrane re-forms

Chromosomes unravel back into threadlike strands

Cytokinesis

Cytokinesis is not a technical phase of mitosis. Rather, it is just the division of the cytoplasm. In animals, a cleavage furrow forms between the two cells, and the cytoplasm pinches inwards as the two cells separate. In plant cells, a cell plate forms in the middle of the cell. The cells do not separate, instead, a sticky middle lamella cements the cells together.

Meiosis

Meiosis has 2 stages: Meiosis I and Meiosis II, each with their own phases.

Meiosis I

This stage is also referred to as reduction division

Synapsis and crossing-over occur here. In synapsis, chromosomes pair up precisely with their homologue to allow for crossing-over. Crossing-over switches pieces of 2 different chromosomes, creating new blends of chromosomes. This allows for genetic diversity.

Homologous chromosomes separate! When this fails to happen, it is called nondisjunction.

Chromosomes line up randomly on the equatorial plate and separate independently.

Each gamete is genetically unique

Meiosis II

This stage is similar to mitosis

Sister chromatids separate

This stage finishes with 4 haploid cells.

In Metaphase I of Meiosis I, the chromosomes line up double file. In Metaphase II in Meiosis II, they are lined up single file.

#4.   Why Females are Twice Males.

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Mitosis and Meiosis These are basically types of cell division. Cell division occurs by mitosis and meiosis. Mitosis is a general cell division found in every living organism. Meiosis is a type of cell division occurring only in the gametes (sperm and ovary). These actually refer to the division of nucleus. Mitosis Mitosis is a general cell division. How it occurs is that there are four phases of…